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INSERT INTO `Entry` VALUES (1,2,'Question','2013-08-22 22:07:56','What is the reason we have seasons on Earth?&nbsp;'),(2,2,'Script','2013-08-22 22:09:01','I think it is because the Earth gets closer to the Sun at certain times of the year and farther away at others.&nbsp;'),(3,3,'Script','2013-08-22 22:12:20','<div style=\"font-family: arial, helvetica, sans-serif; font-size: 15px; line-height: normal; text-align: left; \"><p style=\"margin-bottom: 0px; padding: 0px; \">The Earth\'s axis is tilted at an angle of about 23.5 degrees away from what would be \"upright\" compared to its orbital plane.&nbsp; At the point on the Earth\'s orbit when the Northern hemisphere is tilted toward the Sun, the Northern Hemisphere has summer.&nbsp; At the same time, the Southern Hemisphere is tilted away from the Sun, and it\'s winter there.&nbsp; The opposite seasons occur six months later, when the Southern Hemisphere is tilted toward the sun (S. Hem summer) and the Northern Hemisphere is tilted away from the Sun (N. Hem winter).&nbsp;</p><div><br></div></div>'),(4,3,'CommentA','2013-08-22 22:25:11','That\'s what most people think: see&nbsp;<a href=\"http://www.youtube.com/watch?v=p0wk4qG2mIg\">http://www.youtube.com/watch?v=p0wk4qG2mIg</a>'),(34,4,'Question','2013-08-23 23:00:50','Don\'t heavier objects actually fall faster because they exert their own gravity? (from: http://physics.stackexchange.com/questions/3534/dont-heavier-objects-actually-fall-faster-because-they-exert-their-own-gravity)'),(35,5,'Script','2013-08-23 23:02:07','The common understanding is that, setting air resistance aside, all objects dropped to Earth fall at the same rate. This is often demonstrated through the thought experiment of cutting a large object in half, the halves of which clearly can\'t then fall more slowly just by being sliced in two.<br><br>However, I believe the answer is that when two objects fall together, attached or not, they do \"fall\" faster than an object of less mass alone does. This is because not only does the Earth accelerate the objects toward itself but the objects also accelerate the Earth toward themselves. Considering the formula:<br><br>`F_g=G(m_1m_2)/d_2`<br>We can see that the force of gravity is dependent on BOTH the masses, not just that of the more massive object.<br><br>Of course in everyday situations, we can for all practical purposes treat objects as falling at the same speed. But I\'m hoping not for a discussion of practicality or what\'s measurable or observable, but what we think is actually happening.<br><br>Am I right or wrong?<br><br>What really clinched this for me was considering dropping a small Moon-massed object close to the Earth and a small Earth-massed object close to the Earth. This made me realize that falling isn\'t one object moving toward some fixed frame of reference, but that the Earth is just another object and falling is multiple objects mutually attracting in space.<br>'),(36,7,'Script','2013-08-23 23:26:44','<div>Using your definition of \"falling,\" heavier objects do fall faster, and here\'s one way to justify it: consider the situation in the frame of reference of the center of mass of the two-body system (CM of the Earth and whatever you\'re dropping on it, for example). Each object exerts a force on the other of</div><div><br></div><div>`F=G(m_1m_2)/r_2`</div><div><br></div><div>where `r=x_2−x_1` (assuming `x2&gt;x1`) is the separation distance. So for object 1, you have</div><div><br></div><div>`G(m_1m_2)/r_2=m_1 ddot x_1`</div><div><br></div><div>and for object 2,</div><div><br></div><div>`G(m_1m_2)/r_2=−m_2 ddot x_2`</div><div><br></div><div>Since object 2 is to the right, it gets pulled to the left, in the negative direction. Canceling common factors and adding these up, you get</div><div><br></div><div>`G(m_1+m_2)/r^2=− ddot r`</div><div><br></div><div>So it\'s clear that when the total mass is larger, the magnitude of the acceleration is larger, meaning that it will take less time for the objects to come together. If you want to see this mathematically, multiply both sides of the equation by `dot rdt` to get</div><div><br></div><div>`G(m_1+m_2)/r^2 dr=− dot r d dot r`</div><div><br></div><div>and integrate,</div><div><br></div><div>`G(m_1+m_2)(1/r−1/r_i)=(dot r^2−dot r_i^2)/2`</div><div><br></div><div>Assuming `dot r_i = 0` (the objects start from relative rest), you can rearrange this to</div><div><br></div><div>`sqrt(2G(m_1+m_2)) dt=−sqrt((r_ir)/(r_i−r))dr`</div><div><br></div><div>where I\'ve chosen the negative square root because `dot r &lt; 0`, and integrate it again to find</div><div><br></div><div>`t=1/(sqrt(2G(m_1+m_2)))(sqrt((r_i)(r_f)(r_i−r_f)))+r_i^(3/2)cos^(−1)sqrt(r_f/r_i)`</div><div><br></div><div>where `r_f` is the final center-to-center separation distance. Notice that `t` is inversely proportional to the total mass, so larger mass translates into a lower collision time.</div><div><br></div><div>In the case of something like the Earth and a bowling ball, one of the masses is much larger, `m_1≫m_2`. So you can approximate the mass dependence of `t` using a Taylor series,</div><div><br></div><div>`1/sqrt(2G(m_1+m_2))=1/sqrt(2Gm_1)(1−1/2m_2/m_1+ ...)`</div><div><br></div><div>The leading term is completely independent of `m_2` (mass of the bowling ball or whatever), and this is why we can say, to a leading order approximation, that all objects fall at the same rate on the Earth\'s surface. For typical objects that might be dropped, the first correction term has a magnitude of a few kilograms divided by the mass of the Earth, which works out to `10^(−24)`. So the inaccuracy introduced by ignoring the motion of the Earth is roughly one part in a trillion trillion, far beyond the sensitivity of any measuring device that exists (or can even be imagined) today.</div>'),(37,6,'Script','2013-08-23 23:30:34','<div>The paradox appears because the \"rest frame\" of the Earth is not an inertial reference frame, it is accelerating. Keep yourself in the CM reference frame and, at least for two bodies, there is no paradox. Given an Earth of mass M, a body of mass `m_i` will fall towards the center of mass `x_(CM)=(Mx_M+(m_ix_i))/(M+m_i)` with an acceleration `(GM)/(x_i−x_M)^2`. Note that `ddot x_(CM)=0`</div><div><br></div><div>Really we have only hidden the paradox, because of course `x_(CM)` is different for each `m_i`. But this is a first step to formulate the problem in a decent inertial frame.</div>'),(38,3,'Script','2013-08-23 23:33:50','The paradox resurfaces again if you want to get rid of `(x_i−x_M)`. In most applications, now that you are in a non accelerating reference system, you want to consider distances related to it, ie `x_i−X_(CM)`. The solution is to redefine the mass. As `x_i−x_(CM)=M(x_i−x_M)/(M+m_i)`, we can say that the object `i ` falls into the Mass Center with an acceleration `GM^3/(M+m_i)^2*1/(x_i−x_(CM))^2` You could say that the actual mass of the \"earth at center of mass\" is this correction.'),(39,4,'Script','2013-08-23 23:38:33','Once you are into the trick of changing the value of the mass, you can still stick to the reference frame of the earth. In this reference frame the quotient between force and acceleration is `Mm_i/M+m_i` You can claim that this is the actual mass of the body during the calculation. This is called the reduced mass `m_r` of the system, and you can see that for small `m_i`, it is almost equal to `m_i` itself. You can ever write some of the previous formulae using the reduced mass `m_r` in combination with the original masses, for instance the above `M^3/(M+m_i)^2=Mm_r^2/m^2`, but I am not sure of how useful it is. In any case, you see that you were right about the \"heavier implies faster\" but that it is perfectly managed.'),(40,11,'Script','2013-08-23 23:48:00','<div>For three objects, `m_1` and `m_2` falling into `M`, the question is how to compare the case to `m_1+m_2` falling into `M`. You separate the forces between internal, between 1 and 2, and external, against `M`. Look at the point `x_0=(m_1*x_1+m_2*x_2)/(m_1+m_2` . This point it is not accelerated by the internal forces. And the external forces move them as</div><div><br></div><div>`ddot x_0=1/(m_1+m_2)*(m_1(GM)/(x_1−x_M)^2+m_2(GM)/(x_2−x_M)^2)=(F_1+F_2)/(m_1+m_2)`</div>'),(41,11,'Question','2013-08-23 23:53:30','<div>Coulomb\'s Law states that the fall-off of the strength of the electrostatic force is inversely proportional to the distance squared of the charges.</div><div><br></div><div>Gauss\'s law implies that a the total flux through a surface completely enclosing a charge is proportional to the total amount of charge.</div><div><br></div><div>If we imagine a two-dimensional world of people who knew Gauss\'s law, they would imagine a surface completely enclosing a charge as a flat circle around the charge. Integrating the flux, they would find that the electrostatic force should be inversely proportional to the distance of the charges, if Gauss\'s law were true in a two-dimensional world.</div><div><br></div><div>However, if they observed a `1/r^2` fall-off, this implies a two-dimensional world is not all there is.</div><div><br></div><div>Is this argument correct? Does the `1/r^2` fall-off imply that there are only three spatial dimensions we live in?</div><div><br></div><div>I want to make sure this is right before I tell this to my friends and they laugh at me.</div><div><br></div><div>http://physics.stackexchange.com/questions/93/does-coulombs-law-with-gausss-law-imply-the-existence-of-only-three-spatial<br></div>'),(42,12,'Script','2013-08-23 23:58:08','<div>Yes, absolutely. In fact, Gauss\'s law is generally considered to be the fundamental law, and Coulomb\'s law is simply a consequence of it (and of the Lorentz force law).</div><div><br></div><div>You can actually simulate a 2D world by using a line charge instead of a point charge, and taking a cross section perpendicular to the line. In this case, you find that the force (or electric field) is proportional to `1/r`, not `1/r^2`, so Gauss\'s law is still perfectly valid.</div><div><br></div><div>I believe the same conclusion can be made from experiments performed in graphene sheets and the like, which are even better simulations of a true 2D universe, but I don\'t know of a specific reference to cite for that.</div>'),(43,13,'Script','2013-08-24 00:00:08','<div>I would say yes !</div><div><br></div><div>Actually some theories explaining quantum gravity use also this reasoning: gravity is a very weak interaction at a quantum level because it \"leaks\" into other dimensions, not observable at our scale, but that are present at this scale.</div><div><br></div><div>The mathematical tools are different, but if you just think about gauss\'s law you can imagine one explanation why additional dimensions are present in these theories.</div>'),(44,14,'Script','2013-08-24 00:01:21','<div>It\'s more the other way around, I would say. Gauss\'s law, together with the fact that we live in a world with 3 spatial dimensions, requires that the force between charges falls off as `1/r^2`. But there are perfectly consistent analogues of electrostatics in worlds with 2 or more spatial dimensions, which each have their own ``Coulomb\'s law\" -- with a different falloff of force with distance.</div><div><br></div><div>More to the point, it\'s a lot more obvious that we live in a world with 3 spatial dimensions (look around!) than it is that the force between charges has an inverse-square law. So empirically, as well as theoretically, the number of spatial dimensions is more fundamental than the force law.</div>'),(45,15,'Script','2013-08-24 00:02:45','<p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Loosely speaking, (super)string theory considers additional spatial dimensions that are \"wrapped up\" (have unusual topologies of high curvature, I believe). Now it is of course complete speculation, but if these dimensions do exist, electromagnetism would not spread out much into those dimensions, hence it would appear as if there are only three dimensions still (to a very good approximation).</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Saying that, your argument is more or less sound (though far from bulletproof). It certainly suggests we don\'t live in a 2D world, and that any possible extra dimensions are comparatively very small!</p>'),(46,3,'Question','2013-08-24 00:05:44','If a book is there on the table, no work is done as no distance is covered. If I hold up a book in my hand and my arm is stretched, if no work is being done, where is my energy going?<div><br></div><div>http://physics.stackexchange.com/questions/1984/why-does-holding-something-up-cost-energy-while-no-work-is-being-done<br></div>'),(47,4,'Script','2013-08-24 00:07:08','<p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">While you do spend some body energy to keep the book lifted, it\'s important to differentiate it from physical effort. They are connected but are not the same. Physical effort depends not only on how<strong style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline;\">much</strong>&nbsp;energy is spent, but also on&nbsp;<strong style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline;\">how</strong>&nbsp;energy is spent.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Holding a book in a stretched arm requires&nbsp;<strong style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline;\">a lot</strong>&nbsp;of physical effort, but it doesn\'t take that much energy.</p><ul style=\"margin-bottom: 1em; margin-left: 30px; border: 0px; vertical-align: baseline; list-style-position: initial; list-style-image: initial; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\"><li style=\"margin: 0px 0px 0.5em; padding: 0px; border: 0px; vertical-align: baseline; line-height: 1.3em; word-wrap: break-word;\"><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word;\">In the&nbsp;<strong style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline;\">ideal</strong>&nbsp;case,&nbsp;<em style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline;\">if you manage to hold your arm perfectly steady, and your muscle cells managed to stay contracted without requiring energy input</em>, there wouldn\'t be any energy spent at all because there wouldn\'t be any distance moved.</p></li><li style=\"margin: 0px 0px 0.5em; padding: 0px; border: 0px; vertical-align: baseline; line-height: 1.3em; word-wrap: break-word;\"><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word;\">On&nbsp;<strong style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline;\">real</strong>&nbsp;scenarios, however, you do spend (chemical) energy stored within your body, but&nbsp;<em style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline;\">where</em>&nbsp;is it spent? It is spent on a cellular level. The only way your muscle cells can stay contracted is by receiving electrical impulses. This electrical energy comes from the chemical energy contained within your molecules. What happens on a cellular level is that the cells contract and relax very quickly, thus resulting in a macroscopic contracted muscle. Since the cells are moving and applying a force, they are doing work. Since they are not efficient machines, they are also releasing a lot of heat. So, finally, the Chemical energy stored within your body is released by the cell as both work and heat.*</p></li></ul><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Both on the ideal and the real scenarios we are talking about the physical definition of energy. On your consideration, you ignore the movement of muscle cells, so you\'re considering the ideal case. A careful analysis of the&nbsp;<em style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline;\">real</em>&nbsp;case leads to the conclusion that work is done and heat is released, even though the arm itself isn\'t moving.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">*&nbsp;<em style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline;\">Ultimately, the work done by the cells is actually done on other cells, which eventually dissipates into heat due to friction and non-elasticity. So all the energy you spend is invested in keeping the muscle tension and eventually dissipated as heat.</em></p>'),(48,5,'Script','2013-08-24 00:08:03','<span style=\"font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">This is about how your muscles work -- they\'re an ensemble of small elements that, triggered by a signal from nerves, use chemical energy to go from less energetical long state to more energetical short one. Yet, this obviously is not permanent and there is spontaneous come back, that must be compensated by another trigger. This way there are numerous stretches and releases that in sum gives small oscillations that create macroscopic work on the weight.</span>'),(49,6,'Script','2013-08-26 16:43:58','Perhaps an analogy is in order. Lets hold up the book by using an electromagnet (say we put a piece if steel under it ). If the coils were made of superconducting material it would take no energy input to maintain the position/field strength. But if we use ordinary wire, ohmic loses within the coil must be made up for by externally supplied electrical energy.'),(50,7,'Script','2013-08-26 16:46:41','<div>The reason is that you need to spend energy to keep muscle stretched.</div><div><br></div><div>The first thing you need know is that the work `W=FDeltax` is the energy transfer between objects. Hence, there are no work done on the book when it is put on the table because there are no movement.</div><div><br></div><div>When your arm muscle is stretched, however, it consumes energy continuously to keep this state so you feel tire very fast. This energy comes from the chemical energy in your body and most of them are converted into heat and lost to the surrounding. In this situation, no energy is transferred to the book, so no work is done.</div><div><br></div><div>You can feel the different energy consumption when your arm is stretched in different angle. A particular case is that you put the book on your leg when you sit on a chair so your muscle is relaxed and the energy spent is less.</div><div><br></div><div>There are also a special type of muscle, smooth muscle, requires very little energy to keep its state so that it can always keep it stretched and you won\'t get tire:</div>'),(51,11,'Script','2013-08-26 16:48:03','<p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">When a physicist talks about work, they are using the word in the technical sense of the equation you quote. To a biologist, though, work might be defined as energy expended to carry out a task. In your example, your arm will not naturally stay in the position described. Your body (mostly your muscles) must expend energy to hold your arm (and the book) in a set position, unsupported by anything but your own physiology.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">So, by the biologist\'s definition, your muscles are doing work to hold up the book and your arm (muscle fibers are contracting and relaxing based on a host of chemical processes at the cellular level). But by the physicist\'s technical definition, no work is being done.</p>'),(52,12,'Question','2013-08-26 16:50:26','<span style=\"font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">If you could travel to the center of the Earth (or any planet), would you be weightless there?</span><div><span style=\"font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\"><br></span></div><div><font face=\"Helvetica Neue, Arial, sans-serif\"><span style=\"line-height: 18.1875px;\">http://physics.stackexchange.com/questions/2481/would-you-be-weightless-at-the-center-of-the-earth/2482#2482</span></font><br></div>'),(53,13,'Script','2013-08-26 16:50:55','<span style=\"font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Correct. If you split the earth up into spherical shells, then the gravity from the shells \"above\" you cancels out, and you only feel the shells \"below\" you. When you are in the middle there is nothing \"below\" you.</span>'),(54,14,'Script','2013-08-26 16:51:26','<p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">The simplest way to think about it is that there is mass all around you in the center of the Earth so you get an equal gravitational \"pull\" from all directions. The pulls cancel out so you get no acceleration.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">If one assumes constant density for the Earth (which isn\'t strictly speaking true but it is close enough for this illustration) the gravitational acceleration drops linearly from 1g at the surface to 0 at the center of the Earth. So you\'d get a zero if you stepped on a scale at the center of the Earth.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">The more complicated explanation is that acceleration due to gravity is the derivative of the gravitational potential. This potential is a minimum at the center of the Earth and grows quadratically up to the surface. It then continues to increase at a lower rate. Since at the exact center is flat (like the bottom of a valley), the derivative which is a measure of the rate of change is zero, and there is no acceleration.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Interestingly, even though you would be weightless there, the effects of gravity are highest at the center of the Earth. You get more gravitational time dilation, for example, than you do at the surface.</p>'),(55,15,'Script','2013-08-26 16:52:03','<span style=\"font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">I like answers that appeal to symmetry, so I answer this one with a question: If you were at the center, which way would you fall? That tells us you could stay floating there.</span>'),(57,3,'Script','2013-08-26 16:53:52','<div>You would not be weightless at the center of the Earth. In other words, the Earth does not follow a geodesic. Let me explain.</div><div><br></div><div>The Earth is not spherical, it is an oblate spheroid. The acceleration of a uniform non-spherical body in a spherical gravitational field does not follow an inverse square law. The acceleration of the center of mass does not equal the acceleration at the center of mass. An accelerometer fixed at the center of the Earth would read approx 1.75 pgal (`1.75e-14 m/s^2`), not zero.</div>'),(58,4,'Question','2013-08-26 16:57:50','<span style=\"font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">I can understand why 2 protons will repel each other, because they\'re both positive. But there isn\'t a neutral charge is there? So why do neutrons repel (do they even or Have I been misinformed?)</span><div><span style=\"font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\"><br></span></div><div><font face=\"Helvetica Neue, Arial, sans-serif\"><span style=\"line-height: 18.1875px;\">http://physics.stackexchange.com/questions/78/why-do-neutrons-repel-each-other/701#701</span></font><br></div>'),(59,5,'Script','2013-08-26 16:59:50','<span style=\"font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Neutrons have spin 1/2 and therefore obey the pauli exclusion principle, meaning two neutrons cannot occupy the same space at the same time. When two neutrons\' wavefunctions overlap, they feel a strong repulsive force.</span>'),(60,6,'Script','2013-08-26 17:00:49','<p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Neutrons (and protons) being spin 1/2 fermions, must fit antisymmetric wavefunctions. This \"wavefunction\" doesn\'t always involve waves, though. For nucleons - the generic term for neutron or proton - this wavefunction for the pair is a produce of (1) a spatial part, (2) a spin part, and (3) an isospin part.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">The isospin part is a clever way to describe charge possibilities of otherwise identical particles. We regard neutrons and protons as being in a sense identical. Just as a spin 1/2 particle can be \"up\" or \"down\" along some chosen axis, so is an isospin 1/2 particle can be \"up\" or \"down\" along an abstract mathematical axis - it\'s exactly the same SU(2) math as spin - but it plays out in physical reality as charge. For nucleons it\'s not +1/2 and -1/2 charge but with an offset, so we have +1 (proton) and 0 (neutron). This idea is from Heisenberg in 1932.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Now, how can the overall wavefunction of a pair of particles be antisymmetric? There are three factors - right away we can imagine three possibilites: any one factor being antisymmetric with the other two symmetric. We could also have all three antisymmetric at the same time.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">An antisymmetric spatial wavefunction would have a node, like an atomic p orbital, like the electric potential around a dipole antenna. This is a higher energy state than a simple spherical blog, a Gaussian. Given the range of internuclear forces, this nodal antisymmetric wavefunction has more energy than if the two nucleons just stayed apart. This is a matter of radial or angular kinetic energy has to be either \"zero\" or some quantized value that exceeds \"escape velocity\" So forget that part of the system wavefunction being antisymmetric.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">BTW, we don\'t have separate spatial wavefunctions for the two nucleons - whatever one does, the partner does the exact opposite, like a two-body celestial mechanics problem. They orbit a common barycenter.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">The spin part could be antisymmetric. This is a bit tricky. If particle #1 is up and #2 is down, we can write \"UD\". There is also \"DU\". We form the spin part of the wavefunction for the pair as UD-DU. We could instead choose UD+DU but note this is symmetric. So are UU and DD. Just how UD-DU differs from UD+DU may mystify beginners in quantum mechanics, but it\'s important, and it\'s how physical matter works whether we humans like it or not. (You might also see where the \'u\' and \'d\' quarks got their names. The quark idea came along years after isospin.)</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Neither D nor U is really a wave or a function; they\'re at most just rows and columns in matrices if you must represent them in familiar math. Otherwise quantum physicists just deal with these symbolically. Still the jargon is \"wavefunction\" - we silly humans and our primitive scientific language!</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">The same math applies for isospin. But the physics differs. We\'ve concluded that the spatial part of the system wavefunction must be symmetric, so it\'s up to either the spin part or the isospin part to be antisymmetric. But not both! If spins are symmetric - they\'re parallel. This is experimentally the case - the Deuteron (obtain by distilling \"heavy water\" from water) - so we deduce that the isospin part is antisymmetric. That is, we must have one isospin \"up\" and one isospin \"down\" - neutron and a proton, not two neutrons or two protons.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Just why must the spins of the two nucleons be parallel? The strong force holding them together - the exchange of pions, kaons and other mesons - works better in that case. To explain that takes deeper analysis than I can go into here. When the spins are antiparallel, there\'s not enough force to keep the nucleons together.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">This would be the case though, were to try pushing two neutrons together. They\'d be both isospin \"up\" therefore symmetric isospin part of the wavefunction, therefore requiring an antisymmetric spin part, which leads to the pions and their buddies not getting as good a grip on the neutrons, which drift off going their separate ways.</p>'),(61,7,'Script','2013-08-26 17:01:39','<span style=\"font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">I think this may be a summary of other answers, but there are a couple things going on here. First, neutrons are electrically neutral, so an obvious force is the Van der Waals force. However, due to quantum mechanics and the Pauli exclusion principle (as noted above), neutrons\' wave functions cannot (ish) overlap and so they are subject to the&nbsp;</span><a href=\"http://en.wikipedia.org/wiki/Degenerate_matter#Neutron_degeneracy\" rel=\"nofollow\" style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; cursor: pointer; color: rgb(31, 108, 130); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Neutron Degeneracy Pressure</a><span style=\"font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">. I think if you understand these two concepts, then you should have a fairly good grasp on what is going on between neutrons in a neutron star.</span>'),(62,11,'Script','2013-08-26 17:04:14','<span style=\"font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Neutrons consist of&nbsp;</span><a href=\"http://en.wikipedia.org/wiki/Quarks\" rel=\"nofollow\" style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; cursor: pointer; color: rgb(31, 108, 130); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">quarks</a><span style=\"font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">&nbsp;that are electrically charged, so when two neutrons get close enough to each other the higher electrical multipole moments will become relevant and cause repelling. But also note the magnetic moment and the strong force Cedric mentioned plus the Pauli exclusion mentioned by nibot.</span>'),(64,12,'Script','2013-08-26 17:06:19','<p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">They repel because of the same reason why, say, atoms or molecules repel: because of Pauli principle which is the basis of elastiсity and solidity of matter. Why you cannot move through the walls? Why the dish holds food? Because all these objects are composed of fermions, the particles of substance.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">This is opposed to say, light which can go through another light ray without any interaction. Because light is composed of bosons - the quants of radiation.</p>'),(65,13,'Question','2013-08-26 17:11:01','<p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">My science teacher is always saying the words \"<a href=\"http://en.wikipedia.org/wiki/Weight\" style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; cursor: pointer; color: rgb(31, 108, 130);\">weight</a>&nbsp;of an object\" and \"<a href=\"http://en.wikipedia.org/wiki/Mass\" style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; cursor: pointer; color: rgb(31, 108, 130);\">mass</a>&nbsp;of an object,\" but then my physics book (that I read on my own) tells me completely different definitions from the way these words are used in my science class... so which is right?</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">What is the difference between the weight of an object and the mass of an object?</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word;\"><font face=\"Helvetica Neue, Arial, sans-serif\"><span style=\"line-height: 18.1875px;\">http://physics.stackexchange.com/questions/43195/what-is-the-difference-between-weight-and-mass/43196#43196</span></font><br></p>'),(66,14,'Script','2013-08-26 17:14:21','<p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Weight is the force with which gravity pulls on a mass.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Maybe the simplest way to explain the difference is that on the Moon or on Mars, your&nbsp;<em style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline;\">weight</em>&nbsp;is reduced because gravity is weaker there, but your&nbsp;<em style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline;\">mass</em>&nbsp;is still the same.</p>'),(67,15,'Script','2013-08-26 17:15:27','<div>The mass, strictly the inertial mass, relates the acceleration of a body to the applied force via Newton\'s law:</div><div><br></div><div>`F=ma`</div><div>So if you apply a force of 1 Newton to a mass of 1kg it will accelerate at `1m/s^2`. This is true whether the object is floating in space or in a gravity field e.g. at the Earth\'s surface.</div><div><br></div><div>The weight is the force a body exerts when it is in a gravitational field. The weight depends on the gravitational field. For example the weight of a 1kg mass at the Earth\'s surface is 9.81 Newtons, while at the surface of Mars it\'s about 3.5 Newtons.</div><div><br></div><div>This is possibly a bit too much info: if so ignore this last paragraph. Although weight specifically means the force exerted in a gravitational field, Einstein told us that sitting stationary in a gravitational field is equivalent to being accelerated in the absence of gravity. The inertial mass defined using Newton\'s laws is the same as the gravitational mass defined by the force a body exerts in a gravitational field. So if you take a 1kg mass at the Earth\'s surface, the weight of 9.81 Newtons it exerts is exactly the same as the force you\'d need to accelerate the 1kg mass at `9.81m/s^2`.</div>'),(68,3,'Script','2013-08-26 17:17:08','<p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Yes of course, According to physics the Mass and Weight are different from each other. Following is their main difference,</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\"><strong style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline;\">Mass</strong>:</p><ul style=\"margin-bottom: 1em; margin-left: 30px; border: 0px; vertical-align: baseline; list-style-position: initial; list-style-image: initial; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\"><li style=\"margin: 0px 0px 0.5em; padding: 0px; border: 0px; vertical-align: baseline; line-height: 1.3em; word-wrap: break-word;\"><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word;\">Mass is the amount of matter contained in a body.</p></li><li style=\"margin: 0px 0px 0.5em; padding: 0px; border: 0px; vertical-align: baseline; line-height: 1.3em; word-wrap: break-word;\"><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word;\">Mass of the body is the constant quantity and does not change with the change of position or location.</p></li></ul><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\"><strong style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline;\">Weight</strong>:</p><ul style=\"margin-bottom: 1em; margin-left: 30px; border: 0px; vertical-align: baseline; list-style-position: initial; list-style-image: initial; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\"><li style=\"margin: 0px 0px 0.5em; padding: 0px; border: 0px; vertical-align: baseline; line-height: 1.3em; word-wrap: break-word;\">Weight is the force by which the earth attracts a body toward its centre, Or we can say it is the force on the object which is offered by the gravity</li><li style=\"margin: 0px 0px 0.5em; padding: 0px; border: 0px; vertical-align: baseline; line-height: 1.3em; word-wrap: break-word;\">Weight of the body is the variable quantity and changes with the change in position and location due to the acceleration of the gravity acting on it. Yes they are used at different places and time.</li></ul><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">So these are the basic difference between&nbsp;<strong style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline;\">Weight</strong>&nbsp;and&nbsp;<strong style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline;\">Mass</strong>&nbsp;of the object</p>'),(69,4,'Script','2013-08-26 17:18:59','<div>Mass is a constant for each object, meaning that the mass won\'t change unless the object changes.</div><div><br></div><div>On the other hand Weight changes due to the gravity; any object\'s weight is the force pulling it down to the ground. The force can be determined like so: `F=MA`, where `F` is the force, `M` is the mass and `A` is the acceleration (the gravity in case of weight). So `W=MG`, where `W` is the weight, `M` is the mass and `G` is the gravity.</div><div><br></div><div>Btw, weight is in Newtons, while Mass is in kilograms.</div><div><br></div><div>1Newton=`1Kgm∗m/s^2`</div>'),(71,5,'Script','2013-08-26 17:21:10','<div>To elaborate on John Rennie\'s answer - As he said, the mass is the inertial mass of the body, which isn\'t the same as the weight. The weight is typically defined in context to a gravitational field.</div><div><br></div><div>No doubt you know that the acceleration due to gravity on the earth\'s surface is `9.8m/s^2` (on average). So if your mass on earth is say, 5 kilograms, your weight on earth would be 9.8×5=49N. So the weight that your weighing machine registers is your weight, not your mass. You\'d have to divide by the acceleration due to gravity at that point to calculate the mass.</div><div><br></div><div>So basically your weight basically measures the force that you exert on the weighing scale, not the actual mass of your body. Which is why if you stand on a weighing scale in free fall it\'ll register zero, since you aren\'t exerting any additional force on it.</div>'),(72,6,'Question','2013-08-26 17:30:01','<div>I can\'t seem to figure out the relationship between `E_k` and `p` or `F`. I understand that the units are pretty different. But for example:</div><div><br></div><div>A bullet with a mass of 10.0g is moving at the speed of 1000m/s. A bull with a mass of 400kg is charging at the speed of 5.00m/s Which has the greater kinetic energy?</div><div><br></div><div>5000J for the bullet and 5000J for the bull</div><div><br></div><div>That worked fine but now if you calculate momentum you get:</div><div>`p=10kg*m/s` for the bullet and `p=2000kg*m/s` for the bull.</div><div>Which does not make sense!</div><div><br></div><div>Does the bull hit harder than the bullet or the same amount as the bullet?</div><div><br></div><div>http://physics.stackexchange.com/questions/10710/what-is-the-relationship-between-kinetic-energy-and-momentum/10711#10711<br></div>'),(73,7,'Script','2013-08-26 17:32:18','<div>If you write down the formulas for kinetic energy,</div><div>`E_k=1/2*mv^2`</div><div>and momentum</div><div>`p=mv`</div><div>you see that you can write the energy in terms of momentum via</div><div>`E_k=p^2/(2m)`</div><div>So, if two objects have the same energy `E_k`, they only have the same momentum if they also have the same mass. Since the bull has a much larger mass than the bullet, it must therefore have a much larger momentum than the bullet to arrive at the same kinetic energy.</div><div><br></div><div>Force is change of momentum with time, `F=dot p`. If we assume that the bullet and the bull hit a target and come to rest in the same time, the bull hits much harder, as it has the higher momentum.</div>'),(74,11,'Script','2013-08-26 17:34:37','<div>If we apply a given force `F` for a given time t, it changes the momentum by `F_t`. If in that time the force has moved whatever it\'s pushing by a distance s, it changes the energy by `F_s`. If we push a light thing and a heavy thing equally hard for the same amount of time --so they both have the same momentum-- the light thing will be pushed farther in the same time because it accelerates more, so it has more energy. Equally, if we push them both equally hard until they have the same energy --that is, for the same distance, remembering that in this kind of Physics the bull puts up no resistance other than its mass (and it may even be a Bull Sphere)-- the light thing will have less momentum, because it will take less time to get there.</div><div><br></div><div>When it comes to stopping the bull, you have to keep up a force that\'s trying to stop it for much more time, the bull <i>keeps on coming</i>, whereas the same force will stop the bullet in the same distance.</div><div><br></div><div>There\'s a fourth concept that also has to be considered, which is pressure. Sorry! Force, Energy, Momentum, and Pressure are all different. To apply a force to the small area that a bullet has is different from applying the same force to the big area that the bull has. It\'s more difficult, it needs more pressure, to apply the same force to a smaller area.</div>'),(75,12,'Question','2013-08-26 17:36:33','<span style=\"font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">This is a confused part ever since I started learning electricity. What is the difference between voltage and electromotive force (emf)? Both of them have the same SI unit, right? I would appreciate an answer.</span><div><span style=\"font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\"><br></span></div><div><font face=\"Helvetica Neue, Arial, sans-serif\"><span style=\"line-height: 18.1875px;\">http://physics.stackexchange.com/questions/15402/what-is-the-difference-between-electric-potential-voltage-and-electromotive-for</span></font><br></div>'),(76,13,'Script','2013-08-26 17:53:52','<div>I don\'t like the term \"voltage\". It seems to mean anything measured in volts. I\'d rather say electric potential and electromotive force.</div><div><br></div><div>And the two are fundamentally different.</div><div><br></div><div>Electrostatic field is conservative, that is, over any loop l we have `oint_lvecE ⋅dvecl =0`. In other words, the line integral of electrostatic field does not depend on the path, but only on end points. So we can define point by point a scalar value electrostatic potential&nbsp;`phi`, such that</div><div>`phi_A−phi_B=int_A^BvecE * dvecl` ,</div><div><br></div><div>or</div><div><br></div><div>`q(phi_A−phi_B)=int_A^BqvecE * dvecl` ,</div><div>so `qDeltaphi` equals the work done by electrostatic force.</div><div><br></div><div>In pratical application, electrons (and other carriers) flow in circuits. Since electrostatic field is conservative, it alone cannot move electrons in circles; it can only move them from lower potential to higher potential. You need another kind of force to move them from higher potential to lower ones in order to complete a cycle. This other force could be chemical, magnetic or even electric (vortex electric field, different from electrostatic field), and their equivalent contribution is called electromotive force.</div><div>E.M.F.=`int_(Circuit)vecF/q * dvecl`&nbsp;</div>'),(77,14,'Script','2013-08-26 17:59:24','<div>Anyway the simple answer is e.m.f. is not a force in the mechanical sense. It measures the amount of work to be done for a unit charge to travel in a closed loop of a conducting material.</div><div><br></div><div>Let\'s make it more clear. In static case (ignoring time variation of any magnetic field), electric field at a point can be derived solely from a scalar as the negative of the gradient of this scalar. This scalar at any point is called the \"electric potential\" at that point. If two points are at different potentials then we say there exists a potential difference. Obviously it is the difference in the potentials that matters and not their absolute values. One can therefore arbitrarily assign a value zero for some fixed point who\'s potential may be considered constant and compare the potentials of other points with respect to it. In this way one need not have to always speak of potential difference but simply potentials.</div><div><br></div><div>Now, often this \"electric potential\" at some point in a conductor or a dielectric is called \"voltage\" at that point assigning the value of the voltage to be zero for earth since the potential of earth is constant for all practical purposes.</div><div><br></div><div>If there is no variation of magnetic field then the work done by an unit charge in a closed loop will be 0. But if the magnetic field varies then it will be non zero. Recall the formula</div><div>`nablaxxE=(deltaB)/(deltat)`</div><div>What it really implies is, it is impossible for an electric field, derived solely from a scalar potential, to maintain an electric current in a closed circuit. So an e.m.f. implies presence of some source other then a source which can only produce a scalar potential.</div><div><br></div><div>The following equation tells the whole story:</div><div><br></div><div>`E=−nablaphi−(deltaA)/(deltat)`</div><div>`phi` is the scalar potential and `A` is the vector potential.</div>'),(78,15,'Script','2013-08-26 18:00:20','<p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Actually these are are same thing but usage is at different places.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Whenever we talk about batteries or a DC system, we use the Potential difference, as there is potential difference of 3.7 Volt.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">The phrase \"electro-motive force\" (EMF) is used when a conductor cuts the flux inside the machine (Transformer, Generator, etc)</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Voltage is used as Output from an electrical machine.</p>'),(79,3,'Script','2013-08-26 18:01:17','<span style=\"font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Electromotive force (Note; not a force) is simply the source of voltage in a circuit.</span>'),(80,4,'Script','2013-08-26 18:02:43','<span style=\"font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Voltage is a potential difference, due to the energy dissipation. Emf is a potential difference, due to the energy generation.</span>'),(81,5,'Question','2013-08-26 18:08:58','<p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Let us suppose that we have a known electromagnetic wave-train of finite size propagating in a certain direction. There is a probe charge on its way. This EMW is an external field for the charge. The EMW has a certain energy-momentum (integral over the whole space). After acting on the probe charge the wave continues its way away. In the end we have the energy of the initial wave (displaced somewhere father), the kinetic energy of the charge (hopefully it starts moving), and the energy of the radiated EMF propagating in other directions. Thus the total energy may become different from the initial one. How to show that the total energy is conserved in this case?</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">It is not a Compton scattering. Just a regular electrodynamics problem. How EM energy can change appropriately? Via destructive interference? How to show it if the incident field is a known function of space-time?</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word;\"><font face=\"Helvetica Neue, Arial, sans-serif\"><span style=\"line-height: 18.1875px;\">http://physics.stackexchange.com/questions/9084/energy-conservation-in-electrodynamics/9087#9087</span></font><br></p>'),(82,6,'Script','2013-08-26 18:13:40','<div>The stress energy tensor `T^(munu)` contains all the energy/momentum components of the electromagnetic field and the conservation of these components is expressed by</div><div><br></div><div>`deltanuT^(munu)=0`</div><div>Which states that the change in time of energy/momentum is zero. If the above is non-zero then electromagnetic field energy/momentum is transferred to charged matter and in this case the conservation law becomes.</div><div><br></div><div>`deltanuT^(munu)+nu^(mup)f_p=0`</div><div>Where f is the force density four vector acting on the charge matter. If we talk specifically about the energy, as in your case, then `f_0` is given by `vecJ * vecE` &nbsp; representing charged matter moving up or down a potential field which causes a change in time of the potential energy of the charged matter.</div>'),(83,7,'Script','2013-08-26 18:14:48','<span style=\"font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">The proof that classical electrodynamics conserves energy is found in all sorts of textbooks. I\'d start with Griffiths\'s&nbsp;</span><em style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Introduction to Electrodynamics</em><span style=\"font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">, and go on to Jackson\'s&nbsp;</span><em style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">Classical Electrodynamics</em><span style=\"font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 18.1875px;\">&nbsp;if you want more.</span>'),(84,11,'Script','2013-08-26 18:15:53','<div>In order for energy to be conserved in a model, one must use a translation invariant formalism, then, with other assumptions about the space of models, one can prove Noether\'s theorem. As soon as one says that some part of the interaction is \"external\", one is working in a formalism in which energy will not be conserved. Making the EMW internal to the system is your first step.</div><div><br></div>'),(85,8,'Question','2013-08-26 18:19:12','I mean to ask why there is 4`pi` present in force equations governing electricity? Though all objects in universe are not spherical and circular, the constant of proportionality in both equations contain 4`pi`. Why?<div><br></div><div>http://physics.stackexchange.com/questions/74254/why-is-there-a-factor-of-4-pi-in-certain-force-equations/74259#74259<br></div>'),(86,11,'Script','2013-08-26 18:23:15','<div>I suppose you mean `k_e=1/(4piepsilon_0)`. That comes from the fact that Coulomb\'s law can be stated as</div><div><br></div><div>`F=1/(epsilon_0) * 1/(4pir^2)q_1q_2`</div><div>Now, `epsilon_0` is the electric constant, or the permittivity of free space, and it essentially scales the force. The `4pir^2` comes from the surface of the sphere. I.e., as the EM field goes further away, it becomes diluted over the surface of a sphere.</div>'),(87,12,'Script','2013-08-26 18:27:44','<div>If you want to avoid factors of `pi` in the more fundamental equations like `nabla*E=rho/epsilon_0`, you have to accept them where they belong, for instance in: `E=1/(epsilon_0) * Q/(4pir^2)`.</div><div><br></div><div>As remarked by others, Newton failed to put a factor `4pi` into his gravitation equation (he stipulated `g=GM/r^2`, instead of `g=GM/(4pir^2)`) and as a result we have to live with factors of `pi` in the more fundamental Gauss\' law for gravity, and more importantly, also in Einstein\'s theory of gravity.</div>'),(88,13,'Script','2013-08-26 18:37:11','<div>The physical reason for the appearance of a `4pi` somewhere in the theory is the spherical symmetry of the problem and is discussed more in other answers . Here I want to quote an interesting argument from Arnold Sommerfeld\'s Lectures on Theoretical Physics Vol III, which has a section dedicated to this issue.</div><div><br></div><div>If you remove the `4pi` from the force law you will have it in more fundamental Maxwell\'s equation:</div><div><br></div><div>`nabla*D=4pirho`</div><div>and also it will distorts the energy density into:</div><div>`W=1/(8pi)E*D`</div><div>But Heaviside, as is said in the mentioned book, who fought a life-long battle for the rational units (`4pi` present in the force law), has another interesting argument about advantage of this system to others. He points to the capacity of a capacitor:</div><div><br></div><div>A plate capacitor (with area `A` , plate separation `d`) in this rational units (`4pi` present in the force law) and in other units (with `4pi` present in the Maxwell\'s equations) has a capacity of</div><div>`C=(epsilonA)/d`(rational)</div><div>`C=(epsilonA)/(4pid)`(others)</div><div>and for a spherical capacitor (radius `R`, outer sphere imagined at infinity):</div><div>`C=4piepsilonR`(rational)</div><div>`C=epsilonR`(others)</div><div>We see that with rational units the factor 4π appears for the sphere. For other units it is missing for the sphere and appears for the plane capacitor.</div><div><br></div><div>Heaviside then makes the following striking comparison: In passing from the measurement of distance to the measurement of area one might define as unit of area the area of a circle of radius 1. This would be logically possible. It would however lead to the strange result that a square with the side 1 would have the area 1`pi`. Everyone would then say that π was at the wrong place. We said the same of the factor 4`pi` in the above formulas for capacities.</div>'),(89,14,'Script','2013-08-26 18:39:46','<div>The main reason is that it makes the calculation easier and the results look nicer. For example, suppose the field (or force) is given by</div><div>`E=1/(4pi)f(r)`</div><div>for some function `f(r)`.</div><div><br></div><div>If the system process rotational symmetry, after sum over the density, you will get a factor of `2pi`. If the system process spherical symmetry, you will get a factor of `4pi`. In both case, the resulting equation contains no `pi` factor. It is particular useful in EM as we usually consider a density distribution process some symmetry.</div><div><br></div><div>So, why there is no `pi` in the Newtonian gravity `F=G(Mm)/r^2` ?</div><div><br></div><div>Because there is no such need. You cannot craft a planet like macroscopic object into a infinitely long rod, a disk shape, or some funny shape. Gravity only have significant effect when it accumulates a large mass, at the same time, it become a sphere by its own gravity. So, basically, it is already a good point like particle when we look at it some radius away. The extra `pi` won\'t make any calculation easier.</div>'),(90,15,'Script','2013-08-26 18:44:15','<p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word;\"><font face=\"Helvetica Neue, Arial, sans-serif\"><span style=\"line-height: 18.1875px;\">Some people (myself included) would regard field equations like Gauss\' Law as more \"fundamental\" than force equations. The most obvious reason for this is that Coulomb\'s Force Law only works when the charges in question are held static - it has to be modified once they are allowed to move.</span></font></p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word;\"><font face=\"Helvetica Neue, Arial, sans-serif\"><span style=\"line-height: 18.1875px;\">As the others have stated, the `4pi` has to do with the surface area of a sphere. Gauss\' Law (I\'ll use the integral form to make the surface area connection more apparent) for the electric field tells us:</span></font></p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word;\"><font face=\"Helvetica Neue, Arial, sans-serif\"><span style=\"line-height: 18.1875px;\">`intE * dA= q/epsilon_0`</span></font></p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word;\"><font face=\"Helvetica Neue, Arial, sans-serif\"><span style=\"line-height: 18.1875px;\">In words, this means that if you enclose some charges in an imaginary surface, then the sum of the electric field sticking out of that surface multiplied by the area of the surface is equal to the total charge enclosed by it, multiplied by some constant factor. If you take a single point charge and enclose it with a spherical surface of radius `r`, then it reduces to:</span></font></p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word;\"><font face=\"Helvetica Neue, Arial, sans-serif\"><span style=\"line-height: 18.1875px;\">`4πr^2E=q/epsilon_0`</span></font></p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word;\"><font face=\"Helvetica Neue, Arial, sans-serif\"><span style=\"line-height: 18.1875px;\">Do some rearranging and it\'s easy enough to see how it relates to Coulomb\'s Law.</span></font></p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word;\"><font face=\"Helvetica Neue, Arial, sans-serif\"><span style=\"line-height: 18.1875px;\">There\'s also a form of Gauss\' Law for (Newtonian) gravity:</span></font></p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word;\"><font face=\"Helvetica Neue, Arial, sans-serif\"><span style=\"line-height: 18.1875px;\">`intg*dA=4piGM`</span></font></p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word;\"><font face=\"Helvetica Neue, Arial, sans-serif\"><span style=\"line-height: 18.1875px;\">This field equation actually contains the factor `4pi` already, so when you enclose a mass with a spherical surface the factor cancels on both sides. This is simply because when Newton wrote down his force law for gravity he didn\'t know about things like Gauss\' Law, and so neglected to include the 4`pi` in the force equation. And since then the convention has stuck around, so we\'re left with a slightly confusing hodgepodge of some field equations needing factors of `pi` and some not. In general, if you see a factor of `pi` in a field equation then you\'re probably looking at something gravity-related.</span></font></p>'),(91,15,'CommentQ','2013-08-26 20:41:32','<span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(255, 255, 255);\">Yes, but since coulomb\'s law is only verified for large(compared to the planck-scale) distances, it does not exclude the existence of very small spacial dimensions like they are used in some theories beyond the standard model.</span>'),(92,15,'CommentA','2013-08-26 20:42:10','<span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(250, 250, 250);\">Gauss law is derived from Couloumb\'s law, not the other way around.</span>'),(93,12,'CommentA','2013-08-26 20:43:20','<span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(250, 250, 250);\">No, sorry, but that\'s really not true. The derivation can go both ways - look at the introduction of the Wikipedia article for Gauss\'s law, for example, and notice that the citation to the HRW textbook is for the derivation of Coulomb\'s law from Gauss\'s law. I get the sense that your education in physics is somewhat limited (forgive me if I\'m wrong; it can be hard to tell online), but if you had studied electromagnetism you would know that Gauss\'s law, being one of Maxwell\'s equations, is considered part of the foundation of the subject. Not so with Coulomb\'s law.</span>'),(94,15,'CommentA','2013-08-26 20:44:29','<div>I know that the derivation can go both ways, however, the second derivation (Gauss\'s law-&gt;Coulomb\'s law) is not as strict and fundamental as the first one. Precisely, quoting from the Wikipedia article: \"Strictly speaking, Coulomb\'s law cannot be derived from Gauss\'s law alone, since Gauss\'s law does not give any information regarding the curl of E (see Helmholtz decomposition and Faraday\'s law). However, Coulomb\'s law can be proven from Gauss\'s law if it is assumed, in addition, that the electric field from a point charge is spherically-symmetric (this assumption, like Coulomb\'s law itself,</div><div><br></div>'),(95,12,'CommentA','2013-08-26 20:45:26','<span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(250, 250, 250);\">Maxwell\'s Equations (which include Gauss\'s law) are generally regarded as the fundamental laws of classical electrodynamics. That said, Coulomb\'s law came first historically.</span>'),(96,14,'CommentQ','2013-08-26 20:47:43','<span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(250, 250, 250);\">Good question! It\'s really a bit of biophysics, and it\'s not immediately obvious.</span>'),(97,14,'CommentA','2013-08-26 20:48:28','<span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(250, 250, 250);\">Just like real scenario , what happens in solid objects like table. From where does the molecules get energy to remain in straight and hold up the weight of book</span>'),(98,4,'CommentA','2013-08-26 20:49:34','<span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(250, 250, 250);\">Just like real scenario , what happens in solid objects like table. From where does the molecules get energy to remain in straight and hold up the weight of book</span>'),(99,11,'CommentQ','2013-08-26 20:51:33','<span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(255, 255, 255);\">You forgot to ask for meaning of \"potential\"</span>'),(100,11,'CommentA','2013-08-26 20:52:20','<span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(250, 250, 250);\">your explanation (which repeats what I said regarding useful work) is confusing because it doesn\'t account for the difference in potentials when the circuit is not closed in a loop and which is called alternatively emf or voltage.</span>'),(102,13,'CommentA','2013-08-26 20:55:24','<span class=\"comment-copy\" style=\"margin: 0px; padding: 0px; border: 0px; font-size: 13px; vertical-align: baseline; color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 17px; background-color: rgb(255, 255, 255);\">Well, my explanation may be confusing, but potential and emf are fundamentally different. Even when the circuit is not closed, potential difference is not the same as emf.</span><span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(255, 255, 255);\">&nbsp;</span>'),(103,3,'CommentQ','2013-08-26 21:02:31','Why do You think Your result \"does not make sense\"? I\'m quite comfortable with it.'),(104,6,'CommentQ','2013-08-26 21:05:15','I don\'t understand it. Kinetic energy, force, and momentum are all similar kind of the same thing, right? I think i\'m missing something.'),(105,3,'CommentQ','2013-08-26 21:05:57','<span class=\"comment-copy\" style=\"margin: 0px; padding: 0px; border: 0px; font-size: 13px; vertical-align: baseline; color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 17px; background-color: rgb(250, 250, 250);\">\"\" Kinetic energy, force, and momentum are all similar kind of the same thing, right? \"\" Not at all. Kinetic energy is mass times velocity squared, momentum is mass times velocity. What makes You to throw in force here, is something I do not understand.</span><span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(250, 250, 250);\">&nbsp;</span>'),(106,3,'CommentA','2013-08-26 21:06:54','Thanks a lot! it makes more sense now'),(107,4,'CommentA','2013-08-26 21:09:46','<span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(255, 255, 255);\">The answer is bit over my head, could you explain&nbsp;</span><i style=\"margin: 0px; padding: 0px; border: 0px; font-size: 13px; vertical-align: baseline; color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 17px; background-color: rgb(255, 255, 255);\">your answer</i><span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(255, 255, 255);\">&nbsp;in a slightly more simpler way/more detail.</span>'),(108,7,'CommentQ','2013-08-26 21:12:07','<span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(250, 250, 250);\">When you say \"integral over the space\", is it all space? If the \"electromagnetic wave-train of finite size propagating in a certain direction\" there must be regions of the \'all space\' where EM is null. I think that in the \'physical world\' the limits of integrations must be bounded to some value, depending on the lifetime of the EM radiation.</span>'),(109,5,'CommentQ','2013-08-26 21:13:28','<span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(250, 250, 250);\">Yes, I mean something quite ordinary, like a pulse from a radar or so.</span>'),(110,9,'Question','2013-08-26 21:22:36','<p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 19.59375px; background-color: rgb(253, 253, 253);\">A long time ago, I read a book that started with \"In the beginning there was nothing. But in order for nothing to exist, there has to be something which defines it as such.\".</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 19.59375px; background-color: rgb(253, 253, 253);\">Assuming it\'s true, and things are defined by how different they are from their opposites and vice versa, what exactly defines a thing? For example, are&nbsp;<em style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline;\">something</em>&nbsp;and&nbsp;<em style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline;\">nothing</em>&nbsp;simply different ends of the \"spectrum\" of existence, being defined by their amount of existence (or non-existence)?</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; background-color: rgb(253, 253, 253);\"><font color=\"#111111\" face=\"Helvetica Neue, Arial, sans-serif\"><span style=\"line-height: 19.59375px;\">http://philosophy.stackexchange.com/questions/7966/does-something-require-its-opposite-in-order-to-be-defined</span></font><br></p>'),(111,3,'Script','2013-08-26 21:23:26','<p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 19.59375px; background-color: rgb(253, 253, 253);\">It depends of your point of view. You can define something by opposition, A=-B, but you can define positively A=B or tautologically A=A. Words are defined positively. In politics you have the same issue. Like Paul Sartre says in&nbsp;<em style=\"margin: 0px; padding: 0px; border: 0px; vertical-align: baseline;\">Critic of the Dialectical Reason</em>&nbsp;the jew has defined himself firstly as opposed to the anti-Semitic.Then he defined himself through the figure of the State of Israel. And last, he defined himself by finding his singular character, by asserting his uniqueness.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 19.59375px; background-color: rgb(253, 253, 253);\">In the case of the \"nothing\", you can have a purely logic concept who opposes to the being concept, or a more metaphisical concept (like medieval notion of nothing), where the nothing has it´s own (no)-determinations in itself.</p>'),(112,4,'Script','2013-08-26 21:25:12','<div>This idea crops up in a number of different contexts in Modern Western Philosophy:<br></div><div><br></div><div>1. In Hegels phenomenology he starts with the thesis Nothing whose antithesis is Being, and whose synthesis is Becoming.<br>2. In Saussure linguistics, elements of language are understood not in terms of themselves but of their differences.<br></div><div><br></div><div>However there are many modes of definition - oppositional is only one, and they are not neccessarily mutually exclusive. Substance in Greek antiquity is positively defined by its characteristics, but also in opposition to Void. Substance is what is in common with what on the face of it are differences. The eternity of substance comes from the already existing idea of immortality of the gods.</div><div><br></div><div>In Nagarjunas Buddhist metaphysics, Being is Empty. So here we have a categorical difference in Western Metaphysics that is asserted to be equal. In fact, it is paradoxical in the strong sense of the word.</div><div><br></div><div>In Daoist metaphysics, definitions in the strict sense are not possible. For what can Be is always fuller than our means of description. And this is just as true of Nothing.</div><div><br></div><div>In Shurawardis Sufi metaphysics Being is light, and where light is not is Nothing; but light is not self-subsistent - it is an emanation from Allah - who is outside of Being and outside of Nothing.</div>'),(113,5,'Script','2013-08-26 21:27:06','<p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 19.59375px; background-color: rgb(253, 253, 253);\">I think this is related with the notion of entropy. If something is constant, necessary and omnipresent, it seems to be undefinable as there is nothing to compare it with. A constant, with no entropy, means no information and no knowledge, nothing to say about it.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 19.59375px; background-color: rgb(253, 253, 253);\">However, we don\'t need the \"opposite\". What are the opposites between vacuum, water and air? What is the opposite of red? I think the best example is gravity.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 19.59375px; background-color: rgb(253, 253, 253);\">Magnetism and electrical charges have two polarities, so the force can be attractive or repulsive. In quantum mechanics we can see a lot of symmetry, but gravity is puzzling because it seems not to be symmetric. Both matter and antimatter have the same gravity if I\'m not mistaken.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 19.59375px; background-color: rgb(253, 253, 253);\">However we have fairly good definitions for gravity. We have now gone to the space and we have a notion of \"weightlessness\", but:</p><span style=\"background-color: rgb(253, 253, 253); color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif;\">- there is gravity in space as well, although it\'s small and other forces like inertia may be more significant.</span><br><span style=\"background-color: rgb(253, 253, 253); color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif;\">- gravity was well defined before, we didn\'t need to go to the space to define it.</span><div><br><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 19.59375px; background-color: rgb(253, 253, 253);\">How could we study gravity before? Because it\'s not constant. For instance it is weaker in the mountains. Therefore we don\'t really need the opposite, but variability is definitively interesting and important. With a differential in gravity we can study it\'s causes, define gravity as dependent on those causes, etc.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 19.59375px; background-color: rgb(253, 253, 253);\">If gravity was constant, like the speed of light, we could still know it and have some information about it, but the greater the entropy (contingency, variability, etc.) the more we can know about something.</p></div>'),(114,6,'Question','2013-08-26 21:29:37','<div>I am puzzled a bit. I read the wiki page and an introductory book on logics, but I can\'t quite grasp it yet. The place where i came across them is Van Inwagens \'Material Beings\'.</div><div><br></div><div>Consider the following three sentences:</div><div><br></div><div>(1) x=x</div><div><br></div><div>(2) ∃x x=x</div><div><br></div><div>(3) ∀x x=x</div><div><br></div><div>It seems to me that they are all equally true.</div><div><br></div><div>Also, I can\'t see the difference between the next two sentences:</div><div><br></div><div>(4) If x happens, I will get mad (free)</div><div><br></div><div>(5) ∃x If x happens, I will get mad (bound)</div><div><br></div><div>The only difference I see is to (6):</div><div><br></div><div>(6) ∀x If x happens, I will get mad (bound)</div><div><br></div><div>Clearly, in (6) im constantly mad and in (5) not.</div><div><br></div><div>A good answer would explain the difference between (4) and (5), I feel that that explanation would help me along in understanding what it means for a sentence to contain a free variable.</div><div><br></div><div>http://philosophy.stackexchange.com/questions/7827/what-are-free-variables-and-what-does-it-mean-for-a-statement-to-contain-one<br></div>'),(115,7,'CommentQ','2013-08-26 21:30:15','<span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(250, 250, 250);\">Regarding the first formula (x = x). You say it is true. However, you haven\'t actually made a statement yet until you specify what x is. You say it doesn\'t matter because it\'s true for all x! And I say so the statement you\'re really claiming to be true is the third one you referred to: ∀x(x = x), in which case I agree. :)</span>'),(116,7,'CommentQ','2013-08-26 21:30:30','<span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(253, 253, 253);\">Note that (4) is not a sentence, because a sentence is defined to be a formula with no free variables. So even informally (4) can at most be \"equivalent\" to a formula. Of course, the same is also true for (1). Regarding (1), a constant function is not (necessarily) the same as a constant, even so some languages may allow you to use a constant in place of a (constant) function.</span>'),(117,7,'Script','2013-08-26 21:32:26','<div>First difference:</div><div><br></div><div>- In some sense statement (4) is incomplete, if x indeed is a variable, then the truth value of (4) is undefined.</div><div><br></div><div>- However, statement (5) does have a well defined truth value (even if it is unknown) even though the statement contains a variable.</div><div><br></div><div>a second difference:</div><div><br></div><div>- (5) is logically entailed by one (or more) true statements of (4) for different x\'s.</div><div><br></div><div>- However, one can know/assert/prove a statement of form (5) without knowing/identifying/proving, for any particular \'x\', a statement of the form (4).</div>'),(118,11,'Script','2013-08-26 21:33:12','<div>Variables are a useful concept in various contexts. Their usefulness arises from their ability to refer to objects by a more or less arbitrary name or symbol. In first order logic, the need for this ability arises for the quantifiers ∃ and ∀. Also the lambda abstraction λ needs this ability, and other examples less closely related to logic are probably easy to find. There are alternatives like de Bruijn indices, which allow to avoid the usage of variables.</div><div><br></div><div>Like a bound variable, a free variable refers to an object. Depending on context, more or less assumptions about such an object are made. In the context of first order logic, the only assumption is that the referenced object exists. So in this context, free variables are nearly the same thing as constants, except for the fact that they serve a different purpose. Therefore, the treatment and exact interpretation of (free) variables is often more restricted and slightly more subtle than the treatment of constants.</div><div><br></div><div>For example in Ebbinghaus et. al, officially only the variables \"v_0, v_1, v_2, ...\" exists, despite the fact that the text normally uses \"x, y, z, ...\" when talking about variables. The funny thing is that \"x, y, z, ...\" can mean any of \"v_0, v_1, v_2, ...\", so that the text has to sometimes explicitly exclude the case \"x=y\" (the text uses \"\\equiv\" for equality between objects). The text further defines the subset \"L^S_n\" of the language \"L^S\" for the formulas which only contain \"v_0, ..., v_{n-1}\" freely. S-structures and S-interpretations are two separate notions, where an S-interpretation contains a concrete assignment of objects to the variables \"v_0, v_1, v_2, ...\". For an S-structure, you can ask whether it could satisfy a given set of S-formulas, or whether a set of S-formulas will be satisfied by the S-structure for all possible variable assignments.</div><div><br></div><div>As another example, my references for universal algebra explicitly specify \"X\" as a finite or countable infinite set, whose elements are called variables, and explicitly tag this \"X\" to any set of terms or formulas. What this has in common with the previous example is that there are at most countable infinite different variables. Still, in universal algebra, the free variables are also use as a sort of generic element, although not through abuse of notation but through an explicit construction of corresponding free algebras.</div>'),(120,12,'Script','2013-08-26 21:35:40','<div>Your question amounts to asking what the difference is between saying:</div><div><br></div><div>\"it is the same thing as it\"</div><div><br></div><div>\"something is the same thing as itself\"</div><div><br></div><div>\"everything is the same thing as itself\"</div><div><br></div><div>Answer:</div><div><br></div><div>2 and 3 express different propositions but 1 expresses no proposition at all. It is not \"well formed\", this means that it is not a sentence in first order logic. Stack exchange is not the place to learn first order logic, buy a book by Quine. Intuitively though, it is obvious that 1 is meaningless because it has not been completed and turned into either 2 or 3.</div><div><br></div><div>To answer your Q directly: A free variable in a sentence is a variable in a sentence for which it has not been specified whether the concept of All or the concept of SOME is to apply to it. So in your example: \'x=x\' contains two occurrences of the same free variable \'x\' because it has yet to be specified whether the concept of All is to apply to the \'x\', and so be turned into \'Ax x=x\', or whether the concept of SOME is to apply to the x and so be turned into \'Ex x=x\'</div>'),(121,13,'Question','2013-08-26 21:37:13','<div>Fine-tuning of constants in the standard model is an acknowledged mainstream physical question and not an eccentric one.</div><div><br></div><div>Looking at this carefully what one is saying is why do the parameters of this model the values they are. The anthropic principle says that they are what they are so that life can evolve. Of course they may not be a unique point in parameter space where this happens.</div><div><br></div><div>But why restrict change to constants? Are other parts of the model not changeable?</div><div><br></div><div>There is at least two that are mainstream: The dimension of space, the nature of point particles - perhaps they are not points and have either length or volume. Both of these changes are seen in string theory.</div><div><br></div><div>The jury is still out as to whether string theory was a credible alternative to the standard model. But it certainly did have the credibility to keep a lot of physicists working for sometime to make the theory fully credible.</div><div><br></div><div>But why stop there - are there not other parts of the theory we can change? That is we are fundamentally changing the nature of the laws themselves.</div><div><br></div><div>Purely speculatively, we could envisage a whole class of theories of modified physical laws forming a geometry with curvature, and our theory are extremal point of the curvature.</div><div><br></div><div>This means that the range of possible theories is incredibly vast.</div><div><br></div><div>Is it possible to envisage a universe whose laws are completely different to ours (where life does arise)? It seems to me that in theory this is possible, but in practise we can only consider deformations of our current laws only - as we could never theorise about the consequences of those laws. Is this correct.</div><div><br></div><div>http://philosophy.stackexchange.com/questions/7057/what-kind-of-universe-would-there-be-with-entirely-different-physical-laws<br></div>'),(122,14,'CommentQ','2013-08-26 21:37:51','You can definitely reason about universes with entirely different physical constants, at least and then make evolutionary arguments to explain why ours has the constants it does. However, it is easy to confuse your model of the world with the world when you do this and physicists are prone to this mistake.'),(123,15,'CommentQ','2013-08-26 21:38:29','<div>If we define \"life\" only in terms of our models that describe our universe as we theorise it, than by definition, we cannot have any other universes that would host \"life\". If you can come up with a flexible definition of life, then, the task would be to prove that, despite its general definition, only our universe\'s laws would allow life. If, at least, uniqueness would be proven to be not-provable, then there is room for other universes hosting life. On the positive side, one can prove existence by giving examples (via,say, withsimulation) of alternate laws leading to \"defined\" forms of life.<br></div>'),(124,3,'Script','2013-08-26 21:39:58','<div>That\'s a big question.</div><div><br></div><div>I\'ve asked tough questions that required \'soft\' [read: speculative; non-quantitative] answers on physics.se, and I received helpful replies.</div><div><br></div><div>That aside, I recently wondered whether it were feasible for a human to conceive of a universe with a set of natural laws that were completely and essentially different than our universe\'s natural laws. Presently, I lean toward believing that it is not feasible for a human to do so:</div><div><br></div><div>- Hume postulated that a human\'s imagination is his ability to combine things he has observed. If that\'s true, then it may not be possible for us to think of a universe guided by different natural laws. Although, if we assume that our reasonable [read: mathematical / logical] methods of thinking can be applied to at least one other universe, then I can imagine how we might come across a set of self-consistent rules that could govern another universe (a very long&nbsp;</div><div>session of trial and error, for instance).</div><div><br></div><div>- If you conceive of self consistent sets as a way to model possible universes, then I think you could also conceive of Euclid\'s axioms of geometry as a model of the forms that are possible in our universe. Then, consider that mathematicians have discovered sets of self-consistent non-euclidean geometric axioms that allow for different forms than Euclid\'s axioms allowed. The existence of these non-euclidean sets suggests that it is possible that another universe contains a set of natural laws that are different than the set of natural laws our universe contains.</div><div><br></div><div>-It\'s also worth considering hyperbolic geometry, the first non-euclidean geometry. At first, (I believe) it was thought (by Gauss and other cognitive powerhouses) that hyperbolic geometry represented a discrete non-euclidean set of geometric axioms. However, it was later proven that the axioms of hyperbolic geometry were \'equiconsistent\' with the axioms of euclidean geometry. More specifically, the axioms of hyperbolic geometry just described how forms would look to an observer positioned within a sphere, observing forms cast on the inner surface of the sphere. We know that such a scenario is consistent with the axioms of euclidean geometry because we know that, in our universe, shapes can be cast on the inner surface of spheres. Therefore, the axioms of hyperbolic geometry and Euclidean geometry are not necessarily discrete. So, hyperbolic geometry serves as evidence that, to even the most powerful minds, a distinct self-consistent set can appear to be a logically possible discrete universe, when, in fact, it is not truly discrete.</div><div><br></div><div>-it\'s axiomatic that as one reduces the number of constraints in a consideration, its number of possibilities increases. If you went as far as to say that the laws that govern the other universe do not need to be compatible with our faculties of reason, then you will have cast away all conceivable constraints - including the requirement of conceivability itself. But if you did that, and if you had to do that to allow for the existence of a universe that contained a discrete set of natural laws, then you could say nothing of that universe, since (meaningfully) speaking of a thing requires conceiving of it, and if you could conceive of it, then you wouldn\'t have needed to shed the requirement that it must be conceivable.</div><div><br></div><div>In sum, I tend toward believing that it is not feasible to conceive of a universe that has a set of natural laws that are different than the natural laws our universe has.</div><div><br></div><div>Then again, Stephen Hawking, once speculated that the universe was created by another universe that had universe-creating properties and whose properties did not require it be created. So I\'m hesitant to say that my take is more likely to be right than his is.</div>'),(125,4,'Script','2013-08-26 21:41:09','i think that we are already have Artificial Universe - one existing inside computers. this universe is all about processing information so it\'s very dissimilar to the physical one. it\'s easy to prove that Life can exist there - as far as you believe in Physics that describes \"Nature laws\", you can emulate them inside computers too (of course emulation will just recreate physical universe inside computer, but it\'s just \"worst case\" proof)'),(126,5,'Question','2013-08-26 21:42:45','<div><div>What would it mean to say that mathematics was invented and how would this be different from saying mathematics was discovered?</div><div><br></div><div>Is this even a serious philosophical question or just a meaningless/tautological linguistic ambiguity?</div></div><div><br></div>http://philosophy.stackexchange.com/questions/1/was-mathematics-invented-or-discovered'),(127,6,'Script','2013-08-26 21:43:43','<div>\"Intuitionists\" believe that mathematics is just a creation of the human mind. In that sense you can argue that mathematics is invented by humans. Any mathematical object exists only in our mind and don\'t as such have an existence.</div><div><br></div><div>\"Platonists\", on the other hand, argue that any mathematical object exists and we can only \"see\" them through our mind. Hence in some sense Platonists would vote that mathematics was discovered.</div>'),(128,7,'CommentA','2013-08-26 21:44:43','<font color=\"#444444\" face=\"Helvetica Neue, Arial, sans-serif\" size=\"2\"><span style=\"line-height: 17px;\">True Platonists would argue that anything we learn is in fact remembered. This is the point of Socrates walking Meno\'s slave through a simple Euclidean proof about squares -- i.e., that through dialogue and introspection, our \'innate\' knowledge (memory!) of mathematical Reality can be recovered in some partial way.</span></font><br>'),(129,11,'Script','2013-08-26 21:45:42','<div>My personal point of view is that mathematicians invented the axioms and the rules of operation, the rest are discovered. Mathematicians invented the notations for writing down the concepts which are discovered within the universe of an axiom.</div><div><br></div><div>The concept of numbers exists, but we invent the notation that the glyph \'1\' and the sound /wʌn/ refers to the concept of singular object that we discovered. We invented the rules of matrix multiplication, but the consequences of the way we do matrix multiplications are discovered.</div><div><br></div><div>Most of the time, we deliberately invent a set of axioms that will lead us to discover a set of facts we want to be true. This is certainly true with imaginary numbers, we invented them so that we can discover the solutions to problems we previously were unable or difficult to solve.</div>'),(130,12,'CommentA','2013-08-26 21:46:40','<span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(250, 250, 250);\">why does the concept of number exist, but the concept of complex number invented?</span>'),(131,11,'CommentA','2013-08-26 21:47:23','<span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(253, 253, 253);\">Good point. Complex number is just a notation for writing down a pair of numbers, we invented the rules of complex arithmetic so they can conveniently represents coordinates in a plane and a few common transformations. The concept of a plane and a point in plane exists, but the notation (e.g. complex number) are invented.</span>'),(132,12,'Script','2013-08-26 21:48:35','<div>This is only a partial answer:</div><div><br></div><div>As a mathematician, I have been asked this sort of question from time to time. Like most other mathematicians, I tend to sort of evade the question, because it\'s tricky. Usually, the question is put in the form, \"Are you a platonist?\"</div><div><br></div><div>The reference here is to Plato\'s eternal form that we are able to recognize, and that allows us to recognize the world around us (it is not obvious, afterall, that we should still be able to recognize an amputee as a human when we first see him or her, for example). When forced to continue, I usually respond \"No.\"</div><div><br></div><div>I think the fundamental problem with Platonism is summed up in Brian Davies\'s paper, aptly titled \"Let Platonism Die.\" I also add - if a mathematical \'discovery\' hasn\'t yet been discovered, does it exist? A Platonist would say absolutely. An intuitionist would either say that it does not exist, or it exists only in the sense that some current or future mathematical system, devised and formulated vulgarly by humans, will lead to many more theorems - i.e. it exists only as an extension of what we have already created.</div><div><br></div><div>But ultimately, I don\'t think that this distinction is very important aside from the theistic or neural implications. A Platonist would say that when we recognize a triangle, for example, it is because we are recognizing the Form of a Triangle, some idealized, perfect, transcendental object. This makes a lot of sense, because Platonism obviously has at its roots Plato, who read much into the divine relationship between mathematics and the world espoused by Pythagoras.</div><div><br></div><div>As a final note, I should say that many well-known mathematicians lie on both sides of the fence. The most famous Platonist, I believe, is Roger Penrose, who is most famous for his creation of dozens of non-obvious tessellations and tilings.</div>'),(133,13,'Script','2013-08-26 21:50:23','<div>I think the words \"invention\" and \"discovery\" are a bit poor to describe the birth of mathematic if there is one. It makes no sense to me to say mathematic has appeared as when Christophe Colomb discovered America or was invented as the boomerang.</div><div><br></div><div>The word mathematics might have been invented, the language in which the mathematics are written might have been invented but the abstraction movement from the real word, the structured synthesis that it undertakes, all that give thickness to mathematics themselves (it depends what you call mathematics) are part of mankind. You don\'t ask if beauty has been discovered or invented ?</div><div><br></div><div>My personnal point of view is that the question \"what is mathematics\" would be more serious, I would found even more interesting \"why do we do mathematics\".</div>'),(134,14,'Question','2013-08-26 21:51:25','<div>How does one know one is not dreaming? How could one logically demonstrate to a skeptic that one is \"really\" there, awake and not just dreaming the entire situation/world around him?</div><div><br></div><div>Specifically what I\'m asking is: which if any philosophers have addressed this problem of how one knows one is or is not dreaming?</div><div><br></div><div>Which if any philosophies have attempted to evaluate the sense of claims like \"I am not dreaming\"?</div><div><br></div><div>http://philosophy.stackexchange.com/questions/24/how-does-one-know-one-is-not-dreaming<br></div>'),(135,15,'Script','2013-08-26 21:52:23','Scientifically speaking, as reality is only the interpretations of chemical signals by your brain any proof you could come up with could also just be an interpretation of the chemical signals in your brain. But since my knowledge of this is also just an interpretation of chemical signals in my brain I can not even prove that my knowledge is not just a dream.'),(136,3,'Script','2013-08-26 21:53:41','<div>This is actually an easier question than it seems, largely because it operates on assumptions that are easily conceded to or missed.</div><div><br></div><div>The first assumption is that reality is absolutely compartmentalized; it is not.</div><div><br></div><div>- Where does Greek end and Latin begin? That\'s a harder question than it looks like if you pay attention to language and there\'s a whole book dedicated to studying the indistinction between languages. It\'s called Echolalias.</div><div>- Is Beckett\'s The Unnameable a book in the same way that Joyce\'s Ulysses and Nietzsche\'s Will to Power are books? Is an oeuvre everything a philosopher published or do notes and fragments count to? Are these easy questions? Nope, and these are just some of the unitary ideas Foucault pulls apart in The Archaeology of Knowledge. You could say that these details don\'t matter if this is but a dream, but then apparently at least your dream would itself call for these distinctions in mode of being, so being in a dream right now wouldn\'t matter.</div><div>The second assumption is that perception must be observably identical in dream and reality; they are not. For every instance I ask you what you sense or remember in a dream, there are observable regularities that can be distinguished from the regularities of sense in waking life, lucid dreaming only exacerbating that distinction. Even if you\'re in a dream right now, then as far as your dream goes, there are two distinct realities which negate the need to even remark on the possibility that this is all a dream.</div><div><br></div><div>The third assumption is that absolute necessity exist; it does not. Hume dispelled that idea in his Essay concerning Human Understanding and we haven\'t successfully refuted him since.</div><div><br></div><div>I hope I\'m being helpful.</div>'),(137,4,'CommentA','2013-08-26 21:54:27','<span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(250, 250, 250);\">I think this is a great answer.</span>'),(138,4,'Script','2013-08-26 21:55:06','We simply can\'t. We can\'t even prove that the Universe was created yesterday along with all memories of the past. We can\'t prove the Universe isn\'t just a run of a simulation (see the Simulation Hypothesis). If you put it this way, nothing can actually be proven.'),(139,5,'Script','2013-08-26 21:56:00','<div>Two interesting arguments from recent decades relevant to this are Wittgenstein\'s Private Language Argument and the doctrine of Semantic Externalism.</div><div><br></div><div>Wittgenstein argues in Philosophical Investigations that it is impossible for there to be a language which only referrs to private, inner sensations. Very roughly, the idea is that there is nothing which could count as misapplying a word used to refer only to an internal mental state. Correct and incorrect depend essentially upon external frames of reference as reflected in the responses of others. The argument is targetted at empiricism, but it is clearly applicable to the skeptic who claims we are dreaming, for it would be impossible, if Wittgenstein is right, to ever refer to one\'s own dream experience if dreaming was all one ever knew.</div><div><br></div><div>Semantic externalism is a doctrine associated with Davidson, Putnam, Burge and to some extent, Kripke. This is the doctrine that it is an essential component of language that it is not an internal psychological state, that meanings must be grounded in a shared, external world. When you speak words I must take you as referring to something common to both our worlds, or else there would be no basis for successful communication. A shared, outer world is a precondition of communication. The idea is similar to Wittgenstein\'s.</div><div><br></div><div>I have not presented either argument in any detail, but really have just sketched them in order to answer the question. They are anti-skeptical arguments which apply equally to the claims that we are dreaming, decieved by a demon, or are a brain in a vat. They are widely accepted as effective in proving that any language user cannot have always been dreaming, always deceived about the reference of his words, for language must have been learned in a shared environment. But the arguments cannot really show that I am not dreaming or hallucinating right now. They do however try to show that deception cannot be the norm.</div>'),(140,6,'Question','2013-08-26 22:03:23','<div>It is generally considered beneficial to discover some scientific law or invent an object that is said to further the state of mankind. All inventions and scientific discovery hinges in some way on past knowledge. Since knowledge is so instrumental to any form of advancement, are all facts worth knowing?<br></div><div><br></div>http://philosophy.stackexchange.com/questions/1641/are-all-facts-worth-knowing'),(141,7,'Script','2013-08-26 22:05:13','<div><b>\"Worth Knowing\" is not an objective notion, it is entirely relative</b></div><div><br></div><div>The notion of \"worth knowing\" is entirely dependent on circumstance. It is not worth knowing to me that there is 600 mbps of traffic going through Server X right now, but to the administrator of that server, that\'s probably a huge deal. Not worth knowing to me, very much worth knowing to him.</div><div><br></div><div><b>The weight of knowledge acquisition</b></div><div><br></div><div>Are all facts worth knowing? At zero cost, sure (i.e. omniscience), but as Michael points out, as soon as there is even the slightest burden acquiring any facts, there become a huge (essentially infinite) number of facts which are no longer worth knowing.</div><div><br></div><div><b>The weight of knowledge maintenance</b></div><div><br></div><div>The human brain, being a finite physical entity, actually has limits in terms of the amount of raw data it can store. No one has ever reached that limit yet (as far as we know), but theoretically schools are going to pack more and more knowledge into kids brains and newspapers will be packed with more than country or global news but galactic news, and perhaps one day storage may be an issue. Even if you were somehow privy to all the facts in the universe, you would lack the space to store them all. Some facts would simply take a lifetime to acquire or not even be possible in practice (P = NP).</div><div><br></div><div><b>Your additional questions not already addressed above:</b></div><div><br></div><div><b>Is it possible to quantify the number of facts a person knows?</b></div><div>Theoretically, yes, as the brain is a physical organism it is not outside the scope of observation. In practice, we do not have this technology and we probably won\'t have it for at least another 100 years. You could do it the old fashion way and get a rough count that would provide a relatively consistent measure across the board though; simply have people take every single subject test in the world and count up the questions they got correct. It would be very rough, but have internal consistency. (No, it would not get at little facts like \"my sisters blanket has 2 juice stains on it\", etc, but general stuff, yes).</div><div><b><br></b></div><div><b>If an event that has not yet occurred, can knowledge of that event be considered a fact? That is, is the future deterministic?</b></div><div>If the world is deterministic, then yes, it would be a fact. This is, of course, highly debated however (whether the universe is deterministic or not).</div>'),(142,11,'Script','2013-08-26 22:06:05','All facts are not worth knowing for the simple reason that most of them don\'t make any significant difference. For example, knowing the precise position of every iron atom in the earth\'s core is extraordinarily useless because (1) they won\'t be there for long, and (2) it doesn\'t matter anyway, since nothing really interacts with the earth\'s core in a way that depends in detail on the position of said atoms.'),(143,12,'Script','2013-08-26 22:07:12','<div>Let\'s say I\'m reading Murder on the Orient Express and you offer to tell me who killed Mr Ratchett. Arguably, given my current state (half way through M. Poirot\'s adventure) this is not a fact that it\'s worth my knowing: it would spoil my enjoyment of the book. [Let\'s leave aside the factivity of fictional claims. Pretend I\'m reading something based on a true story, so there really is a fact of the matter at stake.] So this looks like it\'s a fact that it\'s not (right now) worth my learning. Obviously the point of reading through the whole book is to discover exactly this fact so it can\'t simply be that this is a fact not worth learning. It\'s just that learning the fact before the proper time reduces my enjoyment of the reading experience.</div><div><br></div><div>Let\'s take another example. Let\'s say that there is some scientific discovery that, if discovered, would give the discoverer unimaginable power: let\'s say the discovery is some kind of programmable virus that kills all and only the people it is designed to, or some such. Let\'s say that this discovery would have no benefits whatsoever, all it would do is give its discoverer the power to bend the world to her whim. Is this a fact worth knowing? Would the world be better off if this fact was simply never learned? Arguably: yes. There\'s a separate question as to whether there are any such facts with only bad consequences. If there aren\'t, then maybe scientific knowledge is worth knowing, always.</div><div><br></div><div>If you are a Bayesian, then there\'s a nice theorem due to I.J. Good that if you have a choice between learning something and not learning it, the expectation of the option to learn cannot be lower than the expectation of not learning.</div>'),(145,13,'Script','2013-08-26 22:08:57','<div>You are missing an important dimension here. TIME. Not all facts are worth knowing at any given time, not even at the same time, but right time. You can\'t handle it. Say for example, a process or system must evolve like X-&gt;Y-&gt;Z. To get to Y all you need is X, not Z, when you go to Z, you remember or know Y, at that time X becomes ingrained in your mind. Later when you move to Z-&gt;A-&gt;B-&gt;C, you don\'t even need to remember X. \"Last generations knowledge becomes current generation\'s common sense\".</div><div><br></div><div>Answering your Questions</div><div><br></div><div>If an event that has not yet occurred, can knowledge of that event be considered a fact? That is, is the future deterministic? No, Fact has an hit ratio of 100% (it may vary with the way people perceive, that is another topic). No matter how big is your sample size, all you need is one FALSE to bring your hit percentage to something less than 100.</div><div><br></div><div>If knowledge of some scientific principle would allow the bearer to perform evil (genocide, famine, earthquake, etc.), is that knowledge still worth knowing? You need to learn from the past, these events you mentioned has happened before and the root cause is knowing some fact or the other.</div><div><br></div><div>Is there a difference between the effort required to gain a fact and the effort required to retain a fact? Yes, the effort required to gain a fact is &gt; effort required to retain a fact. In other words, you don\'t need anything to retain a fact, you need to give at-least something to gain a fact. People who believe in luck would say \"sometimes that later will be zero\". But all have to agree, you don\'t need to do anything to retain a fact, you just let it there float like a ball filled with air on top of water. If it sinks, then it is not a fact. Same way, if you make effort to retain it, it is not a fact.</div><div><br></div><div>Is it possible to quantify the number of facts a person knows? Yes. You can count the number of stars on the sky, but when you see a star, you wont know whether it is already counted or not. I would rather blame the geometry of the sky than my counting ability and patience to do so.</div>'),(146,14,'Question','2013-08-26 22:10:36','<div><div>Many people who support death penalty are also against abortion, and vice-versa.</div><div><br></div><div>What is the moral difference between the two? In both cases, isn\'t the \"right to life\" of another being violated?</div><div><br></div><div>What are the major ethical or philosophical positions on the problems involved with these sorts of \"somatic rights\"?</div></div><div><br></div>http://philosophy.stackexchange.com/questions/807/if-the-right-to-life-is-denied-in-abortion-isnt-it-also-denied-in-the-use-of'),(147,15,'Script','2013-08-26 22:12:12','<div>Death penalty is a punishment, (in theory) reserved for the most heinous of crimes. While this is often done medically it does not have to be. Firing squads, hanging(thenyc.com has a NSFW Video i am not going to link from syria of mass hanging in last month), and electric chair are still common throughout the world.</div><div><br></div><div>That said there is no moral difference when considering commandment \"Thou shalt not kill\". Taking a life is still taking a life. You can justify the execution but that should not be confused with making it morally better. The commandment is not thou shalt not kill unless you deem the crime heinous. So to support one but oppose the other would seem hipocritical.</div><div><br></div><div>Hammurabi first proposed a set of laws that tried to set punishments that were equal to the crimes committed. Currently most advanced societies have moved to a policy that have lessened the punishment for some crimes, particularly violent crimes. The policies often involve the possiblity of release from prison, and often the potential for the death penalty in certian countries. One could argue that when committing the crime the perpetrator knew the potential penalty and thus committed a form of suicide. Albiet one that will be completed by agents of the government.</div>'),(148,3,'Script','2013-08-26 22:13:13','<div>There is a contradiction if the distinction isn\'t made between the death penalty as a form of punishment or as a necessary means of protecting society. The former, is indefensible if juxtaposed with the belief in a right to life and the latter, in any society capable of producing prisons keeping inmates in near complete isolation, is nearly never the case.</div><div><br></div><div>The reason killing as a form of punishment is not justified by one who adopts the \'right to life\' philosophy is that it doesn\'t protect any other peoples lives. The personalistic norm of John Paul II says a person is an entity towards which the only proper way to relate is love. The notion of \'doing unto others as they would do unto you\' could be used to justify the death penalty is thrown out the door when you consider the personalistic norm, as do many prolifers. The personalistic norm doesn\'t mean surrender yourself or your family to invaders who want to kill you, when any notion of \'proper relations\' has gone out the door.</div><div><br></div><div>The reason killing as a form of punishment is accepted as a plank of the Republican Party is that it is a tough way to deal with crime. That\'s not very philosophical or wise.</div><div><br></div><div>People often conflate these two very different ideas, but I\'d like to hope we\'re moving away from the death penalty (and life sentences) altogether.</div>'),(149,4,'Script','2013-08-26 22:14:10','The real question, in the context you phrased it, is whether a person has the right to waive their \"right to life\" through the commission of a heinous felony or euthanasia. I think this concept reconciles the apparent contradiction in position of those who support abortion and/or the death penalty, but not both.'),(150,5,'Script','2013-08-26 22:15:35','Although many people who hold the beliefs you described, many people who are anti-abortion identify with a \"pro-life\" movement that is also against things like the death penalty due to their \"right to life\" beliefs. These people would agree that the people you describe are holding contradictory views.'),(151,7,'Question','2013-08-26 22:19:52','<div>Searle\'s Chinese Room basically argues that a program cannot make a computer \'intelligent\'.</div><div><br></div><div>Searle summarises the argument as</div><div><br></div><div>Imagine a native English speaker who knows no Chinese locked in a room full of boxes of Chinese symbols (a data base) together with a book of instructions for manipulating the symbols (the program). Imagine that people outside the room send in other Chinese symbols which, unknown to the person in the room, are questions in Chinese (the input). And imagine that by following the instructions in the program the man in the room is able to pass out Chinese symbols which are correct answers to the questions (the output). The program enables the person in the room to pass the Turing Test for understanding Chinese but he does not understand a word of Chinese.</div><div>What are the counter-arguments? Including the standard and not so standard. This notion of \'intelligence\' intrigues me and I wish to see what other people have written said. Point me to your favourite counter-argument.</div><div><br></div><div>I have read Levesque\'s \"Is It Enough to Get the Behaviour Right?\"(http://ijcai.org/papers09/Abstracts/241.html) (full paper here) response. As Levesque is an AI researcher I would like to broaden my appreciation of counter-arguments from other fields.</div><div><br></div><div>http://philosophy.stackexchange.com/questions/1091/what-are-the-retorts-to-searles-chinese-room<br></div>'),(152,11,'Script','2013-08-26 22:20:47','<div>Searle\'s Chinese Room experiment doesn\'t make any sense, but some context is required to bring to light the contradiction inherent in his argument. At the time of its writing, he was attempting to fight an extreme position among AI theorists who supposed that all of the properties of understanding, whatever we agree that entails, could be reduced to or captured by a sufficiently powerful computer acting on symbols by completely formal methods i.e. mathematics.</div><div><br></div><div>In a nutshell, the CR experiment supposes it possible for a Chinese-speaker to pass meaningful messages in their native language to a man in the room who doesn\'t speak it and have them respond intelligently or meaningfully by following a purely algorithmic or formal process, whatever that may entail i.e. using a lookup table, following preset rules, etc.</div><div><br></div><div>This is a strange way to argue for intuition, or rather for intuition having the property of non-reducibility. Because if the man in the room can communicate with the man on the outside despite his lack of understanding, and the semantic gap is bridged by a completely formal process, then how is this anything but semantics being reduced to syntax? Exactly what Searle wants to argue against. Once again, the man on the outside understands what goes in and what comes out of the room, and the only means the man has to communicate is to follow a mechanical recipe for writing.</div>'),(153,12,'Script','2013-08-26 22:21:47','A trivial web search will bring up an enormous amount of material on this. For instance, the Stanford Encyclopedia of Philosophy\'s entry on Searle\'s \"Chinese Room\" thought experiment identifies three major categories of objections and goes into some detail on each (and provides generous citations.) Here is their high-level summary of the categories of possible responses to the argument:'),(154,14,'Script','2013-08-26 22:23:14','<div>I quite like the chapter in Jack Copeland\'s Philosophy of AI book on the Chinese room. Even though it\'s an introductory book, it goes into quite some breadths and depth. The chapter has a summary of the arguments against the Chinese room, and the retorts from Searl and friends, plus counter retorts. Rest of the book also sets the context of the discussions very well and definitely goes into the various ways people understand and use the term \'intelligence\' in the AI debates (you might find some you like)</div><div><br></div><div>A not too standard problem I have with the argument is its general argument form (non-standard problem because it\'s not specific to the Chinese room argument.)</div><div><br></div><div>The Chinese room is a modal argument which is deployed in a lot in philosophy of mind, -- examples of other modal arguments include things like the zombie argument from David Chalmers, the inverted spectrum argument, even brain in vats and Descarte\'s evil demon argument etc. They are generally of the form: it is possible that despite the appearance of X we cannot know *that X* because of an equally plausible alternative understanding of the situation which exclude X). In this case, It is possible that even if the instructions lets the box cogently parlay with a Chinese Turing tester, it is more than equally plausible that no real understanding is occurring (sneakily established by the fact that we\'d all had prior agreement that the person inside has no understanding of Chinese, and he\'s the only one doing anything)</div><div><br></div><div>I am not sure how seriously to take modal arguments in general, because I feel they are invariably sneaky, and often leads to unfruitfulness due to the way things are set up. A successful modal argument will give you no greater reason to believe either of the options, and the rhetoric is usually set up such that you forget one of the options consists of a natural and familiar scenarios (sitting by the fire, seeing a person, receiving red qualia) and the another scenario which is speculative (deceived by evil demons, seeing a zombie, receiving green qualia instead of read). So each camp can\'t really engage the other side to have a fruitful discussion because each has a reason to believe that the other person\'s argument is flawed -- with no real criterion to evaluate either side of the claim. Sneaky IMHO.</div>'),(155,13,'Script','2013-08-26 22:24:54','<div>Searle\'s Chinese Room arguments depends initially on the assumption that, if there is an intelligent system, some component of the system must be intelligent. Carrying this a long way to a conclusion, each lepton and quark in the Universe must be intelligent to some extent. The alternatives to that conclusion are either (a) there is nothing intelligent in the Universe, or (b) intelligence is an emergent phenomenon. I\'m going with the latter.</div><div><br></div><div>Searle then attempts to answer all objections (such as emergent intelligence) by indulging in more or less relevant discussion before claiming it reduces to the original Chinese Room, which he showed was not intelligent. This is an excellent example of begging the question, in the classic definition (assuming the proposition to be proven).</div><div><br></div><div>The most relevant other discussion is the claim that a computer could not be intelligent in the human sense without having human-type senses to sense things like a human. How is a computer to know what a hamburger is without eating one? This seems like a good line of argument, but Searle does not pursue it.</div><div><br></div><div>Searle claims, without support, that intelligence must be biological, and states that we know intentionality is biological in nature. I haven\'t seen this claim anywhere else, so I don\'t think we do know that.</div>'),(156,15,'Question','2013-08-26 22:30:33','<div>I\'ve been reading a conversation between two individuals - A claiming to be atheist and B asking him to prove it, since B does not believe that A is saying the truth and can\'t be sure if A is really an atheist as A claims.</div><div>So I was wondering - is this even a valid argument B has there - to say that B does not believe what A claims since \"many people claim a lot of things that aren\'t really true\".</div><div><br></div><div><b>How can A convince B that A is really an atheist?</b></div><div><b><br></b></div><div>http://philosophy.stackexchange.com/questions/2962/how-to-prove-you-are-an-atheist<br></div>'),(157,3,'Script','2013-08-26 22:31:35','<div><b>As written, the B\'s argument seems an invalid red herring</b></div><div><br></div><div>Assuming the question is:</div><div><br></div><div>Is atheism true or false?</div><div>whether either party believes the premise that it is true is simply irrelevant to the argument. There are many things that are true, but which nobody believes. For instance, it\'s very likely a supernova has occurred which is not yet visible on Earth. A question about the existence of supernova should focus on astronomy, not psychology.</div><div><br></div><div>Obviously, we prefer that people who make claims actually believe them, but usually, we can just trust that what they say they believe is true. If A says they don\'t believe in God, there\'s nothing to be gained by claiming they are mistaken in their own beliefs.</div><div><br></div><div>Was there more to the argument? How did it get to that point?</div>'),(158,4,'CommentA','2013-08-26 22:33:09','Actually, this answer could be called a red herring. It is true they are two different topics. It is a good point, but it does not answer the question as asked'),(159,4,'Script','2013-08-26 22:33:40','Its not possible. There is never a way to \"prove\" that you think this or that. You could always lie. The question is: Why should a person lie about that in such a discussion?'),(160,5,'Script','2013-08-26 22:34:46','<div>It\'s a clever tactic. Atheists may often presents themselves as believing in nothing without proof. Thus, such an atheist, to be consistent needs to prove that they are an atheist, otherwise, they are inconsistent in believing it.</div><div><br></div><div>I believe a more educated end goal for this tack is to point out how hard it is to prove something, and that we often go on things like confidences and convictions or even inductive conclusions, where they present themselves.</div>'),(161,6,'Question','2013-08-26 22:46:07','You think that you will die just because everyone dies. And you would like to know if you are immortal. How can you know if you are immortal or not?<div><br></div><div>http://philosophy.stackexchange.com/questions/4016/how-can-i-know-that-i-am-not-immortal<br></div>'),(162,7,'CommentQ','2013-08-26 22:47:24','Inferring something (inductive reasoning) is considered by most people to be a valid source of knowledge.'),(163,7,'Script','2013-08-26 22:47:42','<span style=\"color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 19.59375px; background-color: rgb(253, 253, 253);\">Inductive inference. All humans have died so far, therefore (in all likelihood) all humans die at some point. You are human, I take it, so there you go.</span>'),(164,11,'CommentA','2013-08-26 22:48:52','This does not prove that one is mortal just makes it veeeery probable.'),(165,11,'Script','2013-08-26 22:49:24','<div>I take it the questioner is aware that inductive inference is what leads most people to think that they will die. But there\'s the problem of induction, mentioned in the comments, stated by Hume - which makes it questionable whether my inference that I will die has the status of knowledge.<br></div><div><br></div><div>Inductive reasoning is premised on the thought that the past resembles the future. But why do we think the that the past will resemble the future in this case? Because it always has done in the past. Inductive inference has to presuppose what it sets out to establish and is therefore circular.</div><div><br></div><div>You might say that this is as good as it gets - and is how we claim to know most things, therefore I know that I will die. But, on a stricter definition of knowledge, this lack of certainty prevents us from claiming knowledge of our own mortality.</div><div><br></div><div>The standard definition of knowledge is justified true belief. I can justify my belief that I am mortal via inductive reasoning but can I can\'t establish its truth that way - the conclusion that I am mortal still requires verification, and when that happens, I probably won\'t exist to experience it for myself. If I were immortal, on the other hand, there would always remain open the possibility that I might die at some point in the future, say, at the age of 325. (Though at the age of 150, I might begin to think the evidence is beginning to point in favour of my immortality.) So it seems that one\'s own mortality can be neither verified nor falsified by that person. Together with the circularity of inductive reasoning, this implies that my own justified inference that I will die doesn\'t have the status of knowledge and cannot have that status in my lifetime.</div>'),(167,12,'Script','2013-08-26 22:51:56','<div>Taking this question to a level above the simple \"we have seen everybody dies, therefore we are going to die\" it\'s not definitely that stupid as it seems.</div><div><br></div><div>What if we are living in a dream and therefore we live as long as we want to (and then move to something else)?</div><div>What if the world is created because of us and we are God?</div><div>There a lot of similar questions that are not that obvious to an open mind.</div><div><br></div><div>In this topic the answer is: we might be, we might be not. We don\'t know for sure that we are going to die. We assume we are going to die. But we can\'t be totally sure (as well as anything else in this reality).</div>'),(168,5,'Question','2013-08-27 16:23:24','<div>I know plants are green due to chlorophyll.</div><div>Surely it would be more beneficial for plants to be red than green as by being green they reflect green light and do not absorb it even though green light has more energy than red light.</div><div>Is there no alternative to chlorophyll? Or is it something else?</div><div><br></div><div>http://biology.stackexchange.com/questions/450/why-do-plants-have-green-leaves-and-not-red<br></div>'),(169,6,'CommentQ','2013-08-27 16:24:45','<span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(253, 253, 253);\">I find it even more puzzling why plants don\'t absorb all of the visible spectrum altogether (resulting in the leaves being black).</span>'),(170,7,'CommentQ','2013-08-27 16:26:00','It is even more strange becouse green algae evolved in water. And red light is absorbed by water. Rhodophyta (red algae) are red due to phycoerythrin but it seems that red color is advantageous for them only in sea depths.'),(171,7,'Script','2013-08-27 16:28:48','<div>Surely it would be even more beneficial for plants to be black instead of red or green, from an energy absorption point of view. And Solar cells are indeed pretty dark.</div><div><br></div><div>But, as Rory indicated, higher energy photons will only produce heat. This is because the chemical reactions powered by photosynthesis require a only certain amount of energy, and any excessive amount delivered by higher-energy photons cannot be simply used for another reaction but will yield heat. I don\'t know how much trouble that actually causes, but there is another point:</div><div><br></div><div>As explained, what determines the efficiency of solar energy conversion is not the energy per photon, but the amount of photons available.</div><div><br></div><div><div>The Irradiance is an energy density, however we are interested in photon density, so you have to divide this curve by the energy per photon, which means multiply it by `lambda`/(hc) (that is higher wavelengths need more photons to achieve the same Irradiance). If you compare that curve integrated over the high energy photons (say, `lambda` &lt; 580 nm) to the integration over the the low energy ones, you\'ll notice that despite the atmospheric losses (the red curve is what is left of the sunlight at sea level) there are a lot more \"red\" photons than \"green\" ones, so making leaves red would waste a lot of potentially converted energy.</div><div><br></div><div>Of course, this is still no explanation why leaves are not simply black - absorbing all light is surely even more effective, no? I don\'t know enough about organic chemistry, but my guess would be that there are no organic substances with such a broad absorption spectrum and adding another kind of pigment might not pay off.</div></div><div><br></div>'),(172,11,'CommentA','2013-08-27 16:30:41','I would also add the fact that blue light reaches earth surface greatly scattered due to Rayleigh scattering, so the absolute amount of energy carried blue light is comparable (or even less) than that of other parts of the visible spectrum.'),(173,11,'CommentA','2013-08-27 16:32:05','<span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(253, 253, 253);\">Also don\'t forget - evolution only produces something which is good enough - not optimal. If green turned out to be good enough, then there would be impetus to develop black leaves.</span><br>'),(174,11,'Script','2013-08-27 16:32:37','<div>I believe it is because of a trade off between absorbing a wide range of photons and not absorbing too much heat. Certainly this is a reason why leaves are not black - the enzymes in photosynthesis as it stands would be denatured by the excess heat that would be gained.</div><div><br></div><div>This may go some of the way towards explaining why green is reflected rather than red as you suggested - reflecting away a higher energy colour reduces the amount of thermal energy gained by the leaves.</div>'),(175,12,'CommentA','2013-08-27 16:33:51','Heat is the least of a plant\'s problems. That is why we can breed black tulips without self-cooking petals'),(176,12,'Script','2013-08-27 16:34:31','<div>There is quite a fun article here which discusses the colours of hypothetical plants on planets around other stars.</div><div><br></div><div>Stars are classified by their spectral type which is dictated by their surface temperatures. The Sun\'s is relatively hot, and it\'s spectral energy distribution peaks in the green region of the spectrum. However the majority of stars in the Galaxy are K and M type stars which emit mainly in the red and infrared.</div><div><br></div><div>This is relevant to this discussion since any photosynthesis on these worlds would have to adapt to these wavelengths of light in order to proceed. On planets around cool stars plant life (or its equivalent) might well be black!</div><div><br></div><div>OK, this is not entirely pie in the sky astrobiologist rubbish. It is actually quite relevant to the search for biosignatures and life on other planets. In order to model the reflectance spectrum of planets we observe (i.e. the light reflected from the primary star) we need to try and take into account any potential vegetation.</div><div><br></div><div>For example, if we take a reflectance spectrum of the Earth, we see a characteristic peak in the red \"the red edge\" which is due to surface plant life.</div>'),(177,13,'CommentA','2013-08-27 16:35:33','Maybe in most cases absorbing green was just the first thing that happened (by evolution) and it was sufficient, but I\'m just speculating...'),(178,13,'Script','2013-08-27 16:36:00','<div>There are two factors at play here. First is the balance between how much energy a plant can collect and how much it can use. It is not a problem of too much heat, but too many electrons. If it were a question of heat, a number of flowers selected for their black pigmentation would have their petals cooked off. ;)</div><div><br></div><div>If a plant does not have enough water, is too cold, is too hot, collects too much light, or has some other condition that prevents the electron transport chain from functioning properly, the electrons pile up in a process called photoinhibition.</div><div><br></div><div>These electrons are then transferred to molecules that they should not be transferred to, creating free radicals, wreaking havok within the plant\'s cells. Fortunately, plants produce other compounds that prevent some of the damage by absorbing and passing around the electrons like hot potatos. These antioxidants are also beneficial to us when we eat them.</div><div><br></div><div>This explains why plants collect the amount of light energy they do, but does not explain why they are green, and not grey or dark red. Surely there are other pigments that would be able to generate electrons for the electron transport chain.</div><div><br></div><div>The answer to that is the same as why ATP is used as the main energy transport molecule in organisms rather than GTP or something else.</div><div><br></div><div>Chlorophyll a and b were just the first things that came about that fulfilled the requirement. Certainly some other pigment could have collected the energy, but that region of parameter space never needed to be explored.</div>'),(179,14,'Question','2013-08-27 16:37:03','<div>All humans can be grouped into ABO and Rh+/- blood groups (at a minimum). Is there any advantage at all to one group or the other? This article hints that there are some pathogens that display a preference to a blood type (for example Schistosomiasis apparently being more common in people with blood group A, although it could be that more people have type A in the areas that the parasite inhabits). Is there any literature out there to support or refute this claim or provide similar examples?</div><div><br></div><div>Beyond ABO-Rh, is there any advantage or disadvantage (excluding the obvious difficulties in finding a donor after accident/trauma) in the 30 other blood type suffixes recognised by the International Society of Blood Transfusions (ISBT)?</div><div><br></div><div>I\'d imagine not (or at least very minimal) but it would be interesting to find out if anyone knows more.</div><div><br></div><div>http://biology.stackexchange.com/questions/714/is-there-any-advantage-to-one-blood-type-over-another<br></div>'),(180,15,'Script','2013-08-27 16:38:30','<div>The less antigens a woman (or in fact a female of any species close enough to humans for this phenomenon) has, the higher are the risks of triggering an immune reaction during her pregnancy, if the child has those antigens.</div><div>The Rhesus incompatibility is probably the most common case of this problem.</div><div><br></div><div>One could thus assume that in populations that are genetically diverse in respect to blood groups, absence of antigens has negative effects on reproduction (unless countered by medicine).</div>'),(181,15,'CommentQ','2013-08-27 16:39:38','Do you mean advantage in an evolutionary context?'),(182,3,'CommentQ','2013-08-27 16:40:45','That reminds me about the sickle-cell disease where the malformed red blood cells show much higher resistance against the malaria plasmodium.'),(183,3,'Script','2013-08-27 16:41:24','<div>These two papers study whether there is a relationship between the ABO blood type of a person and his/her attractiveness to mosquitoes. It was found that people with type O blood are more likely to be bitten than people with other blood types. Additionally, the relationship between blood type and the secretion of oligosaccharides from the skin had any effect on attractiveness to mosquitoes. It was found that type O secretors are very attractive to mosquitoes compared to type O nonsecretors and type A nonsecretors.</div><div><br></div><div>On the flip side, type A individuals are more susceptible to infection by plasmodia (i.e., malaria).</div>'),(184,4,'CommentQ','2013-08-27 16:42:19','Sickle cell is considered autosomal regressive in classical genetics, but this malarial resistance only applies to the heterozygous case (in which blood cells are also normal). The altered cell morphology occurs only in the homozygous recessive genotype.'),(185,4,'Script','2013-08-27 16:43:39','<div>I\'ve been doing a little more digging myself and have found a couple of other advantages:</div><div><br></div><div><b>Risk of Venous-thromboembolism</b> (deep vein thrombosis/pulmonary embolism). Blood group O individuals are at lower risk of the above conditions due to reduced levels of von Willebrand factor and factor VIII clotting factors.</div><div><br></div><div><b>Cholera Infection Susceptibility &amp; Severity</b>. Individuals with blood group O are less susceptible to some strains of cholera (O1) but are more likely to suffer severe effects from the disease if infected.</div><div><br></div><div><b>E. coli Infection Susceptibility &amp; Severity</b>. A study in Scotland indicated that those with the O blood group showed higher than expected infection rates with E. coli O157 and significantly higher fatality rates (78.5% of fatalities had blood group O).</div><div><br></div><div><b>Peptic Ulcers</b> caused by Heliobacter pylori which can also lead to gastric cancer. Group O are again more susceptible to strains of H. pylori.</div><div><br></div><div>Whether blood group antigens are displayed on other body cells or not has been linked to increased or decreased susceptibility to many diseases, notably norrovirus and HIV. This is fully explained in the article that I was above summarising - \"The relationship between blood group and disease\" in addition to extended descriptions of the other two answers.</div>'),(186,4,'Question','2013-08-27 16:45:17','<div>My biology teachers never explained why animals need to breathe oxygen, just that we organisms die if we don\'t get oxygen for too long. Maybe one of them happened to mention that its used to make ATP. Now in my AP Biology class we finally learned the specifics of how oxygen is used in the electron transport chain due to its high electronegativity. But I assume this probably isn\'t the only reason we need oxygen.</div><div><br></div><div>What other purposes does the oxygen we take in through respiration serve? Does oxygen deprivation result in death just due to the halting of ATP production, or is there some other reason as well? What percentage of the oxygen we take in through respiration is expelled later through the breath as carbon dioxide?</div><div><br></div><div>http://biology.stackexchange.com/questions/452/what-does-the-human-body-use-oxygen-for-besides-the-final-electron-acceptor-in-t<br></div>'),(187,5,'CommentQ','2013-08-27 16:45:54','Doesn\'t the body use sunlight and oxygen to create Peroxide in the skin for immune reasons? I heard this once from somewhere'),(188,6,'Script','2013-08-27 16:47:13','<div>Oxygen is actually highly toxic to cells and organisms – reactive oxygen species cause oxidative stress, essentially cell damage and contributing to cell ageing. A lot of anaerobic organisms have never learned to cope with this and die almost immediately when exposed to oxygen. One classical example of this is C. botulinum.</div><div><br></div><div>Oxygen is incorporated in several molecules in the cell (for instance riboses and certain amino acids) but as far as I know, all of this comes into the cell as metabolic products, not in the form of pure oxygen.</div><div><br></div><div>The oxygen (`O_2`) we breathe is (almost) completely transformed into `CO_2` and exhaled. The stoichiometry is (again, almost) 1:1.</div>'),(189,7,'Script','2013-08-27 16:49:02','<div>Superoxide, `O_2^−` is created by the immune system in phagocytes (including neutrophils, monocytes, macrophages, dendritic cells, and mast cells) which use NADPH oxidase to produce it from `O_2` for use against invading microorganisms. However, under normal conditions, the mitochondrial electron transport chain is a major source of `O_2^−`, converting up to perhaps 5% of `O_2` to superoxide. [1]</div><div><br></div><div>As a side note, there are two sides to this coin. While this is a useful tool against microorganisms, the formation of the reactive oxygen species has been incriminated in autoimmune reactions and diabetes (type 1). [2]</div><div><br></div><div><div>[1] Packer L, Ed. Methods in Enzymology, Volume 349. San Diego, Calif: Academic Press; 2002</div><div><br></div><div>[2] Thayer TC, Delano M, et al. (2011) Superoxide production by macrophages and T cells is critical for the induction of autoreactivity and type 1 diabetes,60(8), 2144-51.</div></div>'),(190,11,'Script','2013-08-27 16:55:38','<div>You probably know by now that cytochrome c oxidase, the last complex of the electron transport chain, belongs to a class of enzymes called oxidoreductases, that use oxygen atoms as electron acceptors. One type of oxidoreductases are oxidases, enzymes that (at least in theory [1]) use molecular oxygen--`O_2`, like in air--as their electron acceptor. From what I know, however, sometimes that isn\'t the case: xanthine oxidase, that converts xanthine to uric acid, gets its oxygen atoms from water [2]. Examples of the \"true\" oxidases include L-amino-acid oxidase and cytochrome P450 (aka. CYP family).</div><div><br></div><div>Despite cytochrome P450 being a numerous and important enzyme family, responsible for most of known drugs metabolism and some essential lipids transformations, it probably consumes only a fraction of oxygen that animals breathe in. I wasn\'t able to find any estimations, but would be surprised if it was more than perhaps 0,1%.</div><div><br></div><div><div>[1] Introduction to EC1 class</div><div><br></div><div>[2] Metz, S. &amp; Thiel, W. A Combined QM/MM Study on the Reductive Half-Reaction of Xanthine Oxidase: Substrate Orientation and Mechanism. J. Am. Chem. Soc. 2009, 131, 14885–14902, PMID: 20050623.</div></div>'),(191,12,'Script','2013-08-27 16:57:40','<div>The overwhelming use of oxygen is to provide us (in combination with food) with energy. We have a great need for energy in our cells, which is why we have these lungs, diaphragms, red blood cells, etc.; they assure we get the oxygen to obtain the energy (via the electron transport chain).</div><div><br></div><div>The overall metabolism of glucose (`C_6H_(12)O_6`) is a representative reaction:</div><div><br></div><div>&nbsp;`C_6H_(12)O_6 + 6 O_2 --&gt; 6 CO_2 + 6 H_2O +` energy</div><div><br></div><div>You can see that just as much oxygen goes out as gaseous `CO_2` as came in as gaseous oxygen (`O_2`).</div><div><br></div><div>The energy is temporarily kept in the form of the phosphate bond in ATP molecules so that it can be shuttled around the cell to the multitude of cellular processes that need energy.</div><div><br></div><div>Energy is so essential to the cellular processes that maintain animal cells that lack of that energy, which results quickly when there is oxygen deprivation, soon causes irreversible damage and death.</div>'),(192,13,'Question','2013-08-27 16:59:05','<div>I was walking down a road with these beautifully huge trees when this question occurred to me.</div><div><br></div><div>Large trees with many thick branches have to grow equally in all directions, or they would tip over. Is there some sort of mechanism to ensure this uniform growth? Or is it just a happy coincidence arising from uniform availability of sunlight on all sides? If one branch of a tree becomes too heavy due to many sub-branches, does this somehow trigger growth on the opposite side of the tree?</div><div><br></div><div>I have seen that potted plants in houses tend to grow towards sunlight. My mum often turns pots around by 180 degrees to ensure that plants don\'t bend in any one direction. I assume that this is because sunlight increases the rate of photosynthesis, leading to rapid growth of the meristem. For trees growing in open spaces, this wouldn\'t be a problem. But there are many large trees that grow in the shadow of buildings without bending away from the buildings, even though this is the only direction from which they would receive any sunlight. Is there any explanation for this?</div><div><br></div><div>http://biology.stackexchange.com/questions/1869/how-do-trees-manage-to-grow-equally-in-all-directions<br></div>'),(193,14,'Script','2013-08-27 17:00:16','Growth in plants is tightly controlled by auxins – plant hormones. Auxin itself usually has an inhibitory effect on growth. As far as I know there is no active control to restore plant symmetry once it has gone awry (but I could be wrong!) but the inhibitory effect of auxin synthesised at the meristem and diffusing in all directions causes a symmetrical pattern of inhibition and activation, forming shoots at symmetrical distances around the shoot apical meristem – this is very visible in the symmetry of the romanesco broccoli:<div><br></div><div><div>Furthermore, there are several mechanisms involving auxin which shape the general growth of the plant. The most important ones are:</div><div><br></div><div>Apical dominance which causes the apex (the stem) of the plant to grow more strongly than other parts of the plant, ensuring a general centring of the growth.</div><div><br></div><div>Phototropism causes the plant to grow towards sunlight. Unlike you hypothesised, this isn’t simply due to more photosynthesis and hence faster growth at the front of the plant facing the light, it’s actively controlled.</div><div><br></div><div>Gravitropism is a very interesting effect which causes the plant to grow generally upwards. It’s interesting because the mechanism is actually using gravity: the auxin synthesised at the meristem diffuses downwards in the plant due to gravity, inhibiting the growth in lower regions (but note that in the root apical meristem the effect is somehow reversed).</div><div><br></div><div>Hydrotropism causes the plant to grow towards water.</div><div><br></div><div>All these effects combined cause the plant to grow in a generally upwards, laterally distributed fashion.</div></div>'),(194,15,'CommentA','2013-08-27 17:01:09','This makes sense. The gravity thing is really interesting! Isn\'t there also a hormone which induces growth? I can\'t remember what it\'s called. Maybe that\'s more dominant in the root. Again, just hypothesizing.'),(195,15,'Script','2013-08-27 17:02:35','<div>There are some other good answers which provide part of the picture, but I think there is a fundamental organising principle which has been missed. Konrad has touched on it in his answer.</div><div><br></div><div>The reason trees, and most plants, tend to grow equally in all directions is that they have iteratively generated branching and radial symmetry which is controlled in a feedback loop of the growth promoting hormone auxin and auxin-sensitive auxin transporters. This is an elegant biological algorithm which explains all branching growth.</div><div><br></div><div>The things Konrad identifies (phototropism, gravitropism, etc.) serve as orientation cues which help the plant determine which axes to grow along, but fundamentally the process is about auxin gradients. There are exceptions, as others have pointed out in their answers, and they usually result from severe imbalances in the orientation cues.</div><div><br></div><div>I\'ll try to explain the growth process clearly (and it gives me an opportunity to try my hand at diagramming again ^_^)...</div><div><br></div><div>Auxin is a plant hormone (actually a class of hormones, but mostly when people say auxin, they mean indole-3-acetic acid) which promotes cell elongation and division. The basic principle which allows auxin to act in the organising way it does is that auxin is produced inside cells, and proteins which export auxin from a cell develop on the side of the cell which has the highest auxin concentration (see figure below).<br></div><div><br></div><div><div>So auxin gets transported up the concentration gradient of auxin! Thus if you get an area of high auxin concentration developing somehow, more auxin is then transported towards that area. An area of high auxin concentration relative to the surrounding tissue is called an auxin maximum (plural \'maxima\').</div><div><br></div><div>For most of the life of the plant, auxin is produced pretty much equally in most cells. However, at the very early stages of embryo development, it gets produced preferentially along the embryonic axis (see figure below, part 1). That creates a meristem - a group of cells where cell division is taking place - at the auxin maximum at each end of the embryo. Since this particular meristem is at the apex of the plant, it is called the apical meristem, and it is usually the strongest one in the plant.</div></div><div><br></div><div><div>So by having a meristem at each end, the embryo then elongates as cell division is only taking place at those points. This leads to part 2 of the image above, where the two meristems get so far apart that the auxin gradient is so weak as to no longer have its organising effect (area in the red square). When that happens, the auxin produced in cells in that area concentrates in a chaotic way for a short time until another center of transport is created. This happens, as the first one did, when a particular area of the tissue has a slightly higher concentration of auxin, and so auxin in the surrounding tissue is transported towards it. This leads to part 3 of the figure, in which two new meristems are created on the sides of the plant (called lateral meristems).</div><div><br></div><div>Lateral meristems are where branches occur on plants. If you then imagine this process continuing to iterate over and over, you will see that the branches, as they elongate, will develop meristems at the tips and along the sides. The main stem will also continue elongating, and develop more lateral stems. The root will begin to branch, and those branches will branch, etc. If you can understand how this elegant system works, you understand how plants grow, and why they grow in repeating units as opposed to in a body plan like animals.</div><div><br></div><div>It also explains why, if you cut off the tip of a stem, it promotes branching. By removing the apical meristem, you get rid of the auxin gradient and enable the creating of multiple smaller meristems which each develop into branches.</div><div><br></div><div>So far I\'ve explained regular branching, but the same system causes the radial symmetry which makes trees (usually) grow in all directions equally...</div></div><div><br></div><div><div>Imagine taking a cross section through a stem and looking down all the way through it (as depicted crudely above). Just as auxin gradients act to coordinate growth along the length of the plant, they also coordinate it radially, as the maxima will tend to space themselves out as far from one another as possible. That leads to branches growing in all directions equally (on average).</div><div><br></div><div>I welcome comments on this answer, as I think its so important to understanding plant growth that I\'d like to hone my answer to make it as good as possible.</div></div>'),(196,3,'CommentA','2013-08-27 17:03:36','Giberellic acid is another plant hormone that can induces plant cell growth or elongation, among many other functions.'),(197,4,'Script','2013-08-27 17:04:50','So far, it hasn\'t fallen over yet. The reason it grows that way is because all the light is coming from the right side of the picture: the tree leans roughly to southeast, while the building is to southwest of it and casts a shadow on the center of the yard for much of the day. Also, to the left you can see the branches of other, taller trees that block pretty much all scattered skylight from that direction.'),(198,5,'Script','2013-08-27 17:05:39','As shown above, that is not always the case. most trees grown next to the shady sides of buildings tend to lean away from the building. Some trees can stand more shade than others, and these will send branches into the shade of buildings. They never grow as dense or robust in the shade. The reason for the plants growing toward the light is Phototropism. When light hits the plant stem, the auxins that determine the length of the stem are destroyed, decreasing the amount of growth on that side of the stem. When this happens, the stems bend toward the light source from the imbalance of auxins in the stem. The reason that the shady side is weaker is that all trees rely on photosynthesis for their energy. Of course, the side without direct sunlight will not perform as much photosynthesis, and so has that much less energy to grow with. Also, many large trees are imbalanced, sometimes all of the weight on one side. The roots hold them up.'),(199,6,'Question','2013-08-27 17:06:40','Does someone know why DNA is composed of four nucleobases? In particular, is there an explanation for the number? Why four and not two, or eight?<div><br></div><div>http://biology.stackexchange.com/questions/2874/why-are-there-exactly-four-nucleobases-in-dna<br></div>'),(200,7,'Script','2013-08-27 17:07:23','<div>A now deleted answer put this down to an argument based on amino-acid coding in codons but, as pointed out by Konrad Rudolph in a comment, arguments based on codons cannot be correct because the 4-base system likely pre-dates the evolution of protein translation.</div><div><br></div><div>So why four?</div><div><br></div><div>I suggest that it comes from a combination of factors which make it a \'just right\' fit for RNA-based replicators: more would be bad, and less would also be bad. The four bases are able to copy with high reliability because two features combine to exclude mis-pairings: there are two size classes - meaning any purine-purine or pyrimidine-pyrimidine pairing results in incorrect separation between the strands, excluding A-G and C-U bonds; and there are two classes based on number of OH-bonds - which limits the potential for A-C and G-U bonds. More bases must, inevitably, weaken the strength of these exclusionary approaches and increase the number of mispairings. RNA world replicators will have lacked the sophisticated repair and error checking mechanisms of modern lifeforms so this increase in mispairings would likely not be corrected.</div><div><br></div><div>Of course, a two base system would be even more capable of reducing errors. I suggest that the advantage of the four base system is that it allows for much more complex 3-dimensional structure to form in RNA and thus enabled a broader range of catalytic capabilities. I\'ve only considered even-numbers because only even numbered systems could use the Watson-Crick style of base pairing.</div><div><br></div><div>I\'d also suggesting reading Eörs Szathmáry\'s 2003 paper on this very question.</div>'),(201,11,'Script','2013-08-27 17:08:22','<div>There is a chemical dimension to this question too.</div><div><br></div><div>If you look at the Watson-Crick Base pairs you can see that there isn\'t a lot of wiggle room:</div><div><br></div><div><div>The nucleotide bases have 2 or 3 hydrogen bonds. It\'s probably not sterically reasonable to have 4.</div><div><br></div><div>That means there are only a limited number of ways the base combinations can be complimentary and also specific as they form the double helix. Since A-&gt;T and T-&gt;A hydrogen bond patterns take up both donor-receptor combinations there is one possible 2 H-bond pairing.</div><div><br></div><div>There are 2 ways to configure the three hydrogen bond bases I think, so perhaps six is possible, but I\'m guessing that if you expand the base pairing repetoire, one could start to lose specificity. As it is, if you play with nucleotides enough, you can make non Watson-Crick base pairs and probably lose some of the confidence you have exact 1:1 matches between complimentary base pairings. Wobble Base pairs in RNA and Hoogsteen Base Pairings can already be demonstrated to allow triple helixes and non-perfect match RNA helices.</div></div>'),(202,12,'Script','2013-08-27 17:11:00','<div>Here is a possible answer given by this paper:</div><div><br></div><div>http://www.ncbi.nlm.nih.gov/pubmed/16794952</div><div>or http://www.math.unl.edu/~bdeng1/Papers/DengDNAreplication.pdf</div><div><br></div><div>It gives a Darwinian explanation to the question. It approaches the problem from Claude Shannon\'s theory for communication. It treats DNA replication conceptually and mathematically the same as a data transmission. It concludes that the system of four bases, not two, not six, replicates the most genetic information at the shortest amount of time.</div><div><br></div><div>The communicational analogy goes like this. If you have two data transmission systems, one can transmit, say, 1 MB per second, and the other can do 2 MB per second but cost less than twice as much. The answer is obvious you will buy the second service for a higher rate per cost. As a data service, it does not care what information you consume -- it can be spam, video, audio, etc. All that matters is the transmission rate. As for DNA replication, it is like a data transmission channel when one base is replicated a time along the mother DNA template. It too does not care whether the process is for a bacterium genome, or a plant, or an animal genome. The pay-off is in information and the cost is in time. Unlike your abiotic communication varieties, time is both the sender and the receiver of all messages of life, and different life forms or species are merely time\'s cell phones. So if one system can replicate more information in a unit time than another, the faster one will win the evolutionary arm race. A prey operating on a slow replicator system will not be able to compete with nor to adapt to a predator operating on a fast one.</div><div><br></div><div>Now because the A-T pair has only two weak hydrogen bonds but the C-G pair has three, A and T take a shorter time to complete duplication than the C and G do. Although the replication time is short in some fraction of nano second, but the time adds up quickly for genomes with base pairs in the billions. So having the C-G pair may slow down the replication, but the gain is in information. One base pair gives you 1 bit per base information. Two pairs gives you 2 bits per base information. But, having more base pairs may eventually run into a diminished return in information replication rate if the new bases take too long a time to replicate. Hence the consideration for the optimal rate of replication measured in information bits per base per time. Without information there would be no diversity, no complexity. Without replication in information there would be no life.</div><div><br></div><div>Using a simple transmission/replication rate calculation by Shannon you can calculate the mean rate for the AT-system, the CG-system, the ATCG-system, and for some hypothetical 6-bases, 2n-bases system whose new bases take progressively longer time to replicate. The analysis shows the ATCG-system has the optimal replication rate if the CG bases take 1.65 to 3 times longer to replicate than the AT bases. That is, a base-2 system replicates its bases faster but does not carry more information to have a higher bit rate. Likewise, a base-6 system has a greater per-base information but replicate slower on average to end up with a suboptimal bit rate.</div><div><br></div><div><div>According to a comparison from the paper, the base-4 system is about 40% faster than the A–T only system, and 133% faster than the G–C only system. Assume life on Earth started about 4 billion years ago, then the A-T only system would set back evolution by 1 billion years, the G–C system would do so by 2.3 billion years. For a hypothetical base-6 system, it would do so by 80 million years. In other words, life is where it should be because the base-4 system is able to transmit information through the time bottleneck at the optimal bit rate.</div><div><br></div><div>In conclusion, life is to replicate the most information with the shortest time, and the base-4 system does it the best. If ever there were other systems they would have lost the informatic competition to the base-4 system from the get-go. Darwin\'s principle works at life\'s most basic and most important level.</div><div><br></div><div>There are other explanations, all non-Darwinian. Most are based on the base\'s molecular structures. But these types of explanation border on circular argument -- using observations to explain themselves. They also face this catch-22 problem since there is no way to exhaust all possible bases for replication. However, such lines of exploration are fruitful regardless because more knowledge the better. But without taking information and its replication into consideration it is hard to imagine a sensible answer to the question.</div></div>'),(203,13,'CommentA','2013-08-27 17:12:05','Cool explanation.'),(204,14,'Script','2013-08-27 17:12:54','The 4-bases DNA system with A-T bonds and C-G bonds is the one that evolved to be used by most living creatures on Earth, as mentioned in other answers, because it can encode a triplet table of bases for all aminoacids used, allowing for some aminoacids to have more than one triplet code. There are though slight variations of the system: even though the A-T and C-G bonds don\'t change, the nucleotides can be modified to mark areas of the genome in what is part of the epigenetic code. The most common modification is adding methyl and 5-hydroxymethyl molecules to the bases, although more modifications are still being discovered.'),(205,15,'Question','2013-08-27 17:14:01','Somewhere I have read we share more than 99% of our genes with every other other person and 98% of our genes with chimpanzees. What does this mean? Don\'t we share 50% of our genes with our mother and 50% with our father?<div><br></div><div>http://biology.stackexchange.com/questions/1981/how-many-genes-do-we-share-with-our-mother<br></div>'),(206,3,'Script','2013-08-27 17:18:15','<p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 19.59375px; background-color: rgb(253, 253, 253);\">There is a distinct difference between the \'genes\' that we share, and the genetic code (the DNA) that the genes are made of.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 19.59375px; background-color: rgb(253, 253, 253);\">All humans (excluding genetic disorders) have the same genes, but not everyone has exactly the same code - there are tiny differences between individuals, and these are manifested in the different traits you can observe between people (eye colour, height, etc).</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 19.59375px; background-color: rgb(253, 253, 253);\">So you therefore have 100% of the genes that your mother has.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 19.59375px; background-color: rgb(253, 253, 253);\">However, as stated in the first answer, you inherit the different \'alleles\', or versions, of the genes from your parents, and end up with ~50% of the alleles from each parents (but all the genes).</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 19.59375px; background-color: rgb(253, 253, 253);\">With regard to species differences; many of the genes we inherit have evolved over millions (in fact billions) of years, and thus many of our genes are present in most other organisms (but in very different forms - the code/DNA). Chimpanzees are our closest relatives in evolutionary terms, and thus their genes are very similar to ours in the code (~98% the same). But this only applies to the coding regions! Less than 2% of your genome actually codes for genes - the rest is mostly regulatory (not junk, as it used to be called), and this is where the true inter-species variation lies. So whilst we have ~98% homology in the protein-coding regions, this is MUCH less if you count the whole genome.</p><p style=\"margin-bottom: 1em; padding: 0px; border: 0px; vertical-align: baseline; clear: both; word-wrap: break-word; color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 19.59375px; background-color: rgb(253, 253, 253);\">I hope that helped!</p>'),(207,4,'CommentA','2013-08-27 17:19:04','Actually, recent analysis of 1000 Genomes data shows that each person has some 25-40 homozygous LOF (loss of function) variants not found in either parent. Thus, one could lower the 100% figure to ~99.9% shared.'),(208,4,'Script','2013-08-27 17:20:11','You get 50% of our genes from each respective parent (disregarding mutations for now). But if you shared only 50% with each parent, this would imply that your parents don’t share a single gene:<div>But your parents do share genes – or rather, alleles – which is why the you also share more than 50% of the alleles with each of your parent.<br></div>'),(209,5,'CommentA','2013-08-27 17:21:00','How many kinds of genes are there in humans? If we all have same kind of genes, so is it just the combination of these genes that affect our personality?'),(210,5,'CommentA','2013-08-27 17:21:15','Very good point! I had not thought it before. I had too the same misunderstanding.'),(211,5,'Script','2013-08-27 17:21:30','<div>In every cell of your body, you have two physical copies of every gene (ignoring gametes, / copy number variations), one from your mother, one from your father. (Humans are diploid.) That\'s why it\'s correct to say that you got 50% from either parent (ignoring the 13 mitochondrial genes that are inherited from the mother only).</div><div><br></div><div>The second approach is to look at genes as abstract entities. In the abstract sense, you share 100% of your genes with fellow humans, and more distant species will have a number of different genes. (This difference can be quantified in many different ways.)</div>'),(212,6,'CommentA','2013-08-27 17:23:31','<span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(253, 253, 253);\">I think there are around 23,000 accepted protein-coding genes in humans. We each have a copy of every gene. But at the DNA level the genes can vary ever so slightly, and this is where the different</span><i style=\"margin: 0px; padding: 0px; border: 0px; font-size: 13px; vertical-align: baseline; color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 17px; background-color: rgb(253, 253, 253);\">alleles</i><span style=\"color: rgb(68, 68, 68); font-family: \'Helvetica Neue\', Arial, sans-serif; font-size: 13px; line-height: 17px; background-color: rgb(253, 253, 253);\">&nbsp;come from (slight variations in the same trait). With regard to your personality, that is another kettle of fish: your genes can certainly affect your predisposition to certain mentalities, but it should be fairly clear that it is your life experiences that mostly affect how you develop, and thus what kind of person you become. And \'code\' is a bit layman, but it is \'correct\'</span>'),(213,6,'CommentA','2013-08-27 17:23:50','<font color=\"#444444\" face=\"Helvetica Neue, Arial, sans-serif\" size=\"2\"><span style=\"line-height: 17px;\">Also, the DNA formerly known as junk is called heterochromatin. This was long thought to be inactive but I believe that is has now been shown to contain protein coding genes, not just regulatory elements (I spoke to Gary Karpen in the summer and think this is where I heard it). It is quite an unknown entity because it is so damned hard to sequence thanks to lots of repeating units!</span></font><br>'),(214,6,'Script','2013-08-27 17:25:35','<div>Say x is the percentage of an allele in your mom (or cousin, or brother). Say c is the filial similarity (brother=.5, son=.5, cousins=.125, etc.) of the allele. Say y is the general probability of having that allele in that population (assuming mom is the same species with you). Say f(x,y) is the expected value of the number of allele that you have.</div><div><br></div><div>Then f(x,y)= c * x+ (1-c) *y</div><div><br></div><div>In other word. There is no contradiction in the idea that we are 98% similar with monkeys and yet only 50% similar with our own mom.</div><div><br></div><div>Here, the world similar is used in totally different sense.</div><div><br></div><div>f(x,y) is 99% for most allele. Now let gy(x)=f(x,y) then</div><div><br></div><div>gy\'(x) is c.</div><div><br></div><div>In other word, for every allele your mom have, it\'ll improve the expected value of you having the same allele by half. For each allele your cousin has, it\'ll improve the expected value of you having the same allele by 1/8th. That is for the same y. For most y, similarity is 100% nevertheless.</div><div><br></div><div>Say Ann, Beth, and Cindy has AA, Aa, and aa alleles.</div><div><br></div><div>Then Ann\'s sons have 25% higher expected value of having A alleles than Beth, and Beth have 25% higher expected value of having A alleles than Cindy. I say nothing of actual probability distribution.</div><div><br></div><div>Ann\'s cousins have .0625% higher expected value of A occurrence than Beth\'s cousins and .125% expected value of A occurrence than Cindy\'s cousins</div><div><br></div><div>Disclaimer: We do not take into account that people mate to those who are genetically similar but not to similar.</div><div><br></div><div>Another way to see this is to look at y. For rare genes y is small. Hence.</div><div><br></div><div>50-50 is for genes that are rare and family specific. If your mother is color blind (100% carrier), the expected value of the number of color blind carrier is improved by 50%. It doesn\'t mean you\'ll be color blind. We\'ll have to go to the technicality of dominant vs recessive. But that\'s the idea.</div><div><br></div><div>For genes that are NOT rare, say genes that make you have 2 feet and 2 hand, you still share all your mom\'s genes. That\'s because everybody have that. Your mom have that, and your dad have that, and so is everyone else, including chimps.</div><div><br></div><div>Is this what most directly answer your question?</div><div><br></div><div>Again the issue is rarity. For rare genes P(You have it|Mom have it) is 50%.</div><div><br></div><div>For common genes,</div><div><br></div><div>P(You have it|Mom have it) = P(You have it and Mom have it)/P(Mom have it). //By bayesian rule</div><div><br></div><div>That is 1/1, which is true.</div><div><br></div><div>It\'s just obvious, probability =1, that everybody has it.</div><div><br></div><div>Source: selfish gene by richard dawkins I am a mathematician. Now proof me wrong.</div>'),(215,7,'Question','2013-08-27 17:29:17','<div>When people try to explain evolution, they tell me that evolution is a cumulative result of mutations &amp; natural section of the more superior individuals of a particular species. I think I\'m fairly convinced with this explanation.</div><div><br></div><div>But when I think about it, all of them assume that there was an organism, however simple, that was capable of self replication &amp; occasionally mutate. How did such an organism come into existence? Can anyone explain this?</div><div><br></div><div>http://biology.stackexchange.com/questions/2790/how-did-the-first-self-replicating-organism-come-into-existence<br></div>'),(216,11,'Script','2013-08-27 17:30:39','<div>Evolution or (as Darwin called it) \"descent with modification\" is a theory which explains the origin of the species NOT the origin of life. How the first life arose is completely irrelevant to the theory of evolution. What evolution does explain is how and why we have such variety of life on earth all descending from the same organism.</div><div><br></div><div>What you\'re asking about is not a theory of \"evolution\" but rather a theory of \"abiogenesis.\" Although there are many interesting hypothesis for how abiogenesis happened (e.g. the RNA world, the \"metabolism first\" theory, etc.), the fact is we simply do not know yet how life first arose. What we do know is that life first arose between 3.5 and 3.9 billion years ago. That\'s a really long time ago compared to lots of other important events in natural history (even the Cambrian explosion where modern animal phyla evolved was only half a billion years ago), and so it shouldn\'t be surprising that it\'s a hard problem.</div>'),(217,12,'CommentA','2013-08-27 17:31:32','<font color=\"#444444\" face=\"Helvetica Neue, Arial, sans-serif\" size=\"2\"><span style=\"line-height: 17px;\">We know that organisms with the required properties existed 3.4 billion years ago, by looking at fossils of bacteria. There\'s no unproven assumption there. Evolution describes the most recent 3.5-4 billion years of natural history. To go beyond that you need another theory. That\'s fine, that\'s how science works. You wouldn\'t say that gravity relies on unproved assumptions because it doesn\'t explain electricity or radioactive decay; it\'s not supposed to, it\'s supposed to explain gravity.</span></font><br>'),(218,12,'Script','2013-08-27 17:32:17','<div>We don\'t know how self-replicating molecules first arose (and probably never will know exactly) but the Earth is large and had 500 million years (i.e. the prebiotic Earth timescale) or so to experiment in organic chemistry. The land-sea interface (such as tidal pools) are a good candidate site since these are areas where high concentrations of organic goodies can be found.</div><div><br></div><div>In this context, one focus that researchers have been looking at is self-replicating molecules.</div><div><br></div><div>For example, one lab in Cambridge,UK has come up with tC19Z.</div><div><br></div><div>tC19Z is the name of a RNA enzyme that acts like a self-replicating molecule. It can copy chunks of RNA almost 50% as long as itself. It can also make copies of other RNA enzymes. This molecule is not \"alive\" itself, but clearly demonstrates how greater complexity can arise.</div>'),(219,13,'Script','2013-08-27 17:33:48','<div>They teach us in Physics that the entropy of an isolated system is always increasing or at least constant. Then how can an organism be born under these conditions?</div><div><br></div><div>The sun sends energy to the Earth, allowing for a decrease in entropy on Earth at the expense of the sun\'s entropy.</div><div><br></div><div>But when I think about it, all of them assume that there was an organism, however simple, that was capable of self replication &amp; occasionally mutate. How did such an organism come into existence? Can anyone explain this?</div><div><br></div><div>That organism you\'re talking about is just a molecule that copies itself. Exactly how it has come about is not clear to me but it\'s not hard to imagine the possibility. A vast planet with molecules flying all over being bathed in ultraviolet light and if any molecule anywhere acquires the characteristic of copying itself, it will start growing exponentially and quickly spread all over the world.</div>'),(220,14,'Script','2013-08-27 17:35:35','<div>This is an extremely interesting and extremely fundamental question, indeed, and thus far, biologists have failed at coming up with a satisfying answer.</div><div><br></div><div>We know that all the parts are there, we just don\'t know how they were arranged, or which ones go where.</div><div><br></div><div>The question is, in essence, composed of three sub-questions:</div><div><br></div><div>How did the fundamental building blocks of life come about?</div><div>How did the first self-replicating molecules come about?</div><div>How did cell membranes come about?</div><div>The answer generally takes the form of \"On primordial Earth, a small selection of the billions of organic compounds generated when UV-light hits a mess of carbon dioxide, nitrogen and water where captured in a tide pool where concentration and foam led to random chance producing self-replicating molecules in proto-cells.\"</div><div><br></div><div>This answer, while almost certainly true, is also incredibly dissatisfying, because all it tells us is what deductive logic has already taught us, almost intuitively.</div><div><br></div><div>Incidentally, the fact that all of this happens with a million to one odds isn\'t a problem: The Earth is big, and the time frame for this happening is along the lines of hundreds of millions of years: Anything that might happen once per year by a million to one shot would likely happen hundreds of times in that timeframe.</div><div><br></div><div>In any case, when it comes to evolution, or Darwin\'s Theory of Evolution, or any other theory of evolution, this is all irrelevant.</div><div><br></div><div>Evolution is something that happens in any sufficiently complex (open) system, assuming it has the capacity to change at all.</div><div><br></div><div>It is most easily observed in living organisms, because they are at the right scale, and incredibly diverse, but it happens on all scales of the universe.</div><div><br></div><div>In fact, the easiest way to explain how life first originated, is just to keep counting backwards when you reach the Last Common Ancestor (of All Life on Earth), and propose models for how this proto-bacterium could be even simpler, until you\'re left with `CO_2`, `N_2` and `H_2O`, and other simple molecules.</div><div><br></div><div>At that end of the spectrum it is well-understood that e.g. `H_2O` \"evolves\" from `H_2` and `O_2`, because `H_2O` has a quality that makes it more \"fit\" than either of its components, chemical stability.</div><div><br></div><div>Furthermore, H2 \"evolves\" from free hydrogen by a similar mechanism, and free hydrogen \"evolves\" from protons and electrons, because it has the property of being electrically neutral, which is also a desirable property.</div><div><br></div><div>Of course, at the level of protons and electrons, things get a little muddy, and evolution kind of breaks down as a method for explaining how things come about.</div><div><br></div><div>Edit: For reference: Current Models of Abiogenesis on Wikipedia.</div>'),(222,15,'Script','2013-08-27 17:36:56','<div>While many point to RNA, or a variant of it, as being the first molecule of \"life\" very few people know where it came from. Some suggest that it came from outer space because it\'s uncertain how the material for sugar-phosphate backbones could have developed on earth and that the perhaps these materials found their way here via meteorites. There are several hypotheses as to how an early earth environment may have promoted the properties of these molecules, but it\'s difficult to ascertain what exactly happened.</div><div><br></div><div>Somewhere within that abiogenesis wikipedia article is the mention of the role of deep-sea vents. The deep-sea vents and the currents that surround them basically facilitated a PCR reaction. Some of the early emerging DNA, maybe with some RNA and free nucleotides floating around, could have leveraged such an environment for replication and by sheer number (and some chance) entered into symbiotic relationship with other molecules to form the first cellular structures.</div>'),(223,3,'Question','2013-08-27 17:48:49','The dinosaurs, mammoths, giant plants etc are known to be bigger than modern animals. I wonder why they had been lived and why they are not living now? I really don\'t know much but is it something about oxygen balance or something similar?<div><br></div><div>http://biology.stackexchange.com/questions/1767/what-was-the-reason-for-some-plant-and-animals-to-become-giant-in-course-of-evol<br></div>'),(224,4,'Script','2013-08-27 17:50:13','<div>Thanks for asking an interesting question which made me think.</div><div><br></div><div>The short answer is that something evolves if there is an advantage to the genes involved, and, by \'advantage\' I mean it produces more copies of the genes in the next generation so more individuals with that characteristic will be present in the population.</div><div><br></div><div>As to what particular advantage increased body mass had to any particular species that would depend on the particular species and its environment at the time. There is almost certainly not a \'one-fits-all\' answer.</div><div><br></div><div>Most very big land mammals are herbivores and there is an advantage in having a large digestive tract when you eat a lot of plant material with a fairly low nutrient level. However, when there are also predators around, getting big and slow might be more of a disadvantage than staying small and fast. With evolution there is often a balance of competing forces which are traded of against one another.</div><div><br></div><div>Where large mammals have become isolated on islands with no predators, such as a species of now-extinct elephant on the Flores Island, they tend to become smaller so this suggests avoiding predators by simply being too big for them to kill might be an important factor.</div><div><br></div><div>And of course with many species we have the effect of sex-selection. If females select for the biggest males, there will be a selection pressure towards greater size.</div><div><br></div><div>With plants it could be something else entirely. Trees, for example, have long trunks basically for one of two reasons.</div><div><br></div><div>Firstly to put their leaves and seeds out of the reach of herbivores, so we have a kind of arms race like that between giraffes and acacia trees in Africa.</div><div><br></div><div>Secondly, when they grow in close clumps like woods and forests, there will be competition for sunlight and soil resources like water. A species which can reach higher and put it\'s leaves above those of it\'s neighbours will win the competition and there will also then be pressure on it\'s neighbours to evolve longer trunks. Long trunks mean that water must be pumped a long way to the leaves so the tree with the longest trunks will also need bigger root systems. Again a trade off between making a long trunk and dominating the water supply underground.</div><div><br></div><div>Many trees are also believed to have formed symbiotic associations with fungi in their root systems which help them take in the nutrients and water required to \'service\' a long trunk and high-up leaf crown so they may be the only types of plants capable of doing this anyway.</div><div><br></div><div>One thing you might like to think about is why some whales are very big and some are relatively small (dolphins, porpoises, killer whales). What is the difference in their life-style, food, habitat, and so on, which could drive these evolutionary differences.</div>'),(225,5,'CommentA','2013-08-27 17:51:17','Thank you so much for such a detailed answer.'),(226,5,'Script','2013-08-27 17:51:44','<div>to expand on Rose\'s answer, certain environmental factors are what people cite with unusual size changes in an animal over time as it diversifies into new species.</div><div><br></div><div>Gigantism (and dwarfism) on islands are a particular example. In this case an animal may arrive on an island and some of the predators or competitors that it is used to will not be there. Then a single species will spin off many species, each one will fill one of the niches that are unfilled on the island - eating bugs from bark, breaking open nuts. The lack of large carnivores will allow a nice juicy herbivore to get larger without fear of being eaten. Similarly the Moa was a bird that became an aggressive carnivore in its island habitat, where it\'s very rare to see an aggressive land bird when a big cat or other carnivore is already present. Such large changes in environment can also result in dwarfism - smaller species as smaller animals like squirrels or mice might not be present to consume all the food that are available. The Hobbits recently discovered in Indonesia are an example of island dwarfism.</div><div><br></div><div>That being said, selective pressure can probably make many animals larger or smaller - lots of the dog breeds are smaller than wolves that they descended from. I think steers are as large as we can make them and are larger than native cattle stock.</div><div><br></div><div>As far as the environment that caused dinosaurs to be so large, the higher levels of oxygen on the planet at the time is a common theory for why they were so large. I also think that at least early on, the selection conditions that favored larger animals were probably put in place as animals evolved without any competitors whatsoever (first animals on land or sea). If this were to happen again, its not clear whether dinosaurs would have been so large - perhaps we could have had gigantic mammals or descendants of trilobites.</div>'),(227,6,'Script','2013-08-27 17:59:28','<div>It needs to be recognized that this is an outstanding question in biology, i.e., there is no certain answer.</div><div><br></div><div>This review considers the various theoretical and empirical evidence for phenotypic selection on body size:</div><div><br></div><div><i>Blanckenhorn WU (2000) The Evolution of Body Size: What Keeps Organisms Small? Quarterly Review of Biology 75: 385-407.</i></div><div><i>Some have hypothesized a connection between body size and the evolution of endothermy:</i></div><div><i><br></i></div><div><i>Clarke A, Portner HO (2010) Temperature, metabolic power and the evolution of endothermy. Biological Reviews 85: 703-727.</i></div><div><i>And others have suggested a link to oxygen in the atmosphere:</i></div><div><i><br></i></div><div><i>Harrison JF, Kaiser A, VandenBrooks JM (2010) Atmospheric oxygen level and the evolution of insect body size. Proceedings of the Royal Society B: Biological Sciences 277: 1937.</i></div><div><br></div><div>And I\'ll throw this hypothesis into the ring. The large dinosaurs only went extinct 65 million years ago. That\'s basically yesterday in terms of the geological time scales across which body size may evolve - they (dinosaurs) originally evolved 230 million years ago. That means we\'ve got at least 165 million years to go before you can see if our current diversity of life on earth can evolve similarly large organisms!</div>'),(228,7,'Question','2013-08-27 18:03:49','<span style=\"color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 19.59375px; background-color: rgb(253, 253, 253);\">We all know that DNA acts as a genetic molecule. Does DNA have any other function in the cell other than being a genetic material and carrier of information?</span><div><span style=\"color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 19.59375px; background-color: rgb(253, 253, 253);\"><br></span></div><div><span style=\"background-color: rgb(253, 253, 253);\"><font color=\"#111111\" face=\"Helvetica Neue, Arial, sans-serif\"><span style=\"line-height: 19.59375px;\">http://biology.stackexchange.com/questions/8996/is-there-any-other-function-of-dna</span></font><br></span></div>'),(229,11,'Script','2013-08-27 18:12:37','<div>In eukaryotes DNA has a structural as well coding function. Parts of chromosomes called centromeres bind to proteins and form a scaffold which helps chromosomes attach to each other and correctly segregate during division. Technically this is still related to the transmission of genetic information though.</div><div><br></div><div>Since RNA can act as an enzyme (the so-called ribozymes), scientists have hypothesized that DNA can also have an enzymatic function, but no such function has been found in nature. Artificial DNA enzymes (deoxiribozymes) have been designed to cleave DNA, covalently modify proteins, ligate RNA molecules, and even catalyze reactions between small molecules.</div>'),(230,12,'Script','2013-08-27 18:13:41','DNA has been shown to be important for biofilm formation in certain bacteria. This is extracellular DNA that comes from cell lysis.'),(231,13,'Script','2013-08-27 18:15:19','<div>DNA has the following functions other than genetic material :</div><div><br></div><div>It can function as a molecule that attaches to TLR ( Toll like Receptor ) on or in cells and so can contribute to innate immune response.</div><div>It can act as carbon source. ( We have nucleases enzymes in our intestines.)</div><div>It can be structural component of biofilms.</div><div>It can also influence pH in cells as it is an acid.</div><div>It is a structural component of neutrophil extracellular traps to wrap up microbes.</div>'),(232,14,'Script','2013-08-27 18:16:12','Yes, maintenance of the genome. But not DNA specifically, but DNA and all of its associated proteins.'),(233,15,'Question','2013-08-27 18:19:37','<div>The question is rather straight forward: I have always been curious as to why, but cannot find an explanation online.</div><div><br></div><div>I can imagine that the mechanism is different for each, but why does brain tissue and red blood cells use specifically and only glucose for energy metabolism?</div><div><br></div><div>http://biology.stackexchange.com/questions/2935/why-cant-the-brain-and-red-blood-cells-use-fuels-other-than-glucose<br></div>'),(234,3,'Script','2013-08-27 18:20:22','<span style=\"color: rgb(17, 17, 17); font-family: \'Helvetica Neue\', Arial, sans-serif; line-height: 19.59375px; background-color: rgb(253, 253, 253);\">In the case of red blood cells: human erythrocytes (red blood cells) have no mitochondria. Since the mitochondria are the cellular site for oxidative metabolism of fatty acids, erythrocytes cannot oxidise fatty acids to release energy. The erythrocytes also cannot fully oxidise glucose (to carbon dioxide and water) because this is also a mitochondrial process, so they have to rely upon anaerobic glycolysis. The end product of anaerobic glycolysis is pyruvate, and erythrocytes reduce this to lactate (to recycle the NADH that is produced during glycolysis) and then export this lactate into the blood for further metabolism by the liver.</span>'),(235,4,'Script','2013-08-27 18:21:17','<div>The brain and heart can take advantage of ketone bodies when the amount of glucose is low. These are byproducts of fat metabolism and can be converted to acetyl-coA via the citric acid cycle.</div><div><br></div><div>Overproduction of these products can cause pathological conditions:</div><div><br></div><div>When the rate of synthesis of ketone bodies exceeds the rate of utilization ,their concentration in blood increases , this is known as ketonemia. This is followed by ketonuria- excretion of ketone bodies in urine. The overall picture of ketonemia and ketouria is commonly referred as ketosis. Smell of acetone in breath is a common feature in ketosis.</div>'),(236,5,'Script','2013-08-27 18:22:20','Lipid catabolism provides energy mainly through fatty acid beta-oxidation. This process goes through a spiral of four mitochondrial enzymatic steps: one is catalyzed by the mitochondrial trifunctional3-ketoacyl-CoA thiolase (gene HADH). The activity of this thiolase is very low in neurons, explaining the brain need of alternative energy sources. Yang et al., JBC 1987. As previosly mentioned, developing erythrocytes eliminates mitochondria trough autophagy (mitophagy), therefore they became unable to efficiently perform lipid catabolism.'),(237,9,'Question','2013-09-25 13:06:07','Question'),(238,9,'Script','2013-09-25 13:06:07','Answer'),(246,3,'Script','2013-09-26 16:24:19','123'),(247,16,'CommentA','2013-09-27 12:44:05','123'),(248,16,'Question','2013-09-27 15:56:10','2'),(249,16,'Script','2013-09-27 15:56:10','3'),(250,3,'Script','2013-09-27 15:56:39','4'),(251,4,'Question','2013-09-27 16:15:54','2'),(252,4,'Script','2013-09-27 16:15:54','2'),(253,3,'Script','2013-09-27 16:18:16','3'),(254,3,'CommentJ','2013-09-30 10:13:53','123'),(255,3,'Question','2013-09-30 13:28:20','333'),(258,3,'Question','2013-09-30 13:30:18','444'),(259,3,'Script','2013-09-30 13:30:18','444'),(310,3,'Question','2013-10-01 13:54:25','1'),(311,3,'Script','2013-10-01 13:54:25','1');
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(9,'stu3','$6$rounds=60000$XmQ7vWV.0A6y2kEO$apwoGopl.lL5n1jDxiJ6cE9IQh/hgX1SMEm1G1oUS5uKbXcbnQ3pE1h4S1tSJfy/sTUIPlGwJ.2HH.n2Cx8S71','Student','s3@example.com','Student','Three','student3','2013-09-27 16:01:00'),
(10,'stu4','$6$rounds=60000$XmQ7vWV.0A6y2kEO$apwoGopl.lL5n1jDxiJ6cE9IQh/hgX1SMEm1G1oUS5uKbXcbnQ3pE1h4S1tSJfy/sTUIPlGwJ.2HH.n2Cx8S71','Student','s4@example.com','Student','Four','student4','2013-09-27 15:57:49'),
(11,'stu5','$6$rounds=60000$XmQ7vWV.0A6y2kEO$apwoGopl.lL5n1jDxiJ6cE9IQh/hgX1SMEm1G1oUS5uKbXcbnQ3pE1h4S1tSJfy/sTUIPlGwJ.2HH.n2Cx8S71','Student','s5@example.com','Student','Five','student5','2013-09-27 16:15:39'),
(12,'stu6','$6$rounds=60000$XmQ7vWV.0A6y2kEO$apwoGopl.lL5n1jDxiJ6cE9IQh/hgX1SMEm1G1oUS5uKbXcbnQ3pE1h4S1tSJfy/sTUIPlGwJ.2HH.n2Cx8S71','Student','s6@example.com','Student','Six','student6','0000-00-00 00:00:00'),
(13,'stu7','$6$rounds=60000$XmQ7vWV.0A6y2kEO$apwoGopl.lL5n1jDxiJ6cE9IQh/hgX1SMEm1G1oUS5uKbXcbnQ3pE1h4S1tSJfy/sTUIPlGwJ.2HH.n2Cx8S71','Student','s7@example.com','Student','Seven','student7','2013-09-27 16:12:39'),
(14,'stu8','$6$rounds=60000$XmQ7vWV.0A6y2kEO$apwoGopl.lL5n1jDxiJ6cE9IQh/hgX1SMEm1G1oUS5uKbXcbnQ3pE1h4S1tSJfy/sTUIPlGwJ.2HH.n2Cx8S71','Student','s8@example.com','Student','Eight','student8','0000-00-00 00:00:00'),
(15,'stu9','$6$rounds=60000$XmQ7vWV.0A6y2kEO$apwoGopl.lL5n1jDxiJ6cE9IQh/hgX1SMEm1G1oUS5uKbXcbnQ3pE1h4S1tSJfy/sTUIPlGwJ.2HH.n2Cx8S71','Student','s90@example.com','Student','Nine','student9','0000-00-00 00:00:00'),
(16,'stu10','$6$rounds=60000$XmQ7vWV.0A6y2kEO$apwoGopl.lL5n1jDxiJ6cE9IQh/hgX1SMEm1G1oUS5uKbXcbnQ3pE1h4S1tSJfy/sTUIPlGwJ.2HH.n2Cx8S71','Student','s10@example.com','Student','Ten','student10','2013-10-01 14:48:52');
/*!40000 ALTER TABLE `User` ENABLE KEYS */;