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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

#61

Post by aurareturn »

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"…

  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" as the Sun formed.
From the Big Bang originally. We don’t know what caused the Big Bang.

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

#62

Post by whereismyacc »

kgwgk wrote: post #52
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.

> 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.

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

I know very little about physics but I thought that the leading interpretations of quantum physics say that the probability distribution is all we can know about a system. The entropy is not due to due to a lack of information about the quantum state, but because the outcomes are inherently stochastic?

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

#64

Post by mr_mitm »

kgwgk wrote: post #52
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.

> 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.

Just look at the definition of entropy. Knowledge about a system never enters the equation.

S := -k_B sum p_i ln (p_i)

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

#65

Post by tsimionescu »

holowoodman wrote: post #30
bandrami wrote: post #27

Earlier quoted context omitted.

Was that de Broglie's thing? I always thought it didn't get a fair shake

Virtual particles and related effects are actually widely accepted and experimentally proven (at least partially). Current physics wouldn't really work without them, or at least something that looks the same. https://en.wikipedia.org/wiki/Casimir_effect https://en.wikipedia.org/wiki/Zero-point_energy https://en.wikipedia.org/wiki/Virtual_particle https://en.wikipedia.org/wiki/Hawking_radiation The gist of it is, that…

The Wikipedia article that you quote is quite explicit that, while virtual particles are a widely accepted mathematical tool, they're actual existence of elements of reality is very much not widely accepted, and definitely nowhere close to "experimentally verified". It's in fact considered impossible to verify experimentally, even in principle.

Note that there are many very widely used physical theories that include mathematical elements that are not necessarily assigned any physical meaning. The Poynting vector in classical electrodynamics, for example, carries no widely accepted physical meaning, even though it appears in many well verified and used calculations. This doesn't make the theory suspect or anything, I'm not trying to imply that - simply that virtual particles being "real" or not is a mostly philosophical question that has no widely accepted consensus.

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

#66

Post by kgwgk »

whereismyacc wrote: post #62
kgwgk wrote: post #52

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. 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.

> There are no probability distributions over possible states when there is perfect knowledge of the state. I know very little about physics but I thought that the leading interpretations of quantum physics say that the probability distribution is all we can know about a system. The entropy is not due to due to a lack of information about the quantum state, but because the outcomes are inherently stochastic?

Entropy is about the state - not about “outcomes”.

“All we can know” is the precise state - at least in principle - and entropy is zero in that case.

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

#67

Post by kgwgk »

mr_mitm wrote: post #64
kgwgk wrote: post #52

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. 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.

Just look at the definition of entropy. Knowledge about a system never enters the equation. S := -k_B sum p_i ln (p_i)

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!

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

#68

Post by JPLeRouzic »

lewdwig wrote: post #41
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.

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?

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

#69

Post by sat_solver »

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.

You're thinking of information entropy, which is not the same concept as entropy in physics. An ice cube in a warm room can be described using a minimum description length as "ice cube in a warm room" (or a crystal structure inside a fluid space), but if you wait until the heat death of the universe, you just have "a warm room" (a smooth fluid space), which will have an even shorter mdl. Von Neuman should never have repurposed the term entropy from physics. Entropy confuses a lot of people, including me.

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

#70

Post by mr_mitm »

kgwgk wrote: post #67
mr_mitm wrote: post #64

Earlier quoted context omitted.

Just look at the definition of entropy. Knowledge about a system never enters the equation. S := -k_B sum p_i ln (p_i)

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.

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