6 ms·

> For example, consider quantum gravity. Some people say: “Why do we even need to quantize gravity given that we don’t even have experimental evidence for it?

> For example, consider quantum gravity. Some people say: “Why do we even need to quantize gravity given that we don’t even have experimental evidence for it? (Disclaimer not a physicist or mathematician, just curious about the subject, so sorry if my question offends someone) Is it possible that gravity does not exist at the quantum level, but instead emerges from quantum mechanics at larger scales? What I understantood from Stephen Wolfram's physics is that the laws of physics emerge from trivial structure/rules as some kind of computation. I wonder if "large physics" is simply something that is computed from "small physics".

guerrilla 5y ago
That's a theory [1] people are working on. I'm still waiting for PBS Space Time to explain it to me. 1. https://en.m.wikipedia.org/wiki/Entropic_gravity https://en.m.wikipedia.org/wiki/Entropic_gravity
- tarruda 5y ago
  Awesome, thanks for sharing
- avaldes 5y ago
  Interesting counterpoint from Lubos Motl https://motls.blogspot.com/2011/08/once-more-gravity-is-not-entropic-force.html?m=1 https://motls.blogspot.com/2011/08/once-more-gravity-is-not-...
- raattgift 5y ago
  I'm not PBS Space Time, but I'll give it a shot in two parts. First, let's start with entropy as a measure of disorder. The more disordered a system is, the higher the entropy. Next let's consider an ordinary elastic or rubber band. When it is in its most relaxed state it has a certain measure of entropy. But as we stretch it, the elastic molecules tend to become less jumbled and more aligned: they have lower entropy because we have done some external work in stretching the band, and that work meets the requirement of the second law of thermodynamics. The more-aligned rubber band molecules pull against one another along the direction of stretching, because they prefer to be in the more jumbled-up, higher-entropy, relaxed state, in line with the second law. This pulling is an example of an entropic force. Notably, the entropic force increases as we separate the molecules along a single direction: as you stretch an elastic band it gets harder to stretch it further. (It works in more than one dimension too: we can stretch a sheet of rubber, or pulling a ballon in several directions, and so on, and there will be an entropic force felt along all the directions of stretching). Entropic gravity treats gravitation as an entropic force. Masses want to move towards each other because they have lower entropy when there is less space between them. And as one introduces more and more space between them, it gets harder and harder to separate the masses than a strict inverse square law (a quadratic function) of attraction describes. When masses are very far apart, perhaps many thousands of light-years, the decrease in mutual attraction tends to fall off linearly rather than quadratically. In comparison with General Relativity in the absence of dark matter, it gets harder to separate the edges of dark-matter-free galaxies from their cores. And at larger scales, compared to dark-matter-free General Relativity, it also gets harder to separate galaxies within a cluster from one another. (One might notice a problem here: galaxy clusters are flinging away from each other according to redshift surveys like SDSS, so one has to break the entropic force at some distance scale, roughly analogous to how one might break an overstretched elastic band or balloon). Entropic gravity theories (there are numerous individual theories partially written down) aim to eliminate the need for dark matter and dark energy, but also know they must recover the behaviour of General Relativity perfectly within our solar system (where it is very precisely tested and completely accurate to the level of precision) and in various systems inside our galaxy -- eclipsing binaries, including where one or more of the objects are compact or relativistic objects like neutron stars (especially millisecond pulsars) or black holes. The physical intuition behind entropic gravity is that the vacuum -- a state completely devoid of Standard Model particles -- can be in almost any sort of configuration without real particles appearing. "Vacuum energy" is even used in the standard cosmology as an intuition about the cosmological constant, and particularly in trying to understand cosmic inflation in the very early universe. In classical General Relativity the entropy of the vacuum is as high as one can get. It is maximally disordered. One way of understanding this is to think about Boltzmann's measure of entropy as the log relationship between the number of microstates and the number of macrostates. Let's think of how this works. Refining our idea of entropy as a measure of disorder, we can now try to think of it in terms of how sensitively it reacts to a change in the order. Forgive a small abuse of Boltzmann-entropy terminology here: if we take a macrostate to be one m^3 (cubic metre), and treat every mm^3 (cubic millimetre) inside as a microstate, we can consider systems with different entropies. If you have a m^3 of water, you can swap any two mm^3 and not make any practical difference to its characteristics at the m^3 scale: it stays liquid, the absorption/emission spectrum doesn't change. Similarly if we instead do this operation on a m^3 box filled with an inert gas. But if we take a m^3 box filled with a curled up human and swap two of the enclosed mm^3 microstates arbitrarily, there is a good chance we quickly end up with a dead human in a box instead of a live human in a box. The mm^3s we swap could be air and heart valve. Or fingernail and alveolus. The result is that the human's emission spectrum -- heat from metabolism -- cools off rapidly. Algor mortis. We can swap lots of ever-finer sections of the water or inert gas around within the enclosing box and not notice much difference, until we get down to the sub-molecular level. But if we do this with a human or a tree we get a dead organism. As we swap around at the sub-molecular level, in water we end up maybe getting diffuse bubbles of hydrogen peroxide, oxygen, or hydrogen gasses. These have different spectroscopic features. But if we do that to a tree or to a human we are breaking starches apart into sugars, disaccharides into monosaccharides, breaking down fats, and so on. As well as producing bubbles of gasses. Result: dead organism. If we get down to making many swaps at the subatomic level both water and the inert gas end up undergoing nuclear reactions. In our water, by swapping nucleons about, we may get deuterium or tritium, radioactive isotopes of oxygen, and so on. If we start swapping quarks around maybe we suddenly get antiprotons or antineutrons. And so forth. We have something that is visibly not just one m^3 of water. But what about vacuum? At the subatomic level, no matter how small you get with the microstates and how vigorously you swap them, a m^3 of classical vacuum remains an m^3 of classical vacuum. You swap the smallest units (much smaller than (planck length)^3) of empty space with similar units of empty space and you still have empty space by any measurement. This means that classical vacuum has the ultimate entropy. And this is why an expanding universe has enormously-increasing entropy: more and more classical vacuum keeps appearing. end 1/2
  - raattgift 5y ago
    2/2 I just explained that classical vacuum has an important property: it is uniformly empty at every scale. It therefore has the maximum possible entropy, because a chunk of classical vacuum is indistinguishable from any other similar chunk of classical vacuum, whether one measures the chunks in cubic picometres or in cubic lightyears. The smallest microstates can be arbitrarily shuffled and you will observe no changes in any observable features of the largest macrostate. But maybe a quantum vacuum (or string vacuum) is different from a classical vacuum at some level: as you make more and more vigorous swaps at the tiniest scales, perhaps there is a transition from the macrostate of apparent vacuum to something with real particles inside (that might radiate away, for instance, with some striking a detector outside the m^3, similarly to how our nucleon-swapping in water produces unstable isotopes that decay and radiate in the process). We could then ascribe a measure of entropy to such a false vacuum that is different from the perfect entropy of classical vacuum. That in turn has some interesting consequences. That false vacuum is (usually) the source of the entropic force in entropic gravity theories. As one makes the false vacuum purer, by moving dust and gas and light and so forth out of it, its entropy decreases. The decrease in entropy generates an entropic force trying to pull radiation, dust, gas, stars, and so forth back into it in order to return to a higher-energy state. By analogy with the rubber band, lifting matter out of a system stretches the vacuum; the stretched vacuum then generates an entropic force on the nearby vacuum and matter trying to relax into a less-stretched state. False vacuum therefore is in effect very slightly attractive. (One sees the problem: an expanding universe has more and more of this false vacuum appearing, doesn't it? So we need a counter-term, which could be a form of dark energy, or a phase transition at which a sufficiently empty patch of false vacuum becomes very slightly repulsive, but a merely-very-very-empty patch of false vacuum remains more attractive than the patches of interplanetary space inside our solar system out at least as far as Pluto.) A few years ago Erik Verlinde, a well-known theoretical physicist and string theorist, was very excited about being able to describe a model of a spherically symmetrical galaxy in his entropic gravity theory, and the entropic force binding this model galaxy arising from interactions at the scale of strings (as in string theory). The galaxy at some levels resemble elliptical galaxies in our sky, whose outermost stars move faster than would be allowed without some inward pull. In his theory, the inward pull is entropic throughout the modelled galaxy, so gravitation falls off more slowly than the inverse square law for the outermost stars. Thus there was -- at first glance -- no need for dark matter in a halo "suspending" the outermost stars. It was clever, especially as he made a connection with a version of MOND. This programme seems to have stalled since not long after it was presented in 2011. But then so have many other theories that seek to explain the fine details of galaxy rotation curves compared to what one expects with General Relativity where only the Standard Model particles generate the stress-energy tensor. Verlinde's approach noteworthily would also have generated the Standard Model particles out of the same strings that the false vacuum arose from, "just like that", as soon as the "string landscape problem" were solved, and he hoped that one could get there not all at once but by a process of small extensions upon a model that worked for the perfectly spherical galaxy model. Dark matter approaches do not depend on the shape of a galaxy or number of individual galaxies within a cluster but rather the distribution of dark energy, governed by a few key features: it does not move relativistically ("cold"), and it does not radiate or clump ("dark" and "collisionless"). CDM (cold dark matter) approaches explain whole clusters of galaxies (of different shapes and sizes) moving around within a cluster, and one generally only has to set down an initial distribution of CDM from the time before galaxies first formed [1]. It is also not sensitive to the exact details of the vacuum, merely that its bulk property is that it is not attractive of matter. Verlinde's programme turned out to be sensitive to the spherical symmetry, and did not work well with spirals and other shapes, much less arbitrarily-populated galaxy clusters; I do not think much progress was made on an initial-values formulation for the individual-galaxy model(s) that were eventually proposed by his team. Additionally, there was as far as I know no traction on explanations for why galaxy clusters stay gravitationally bound but much larger scale clusters do not, except by directly importing the cosmological constant. As far as I know there aren't many people working on the dynamics of galaxy clusters who are convinced that Verlinde's work will do any better (or be at least as easy to work with as) General Relativity equipped with some bulk behaviour model of dark matter. While Verlinde's team's work -- or some other entropic gravity theory -- might produce a clear connection between the very very very small scale physics of strings and the bulk behaviour of galaxies in an expanding universe, it looks less and less likely to do so without importing some bulk behaviour from the standard cosmology. - -- [1] https://map.gsfc.nasa.gov/universe/bb_cosmo_struct.html https://map.gsfc.nasa.gov/universe/bb_cosmo_struct.html
- bozzcl 5y ago
  I like the theory that gravity is an emergent property of relativity. I tried to write an explanation here, but I don’t think I understand it well enough. Funnily enough, PBS Space Time has a video about it. https://invidious.tube/watch?v=UKxQTvqcpSg https://invidious.tube/watch?v=UKxQTvqcpSg

guerrilla 5y ago

That's a theory [1] people are working on. I'm still waiting for PBS Space Time to explain it to me. 1. https://en.m.wikipedia.org/wiki/Entropic_gravity https://en.m.wikipedia.org/wiki/Entropic_gravity

tarruda 5y ago
Awesome, thanks for sharing
avaldes 5y ago
Interesting counterpoint from Lubos Motl https://motls.blogspot.com/2011/08/once-more-gravity-is-not-entropic-force.html?m=1 https://motls.blogspot.com/2011/08/once-more-gravity-is-not-...
raattgift 5y ago
I'm not PBS Space Time, but I'll give it a shot in two parts. First, let's start with entropy as a measure of disorder. The more disordered a system is, the higher the entropy. Next let's consider an ordinary elastic or rubber band. When it is in its most relaxed state it has a certain measure of entropy. But as we stretch it, the elastic molecules tend to become less jumbled and more aligned: they have lower entropy because we have done some external work in stretching the band, and that work meets the requirement of the second law of thermodynamics. The more-aligned rubber band molecules pull against one another along the direction of stretching, because they prefer to be in the more jumbled-up, higher-entropy, relaxed state, in line with the second law. This pulling is an example of an entropic force. Notably, the entropic force increases as we separate the molecules along a single direction: as you stretch an elastic band it gets harder to stretch it further. (It works in more than one dimension too: we can stretch a sheet of rubber, or pulling a ballon in several directions, and so on, and there will be an entropic force felt along all the directions of stretching). Entropic gravity treats gravitation as an entropic force. Masses want to move towards each other because they have lower entropy when there is less space between them. And as one introduces more and more space between them, it gets harder and harder to separate the masses than a strict inverse square law (a quadratic function) of attraction describes. When masses are very far apart, perhaps many thousands of light-years, the decrease in mutual attraction tends to fall off linearly rather than quadratically. In comparison with General Relativity in the absence of dark matter, it gets harder to separate the edges of dark-matter-free galaxies from their cores. And at larger scales, compared to dark-matter-free General Relativity, it also gets harder to separate galaxies within a cluster from one another. (One might notice a problem here: galaxy clusters are flinging away from each other according to redshift surveys like SDSS, so one has to break the entropic force at some distance scale, roughly analogous to how one might break an overstretched elastic band or balloon). Entropic gravity theories (there are numerous individual theories partially written down) aim to eliminate the need for dark matter and dark energy, but also know they must recover the behaviour of General Relativity perfectly within our solar system (where it is very precisely tested and completely accurate to the level of precision) and in various systems inside our galaxy -- eclipsing binaries, including where one or more of the objects are compact or relativistic objects like neutron stars (especially millisecond pulsars) or black holes. The physical intuition behind entropic gravity is that the vacuum -- a state completely devoid of Standard Model particles -- can be in almost any sort of configuration without real particles appearing. "Vacuum energy" is even used in the standard cosmology as an intuition about the cosmological constant, and particularly in trying to understand cosmic inflation in the very early universe. In classical General Relativity the entropy of the vacuum is as high as one can get. It is maximally disordered. One way of understanding this is to think about Boltzmann's measure of entropy as the log relationship between the number of microstates and the number of macrostates. Let's think of how this works. Refining our idea of entropy as a measure of disorder, we can now try to think of it in terms of how sensitively it reacts to a change in the order. Forgive a small abuse of Boltzmann-entropy terminology here: if we take a macrostate to be one m^3 (cubic metre), and treat every mm^3 (cubic millimetre) inside as a microstate, we can consider systems with different entropies. If you have a m^3 of water, you can swap any two mm^3 and not make any practical difference to its characteristics at the m^3 scale: it stays liquid, the absorption/emission spectrum doesn't change. Similarly if we instead do this operation on a m^3 box filled with an inert gas. But if we take a m^3 box filled with a curled up human and swap two of the enclosed mm^3 microstates arbitrarily, there is a good chance we quickly end up with a dead human in a box instead of a live human in a box. The mm^3s we swap could be air and heart valve. Or fingernail and alveolus. The result is that the human's emission spectrum -- heat from metabolism -- cools off rapidly. Algor mortis. We can swap lots of ever-finer sections of the water or inert gas around within the enclosing box and not notice much difference, until we get down to the sub-molecular level. But if we do this with a human or a tree we get a dead organism. As we swap around at the sub-molecular level, in water we end up maybe getting diffuse bubbles of hydrogen peroxide, oxygen, or hydrogen gasses. These have different spectroscopic features. But if we do that to a tree or to a human we are breaking starches apart into sugars, disaccharides into monosaccharides, breaking down fats, and so on. As well as producing bubbles of gasses. Result: dead organism. If we get down to making many swaps at the subatomic level both water and the inert gas end up undergoing nuclear reactions. In our water, by swapping nucleons about, we may get deuterium or tritium, radioactive isotopes of oxygen, and so on. If we start swapping quarks around maybe we suddenly get antiprotons or antineutrons. And so forth. We have something that is visibly not just one m^3 of water. But what about vacuum? At the subatomic level, no matter how small you get with the microstates and how vigorously you swap them, a m^3 of classical vacuum remains an m^3 of classical vacuum. You swap the smallest units (much smaller than (planck length)^3) of empty space with similar units of empty space and you still have empty space by any measurement. This means that classical vacuum has the ultimate entropy. And this is why an expanding universe has enormously-increasing entropy: more and more classical vacuum keeps appearing. end 1/2
- raattgift 5y ago
  2/2 I just explained that classical vacuum has an important property: it is uniformly empty at every scale. It therefore has the maximum possible entropy, because a chunk of classical vacuum is indistinguishable from any other similar chunk of classical vacuum, whether one measures the chunks in cubic picometres or in cubic lightyears. The smallest microstates can be arbitrarily shuffled and you will observe no changes in any observable features of the largest macrostate. But maybe a quantum vacuum (or string vacuum) is different from a classical vacuum at some level: as you make more and more vigorous swaps at the tiniest scales, perhaps there is a transition from the macrostate of apparent vacuum to something with real particles inside (that might radiate away, for instance, with some striking a detector outside the m^3, similarly to how our nucleon-swapping in water produces unstable isotopes that decay and radiate in the process). We could then ascribe a measure of entropy to such a false vacuum that is different from the perfect entropy of classical vacuum. That in turn has some interesting consequences. That false vacuum is (usually) the source of the entropic force in entropic gravity theories. As one makes the false vacuum purer, by moving dust and gas and light and so forth out of it, its entropy decreases. The decrease in entropy generates an entropic force trying to pull radiation, dust, gas, stars, and so forth back into it in order to return to a higher-energy state. By analogy with the rubber band, lifting matter out of a system stretches the vacuum; the stretched vacuum then generates an entropic force on the nearby vacuum and matter trying to relax into a less-stretched state. False vacuum therefore is in effect very slightly attractive. (One sees the problem: an expanding universe has more and more of this false vacuum appearing, doesn't it? So we need a counter-term, which could be a form of dark energy, or a phase transition at which a sufficiently empty patch of false vacuum becomes very slightly repulsive, but a merely-very-very-empty patch of false vacuum remains more attractive than the patches of interplanetary space inside our solar system out at least as far as Pluto.) A few years ago Erik Verlinde, a well-known theoretical physicist and string theorist, was very excited about being able to describe a model of a spherically symmetrical galaxy in his entropic gravity theory, and the entropic force binding this model galaxy arising from interactions at the scale of strings (as in string theory). The galaxy at some levels resemble elliptical galaxies in our sky, whose outermost stars move faster than would be allowed without some inward pull. In his theory, the inward pull is entropic throughout the modelled galaxy, so gravitation falls off more slowly than the inverse square law for the outermost stars. Thus there was -- at first glance -- no need for dark matter in a halo "suspending" the outermost stars. It was clever, especially as he made a connection with a version of MOND. This programme seems to have stalled since not long after it was presented in 2011. But then so have many other theories that seek to explain the fine details of galaxy rotation curves compared to what one expects with General Relativity where only the Standard Model particles generate the stress-energy tensor. Verlinde's approach noteworthily would also have generated the Standard Model particles out of the same strings that the false vacuum arose from, "just like that", as soon as the "string landscape problem" were solved, and he hoped that one could get there not all at once but by a process of small extensions upon a model that worked for the perfectly spherical galaxy model. Dark matter approaches do not depend on the shape of a galaxy or number of individual galaxies within a cluster but rather the distribution of dark energy, governed by a few key features: it does not move relativistically ("cold"), and it does not radiate or clump ("dark" and "collisionless"). CDM (cold dark matter) approaches explain whole clusters of galaxies (of different shapes and sizes) moving around within a cluster, and one generally only has to set down an initial distribution of CDM from the time before galaxies first formed [1]. It is also not sensitive to the exact details of the vacuum, merely that its bulk property is that it is not attractive of matter. Verlinde's programme turned out to be sensitive to the spherical symmetry, and did not work well with spirals and other shapes, much less arbitrarily-populated galaxy clusters; I do not think much progress was made on an initial-values formulation for the individual-galaxy model(s) that were eventually proposed by his team. Additionally, there was as far as I know no traction on explanations for why galaxy clusters stay gravitationally bound but much larger scale clusters do not, except by directly importing the cosmological constant. As far as I know there aren't many people working on the dynamics of galaxy clusters who are convinced that Verlinde's work will do any better (or be at least as easy to work with as) General Relativity equipped with some bulk behaviour model of dark matter. While Verlinde's team's work -- or some other entropic gravity theory -- might produce a clear connection between the very very very small scale physics of strings and the bulk behaviour of galaxies in an expanding universe, it looks less and less likely to do so without importing some bulk behaviour from the standard cosmology. - -- [1] https://map.gsfc.nasa.gov/universe/bb_cosmo_struct.html https://map.gsfc.nasa.gov/universe/bb_cosmo_struct.html
bozzcl 5y ago
I like the theory that gravity is an emergent property of relativity. I tried to write an explanation here, but I don’t think I understand it well enough. Funnily enough, PBS Space Time has a video about it. https://invidious.tube/watch?v=UKxQTvqcpSg https://invidious.tube/watch?v=UKxQTvqcpSg