Would equilibrium negate gravity.

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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.
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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 »
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Offline 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 µm 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).

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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.
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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 µm 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.
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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.

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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.
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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?

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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.

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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.
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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. 

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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?
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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.

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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 »

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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.
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]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.

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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.

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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.

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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!"

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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.

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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.

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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.

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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.

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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! :)

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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.

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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 »

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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.
"The more things change, the more they remain the same."

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Offline PmbPhy

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Re: Would equilibrium negate gravity.
« Reply #25 on: 23/02/2015 23:37:44 »

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.
In the case I was thinking of the spacetime was flat everywhere and thus no mass present anywhere. That's how the space is homogeneous. However if one changes their frame of reference to one that is uniformly accelerating then in that frame there's a uniform gravitational field which was "produced" merely by changing frames of reference.
« Last Edit: 25/02/2015 21:10:04 by evan_au »

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Offline Ethos_

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Re: Would equilibrium negate gravity.
« Reply #26 on: 24/02/2015 00:18:06 »

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.
In the case I was thinking of the spacetime was flat everywhere and thus no mass present anywhere. That's how the space is homogeneous. However if one changes their frame of reference to one that is uniformly accelerating then in that frame there's a uniform gravitational field which was "produced" merely by changing frames of reference.
OK..........It appears that I was unaware of how the word "homogeneous" was being used here. My understanding is lacking many times because I'm not always acquainted with current terminology. At any rate, your example about changes in frames is certainly a good one. Any uniform acceleration will translate into a gravitational field.
« Last Edit: 25/02/2015 21:10:59 by evan_au »
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Online yor_on

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Re: Would equilibrium negate gravity.
« Reply #27 on: 24/02/2015 18:08:17 »
"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."

Let me guess Jeffrey :)

That's about HUP, isn't it? (Heisenberg's uncertainty principle) ?

Whether gravity is measurable or not, it would still exist as a property, backtracking this universe we live in into another configuration. Unless we jump into a configuration where gravity is non existent. Keep on thinking.
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Eh, Jeffrey, that last should not be taken as a reprimand. I mean it.
« Last Edit: 24/02/2015 18:09:58 by yor_on »
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Re: Would equilibrium negate gravity.
« Reply #28 on: 24/02/2015 18:22:50 »
Let me now argue against it, and for it.
If HUP defines a universe, Planck scale too should exist as a limit.

If HUP and Planck scale are wrong, no motion exist.
=

And yes, that is now, and here.
Doing so we change the stipulations of the game into one where you have to prove a universe consisting of motion, from one where it doesn't exist.
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the main point is that life is a mystery, so is a universe :)
Let's get as far as we can explaining it.
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Look at it this way, assume a static universe. Does it need HUP to represent what we observe?
Why?

And what about Planck scales?
Why do scaling exist?
« Last Edit: 24/02/2015 18:42:01 by yor_on »
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Offline PmbPhy

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Re: Would equilibrium negate gravity.
« Reply #29 on: 24/02/2015 20:43:49 »
Quote from: yor_on
If HUP defines a universe, ...
That is incorrect. Where did you get that idea from?

If HUP and Planck scale are wrong, no motion exist.

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Offline JohnDuffield

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Re: Would equilibrium negate gravity.
« Reply #30 on: 25/02/2015 12:15:31 »
...As I said, you made the false statement If space is homogeneous, there is no gravity. which makes you wrong.
I'm not wrong.

In the case I was thinking of the spacetime was flat everywhere and thus no mass present anywhere. That's how the space is homogeneous.
Whereupon there is no gravity. So I'm right.

However if one changes their frame of reference to one that is uniformly accelerating then in that frame there's a uniform gravitational field which was "produced" merely by changing frames of reference.
The word "produced" has to be in quotes because one is merely emulating gravity. Objects appear to fall down within your spaceship, but objects outside don't fall towards your spaceship. You haven't really produced a gravitational field. You're just accelerating through homogeneous space, and it isn't the same as standing on a planet in inhomogeneous space. Einstein referred to the latter as a gravitational field of quite special form, but we would nowadays call it a true gravitational field, or just a gravitational field. Nobody would say a rocket creates a gravitational field. See this quote by Synge:

"I have never been able to understand this principle…Does it mean that the effects of a gravitational field are indistinguishable from the effects of an observer’s acceleration? If so, it is false. In Einstein’s theory, either there is a gravitational field or there is none, according as the Riemann tensor does not or does vanish. This is an absolute property; it has nothing to do with any observers world line … The Principle of Equivalence performed the essential office of midwife at the birth of general relativity, but, as Einstein remarked, the infant would never have gone beyond its long clothes had it not been for Minkowski’s concept [of space-time geometry]. I suggest that the midwife be buried with appropriate honours and the facts of absolute space-time faced."

You refer to it in http://arxiv.org/abs/physics/0204044.

Quote from: Ethos_
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.
I'm afraid it isn't true. Gravity is not present if energy is homogeneous. For example, see the plot of gravitational potential on Wikipedia. At the bottom of the "upturned hat" there's a small region where there is no spacetime tilt and so no gravity. If you were in a void at the centre of a spherical body, you don't fall down. In similar vein if you're midway between two co-orbiting stars, you don't fall towards either of them.
« Last Edit: 25/02/2015 12:17:28 by JohnDuffield »

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Re: Would equilibrium negate gravity.
« Reply #31 on: 25/02/2015 12:49:32 »
Another situation where there was no gravity was in the early universe. It was very small and very dense, but it didn't collapse into a black hole. Instead it expanded. That's because the spatial energy density was homogeneous, and therefore there was no gravitational field. Again, see Einstein's Leyden Address where he said "empty space" in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gmn). A gravitational field is a place where space is inhomogeneous. Where space is homogeneous, there is no gravitational field.
« Last Edit: 25/02/2015 21:15:25 by evan_au »

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Offline jeffreyH

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Re: Would equilibrium negate gravity.
« Reply #32 on: 25/02/2015 13:41:54 »
Another situation where there was no gravity was in the early universe. It was very small and very dense, but it didn't collapse into a black hole. Instead it expanded. That's because the spatial energy density was homogeneous, and therefore there was no gravitational field. Again, see Einstein's Leyden Address where he said "empty space" in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gmn). A gravitational field is a place where space is inhomogeneous. Where space is homogeneous, there is no gravitational field.

In the cavity at the centre of a mass the space will be homogeneous at an infinitesimally small point in spacetime. How does that help John? Everywhere away from this point GRAVITY DOES NOT CANCEL.
« Last Edit: 25/02/2015 21:16:36 by evan_au »
Fixation on the Einstein papers is a good definition of OCD.

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Offline PhysBang

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Re: Would equilibrium negate gravity.
« Reply #33 on: 25/02/2015 14:17:19 »
Another situation where there was no gravity was in the early universe. It was very small and very dense, but it didn't collapse into a black hole. Instead it expanded. That's because the spatial energy density was homogeneous, and therefore there was no gravitational field.
I disagree.

Much of cosmological physics of the early universe is about the incredibly strong gravitational forces of the early universe because of the immense energy density. Homogeneity has nothing to do with this, it merely shapes the form of the gravitational field. The universe did not collapse because it had, as an initial condition as far as we can tell, a rate of expansion that far outpaced the force of gravity. Gravity slowed this rate of expansion immensely.

Quote
Again, see Einstein's Leyden Address where he said "empty space" in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gmn). A gravitational field is a place where space is inhomogeneous. Where space is homogeneous, there is no gravitational field.
I see this repeated again and again, crying that people are ignoring Einstein, when he has been shown on many occasions that Einstein himself used a homogeneous cosmological model, one with gravity. (The last time he asked for citations, was given many citations and then he complained that he got such a reference.)

These are JohnDuffield's personal views (as expressed in the book he sells).
« Last Edit: 25/02/2015 21:20:37 by evan_au »

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Re: Would equilibrium negate gravity.
« Reply #34 on: 25/02/2015 14:47:32 »
I disagree.
The FLRW metric " starts with the assumption of homogeneity and isotropy of space". Einstein said a gravitational field is a place where space is neither homogeneous nor isotropic. So there's no overall gravitational field in the early universe. It's really simple.

Much of cosmological physics of the early universe is about the incredibly strong gravitational forces of the early universe because of the immense energy density.
Well I'm sorry, but that's wrong. You must surely know that gravitational force is the first derivative of gravitational potential? If the gravitational potential is all the same because space is homogeneous, there is no gravitational force. This is basic stuff PhysBang.

Homogeneity has nothing to do with this, it merely shapes the form of the gravitational field.
Not so. Go and read what Einstein said. A gravitational field is a place where space is inhomogeneous.

The universe did not collapse because it had, as an initial condition as far as we can tell, a rate of expansion that far outpaced the force of gravity. Gravity slowed this rate of expansion immensely.
No it didn't. Gravity alters the motion of space and matter through space. It doesn't suck space in. We do not live in some Chicken-Little world wherein the Earth's gravity is making the sky fall in.

The last time he asked for citations, was given many citations and then he complained that he got such a reference.
You haven't given any citations.
« Last Edit: 25/02/2015 21:25:38 by evan_au »

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Re: Would equilibrium negate gravity.
« Reply #35 on: 25/02/2015 15:31:15 »
The LFRW metric " starts with the assumption of homogeneity and isotropy of space". Einstein said a gravitational field is a place where space is neither homogeneous nor isotropic. So there's no overall gravitational field in the early universe. It's really simple.
Yes, so simple that every physicist missed that there is no gravity in the very models they use to approximate the gravity of the universe.

There are hundreds of observational papers that relate observations to the LFRW metric.

Of course, this is not likely be the case when you make these claims about Einstein, since we know that you have had the opportunity to review Einstein's own homogeneous space models.

Heck, here's another citation where Einstein uses a homogeneous space for you to deny: http://adsabs.harvard.edu/abs/1932PNAS...18..213E
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Much of cosmological physics of the early universe is about the incredibly strong gravitational forces of the early universe because of the immense energy density.
Well I'm sorry, but that's wrong. You must surely know that gravitational force is the first derivative of gravitational potential? If the gravitational potential is all the same because space is homogeneous, there is no gravitational force. This is basic stuff PhysBang.
Really? Then, please, show us the equations that establish this. I am sure that your Nobel prize will come soon after you show that the 2011 Nobel prize was given in error, since the recipients of that prize all use the LFRW model. You can find it all over their work.

http://www.nobelprize.org/nobel_prizes/physics/laureates/2011/press.html

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Homogeneity has nothing to do with this, it merely shapes the form of the gravitational field.
Not so. Go and read what Einstein said. A gravitational field is homogeneous space.
Yes, in one, and only one source, Einstein says something like that. But in all the science he did, he did nothing like that.

Can you show us how to do a gravity problem using inhomogeneous space? I know that the existing science can land objects on distant bodies.
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You haven't given any citations.
See: http://www.thenakedscientists.com/forum/index.php?topic=54281.msg450815#msg450815
« Last Edit: 25/02/2015 21:31:39 by evan_au »

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Re: Would equilibrium negate gravity.
« Reply #36 on: 25/02/2015 16:21:28 »
Yes, so simple that every physicist missed that there is no gravity in the very models they use to approximate the gravity of the universe.
The fact remains that Einstein described a gravitational field as inhomogeneous space, and the FLRW metric starts with the assumption that space is homogeneous. 

By the way it's the FLRW metric, I made a typo, and have now corrected it.

...that physicists know that the LFRW metric doesn't work and they are merely keeping up appearances...
No I don't. But I will say that there are dark-matter particle physicists who are deliberately trying to present their work as "the only game in town", even if that means censoring Einstein.   

Heck, here's another citation where Einstein uses a homogeneous space for you to deny: http://www.pnas.org/content/18/3/213.full.pdf+html
There's no mention of homogeneous.

please, show us the equations that establish this. I am sure that your Nobel prize will come soon after you show that the 2011 Nobel prize was given in error, since the recipients of that prize all use the LFRW model. You can find it all over their work.
I can't show you any equations that "establish this". But I can show you what Einstein said. What's the problem?

Yes, in one, and only one source, Einstein says something like that. But in all the science he did, he did nothing like that.
Yes he did. I recommend you search the Einstein digital archive on homogeneity. Do not be tempted to dismiss what Einstein actually said in favour of vague references do not support your argument.

Can you show us how to do a gravity problem using inhomogeneous space? I know that the existing science can land objects on distant bodies; you have a lot of talk and obvious falsehoods, but nothing that anyone can use to do physics.
You do it the way you do it now. See http://iopscience.iop.org/0256-307X/25/5/014. Inhomogeneous space is the physical reality that underlies curved spacetime. If you plot the inhomogeneity, it's curved. What you think of as curved spacetime is describing the state of inhomogeneous space. That's what Einstein said. Stop doubting it.   

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You haven't given any citations. .
And no, you haven't given any citations. I have. I've given you references and I've quoted what Einstein said. Your "citations" are vague.

http://www.thenakedscientists.com/forum/index.php?topic=54281.msg450815#msg450815
That's a link to a thread about ether. It demonstrates nothing. It's just another vague citation. 
« Last Edit: 25/02/2015 21:37:53 by evan_au »

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Re: Would equilibrium negate gravity.
« Reply #37 on: 25/02/2015 17:05:00 »
The fact remains that Einstein described a gravitational field as inhomogeneous space, and the FLRW metric starts with the assumption that space is homogeneous. 
The facts are 1) Einstein said something like, "space is inhomogeneous" in one public talk, 2) Einstein referred to spacetime when making his claim more specific, 3) Einstein used homogeneous space in his models outlining the behavior of gravity on a cosmological scale, 3) you refuse to acknowledge anything that Einstein ever wrote about cosmology, 4) you appear to have no physics to support your claim, just one quotation from Einstein taken out of context.

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there are dark-matter particle physicists who are deliberately trying to present their work as "the only game in town",
If you have an alternative model, then let us see it and your predictions.

 
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even if that means censoring Einstein.   
The only person censoring Einstein here is you: you do not want to allow the scientific content of Einstein's work to enter this discussion, only the cherry-picked sentences that you have chosen.
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Heck, here's another citation where Einstein uses a homogeneous space for you to deny: http://www.pnas.org/content/18/3/213.full.pdf+html
There's no mention of homogeneous.
No, that word does not appear there. However, even those who do not recognize the FLRW metric at the bottom of the first page can read the first sentence of the paper, "In a recent note in the Gottinger Nachrichten, Dr. O. Heckmann has pointed out that the non-static solutions of the field equations of the general theory of relativity with constant density do not necessarily imply a positive curvature of three-dimensional space, but that this curvature may also be negative or zero." (Emphasis added.) They can then go on to read the rest of the paper.
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please, show us the equations that establish this. I am sure that your Nobel prize will come soon after you show that the 2011 Nobel prize was given in error, since the recipients of that prize all use the LFRW model. You can find it all over their work.
I can't show you any equations that "establish this". But I can show you what Einstein said. What's the problem?
The problem is that you are directly contradicting scientific claims that have been established with a great deal of empirical evidence, and that your only support is one sentence from one public lecture from a man who explicitly engaged in the scientific activity that you deny is possible.
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Yes, in one, and only one source, Einstein says something like that. But in all the science he did, he did nothing like that.
Yes he did. I recommend you search the Einstein digital archive on homogeneity. Do not be tempted to dismiss what Einstein actually said in favour of vague references do not support your argument.
You produced a vague search. The results that appear to be on topic do not support your position. Indeed, if we were going to abandon looking at Einstein's physics (e.g., the document I provided above where Einstein literally uses the FLRW metric), and do your kind of textual analysis, we would be forced to accept that your citation is an error and that someone wrote down, "inhomogeneous space," when Einstein actually said, "homogeneous space."

I find it hard to believe that you actually read the citations that you provide.
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Can you show us how to do a gravity problem using inhomogeneous space? I know that the existing science can land objects on distant bodies; you have a lot of talk and obvious falsehoods, but nothing that anyone can use to do physics.
You do it the way you do it now.
We do not do it that way now.

You have accused all physicists of ignoring Einstein. This means that they are not actually using "inhomogeneous space" now to do physics and you know this.

You cannot have your cake and eat it too. Either the way people do physics now is how Einstein would do it or not.

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See http://iopscience.iop.org/0256-307X/25/5/014. Inhomogeneous space is the physical reality that underlies curved spacetime. If you plot the inhomogeneity, it's curved. What you think of as curved spacetime is describing the state of inhomogeneous space. That's what Einstein said. Stop doubting it. 
Ah, you have a paper by some Chinese scientists, that is not cited by others, that reproduces one, and only one, physical phenomena associated with gravity with "inhomogeneous space".  Can you use their mathematics to calculate the trajectory of any physical object? If you are unable to do this, why should we believe your textual analysis in the place of physics? As you demonstrated above with your own "homogeneity" search, the evidence is that Einstein believed that space is homogenous. Your kind of textual analysis should say that this obscure paper is incorrect.
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you haven't given any citations. I have. I've given you references and I've quoted what Einstein said.
You provided two quotations. Just two. And then you claimed that people were ignoring Einstein when they used the same science that Einstein did in his scientific work.
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Your "citations" are vague.
Really? A citation to one of Einstein's papers where he does nothing but discuss a homogeneous solution in GR is vague but a link to a word search is not vague?

Quote
See:
http://www.thenakedscientists.com/forum/index.php?topic=54281.msg450815#msg450815

That's a link to a thread about ether. It demonstrates nothing. It's just another vague citation.
It's another link to you being provided with citations that you then comment on but essentially ignore. This should be good evidence to you that you may be on the wrong track with your reasoning.
« Last Edit: 25/02/2015 21:49:04 by evan_au »

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Offline Bill S

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Re: Would equilibrium negate gravity.
« Reply #38 on: 25/02/2015 18:59:48 »
One of the problems with picking isolated quotes from Einstein is that, as Hans Ohanian, "Einstein's Mistakes", clearly shows, Einstein made a lot of errors, notwithstanding the fact that he often reached correct conclusions in spite of them. Einstein's work has to be viewed in broader perspective.
There never was nothing.

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Re: Would equilibrium negate gravity.
« Reply #39 on: 25/02/2015 19:40:18 »
One of the problems with picking isolated quotes from Einstein is that, as Hans Ohanian, "Einstein's Mistakes", clearly shows, Einstein made a lot of errors, notwithstanding the fact that he often reached correct conclusions in spite of them. Einstein's work has to be viewed in broader perspective.
Nobody's perfect Bill, and IMHO Einstein did make some mistakes. Cosmology seemed to be a bit of an issue, that's where he made his "greatest blunder". But I think he got things right when it comes to gravity. And that if you find something Einstein said in say 1916 or 1920, then if it doesn't square with what something somebody else says, it's something you should look into. The reason why light curves is a great example. You typically read that "light curves because spacetime is curved", but actually, Einstein never said that. Pete's essay http://arxiv.org/abs/physics/0204044 addresses this. It's well worth reading:

"There exists some confusion, as evidenced in the literature, regarding the nature of the gravitational field in Einstein's General Theory of Relativity. It is argued here the this confusion is a result of a change in interpretation of the gravitational field. Einstein identified the existence of gravity with the inertial motion of accelerating bodies (i.e. bodies in free-fall) whereas contemporary physicists identify the existence of gravity with space-time curvature (i.e. tidal forces). The interpretation of gravity as a curvature in space-time is an interpretation Einstein did not agree with".     

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Re: Would equilibrium negate gravity.
« Reply #40 on: 26/02/2015 17:19:04 »
Heh, if you look at HUP it defines a place where the probability of motion always exist, as 'fluctuations'. If you use a zero temperature then it is argued that due to HUP no such thing is reach able. Now the next question might be what a temperature is thought to exist by. Well, as far as I know those fluctuations imply a motion of sorts, maybe another argument for something setting a limit to me, not allowing me to 'scale away' the idea of a 'motion'? What I meant by the last statement was this Pete .That if we assume HUP to be wrong, and as it includes other, for me connected entities, as me using Plank scale for defining some 'discrete limit' to for example a 'motion', then 'no motion' could be a possibility, but first if those two are proven incorrect.

It depends on how you define that 'limit of Planck scale' though, as if one Planck length in one Plank time is something static, or if it too can be described as a motion. I think it is about motion then too, but I haven't always thought that way. It's more or less a thought game, but interesting to me. In a way it becomes a 'grain of time', as it is the definition of lights propagation in one Planck time, and I get two choices here, either this 'grain' exist in a 'locally measured flat space' (very theoretically now, impossible to test) or it also exist in some 'global representation' of a universe, making it a very flat common universe, scaling it down. the last one is a 'discrete universe' in the way we normally think about it I guess. The first is a strictly local definition of what you would 'measure'. It can't be anything else than thought games, as there is no way to measure at that scale that I know of. Although, any which way I define it, there should be something complementary to it, as I think not using the arrow we define.

One more thing, it seems also as having the ability to reduce geometry to 'properties', as in the example of infinitely splitting a circle into smaller and smaller chunks of length, each one of those becoming 'straighter and straighter', reducing the circle we saw into a property belonging to each of those 'straight bits'. And it also has to do with the question of if there is a discreteness, or not? If there is, can I still define it (the original circle) as becoming a property of this 'straight line' I 'measure' it to be? It's interesting.

I can see one possible way around it, but it's weird. That would be if everything was fractal in some mathematical manner, then it should keep its geometry unchanged. But I can't make that one work for me :) And one last crazy thought. This 'jitteryness' we define to down under, could that also be described as a limit for a local arrow? Because that's what I would like it to be :) What happens as you reach a scale where the arrow loses its consistency? Thinking of it, defining a size to the universe is to my eyes a meaningless occupation, defining it through scales though, that's what I call meaningful. That should make me a novelty, astronomically speaking :)

http://www.physlink.com/Education/AskExperts/ae380.cfm
« Last Edit: 26/02/2015 18:56:21 by yor_on »
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Re: Would equilibrium negate gravity.
« Reply #41 on: 26/02/2015 17:23:12 »
Quote from: Bill S
... as Hans Ohanian, "Einstein's Mistakes", clearly shows, Einstein made a lot of errors, ...
That's quite a misleading statement, Bill. Everyone makes mistakes. If any one persons mistakes were listed out then there'd be a long list of them. Einstein didn't make more or less mistake than anybody else. That book angered a lot of Einstein historians for that reason.

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Re: Would equilibrium negate gravity.
« Reply #42 on: 26/02/2015 18:19:31 »
Einstein was in terms of probability, rather improbable, I would say :) some of the stuff he gets credit for was also 'shared' in the motto of others too thinking in similar terms. But he made outstanding work, both in his own mind-concepts, relativity, explaining Brownian motion, lights duality, etc, as in being one of the founders of modern quantum mechanics, although one main reason for the last was as much due to him thinking up new obstacles to disprove it (as entanglements).
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Re: Would equilibrium negate gravity.
« Reply #43 on: 26/02/2015 18:51:37 »
Quote from: yor_on
... although one main reason for the last was as much due to him thinking up new obstacles to disprove it (as entanglements).
I disagree. He's the one who explained the photoelectric effect by quantizing light by postulating that it consisted of quanta.

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Re: Would equilibrium negate gravity.
« Reply #44 on: 26/02/2015 19:07:47 »
Yes, I stand corrected Pete. Quanta is indeed what QM is about. The thing is, the more we admit the pure awesomeness of his ideas, the worse it will sound to those not enjoying this company. And somehow that population has grown, as a guess a result from his dissatisfaction with the premises of 'spooky action at a distance', as well as the idea of probabilities meaning something in its own, as a set of rules defining a universe. But he was the sole best thinker I've read, and seen discussed, all the same.
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