Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

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

quantamagazine.org

Post reply on HN
Print view View original

241–250 of 298 posts

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

#241

Post by forgotoldacc »

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…

This sounds really dumb, so forgive me. But one thing that's always felt weird to me about gravity is how we consider things to be one body.

Like yes, when we look at earth from incredibly far away, it's a pale blue dot. But all those oceans on it are flowing and separate from the solid ground underneath. Those large boulders on earth that would have their own (tiny) gravitational pull on their own in space are just part of earth's single gravitational force. All the airplanes in the sky are subject to the pull of the earth, but they're also a part of the gravitational pull that pulls other things to earth.

When shaking cereal, the big flakes rise to the top, but tiny bits of dust from each flake also separates and settle at the bottom. But earth, as a whole, has big bits and little bits everywhere all flowing freely. And gravity seemingly treats all those bits as a single object. But with sufficient distance between objects (e.g. different planets), it treats them separately. And with greater distance (e.g. galactic scale), it treats them as one again.

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

#242

Post by WalterBright »

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…

> 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 which gets filled in with peanuts.

I always thought it was because the smaller nuts can fall into smaller spaces, while the larger nuts cannot.

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

#243

Post by klodolph »

forgotoldacc wrote: post #241
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…

This sounds really dumb, so forgive me. But one thing that's always felt weird to me about gravity is how we consider things to be one body. Like yes, when we look at earth from incredibly far away, it's a pale blue dot. But all those oceans on it are flowing and separate from the solid ground underneath. Those large boulders on earth that would have their own (tiny) gravitational pull on their own in space are just…

There’s a bit of math you can do where you calculate the gravitational effect of standing on a sphere. You can redistribute the mass within the sphere and it won’t affect the gravitational pull, assuming it’s radially symmetrical.

One of those unintuitive results.

It probably only works in classical gravity but it’s still neat.

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

#244

Post by nrds »

forgotoldacc wrote: post #241
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…

This sounds really dumb, so forgive me. But one thing that's always felt weird to me about gravity is how we consider things to be one body. Like yes, when we look at earth from incredibly far away, it's a pale blue dot. But all those oceans on it are flowing and separate from the solid ground underneath. Those large boulders on earth that would have their own (tiny) gravitational pull on their own in space are just…

> And gravity seemingly treats all those bits as a single object.

It does not; but when you learned about gravity in school or wherever, you were only presented with scenarios in which it was valid to do so. Specifically, for Newtonian gravity, an object whose density is spherically symmetrical may be treated as a point mass at its center (w/r/t other point masses farther from the center than any point in the first object); this can be seen by integrating Newton's equation.

There's no reason to attribute this to special behavior by gravity with respect to "objects" which you identify. You could decompose a (spherically symmetrical) mass into several different spherically symmetrical "objects" and sum up their influence on a test mass, and the result is the same as if you had treated the original mass as a point mass. The hypothesis that gravity is "treating" the object one way or another now fails to distinguish all these possibilities; it's no longer physically meaningful.

At sufficiently large distances, or for sufficiently large ratio of large to small mass, the difference between, say, a planet and a spherically symmetrical mass is so small with respect to the main effect that it can simply be ignored.

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

#245

Post by fnordpiglet »

I’ve been a believer in entropic gravity for a long time and believe it’s due to quantum foam. In a region of space with nothing in it the quantum foam in that space would be perfectly uniformly random. With mass and energy the state of the space would be biased and less random. This creates an entropic gradient. Further this doesn’t just explain gravity but explains why the space between galaxies seems to demonstrate negative energy and space expansion. I’m glad to see more research into the idea of entropic gravity as it’s IMO a more reasonable explanation than most other gravity theories I’ve heard.

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

#246

Post by zmgsabst »

hackinthebochs wrote: post #145
zmgsabst wrote: post #132

Earlier quoted context omitted.

The structure that most words describe is objective, so you haven’t distinguished math as a language. (Nor is mathematics entirely “objective”, eg, axiom of choice.) And the number of terms in your chosen language with your chosen basis isn’t objective: that’s an intrinsic fact to your frame. The complexity of terms is not fixed — that’s simply wrong mathematically. They’re dependent on our chosen basis. Your definit…

This is getting tedious. The point about mathematics was simply that it carries and objectivity that natural language does not carry. But the point about natural language was always a red-herring; not sure why you introduced it. >You can’t make the whole class simplified at once Yes, this is literally my point. The further point is that the relative complexities of two systems will not switch orders regardless of bas…

I didn’t introduce it, you did — by positing that formal language is more objective, as you’ve again done here. My original point was that mathematics is human language.

> The further point is that the relative complexities of two systems will not switch orders regardless of basis, except perhaps in degenerate cases.

Two normal bases: Fourier and wavelet; two normal signals: a wave and an impulse.

They’ll change complexity between the two bases despite everything being normal — the wave simple and impulse complex in Fourier terms; the wave complex and impulse simple in wavelet terms.

That our choice of basis makes a difference is literally why we invented wavelets.

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

#247

Post by Agentlien »

klodolph wrote: post #243
forgotoldacc wrote: post #241

Earlier quoted context omitted.

This sounds really dumb, so forgive me. But one thing that's always felt weird to me about gravity is how we consider things to be one body. Like yes, when we look at earth from incredibly far away, it's a pale blue dot. But all those oceans on it are flowing and separate from the solid ground underneath. Those large boulders on earth that would have their own (tiny) gravitational pull on their own in space are just…

There’s a bit of math you can do where you calculate the gravitational effect of standing on a sphere. You can redistribute the mass within the sphere and it won’t affect the gravitational pull, assuming it’s radially symmetrical. One of those unintuitive results. It probably only works in classical gravity but it’s still neat.

That actually sounds really intuitive to me. If you start with a uniform mass and then redistribute mass symmetrically it makes sense that the change in the two sides would cancel out.

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

#248

Post by JumpCrisscross »

forgotoldacc wrote: post #241
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…

This sounds really dumb, so forgive me. But one thing that's always felt weird to me about gravity is how we consider things to be one body. Like yes, when we look at earth from incredibly far away, it's a pale blue dot. But all those oceans on it are flowing and separate from the solid ground underneath. Those large boulders on earth that would have their own (tiny) gravitational pull on their own in space are just…

> gravity seemingly treats all those bits as a single object

Understanding the tides depends very much on disabusing oneself of this scaffolding.

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

#249

Post by KingMob »

the__alchemist wrote: post #149
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.

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.

I don't know anything about Wolfram's theory, but one general way to address this is to compare the Akaike information criterion (or similar measures).

The metrics attempt to balance the ability of a model to fit data with the number of parameters required. For equally well-fitting models, they prefer the one with fewer params.

If Wolfram's theory fits as well but has fewer params, it should be preferred. I'm not sure if fewer "concepts" counts, but it's something to consider.

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

#250

Post by hackinthebochs »

zmgsabst wrote: post #246
hackinthebochs wrote: post #145

Earlier quoted context omitted.

This is getting tedious. The point about mathematics was simply that it carries and objectivity that natural language does not carry. But the point about natural language was always a red-herring; not sure why you introduced it. >You can’t make the whole class simplified at once Yes, this is literally my point. The further point is that the relative complexities of two systems will not switch orders regardless of bas…

I didn’t introduce it, you did — by positing that formal language is more objective, as you’ve again done here. My original point was that mathematics is human language. > The further point is that the relative complexities of two systems will not switch orders regardless of basis, except perhaps in degenerate cases. Two normal bases: Fourier and wavelet; two normal signals: a wave and an impulse. They’ll change comp…

Yes, that is a degenerate case. We can always encode an arbitrary amount of data into the basis functions to get a maximally simple representation for some target signal. If the signal is simple (carries little information) or the basis functions are constructed from the target signal, you can get this kind of degeneracy. But degenerate cases do not invalidate the rule for the general case.
Post reply on HN