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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

#71

Post by lewdwig »

JPLeRouzic wrote: post #68
lewdwig wrote: post #41

Earlier quoted context omitted.

The problem with emergent theories like this is that they _derive_ Newtonian gravity and General Relativity so it’s not clear there’s anything to test. If they are able to predict MOND without the need for an additional MOND field then they become falsifiable only insofar as MOND is.

Please, how is the article related to MOND's theories?

In general, they’re not. But if the only thing emergent theories predict is Newtonian dynamics and General Relativity then that’s a big problem for falsifiability. But if they modify Newtonian dynamics in some way, then do we have something to test.

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

#72

Post by IsTom »

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…

If you want to only have one possible past (i.e. can't destroy information) then when you end up in one branch of quantum state you need to "store" enough information to separate you form other branches and you really do need to have multiple possible microstates to differentiate them. If you look post-factum obviously you did end up in a specific state, but statistics do their work otherwise.

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

#73

Post by spacecadet »

meindnoch wrote: post #49
prof-dr-ir wrote: post #36

Earlier quoted context omitted.

Good question. You are absolutely right that entropy is always fundamentally a way to describe are our lack of perfect knowledge of the system [0]. Nevertheless there is a distinct "reality" to entropic forces, in the sense that it is something that can actually be measured in the lab. If you are not convinced then you can look at: https://en.wikipedia.org/wiki/Entropic_force and in particular the example that is alw…

Sure, I'm not denying that entropy exists as a concept, that can be used to explain things macroscopically. But like you said, it's origins are statistical. To me, temperature is also a similar "made up" concept. We can only talk about temperature, because a sufficiently large group of particles will converge to a single-parameter distribution with their velocities. A single particle in isolation doesn't have a tempe…

Im an idiot, let's get that out of the way first. I think that your temperature analogy answered your own question.

I guess my question in turn is, if we imagine a universe at the end of time(?), one that maybe dominated by a few black holes and not much else. Would an observer experience gravity if place sufficiently far enough way? Or even further, if nothing is left in the universe at all. Assuming that doesn't cause a big crunch, rip, or whatever...

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

#74

Post by kgwgk »

mr_mitm wrote: post #70
kgwgk wrote: post #67

Earlier quoted context omitted.

p_i Edit to add lots of words: In the definition of entropy S := -k_B sum p_i ln (p_i) knowledge about the system enters the equation in the p_i terms. The other term is a constant so it’s not like there are many other choices to link the entropy to the system!

Please communicate in full sentences with me. I can only guess that your objection is something about probabilities. A microstate has a probability independent of my knowledge of the system just like the probability of having a royal flush doesn't change after drawing five cards. The probability of me ending the game with a royal flush might, but that is not what we mean by these probabilities.

The same microstate will have different probabilities depending on what are the constraints or measurements used in _your_ description of the system.

If you choose to describe the system using its microstate - and you know it - there are no probabilities anywhere.

You can of course know something and choose to ignore it - the entropy is still a reflection of the uncertainty (actual or for the sake of a lower-resolution model).

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

#75

Post by bmitc »

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…

> It's a made-up thing by humans.

All of physics is made up by humans.

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

#76

Post by geon »

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…

It has been suggested that time too is derived from entropy. At least the single-directionality of it. That’d make entropy one of the most real phenomena in physics.

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

#77

Post by kgwgk »

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…

> Physical entropy governs real physical processes

> the measure is not the same thing as the phenomenon it describes.

There is some tension between those claims.

The latter seems to support the parent comment’s remark questioning whether a “fundamental physical interaction could follow from entropy”.

It seems more appropriate to say that entropy follows from the physical interaction - not to be confused with the measure used to describe it.

One may say that pressure is an entropic force and physical entropy governs the real physical process of gas expanding within a piston.

However, one may also say that it’s the kinetic energy of the gas molecules what governs the physical process - which arguably is a more fundamental and satisfactory explanation.

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

#78

Post by simiones »

prof-dr-ir wrote: post #36
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…

Good question. You are absolutely right that entropy is always fundamentally a way to describe are our lack of perfect knowledge of the system [0]. Nevertheless there is a distinct "reality" to entropic forces, in the sense that it is something that can actually be measured in the lab. If you are not convinced then you can look at: https://en.wikipedia.org/wiki/Entropic_force and in particular the example that is alw…

> You are absolutely right that entropy is always fundamentally a way to describe are our lack of perfect knowledge of the system [0].

> [0] Let me stress this: there is no entropy without probability distributions, even in physics.

The second item doesn't entail the first. Probabilities can be seen as a measure of lack of knowledge about a system, but it isn't necessarily so. A phenomenon can also be inherently/fundamentally probabilistic. For example, wave function collapse is, to the best of our knowledge, an inherently non-deterministic process. This is very relevant to questions about the nature of entropy - especially since we have yet to determine if it's even possible for a large system to be in a non-collapsed state.

If it turns out that there is some fundamental process that causes wave function collapse even in perfectly isolated quantum systems, then it would be quite likely that entropy is related to such a process, and that it may be more than a measure of our lack of knowledge about the internal state of a system, and instead a measurement of the objective "definiteness" of that state.

I am aware that objective collapse theories are both unpopular and have some significant hurdles to overcome - but I also think that from a practical perspective, the gap between the largest systems we have been able to observe in pure states versus the smallest systems we could consider measurement devices is still gigantic and leaves us quite a lot of room for speculation.

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

#79

Post by kgwgk »

dekken_ wrote: post #35
HelloNurse wrote: post #29

Earlier quoted context omitted.

But "disordered" and "ordered" states are just what we define them to be: for example, cords are "tangled" only because we would prefer arrangements of cords with less knots, and knots form because someone didn't handle the cords carefully. Physical processes are "real", but entropy is a figment.

I believe you are correct. Entropy is not a physical quantity, it is a measure of how far a system is from equilibrium. Lots of people talk about order/disorder or macro and micro states, not realizing these are things we've invented and aren't physical in nature.

> Entropy is not a physical quantity, it is a measure of how far a system is from equilibrium.

That’s funny because the original thermodynamic entropy is defined only for systems in equilibrium.

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

#80

Post by zmgsabst »

hackinthebochs wrote: post #37
ludwik wrote: post #31

Earlier quoted context omitted.

> there are far more ways for a system to be disordered than ordered I'm a complete layman when it comes to physics, so forgive me if this is naive — but aren't "ordered" and "disordered" concepts tied to human perception or cognition? It always seemed to me that we call something "ordered" when we can find a pattern in it, and "disordered" when we can't. Different people or cultures might be able to recognize patter…

Think minimum description length. Low entropy states require fewer terms to fully describe than high entropy states. This is an objective property of the system.

“Number of terms” is a human language construct.
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