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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

#31

Post by ludwik »

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…

> 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 patterns in different states. So while I agree that "there are more ways for a system to be disordered than ordered," I would have thought that's a property of how humans perceive the world, not necessarily a fundamental truth about the universe

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

#32

Post by killerstorm »

whycome wrote: post #11
omeysalvi wrote: post #7

"There’s some kind of gas or some thermal system out there that we can’t see directly" - The Ether is back on the menu boys

Caloric. Dark matter. Cosmological constant. We like placeholders for the unknown.

Isn't that how equations get solved?

Pretty much anything known entered through such placeholder, it's just that equations could be connected more easily.

It's not like Higgs field is something you can directly observe

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

#33

Post by [deleted] »

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…

[deleted]

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

#34

Post by jstanley »

killerstorm wrote: post #32
whycome wrote: post #11

Earlier quoted context omitted.

Caloric. Dark matter. Cosmological constant. We like placeholders for the unknown.

Isn't that how equations get solved? Pretty much anything known entered through such placeholder, it's just that equations could be connected more easily. It's not like Higgs field is something you can directly observe

Maybe, (I don't know), but it's easy to accidentally come up with a theory of "mysterious stuff" that appears to explain something, but neither constrains your expectation nor provides predictions.

Phlogiston is the classic example. https://www.lesswrong.com/posts/RgkqLqkg8vLhsYpfh/fake-causa...

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

#35

Post by dekken_ »

HelloNurse wrote: post #29
antonvs wrote: post #24

Earlier quoted context omitted.

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…

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.

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

#36

Post by prof-dr-ir »

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 always used in a first class on this topic:

https://en.wikipedia.org/wiki/Ideal_chain

So when viewed in this way entropy is not just a "made-up thing", but an effective way to describe observed phenomena. That makes it useful for effective but not fundamental laws of physics. And indeed the wiki page says that entropic forces are an "emergent phenomenon".

Therefore, any reasonable person believing in entropic gravity will automatically call gravity an emergent phenomenon. They must conclude that there is a new, fundamental theory of gravity to be found, and this theory will "restore" the probabilistic interpretation of entropy.

The reason entropic gravity is exciting and exotic is that many other searches for this fundamental theory start with a (more or less) direct quantization of gravity, much like one can quantize classical mechanics to arrive at quantum mechanics. Entropic gravity posits that this is the wrong approach, in the same way that one does not try to directly quantize the ideal gas law.

[0] Let me stress this: there is no entropy without probability distributions, even in physics. Anyone claiming otherwise is stuck in the nineteenth century, perhaps because they learned only thermodynamics but not statistical mechanics.

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

#37

Post by hackinthebochs »

ludwik wrote: post #31
antonvs wrote: post #24

Earlier quoted context omitted.

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…

> 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.

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

#38

Post by enriquto »

Quarrel wrote: post #20
enriquto wrote: post #3

Earlier quoted context omitted.

Sounds fun! Would this imply that cold objects have weaker gravity?

It does, but because you have to divide the energy change by c^2, it is really really hard to detect it, and mostly overwhelmed by other effects of the heating/cooling.

why do the units matter here? Under this theory, will a body at absolute zero have no observable mass? No attractive field around it, no inertia if you try to move it.

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

#39

Post by tsimionescu »

mjburgess wrote: post #26
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…

Even if we take that view, gravity is still basically a similar case. What we call "gravity" is really an apparent force, that isnt a force at all when seen from a full 4d pov. Imagine sitting outside the whole universe from t=0,t=end and observing one whole block. Then the trajectories of matter, unaffected by any force at all, are those we call gravitational. From this pov, it makes a lot more sense to connect grav…

> From this pov, it makes a lot more sense to connect gravity with some orderly or disorderly features of these trajectories.

On the contrary, entropic gravity works pretty well for the Newtonian view of gravity as a force, and not the GR view of gravity as a deformation of space time and analogous to acceleration. Acceleration is a very elementary concept, one you find even in microscopic descriptions. Gravity being essentially the same thing makes it far more elementary than a concept like entropy, which only applies to large groups of particles.

So, if the GR picture is the right one, if gravity and acceleration are essentially the same thing, its very hard to see how that aligns with gravity being an emergent phenomenon that only happens at large scales. However, if gravity is just a tendency for massive objects to come together, as in the Newtonian picture, that is perfectly easy to imagine as an entropic effect.

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