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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

#221

Post by Gravityloss »

kgwgk wrote: post #197
floxy wrote: post #191

Earlier quoted context omitted.

> 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?

If you saw a comet coming from the sun, or a meteorite coming from the moon, etc. you would also find that suspicious.

Comets are in elliptical orbits around the sun so literally half the time they're traveling away from the sun.

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

#222

Post by Gravityloss »

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?

Classical gravity doesn't work like that. An apple does not spiral into a black hole in space there. It's in an elliptical orbit. (A circle is a special ellipse.)

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

#223

Post by npunt »

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 :…

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 required more insight and effort to discover? In other words, Theory A used a different process that we know has a higher likelihood of novel discovery.

(Maybe this is a restatement of the simplicity argument, in that Theory A requires fewer predictions to discover it, ergo it is simpler)

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

#224

Post by BenjiWiebe »

dudeinjapan wrote: post #198
franktankbank wrote: post #176

Earlier quoted context omitted.

No you are right. You can't invoke gravity in an analogy trying to explain gravity.

The same effect could be replicated in a zero-gravity environment using an alternative background force (centrifugal force, vacuum suction, electromagnetism, etc.)

Wouldn't that mean that if gravity was like this analogy, that gravity would still need an external background force to work? And how would you explain the presence of that force?

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

#225

Post by nkrisc »

hatsunearu wrote: post #214
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…

My interpretation of entropy is that if you have X states that are equally probable, but not all states are distinct from each other in some sense, then the next state will likely be one where the states satisfying that condition is most numerous. For example, if you flip N coins, there are 2^N states available once the flip is done. Each outcome has an 1/2^N probability of outcome. There's only one state where all o…

A great video and practical demonstration of the concept for anyone interested: https://youtu.be/VCXqELB3UPg?si=lwRxmXLctY1ENu0U

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

#226

Post by fc417fc802 »

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…

Exactly! Temperature isn't fundamental. It's a statistical measure made up by humans. It's certainly a very useful abstraction but it isn't a fundamental property. It describes an emergent pattern observed in larger systems. Same for entropy (and also AFAIK angular momentum).

It's entirely possible I'm wrong about any of the above but if so I've yet to encounter a convincing line of reasoning.

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

#227

Post by woopsn »

antonvs wrote: post #24
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…

Your perspective is incorrect. Physical entropy governs real physical processes. Simple example: why ice melts in a warm room. More subtle example: why cords get tangled up over time. Our measures of entropy can be seen as a way of summarizing, at a macro level, the state of a system such as that warm room containing ice, or a tangle of cables, but the measure is not the same thing as the phenomenon it describes. Bol…

You need some additional assumptions. Only near equilibrium / thermodynamic limit is system linear in entropy. What governs physical processes such as you mention is conservation, dynamics pushing equipartition of energy - but outside that regime these are no longer "theorems".

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

#228

Post by bawana »

the statistical mechanical definition of entropy relies on the number of possible arrangements of particles in a system. In a closed system entropy approaches an equilibrium which has been sensationally described as the 'heat death of the universe'. But since we know our universe is expanding, the number of possible arrangements is also increasing so entropy may never reach equilibrium. If the universe is expanding faster than its components redistribute, then entropy could be decreasing. With this in mind, any theory involving entropy as a component of gravity would suggest a changing gravity over time

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

#230

Post by quantadev »

nathan_compton wrote: post #123
mr_mitm wrote: post #53

Earlier quoted context omitted.

What are some of the most problematic assumptions in your opinion?

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.
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