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Author Topic: Would equilibrium negate gravity.  (Read 12471 times)

Offline jeffreyH

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Would equilibrium negate gravity.
« on: 23/02/2015 00:20:54 »
If all the matter in the universe was at the same temperature/energy state and there was a more or less uniform density distribution would gravity still exist? If you say yes then please state the physical reasons.


 

Offline alancalverd

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Re: Would equilibrium negate gravity.
« Reply #1 on: 23/02/2015 00:25:21 »
Yes. Gravity is a function of mass, which is independent of temperature.

"More or less" may be meaningful to some people but scientifically, it's an oxymoron. Either you are talking about uniform density, or you aren't.

In the case of a medium of truly uniform density, the net gravitational force on any voxel of that medium would be zero. If there were any fluctuations of density, the local net force in and around a more (or less) dense voxel would not be zero.
« Last Edit: 23/02/2015 00:32:39 by alancalverd »
 

Online chiralSPO

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Re: Would equilibrium negate gravity.
« Reply #2 on: 23/02/2015 00:37:00 »
I don't see anything about this setup that would prevent gravity (the fundamental force) from existing. Perhaps you mean that there wouldn't be any net gravitational effect on any of the matter?

If everything were static and perfectly evenly distributed, then I suppose there wouldn't be any gravitational effects observed. However, systems are usually not so well behaved. Everything moves, and there will eventually be some disturbances in this "uniformity" that will throw everything out of balance (even if it is so improbable that it takes millions or billions or quintillions of years...)

I guess the outcome depends on what temperature and density you have in mind (and what form the matter is (are we starting with a uniform distribution of subatomic particles, H atoms, H2 molecules, 100 Ám droplets of water, planets...) I am assuming we are starting with H atoms for this next paragraph:

If the universe is very hot and sparse, then this state could persist for a long time, but if it is cool and dense enough for electrostatic forces to draw the matter into a condensed phase, and the uniformity will deteriorate (I believe this is what happened in our universe, after only a few hundred million years).
 

Offline jeffreyH

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Re: Would equilibrium negate gravity.
« Reply #3 on: 23/02/2015 00:38:31 »
A very quick reply and an excellent response. Sorry about the more or less remark but I was thinking that there couldn't in reality ever be a perfectly uniform density without absolute zero motion. Absolute zero motion is not possible in practice.

Thanks again for the response.
 

Offline jeffreyH

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Re: Would equilibrium negate gravity.
« Reply #4 on: 23/02/2015 00:43:24 »
I don't see anything about this setup that would prevent gravity (the fundamental force) from existing. Perhaps you mean that there wouldn't be any net gravitational effect on any of the matter?

If everything were static and perfectly evenly distributed, then I suppose there wouldn't be any gravitational effects observed. However, systems are usually not so well behaved. Everything moves, and there will eventually be some disturbances in this "uniformity" that will throw everything out of balance (even if it is so improbable that it takes millions or billions or quintillions of years...)

I guess the outcome depends on what temperature and density you have in mind (and what form the matter is (are we starting with a uniform distribution of subatomic particles, H atoms, H2 molecules, 100 Ám droplets of water, planets...) I am assuming we are starting with H atoms for this next paragraph:

If the universe is very hot and sparse, then this state could persist for a long time, but if it is cool and dense enough for electrostatic forces to draw the matter into a condensed phase, and the uniformity will deteriorate (I believe this is what happened in our universe, after only a few hundred million years).

Very good points. As I said to Alan I couldn't see a situation where there would be perfect density and no motion at all. Your last paragraph is of most interest.
 

Offline PhysBang

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Re: Would equilibrium negate gravity.
« Reply #5 on: 23/02/2015 01:37:11 »
Einstein's first preferred cosmology was one that, on the largest scales, was evenly distributed and did not move. To do this, he introduced a more general version of General Relativity than the initial one he initially used. However, a few people pointed out that even in this more general version, it was still unlikely that everything would be so perfectly balanced that gravity wouldn't cause things to move.
 

Offline jeffreyH

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Re: Would equilibrium negate gravity.
« Reply #6 on: 23/02/2015 01:42:50 »
Einstein's first preferred cosmology was one that, on the largest scales, was evenly distributed and did not move. To do this, he introduced a more general version of General Relativity than the initial one he initially used. However, a few people pointed out that even in this more general version, it was still unlikely that everything would be so perfectly balanced that gravity wouldn't cause things to move.

That's interesting and I believe the critics were right. That is a delicate balance to maintain.
 

Offline PmbPhy

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Re: Would equilibrium negate gravity.
« Reply #7 on: 23/02/2015 03:26:00 »
Quote from: jeffreyH
If all the matter in the universe was at the same temperature/energy state and there was a more or less uniform density distribution would gravity still exist? If you say yes then please state the physical reasons.
Of course it would. Its difficult to state any physical reason because there's no reason I can conceive of which would suggest such a thing. The only relationship between gravity and temperature is the fact that when you warm up a body it causes an increase in the internal energy of the body and that energy input causes an increase in mass which then causes an increase in the gravitational field generated by the mass.

In fact my question to you is why would you think otherwise?
 

Offline PmbPhy

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Re: Would equilibrium negate gravity.
« Reply #8 on: 23/02/2015 03:32:26 »
Quote from: alancalverd
"More or less" may be meaningful to some people but scientifically, it's an oxymoron.
That's not quite right. The phrase more or less means

1) to a varying or undetermined extent or degree

2) with small variations

Quote from: alancalverd
Either you are talking about uniform density, or you aren't.
This too is incorrect. Consider the example of the terms use given by Merriam-Webster's Dictionary
Quote
Def 1) they were more or less willing to help

Def 2) contains 16 acres more or less
According to you these examples are wrong because (1) the either were or were not willing to help or (2) it contained 15 acres or it didn't!

In Jeff's example it's easy to see what he meant: the field is uniform to a high degree of accuracy and if any variation was there then it was small.
 

Offline alancalverd

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Re: Would equilibrium negate gravity.
« Reply #9 on: 23/02/2015 08:26:22 »
Pedantry is important here because, as all respondents have pointed out, the uniform universe is inherently unstable, so even the colloquial "more or less" is inappropriate: any inhomogeneity, however small, would lead to total collapse.   

The physics of that collapse is interesting: it predicts persistent gravitational waves, localised coalescence, and a whole bunch of (potentially) observable cosmological phenomena within a Schwartschild radius.

Now any infinite set can be considered "more or less" uniform if all its members are finite, because the ratio of local fluctuation to total set content tends to zero as the set tends to infinity. It is therefore not surprising that the observable universe coalesced from a uniform infinite background. Much more intellectually satisfactory than the Big Bang (something from nothing) or continuous creation (stuff from somewhere else) model of cosmogenesis.
 

Offline JohnDuffield

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Re: Would equilibrium negate gravity.
« Reply #10 on: 23/02/2015 11:54:35 »
I presume everybody knows that the Big Bang is said to involve a singularity? And that the universe is likened to a black hole? Well, if you read Kevin Brown's Formation and Growth of Black Holes, you can read about the frozen-star black hole. What we call a black hole used to be called a frozen star in Oppenheimer's time:

"Historically the two most common conceptual models for general relativity have been the "geometric interpretation" (as originally conceived by Einstein) and the "field interpretation" (patterned after the quantum field theories of the other fundamental interactions). These two views are operationally equivalent outside event horizons, but they tend to lead to different conceptions of the limit of gravitational collapse. According to the field interpretation, a clock runs increasingly slowly as it approaches the event horizon (due to the strength of the field), and the natural "limit" of this process is that the clock asymptotically approaches "full stop" (i.e., running at a rate of zero)..."

Kevin Brown doesn't favour it, but I think it's right. And I think it's very interesting to think of the early universe as something like this frozen-star black hole. There's no motion, no gravity, no light. It's quite a strange place. I am reminded of the gravastar. That's a bit like the frozen-star black hole. But note this: This region is called a "gravitational vacuum", because it is a void in the fabric of space and time. Like, the early universe was some kind of void in the fabric of space and time. There's something about that, that I like. 
 

Offline PhysBang

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Re: Would equilibrium negate gravity.
« Reply #11 on: 23/02/2015 13:49:06 »
I presume everybody knows that the Big Bang is said to involve a singularity? And that the universe is likened to a black hole?
This seems to be another of your pet ideas. Do you have a citation to the idea that the universe is like a black hole?
Quote
Kevin Brown doesn't favour it, but I think it's right. And I think it's very interesting to think of the early universe as something like this frozen-star black hole. There's no motion, no gravity, no light.
That is ridiculous. The early universe was filled with motion, gravity, and light. See, for example, Kolb & Turner's "The Early Universe", or even Weinberg's "The First Three Minutes". Even the WMAP pages, to which you yourself produced a link, should cover this somewhere since their entire project is about estimating the amount of light, motion, and gravity in the early universe.
 

Offline JohnDuffield

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Re: Would equilibrium negate gravity.
« Reply #12 on: 23/02/2015 14:05:37 »
This seems to be another of your pet ideas. Do you have a citation to the idea that the universe is like a black hole?
It isn't my pet idea. Google it. There's loads of articles out there, such as this one on Baez:

"The standard Big Bang models are the Friedmann-Robertson-Walker (FRW) solutions of the gravitational field equations of general relativity.  These can describe open or closed universes.  All of these FRW universes have a singularity at their beginning, which represents the Big Bang.  Black holes also have singularities.  Furthermore, in the case of a closed universe no light can escape, which is just the common definition of a black hole.  So what is the difference?"

That is ridiculous. The early universe was filled with motion, gravity, and light. See, for example, Kolb & Turner's "The Early Universe", or even Weinberg's "The First Three Minutes".
Nobody knows for sure about the early universe. We're confident that the universe is expanding, and we can wind things back, but the further back we go, the more speculative things become.   

Even the WMAP pages, to which you yourself produced a link, should cover this somewhere since their entire project is about estimating the amount of light, motion, and gravity in the early universe.
There was no overall gravity in the early universe. It was small and dense, but it didn't collapse into a black hole, now did it? If space is homogeneous, there is no gravity. By the way, I think this article is interesting:

Physicist Paul Steinhardt Slams Inflation, Cosmic Theory He Helped Conceive

If the universe started out as something akin to a frozen-star black hole rather than a point-singularity black hole, everything would have started out uniform and homogeneous. There's no need to propose a theory of inflation to smooth things out.   
« Last Edit: 23/02/2015 14:09:09 by JohnDuffield »
 

Offline PhysBang

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Re: Would equilibrium negate gravity.
« Reply #13 on: 23/02/2015 14:55:20 »
This seems to be another of your pet ideas. Do you have a citation to the idea that the universe is like a black hole?
It isn't my pet idea. Google it. There's loads of articles out there, such as this one on Baez:

"The standard Big Bang models are the Friedmann-Robertson-Walker (FRW) solutions of the gravitational field equations of general relativity.  These can describe open or closed universes.  All of these FRW universes have a singularity at their beginning, which represents the Big Bang.  Black holes also have singularities.  Furthermore, in the case of a closed universe no light can escape, which is just the common definition of a black hole.  So what is the difference?"
You appear to have stopped reading at that last sentence. Given the full content of that article, there appears to be no significant similarities between the universe and a black hole. When I wrote that you ignored context, this is the kind of thing I meant.
Quote
That is ridiculous. The early universe was filled with motion, gravity, and light. See, for example, Kolb & Turner's "The Early Universe", or even Weinberg's "The First Three Minutes".
Nobody knows for sure about the early universe. We're confident that the universe is expanding, and we can wind things back, but the further back we go, the more speculative things become.   
OK, so your speculation, devoid of any scientific evidence or theory, is that there was no motion, light, or gravity and the speculation of the community of astrophysics, backed with quite a lot of evidence and theory, is that there was motion, light, and gravity. I will stick to the people with the evidence and  hundreds of papers and books on the subject.
Quote
]There was no overall gravity in the early universe. It was small and dense, but it didn't collapse into a black hole, now did it? If space is homogeneous, there is no gravity.
Yes, we all now know about your pet idea, explicitly rejected by Einstein, that there is no gravity in homogeneous space. Again, I will stick with the people with evidence. If you can produce a mathematical proof that there is no gravity in homogeneous space, then I would consider that you had anything but your own religious beliefs to support your position.
 

Offline PmbPhy

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Re: Would equilibrium negate gravity.
« Reply #14 on: 23/02/2015 15:22:33 »
Quote from: PhysBang
Yes, we all now know about your pet idea, explicitly rejected by Einstein, that there is no gravity in homogeneous space. Again, I will stick with the people with evidence. If you can produce a mathematical proof that there is no gravity in homogeneous space, then I would consider that you had anything but your own religious beliefs to support your position.
I believe that there is no spacetime curvature if space is homogeneous. However you can have gravitational fields in the absence of spacetime curvature so he's wrong, again.
 

Offline PhysBang

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Re: Would equilibrium negate gravity.
« Reply #15 on: 23/02/2015 15:45:43 »
One can have global curvature in homogeneous models. In the standard Big Bang models (without the cosmological constant), a recollapsing universe is closed under global curvature and has a finite volume.
 

Offline JohnDuffield

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Re: Would equilibrium negate gravity.
« Reply #16 on: 23/02/2015 16:48:44 »
I believe that there is no spacetime curvature if space is homogeneous. However you can have gravitational fields in the absence of spacetime curvature so he's wrong, again.
Perhaps your memory is letting you down. In post #35 here I said this:

"Jeffrey, imagine you're standing on a big flat board. You roll a bowling ball across it, and it goes straight. There's no curvature. But now imagine I tilt the big flat board. Now when you roll the bowling ball across it, it curves because of the slope. However the board isn't curved, it's still flat, but it's tilted. Spacetime is like the board, the path of light is like the path of the bowling ball. Once you've got that just think of the board as being tilted and curved, like something in a skateboard park".

You said this:

"That is a wonderful analogy John!"
 

Offline PmbPhy

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Re: Would equilibrium negate gravity.
« Reply #17 on: 23/02/2015 16:52:27 »
Quote from: PhysBang
One can have global curvature in homogeneous models.
I don't know what that means. Curvature is, by definition, a local phenomena. See page 2 of Differential Geometry by Erwin Kreyszig

Quote from: PhysBang
In the standard Big Bang models (without the cosmological constant), a recollapsing universe is closed under global curvature and has a finite volume.
The curvature is local in that model. You seem to be thinking that since the local curvature is the same everywhere then the curvature is global. There's no such concept of global curvature in GR/cosmology or differential geometry.
 

Offline PmbPhy

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Re: Would equilibrium negate gravity.
« Reply #18 on: 23/02/2015 16:54:05 »
I believe that there is no spacetime curvature if space is homogeneous. However you can have gravitational fields in the absence of spacetime curvature so he's wrong, again.
Perhaps your memory is letting you down. In post #35 here I said this:

"Jeffrey, imagine you're standing on a big flat board. You roll a bowling ball across it, and it goes straight. There's no curvature. But now imagine I tilt the big flat board. Now when you roll the bowling ball across it, it curves because of the slope. However the board isn't curved, it's still flat, but it's tilted. Spacetime is like the board, the path of light is like the path of the bowling ball. Once you've got that just think of the board as being tilted and curved, like something in a skateboard park".

You said this:

"That is a wonderful analogy John!"
So what? It is a wonderful analogy. It's wonderful because I thought about it back in 1999. :)  However it doesn't prove your point whatsoever.
 

Offline PmbPhy

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Re: Would equilibrium negate gravity.
« Reply #19 on: 23/02/2015 17:02:18 »
Quote from: alancalverd
Yes. Gravity is a function of mass, which is independent of temperature.
That's wrong. Mass is most definitely a function of temperature. At least in relativity it is. When a body heats up it can only do so when heat is added. When this is done the internal thermal energy increases. Since all forms of energy increase the mass of a body then its mass will increase as a result.
 

Offline PhysBang

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Re: Would equilibrium negate gravity.
« Reply #20 on: 23/02/2015 17:04:43 »
You are correct. While I shouldn't have done so, I meant global in the loose sense that the entire spacetime shows the same curvature.
 

Offline PmbPhy

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Re: Would equilibrium negate gravity.
« Reply #21 on: 23/02/2015 17:31:42 »
You are correct. While I shouldn't have done so, I meant global in the loose sense that the entire spacetime shows the same curvature.
Thank you for correcting your statement. I deeply admire people who can do that. Bravo my good man! :)
 

Offline JohnDuffield

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Re: Would equilibrium negate gravity.
« Reply #22 on: 23/02/2015 20:24:59 »
So what? It is a wonderful analogy. It's wonderful because I thought about it back in 1999.
So you say. You also said (in post #38 here that you set it aside because you weren't 100% sure there weren't any problems with that analogy. When it's a wonderful analogy? You know what? I don't believe you.

However it doesn't prove your point whatsoever.
You said you can have gravitational fields in the absence of spacetime curvature so he's wrong, again. But I'm not, am I? Again, you're saying I've got something wrong when you know I haven't. You know full well that spacetime curvature relates to the tidal force rather than gravitational force.
 

Offline PmbPhy

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Re: Would equilibrium negate gravity.
« Reply #23 on: 23/02/2015 22:19:34 »
Quote from: JohnDuffield
(snipped nonsense/gibberish)You know what? I don't believe you.
So what? Nobody here cares if you believe me or anybody else in fact. Just ask them and they'll tell you.

Quote from: JohnDuffield
You said you can have gravitational fields in the absence of spacetime curvature so he's wrong, again. But I'm not, am I?
As I said, you made the false statement If space is homogeneous, there is no gravity. which makes you wrong.

Quote from: JohnDuffield
You know full well that spacetime curvature relates to the tidal force rather than gravitational force.
So what? I explained to you that in a homogeneous spacetime, i.e. one without spacetime curvature, there can still be a gravitational field. A uniform gravitational field, i.e. a gravitational field with zero spacetime curvature, the space is homogeneous. However there's a gravitational field present. This is a well known fact. Perhaps that's why you don't know it, i.e. because it's so well known. Lol!
« Last Edit: 25/02/2015 11:47:24 by Georgia »
 

Offline Ethos_

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Re: Would equilibrium negate gravity.
« Reply #24 on: 23/02/2015 23:26:41 »

So what? I explained to you that in a homogeneous spacetime, i.e. one without spacetime curvature, there can still be a gravitational field. A uniform gravitational field, i.e. a gravitational field with zero spacetime curvature, the space is homogeneous. However there's a gravitational field present. This is a well known fact. Perhaps that's why you don't know it, i.e. because it's so well known. Lol!
Agreed Pete.................It is astounding that anyone could deny such an obvious fact. Like the old saying; "Where there is smoke, there is fire." Same with mass and gravity. "Where there is mass, there is a gravitational field." Even though that mass may be represented as an homogeneous space/time field of mass and energy, gravity will always be present.
 

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Re: Would equilibrium negate gravity.
« Reply #24 on: 23/02/2015 23:26:41 »

 

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