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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

#51

Post by mjanx123 »

dist-epoch wrote: post #4

We all know that life on Earth gets it's energy from the Sun. But we also know that's an approximation we tell kids, really life gets low entropy photons from the Sun, does it's thing, and then emits high entropy infrared waste heat. Energy is conserved, while entropy increases. But where did the Sun got it's low entropy photons to start with? From gravity, empty uniform space has low entropy, which got "scooped up"…

The photons do not have entropy.

The photons from Sun are hot, the space around Sun is cold, the system has a low entropy.

If the space around Sun was as hot as the photons, the entropy would be high.

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

#52

Post by kgwgk »

logicchains wrote: post #22
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 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.

> Entropy isn't a function of imperfect knowledge. It's a function of the possible states of a system and their probability distributions.

There are no probability distributions over possible states when there is perfect knowledge of the state.

> Quantum mechanics

Entropy is also zero for a pure quantum state. You won’t have entropy without imperfect knowledge.

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

#53

Post by mr_mitm »

MathMonkeyMan wrote: post #6

Entropic gravity is a compelling framework. I think that most Physicists admit that it would be nice to believe that the yet unknown theory of everything is microscopic and quantum mechanical, and that the global and exquisitly weak force of gravity emerges from that theory as a sort of accounting error. But there are so many potential assumptions baked into these theories that it's hard to believe when they claim, "…

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

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

#54

Post by whereismyacc »

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 sounds like you're talking about information entropy which to my understanding is analogue to but not the same as entropy in physics?

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

#55

Post by gattr »

dist-epoch wrote: post #13
uncircle wrote: post #8

Earlier quoted context omitted.

Your question fascinated me. Googling "where did the Sun got its low entropy" I also came across these explanations: "Solar energy at Earth is low-entropy because all of it comes from a region of the sky with a diameter of half a degree of arc." also, from another reply: "Sunlight is low entropy because the sun is very hot. Entropy is essentially a measure of how spread out energy is. If you consider two systems with…

I read the gravity explanation for the sun low entropy in the "Road to Reality" book from Roger Penrose. Asked Gemini to summarize the argument (scroll to end) https://g.co/gemini/share/bd9a55da02b6

It's also in his previous book "The Emperor's New Mind: Concerning Computers, Minds and The Laws of Physics", along with a lot more. Strongly recommended (even though after reading a lot of Greg Egan, my views on consciousness somewhat shifted towards "classical computation can do it, too".)

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

#56

Post by Keyframe »

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

Right, but you can push unknowns into tmp vars only so much before you have to introduce constraints, otherwise it's all downright undetermined. You have to inject a structure into the placeholder soup or you're just pushing ambiguity around with no real net gain.. which is also fun to play around, question is will you get a paper out of it or even paid if you play like that to no end.

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

#57

Post by mr_mitm »

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…

You only hear these terms in layman explanations. Physics has precise definitions for these things. When we say "ordered", we mean that a particular macrostate has only few possible microstates.

Check this Wikipedia article for a quick overview: https://en.wikipedia.org/wiki/Microstate_(statistical_mechan...

Details can be found in any textbook on statistical mechanics.

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

#58

Post by aurareturn »

logicchains wrote: post #22
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 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.

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

#59

Post by tsimionescu »

meindnoch wrote: post #44
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…

>Simple example: why ice melts in a warm room. Ice melting is simply the water molecules gaining enough kinetic energy (from collisions with the surrounding air molecules) that they break the bonds that held them in the ice crystal lattice. But at the microscopic level it's still just water molecules acting according to Newton's laws of motion (forgetting about quantum effects of course). Now, back on the topic of th…

> But where is entropy in this picture? How do you even define entropy for a system of 2 particles?

The answer is that this doesn't happen in a system with only 2 particles. The idea of gravity as an entropic phenomenon is that you introduce some other kind of particle that permeates spacetime, so there is no system that only contains 2 particles. You may use some idea like virtual particles from quantum field theory, or you may define "quanta of space time" as something that is not technically a particle but basically works like one in a handwavy sense.

But the basic point of these entropy based theories is to explain gravity, and typcilaly spacetime itself, as an emergent result of a collection of numerous objects of some kind. This necessarily means that they don't make sense if applied to idealized systems with very few objects - which is why they typically posit such isolated systems simply can't actually exist in reality.

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

#60

Post by aeonik »

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…

My take, for what it's worth,

Entropy isn’t always the driver of physical change, sometimes it’s just a map.

Sometimes that map is highly isomorphic to the physical process, like in gas diffusion or smoke dispersion. In those cases, entropy doesn't just describe what happened, it predicts it. The microstates and the probabilities align tightly with what’s physically unfolding. Entropy is the engine.

But other times, like when ice melts, entropy is a summary, not a cause. The real drivers are bond energies and phase thresholds. Entropy increases, yes, but only because the system overcame physical constraints that entropy alone can’t explain. In this case, entropy is the receipt, not the mechanism.

So the key idea is this: entropy’s usefulness depends on how well it “sees” the real degrees of freedom that matter. When it aligns closely with the substrate, it feels like a law. When it doesn't, it’s more like coarse bookkeeping after the fact.

The second law of thermodynamics is most “real” when entropy is the process. Otherwise, it’s a statistical summary of deeper physical causes.

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