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

quantamagazine.org

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

#201

Post by [deleted] »

hellohello2 wrote: post #186
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…

Not a physicist either but this passage from the Feynman lectures seem related to what you are describing: https://www.feynmanlectures.caltech.edu/I_07.html "Many mechanisms for gravitation have been suggested. It is interesting to consider one of these, which many people have thought of from time to time. At first, one is quite excited and happy when he “discovers” it, but he soon finds that it is not correct. It wa…

[deleted]

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

#202

Post by immibis »

meindnoch wrote: post #15

I don't get it. To me, entropy is not a physical thing, but a measure of our imperfect knowledge about a system. We can only measure the bulk properties of matter, so we've made up a number to quantify how imperfect the bulk properties describe the true microscopic state of the system. But if we had the ability to zoom into the microscopic level, entropy would make no sense. So I don't see how gravity or any other fu…

Entropy can be defined as the logarithm of the number of microstates in a macrostate. Since transition between microstates is reversible, and therefore one-to-one (can't converge on any particular microstate, can't go in cycles, have to be something like a random walk) we're more likely to end up in a macrostate that holds a larger number of microstates.

For example, there are many more ways your headphone cord can be tangled than untangled, so when you pull it out of your pocket, and it's in a random state, then it's very likely to be tangled.

If entropy causes gravity, that means there are more somehow more microstates with all the mass in the universe smooshed together than microstates with all the mass in the universe spread apart.

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

#203

Post by immibis »

floxy wrote: post #191
Gravityloss wrote: post #89

Earlier quoted context omitted.

Exactly. The coin flipping example is a very nice way to put it. It works since the coins are interchangeable, you just count the number of heads or tails. If the coins were of different color and you took that into account, then it wouldn't work. It's not intuitive to me what gravity has to do with entropy though, as it's classically just a force and completely reversible (unlike entropy)? Ie if you saw a video of u…

> Ie if you saw a video of undisturbed objects only affected by gravity, you couldn't tell if the video was reversed. How does that work with things like black holes? If you saw an apple spiral out of a black hole, wouldn't you suspect that you were watching a reversed video? Even if you take account the gravitational waves?

That's the question of why time only goes forwards. It seems to be that the universe started in an extremely low-entropy state. It will go towards high entropy. In a high entropy state (e.g. heat death, or a static black hole), there's no meaningful difference between going forwards or backwards in time - if you reverse all the velocities of the particles, they still just whizz around randomly (in the heat death case) or the black hole stays a black hole.

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

#204

Post by AnotherGoodName »

hellohello2 wrote: post #186
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…

Not a physicist either but this passage from the Feynman lectures seem related to what you are describing: https://www.feynmanlectures.caltech.edu/I_07.html "Many mechanisms for gravitation have been suggested. It is interesting to consider one of these, which many people have thought of from time to time. At first, one is quite excited and happy when he “discovers” it, but he soon finds that it is not correct. It wa…

It also doesn't account for time dilation in a gravity well however i still think the general idea has some merit if you think of it as being bombarded by massless ‘action potentials’ on all sides with mass absorbing that field to some to enable translation in space time.

I get this is vague spitballing but essentially an ‘action potential’ would allow mass to move. Higher temperature mass interacts more, lower temperature interacts less. Mass with momentum would be biased to absorb more from one side so it travels in a specific direction in space more than others (the idea i’m getting at is that all movement in space only occurs with interaction with this field), this also would counteract issues with moving mass interacting more on a specific side - the very bias of mass with momentum to absorb more on one side means that from that masses point of view it has the same action potentials interacting from all sides. Mass shielded behind mass receives fewer action potentials so experiences exactly the effect that you can call time dilation. Mass shielding other mass from action potentials also means that mass accelerates towards other mass.

Essentially its the above but instead of a massive particle hitting other mass from all sides it’s a field that allows mass to experience a unit of time.

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

#205

Post by ojo-rojo »

the__alchemist wrote: post #149
pif wrote: post #40

As an experimental physicist, I refuse to get excited about a new theory until the proponent gets to an observable phenomenon that can fix the question.

This is why I'm skeptical of theories like Wolfram's: It feels like an overfit based on this: It produces all sorts of known theories (special relativity, parts of QM, gravity etc), but doesn't make new testable predictions, or new fundamentals. When I see 10 predictions emerge from the theory, and they all happen to be ones we already known of... Overfit.

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 :)

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

#206

Post by mensetmanusman »

aurareturn wrote: post #58
logicchains wrote: post #22

Earlier quoted context omitted.

Entropy isn't a function of imperfect knowledge. It's a function of the possible states of a system and their probability distributions. Quantum mechanics assumes, as the name implies, that reality at the smallest level can be quantised, so it's completely appropriate to apply entropy to describing things at the microscopic scale.

If we knew the exact state of all particles in an enclosed system, we can calculate what future states will be exactly. No need to calculate possible states.

Quantum uncertainty actually says no to this. There is an ‘error’ in any propagating probability field.

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

#207

Post by Ma8ee »

meindnoch wrote: post #15

I don't get it. To me, entropy is not a physical thing, but a measure of our imperfect knowledge about a system. We can only measure the bulk properties of matter, so we've made up a number to quantify how imperfect the bulk properties describe the true microscopic state of the system. But if we had the ability to zoom into the microscopic level, entropy would make no sense. So I don't see how gravity or any other fu…

Entropy is certainly a physical “thing”, in the sense that it affects the development of the system. You can equally well apply your argument that it isn’t a physical thing because it doesn’t exist on a microscopic scale to temperature. Temperature doesn’t exist when you zoom in on single particles either.

There’s no reason to involve our knowledge of the system. Entropy is a measure of the number of possible micro states for a given system, and that number exists independently of us.

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

#208

Post by kgwgk »

Ma8ee wrote: post #207
meindnoch wrote: post #15

I don't get it. To me, entropy is not a physical thing, but a measure of our imperfect knowledge about a system. We can only measure the bulk properties of matter, so we've made up a number to quantify how imperfect the bulk properties describe the true microscopic state of the system. But if we had the ability to zoom into the microscopic level, entropy would make no sense. So I don't see how gravity or any other fu…

Entropy is certainly a physical “thing”, in the sense that it affects the development of the system. You can equally well apply your argument that it isn’t a physical thing because it doesn’t exist on a microscopic scale to temperature. Temperature doesn’t exist when you zoom in on single particles either. There’s no reason to involve our knowledge of the system. Entropy is a measure of the number of possible micro s…

> Entropy is a measure of the number of possible micro states for a given system, and that number exists independently of us.

That number also exists independently of the system! I can imagine any system and calculate the corresponding number.

(And for an even more philosophical question, does the “system” really exist independently of us? What separates the “system” from anything else? Is every subset of the universe a “system”?)

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

#209

Post by [deleted] »

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…

[deleted]

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

#210

Post by generalizations »

ojo-rojo wrote: post #205
the__alchemist wrote: post #149

Earlier quoted context omitted.

This is why I'm skeptical of theories like Wolfram's: It feels like an overfit based on this: It produces all sorts of known theories (special relativity, parts of QM, gravity etc), but doesn't make new testable predictions, or new fundamentals. When I see 10 predictions emerge from the theory, and they all happen to be ones we already known of... Overfit.

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 :…

A positive example would be the periodic table - the pattern to the elements made sense, but also exposed 'holes' in the table which were later filled in as we discovered additional elements. Wolfram may be closer to inventing epicycles to explain orbits - which is interesing and technically challenging and probably tickles his mind, but doesn't actually generate new knowledge.
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