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

281–290 of 298 posts

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

#281

Post by Ma8ee »

fc417fc802 wrote: post #276
Ma8ee wrote: post #272

Earlier quoted context omitted.

With the same line of reasoning you can argue (and some have) that numbers aren’t fundamental and are only made up by humans. In my view, the number of swans in the pond is three whether humans exist or not.

I think that's conflating distinct concepts. Numbers aren't a physical property. Whether or not they're a fundamental concept is largely a philosophical question. Meanwhile temperature (for example), while physical, is a statistical property of a macroscopic system. It isn't fundamental - rather it's an abstraction over a (very) large set - but it is nonetheless a quantification of physical characteristics. Whereas a…

The existence of a rock is definitely a physical property, and thus is the number of rocks in a collection a physical property, at least as much as temperature is it for a gas.

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

#282

Post by kgwgk »

Gravityloss wrote: post #279
kgwgk wrote: post #266

Earlier quoted context omitted.

Sure, nothing in the laws of physics prevents a celestial body from distancing itself from the Sun or from the Moon. But it would like suspicious! Wouldn't you suspect that you were watching a reversed video?

You don't understand my point. If we watch purely gravity interactions, the video can be reversed and you wouldn't be able to detect it. If a meteor crashes into the moon, there's other effects than gravity that makes the video not reversible. Ie it's not only gravity. That's the point. In other words, if a comet approached the moon at high speed, missed and slingshotted in another direction, it would be traveling aw…

I think you don’t understand my point.

Someone wrote “If you saw an apple spiral out of a black hole, wouldn't you suspect that you were watching a reversed video?”

I replied “If you saw a comet coming from the sun, or a meteorite coming from the moon, etc. you would also find that suspicious.”

I don’t know what part do you object to (if any).

> If a meteor crashes into the moon, there's other effects than gravity that makes the video not reversible.

If a meteor (or an apple) is still in a crashing trajectory when you stop recording there are no effects other than gravity. The video is reversible - it just looks weird when you play it in reverse (because the meteor seems to be coming from the Moon and the apple seems to be coming from the black hole if you try to imagine where they were before).

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

#284

Post by Gravityloss »

kgwgk wrote: post #282
Gravityloss wrote: post #279

Earlier quoted context omitted.

You don't understand my point. If we watch purely gravity interactions, the video can be reversed and you wouldn't be able to detect it. If a meteor crashes into the moon, there's other effects than gravity that makes the video not reversible. Ie it's not only gravity. That's the point. In other words, if a comet approached the moon at high speed, missed and slingshotted in another direction, it would be traveling aw…

I think you don’t understand my point. Someone wrote “If you saw an apple spiral out of a black hole, wouldn't you suspect that you were watching a reversed video?” I replied “If you saw a comet coming from the sun, or a meteorite coming from the moon, etc. you would also find that suspicious.” I don’t know what part do you object to (if any). > If a meteor crashes into the moon, there's other effects than gravity th…

Well yes for example if there was a cannon firing an apple from the moon, then it would travel the same trajectory, just in reverse.

But we know there are no apple firing cannons on the moon.

Ie if an object was coming from the moon and its past trajectory intersected with the moon's surface, you could say "this is reverse video".

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

#285

Post by kgwgk »

Gravityloss wrote: post #284
kgwgk wrote: post #282

Earlier quoted context omitted.

I think you don’t understand my point. Someone wrote “If you saw an apple spiral out of a black hole, wouldn't you suspect that you were watching a reversed video?” I replied “If you saw a comet coming from the sun, or a meteorite coming from the moon, etc. you would also find that suspicious.” I don’t know what part do you object to (if any). > If a meteor crashes into the moon, there's other effects than gravity th…

Well yes for example if there was a cannon firing an apple from the moon, then it would travel the same trajectory, just in reverse. But we know there are no apple firing cannons on the moon. Ie if an object was coming from the moon and its past trajectory intersected with the moon's surface, you could say "this is reverse video".

That was exactly my point, that an apple coming from a black hole may seem problematic but so does an apple coming from the Sun. I don't see an essential difference regarding the "reversibility" of the physical process (but I could be wrong).

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

#286

Post by nathan_compton »

quantadev wrote: post #270
nathan_compton wrote: post #265

Earlier quoted context omitted.

I think that might oversimplify things, in addition to being a model-laden description. There are empty-space solutions to GR (and indeed many a physicist has found the intuition that there is nothing but empty space appealing, cf geometrodynamics) and in these situations spacetime itself interacts with itself, no particles at all. In any case, I would say this is a somewhat bold description of what we know about gra…

You can disprove my statement by naming something gravity does other than add a shape to spacetime, if you want. Saying "I think you're probably wrong, but I just can't say why" is not an argument.

I'm not trying to disprove what you are saying, I'm just providing some context. Although, point of fact, I think its a weird framing, since even GR, which you seem to be talking about, casts gravity as reciprocal with matter. As the old saying goes: spacetime tells matter how to move, matter tells spacetime how to bend. If that isn't an interaction, I don't know what is.

But, that aside, the idea that gravity is spacetime is really just the theory of general relativity and that is clearly not the final word on the subject. Other approaches to gravity may admit alternative ontologies.

Again, I'm not trying to prove or disprove anything. Just saying that you are kind of oversimplifying an open scientific question.

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

#287

Post by Gravityloss »

kgwgk wrote: post #285
Gravityloss wrote: post #284

Earlier quoted context omitted.

Well yes for example if there was a cannon firing an apple from the moon, then it would travel the same trajectory, just in reverse. But we know there are no apple firing cannons on the moon. Ie if an object was coming from the moon and its past trajectory intersected with the moon's surface, you could say "this is reverse video".

That was exactly my point, that an apple coming from a black hole may seem problematic but so does an apple coming from the Sun. I don't see an essential difference regarding the "reversibility" of the physical process (but I could be wrong).

Maybe this thought experiment makes it clear. There's a cannon firing straight up from the lunar pole. One cannot observe the explosion charge. The ball goes straight up, and then falls down and goes back to the barrel. The cannon is filmed from afar firing and you can see the whole ball travel.

Then you are shown two films, the normal and a reversed one. Would you be able to tell which one is which, and if so, how?

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

#288

Post by quantadev »

nathan_compton wrote: post #286
quantadev wrote: post #270

Earlier quoted context omitted.

You can disprove my statement by naming something gravity does other than add a shape to spacetime, if you want. Saying "I think you're probably wrong, but I just can't say why" is not an argument.

I'm not trying to disprove what you are saying, I'm just providing some context. Although, point of fact, I think its a weird framing, since even GR, which you seem to be talking about, casts gravity as reciprocal with matter. As the old saying goes: spacetime tells matter how to move, matter tells spacetime how to bend. If that isn't an interaction, I don't know what is. But, that aside, the idea that gravity is spa…

Shape of space simply means the "Metric Tensor", and saying it's the sum total of what General Relativity is describing isn't some "bold" or "weird" framing (your words); it's both complete and correct.

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

#289

Post by kgwgk »

Gravityloss wrote: post #287
kgwgk wrote: post #285

Earlier quoted context omitted.

That was exactly my point, that an apple coming from a black hole may seem problematic but so does an apple coming from the Sun. I don't see an essential difference regarding the "reversibility" of the physical process (but I could be wrong).

Maybe this thought experiment makes it clear. There's a cannon firing straight up from the lunar pole. One cannot observe the explosion charge. The ball goes straight up, and then falls down and goes back to the barrel. The cannon is filmed from afar firing and you can see the whole ball travel. Then you are shown two films, the normal and a reversed one. Would you be able to tell which one is which, and if so, how?

> Maybe this thought experiment makes it clear.

Everything was already clear, I think.

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

#290

Post by fc417fc802 »

Ma8ee wrote: post #281
fc417fc802 wrote: post #276

Earlier quoted context omitted.

I think that's conflating distinct concepts. Numbers aren't a physical property. Whether or not they're a fundamental concept is largely a philosophical question. Meanwhile temperature (for example), while physical, is a statistical property of a macroscopic system. It isn't fundamental - rather it's an abstraction over a (very) large set - but it is nonetheless a quantification of physical characteristics. Whereas a…

The existence of a rock is definitely a physical property, and thus is the number of rocks in a collection a physical property, at least as much as temperature is it for a gas.

Numbers aren't "existence" they're a property of an abstract set. The observer draws a fairly arbitrary mental line around a cluster of objects. Whether the set numbers two, three, or four cannot be determined by observing one of the rocks in isolation.

Fundamental physical properties don't depend on other members of an arbitrary set as determined by an observer.

> at least as much as temperature is

But here I've been explicitly claiming that temperature isn't. It is a statistical abstraction over a physical property but it is not itself a physical property in any inherent sense. That's ... kind of the entire point I've been trying to make about an entire class of concepts.

Post reply on HN