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Is gravity just entropy rising? Long-shot idea gets another look

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Re: Is gravity just entropy rising? Long-shot idea gets another look

#261

Post by raattgift »

raattgift wrote: post #237
abetusk wrote: post #160

Entropic gravity is like the "brazil nut effect" [0] [1]. The idea is that if you shake a glass full of different sized nuts, the large ones will rise to the top. From what I understand, this is because larger objects have more mass, moving slower when shaked, so as the larger (brazil nuts) don't move as much relative to the smaller ones (peanuts), and because of gravity, there's a cavity left under the brazil nut wh…

No, here "entropic" is as in the entropic force that returns a stretched rubber band to its unstretched condition, which (as it tends to be scrunched a bit) is at a higher entropy. https://en.wikipedia.org/wiki/Rubber_band_experiment "The stretching of the rubber band is an isobaric expansion (A → B) that increases the energy but reduces the entropy" [apologies for any reversed signs below, I think I caught them all]…

The local theory part of the Carney et al paper (preprint https://arxiv.org/abs/2502.17575>) is interesting in that it isn't obviously related to string theory / holographic entropic gravity. Instead masses induce a spin polarization near them which is a lower entropy state. Two masses with two polarized spin-clouds will attract each other as the system tries to thermalize to a higher-entropy state. With careful choices of parameters, they can generate any central force, and they explore a particular choice which corresponds to Newtons 1/r^2 mutual attraction.

The paper cannot deal with fast-moving masses at all: it's not just the relativstic regime (where speeds are significant fractions of c) but rather the masses must move more slowly than the thermalization. This is hugely restrictive.

Finally, comparing themselves to the traditional approach of quantizing perturbations (e.g. turning classical (General Relativity) gravitational waves into lots of spin-2 gravitons) the authors write:

  The gravitational interactions we observe at accessible
  length scales could in principle emerge in many ways from
  physics at the Planck scale ρ ∼ mPl/ℓ3 Pl ∼ 10104 J/cm3.
  Perhaps the simplest is that gravitational perturbations
  are quantized as gravitons, i.e., as another quantum field
  theory like the gauge bosons of the other fundamental
  forces in nature. This is a perfectly good effective quan-
  tum field theory; nothing in principle forces us to aban-
  don this picture until energies near the Planck scale.
They also say that while their starting point was being very different from the holographic picture:

  we find that the models have a range of free parameters,
  and in some parameter regimes become indistinguishable
  from standard virtual graviton exchange
Some of this will necessarily by driven by the need to be compatible with General Relativity in the weak field limit. They are not compatible with strong gravity in General Relativity at present.

So while the idea is kinda interesting, I think they are putting the cart before the horse in asking what their model says about things like the interaction between gravitation and entanglement. That's simply unmeasurable by experiment right now whereas the very-well-understood relativistic precession of Mercury's perihelion is completely out of scope for this initial paper.

Re: Is gravity just entropy rising? Long-shot idea gets another look

#262

Post by raattgift »

Agentlien wrote: post #257
klodolph wrote: post #256

Earlier quoted context omitted.

It works for some types of symmetry but not others!

Interesting! Now, that sounds like a more unintuitive result to me. Can you give examples of symmetries for which it doesn't work?

The (Newtonian) Shell Theorem is fairly sensitive to spherical symmetry. In General Relativity one can write down a metric wherein inside any boundary surface there is flat spacetime. It's easiest to do this for a spherical boundary, but one can work out a metric which is axisymmetric (e.g. oblate and spinning or prolate and tidally deformed) and probably all sorts of other weird shapes following ideas from Gauss's Law for Gravitation. Writing down a metric for that is hard though -- really hard if the idea is to make it time-independent, and really really hard if the idea is to make it time-dependent but static (as in a complex Gaussian surface doesn't relax into a more spherical shell). For example, bumps raised on each other by binary black holes will vanish after merger (or if they fly away on hyperbolic trajectories, having "grazed" each other), leaving you with a spherical horizon (if nonspinnning) or an oblate one (if spinning).

Essentially to break spherical symmetry (or axisymmetry where there's spin) and keep it broken you have to introduce something like a dark energy. One can do that outside (retaining flat space inside) or inside (leading to the equivalent direction-dependent attraction of outside objects).

Re: Is gravity just entropy rising? Long-shot idea gets another look

#263

Post by Gravityloss »

kgwgk wrote: post #258
Gravityloss wrote: post #254

Earlier quoted context omitted.

Yea and those happen when other forces than gravity come into play. When matter starts colliding.

Every object has been subject to forces other gravity at some point. And the point is that sometimes comets do indeed fall into the Sun. If you object to people calling that a comet that’s fine - we can use whatever name you want.

The point is that the gravity interactions are time reversible. Not so with friction etc.

Re: Is gravity just entropy rising? Long-shot idea gets another look

#264

Post by raattgift »

cryptonector wrote: post #135
cwharris wrote: post #92

This seems backwards. Entropy is a dispersive force — it favors distribution and disorder. But the universe clumps. Planets, stars, galaxies — all of them are low-entropy configurations. So how did scattered dust particles form the planet we’re standing on… through entropy? If gravity is just emergent from entropy, then it should be fighting against planet formation, not causing it. There’s a missing piece here — may…

You have it backwards. The lowest entropy state of the universe would be if there were no attractive forces, only repellent forces, as then all particles would be forced into something of an expanding lattice, but with all particles equidistant from all nearest neighbors (of the same type). It is gravity which disrupts this and causes clumping, and that _increases_ entropy. I know it's confusing because normally one…

If everything must be constrained to the lattice points, yes. However, empty space has high Boltzmann entropy: you can cut a patch of empty space from here and swap it for the same volume of empty space from there, and the two coarse grain macrostates will be indistinguishable.

Expanding de Sitter quasi-vacuum has tremendous growth in entropy. Gibbons and Hawking gives this (for 3+1d de Sitter) as a quarter of the horizon area: S_H = \frac{Area_{H}}{4} \sim H^{-2} with the "quasi-" giving us increasing growth in the horizon area as DoFs exit the horizon compared to classical pure de Sitter vacuum.

I'm not sure how confining some species of matter to expanding lattice is different from quasi-vacuum in the limit where the lattice spacing is large. I guess you have to abolish continuum spacetime in favour of a taxicab geometry with an analogue of dark energy? Otherwise, how does it differ from an isotropic homogeneous FLRW dust?

Re: Is gravity just entropy rising? Long-shot idea gets another look

#265

Post by nathan_compton »

quantadev wrote: post #230
nathan_compton wrote: post #123

Earlier quoted context omitted.

I'm not an expert in this field but I think reproducing realistic gravitational interactions seems to require a lot of fiddly set up with heat baths etc.

Gravity doesn't interact, other than to simply set the shape of the spacetime in which particles move.

I think that might oversimplify things, in addition to being a model-laden description. There are empty-space solutions to GR (and indeed many a physicist has found the intuition that there is nothing but empty space appealing, cf geometrodynamics) and in these situations spacetime itself interacts with itself, no particles at all.

In any case, I would say this is a somewhat bold description of what we know about gravity.

Re: Is gravity just entropy rising? Long-shot idea gets another look

#266

Post by kgwgk »

Gravityloss wrote: post #263
kgwgk wrote: post #258

Earlier quoted context omitted.

Every object has been subject to forces other gravity at some point. And the point is that sometimes comets do indeed fall into the Sun. If you object to people calling that a comet that’s fine - we can use whatever name you want.

The point is that the gravity interactions are time reversible. Not so with friction etc.

Sure, nothing in the laws of physics prevents a celestial body from distancing itself from the Sun or from the Moon. But it would like suspicious! Wouldn't you suspect that you were watching a reversed video?

Re: Is gravity just entropy rising? Long-shot idea gets another look

#267

Post by amy214 »

quantadev wrote: post #230
nathan_compton wrote: post #123

Earlier quoted context omitted.

I'm not an expert in this field but I think reproducing realistic gravitational interactions seems to require a lot of fiddly set up with heat baths etc.

Gravity doesn't interact, other than to simply set the shape of the spacetime in which particles move.

It's also worth pointing out gravitation waves are a thing (have been detected by LaWD or Laser Wave of Gravitation Detector). Does not mean it's not changing the spacetime shape, indeed, these are a type of ripple in spacetime as might occur if Sally Struthers did a jumping jack.

Re: Is gravity just entropy rising? Long-shot idea gets another look

#268

Post by xenadu02 »

npunt wrote: post #223
ojo-rojo wrote: post #205

Earlier quoted context omitted.

But that means we'd prefer whichever theory our species had landed on first. Basing our preference for a theory on that timing seems kind of arbitrary to me. If they're the same in other respects, I'd take a look at both sides to see if there are other compelling reasons to focus on one or the other, such as which is simpler. Of course if they make different predictions that'd be even better, time to get to testing :…

Not quite apples to apples tho because you have to take into consideration what was known at the time each theory was developed (the input), not just the output. Theory A: fits 7 known predictions but also makes a not-yet-verified prediction Theory B: fits 8 known predictions and offers no new ones In this example wouldn't Theory A be better, because all else equal it is less likely the product of overfitting and req…

> In this example wouldn't Theory A be better, because all else equal it is less likely the product of overfitting and required more insight and effort to discover?

No, Theory A might simply be a dead end with no new insights to offer. And alas: the universe does not care about insights, efforts, or simplicity.

All else equal if Theory B is easier to teach - easier for more people to understand - it might have value for that reason. It might also be valuable to teach multiple ways to understand the same underlying phenomenon.

> In other words, Theory A used a different process that we know has a higher likelihood of novel discovery.

How would we measure "likelihood of novel discovery"?

Now to call myself out here: the best way to answer any of these questions is to probe both theories at their limits to find differences in predictions that we can test. It may be that we don't have the right equipment or haven't designed experiments sufficient to do that currently.

Remember that Einstein's GR was validated by its prediction and the Eddington experiment, though his initial 1911 prediction was wrong and he later refined it in 1915. The 1919 Eddington measurements validated the theory.

We should remember though: That only worked out because the 1912 attempt to make the observations (which would have invalidated Einstein) got rained out. Who knows how Einstein's career would have turned out if the 1912 observations had succeeded. Perhaps people would have said he simply over-fit his theory to fit observation.

Re: Is gravity just entropy rising? Long-shot idea gets another look

#269

Post by ajkjk »

antonvs wrote: post #185
ajkjk wrote: post #113

Earlier quoted context omitted.

As the other replier said, despite your dismissiveness, the knowledge about the system is in the probabilities, so it's right there in the equation. Suppose you flip a coin. Before flipping the coin, your knowledge is "heads or tails". After flipping it, your knowledge becomes one of either heads or tails. The amount of information you gained by resolving your imperfect knowledge is the entropy of the distribution. T…

You’re glossing over an important point: your knowledge of the future state of the system is “heads or tails”. One of the things entropy tells us how a system is likely to evolve in future. But looking at this another way, entropy actually helps dictate how it will evolve in future. And we can prove that mathematically.

What are you talking about?

Entropy doesn't tell us that at all. Entropy tells us how much data is encodes between two ways of describing a value. It has nothing to do with time at all.

Re: Is gravity just entropy rising? Long-shot idea gets another look

#270

Post by quantadev »

nathan_compton wrote: post #265
quantadev wrote: post #230

Earlier quoted context omitted.

Gravity doesn't interact, other than to simply set the shape of the spacetime in which particles move.

I think that might oversimplify things, in addition to being a model-laden description. There are empty-space solutions to GR (and indeed many a physicist has found the intuition that there is nothing but empty space appealing, cf geometrodynamics) and in these situations spacetime itself interacts with itself, no particles at all. In any case, I would say this is a somewhat bold description of what we know about gra…

You can disprove my statement by naming something gravity does other than add a shape to spacetime, if you want. Saying "I think you're probably wrong, but I just can't say why" is not an argument.
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