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Author Topic: Can we relate time dilation to escape velocity?  (Read 11437 times)

jeffreyH

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Can we relate time dilation to escape velocity?
« on: 08/03/2015 18:37:14 »
For a mass m we can define a function ve = f(r) for escape velocity. We should then also be able to define a function td = f(r). Can we then define a function td = f(ve)? What would this relate to? Everything is relative so there is no universal frame of reference.

jeffreyH

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Re: Can we relate time dilation to escape velocity?
« Reply #1 on: 09/03/2015 01:49:50 »
I found these two consecutive posts on physicsforums.

1) The time dilation caused by gravity on the surface of a planet is equal to the time dilation for an object moving at the planet's escape velocity in space. This can be proved using the Schwarzschild metric. GR doesn't explain why this is true. It seems to be an odd coincidence.

2) Meaning, I assume, in free space, far away from all gravitating bodies?

Is this correct?

Ethos_

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Re: Can we relate time dilation to escape velocity?
« Reply #2 on: 09/03/2015 21:03:59 »
I found these two consecutive posts on physicsforums.

1) The time dilation caused by gravity on the surface of a planet is equal to the time dilation for an object moving at the planet's escape velocity in space. This can be proved using the Schwarzschild metric. GR doesn't explain why this is true. It seems to be an odd coincidence.

2) Meaning, I assume, in free space, far away from all gravitating bodies?

Is this correct?
It's really very simple Jeff. Think of this in terms of the black hole. We are all familiar with the escape velocity of a black hole as being equal to c.

Understanding this, the fact that escape velocity for any mass in question is caused by the gravitational influence, and as such is directly proportional  to the time dilation associated with that mass. That fact should come as no surprise.

jeffreyH

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Re: Can we relate time dilation to escape velocity?
« Reply #3 on: 09/03/2015 21:58:38 »
I found these two consecutive posts on physicsforums.

1) The time dilation caused by gravity on the surface of a planet is equal to the time dilation for an object moving at the planet's escape velocity in space. This can be proved using the Schwarzschild metric. GR doesn't explain why this is true. It seems to be an odd coincidence.

2) Meaning, I assume, in free space, far away from all gravitating bodies?

Is this correct?
It's really very simple Jeff. Think of this in terms of the black hole. We are all familiar with the escape velocity of a black hole as being equal to c.

Understanding this, the fact that escape velocity for any mass in question is caused by the gravitational influence, and as such is directly proportional  to the time dilation associated with that mass. That fact should come as no surprise.

Well, that being said, I am asking if the same escape velocity for any size of mass will give the same time dilation. This requires a variation in density.

EDIT: This should relate to black hole entropy.

http://en.wikipedia.org/wiki/Black_hole_thermodynamics

This also involves the Bekenstein Bound

http://en.wikipedia.org/wiki/Bekenstein_bound
« Last Edit: 09/03/2015 22:46:10 by jeffreyH »

Ethos_

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Re: Can we relate time dilation to escape velocity?
« Reply #4 on: 09/03/2015 22:47:02 »

This requires a variation in density.
That's very true Jeff but as densities are different for rocky planets like earth, and gas giants like Jupiter, these differences in density also effect where the surface of each is found. And consequently, the radius of each which is used to calculate the escape velocities of those bodies.

jeffreyH

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Re: Can we relate time dilation to escape velocity?
« Reply #5 on: 09/03/2015 23:58:32 »
Distance traveled in free fall is:

s = (1/2)gt^2

Since g = GM/r^2

We can say that

S = (1/2)(GM/r^2)t^2

Then 2Sr/t^2 = GM/r

Ve = SQRT(2GM/r)

So:

4Sr/t^2 = 2GM/r

The factor of 4 is now present

SQRT(4Sr/t^2) = SQRT(2GM/r)

Since g = GM/r^2

Then

Ve = SQRT(2gr) for any mass.
« Last Edit: 10/03/2015 00:00:49 by jeffreyH »

alancalverd

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Re: Can we relate time dilation to escape velocity?
« Reply #6 on: 10/03/2015 00:16:38 »
But that all assumes point masses.

jeffreyH

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Re: Can we relate time dilation to escape velocity?
« Reply #7 on: 10/03/2015 00:25:55 »
But that all assumes point masses.

Yes and that is a major problem. Which highlights just why GR isn't a piece of cake.

Bill S

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Re: Can we relate time dilation to escape velocity?
« Reply #8 on: 10/03/2015 00:53:33 »
Quote from: Ethos
We are all familiar with the escape velocity of a black hole as being equal to c.

If that were the case, wouldn't light escape?

Ethos_

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Re: Can we relate time dilation to escape velocity?
« Reply #9 on: 10/03/2015 01:05:43 »
Quote from: Ethos
We are all familiar with the escape velocity of a black hole as being equal to c.

If that were the case, wouldn't light escape?
Yes of course...........I should have said "escape velocity greater than c." But greater by only the smallest degree.
« Last Edit: 10/03/2015 01:08:15 by Ethos_ »

jeffreyH

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Re: Can we relate time dilation to escape velocity?
« Reply #10 on: 10/03/2015 11:59:42 »
With Ve = SQRT(2gr) if we hold g constant and vary r as in a change in density then we have a differing value for Ve. So gravitational acceleration and escape velocity are not proportional. This means that while for the earth the value of g at the surface is much less than Ve there can be situations in which g is greater than Ve near very dense objects. In theory this can result in superluminal acceleration.

Ethos_

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Re: Can we relate time dilation to escape velocity?
« Reply #11 on: 10/03/2015 15:11:26 »
With Ve = SQRT(2gr) if we hold g constant and vary r as in a change in density then we have a differing value for Ve. So gravitational acceleration and escape velocity are not proportional. This means that while for the earth the value of g at the surface is much less than Ve there can be situations in which g is greater than Ve near very dense objects. In theory this can result in superluminal acceleration.
I think you've misunderstood my use of the word proportional Jeff. I was not saying they were equal, Webster's defines proportional as:

"to arrange the parts of (a whole) so as to be harmonious. a ratio"

jeffreyH

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Re: Can we relate time dilation to escape velocity?
« Reply #12 on: 10/03/2015 17:44:43 »
With Ve = SQRT(2gr) if we hold g constant and vary r as in a change in density then we have a differing value for Ve. So gravitational acceleration and escape velocity are not proportional. This means that while for the earth the value of g at the surface is much less than Ve there can be situations in which g is greater than Ve near very dense objects. In theory this can result in superluminal acceleration.
I think you've misunderstood my use of the word proportional Jeff. I was not saying they were equal, Webster's defines proportional as:

"to arrange the parts of (a whole) so as to be harmonious. a ratio"

Well there is a proportionality and it is balanced but not like the inverse square nature of the gravitational field. Add time dilation to the mix and stir. Can we say time dilation is a function of Ve, g or both?

JohnDuffield

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Re: Can we relate time dilation to escape velocity?
« Reply #13 on: 10/03/2015 17:54:08 »
It isn't a function of g, because that denotes the "local slope" of gravitational potential. Gravitational time dilation denotes the depth of gravitational potential. And to escape it, you need to "take a run at it", wherein your escape velocity is related to the gravitational time dilation.

Quote from: jeffreyH
The time dilation caused by gravity on the surface of a planet is equal to the time dilation for an object moving at the planet's escape velocity in space. This can be proved using the Schwarzschild metric. GR doesn't explain why this is true. It seems to be an odd coincidence.
It's no coincidence. The thing is, GR doesn't actually explain why matter falls down. It doesn't actually say why an object acquires some given speed, which is escape velocity when you flip things round. However it's very easy to understand if you think about the wave nature of matter and stuff electron diffraction and spin. Just simplify the electron to light going round and round, then simplify it further to light going round a square path. The horizontals bend in the gravitational gradient, and the electron falls down. It's similar if you accelerate the electron in gravity-free space. Sadly you never seem to see any texts which combine relativity with the wave nature of matter.
« Last Edit: 10/03/2015 18:02:14 by JohnDuffield »

jeffreyH

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Re: Can we relate time dilation to escape velocity?
« Reply #14 on: 10/03/2015 21:51:05 »
It isn't a function of g, because that denotes the "local slope" of gravitational potential. Gravitational time dilation denotes the depth of gravitational potential. And to escape it, you need to "take a run at it", wherein your escape velocity is related to the gravitational time dilation.

Quote from: jeffreyH
The time dilation caused by gravity on the surface of a planet is equal to the time dilation for an object moving at the planet's escape velocity in space. This can be proved using the Schwarzschild metric. GR doesn't explain why this is true. It seems to be an odd coincidence.
It's no coincidence. The thing is, GR doesn't actually explain why matter falls down. It doesn't actually say why an object acquires some given speed, which is escape velocity when you flip things round. However it's very easy to understand if you think about the wave nature of matter and stuff electron diffraction and spin. Just simplify the electron to light going round and round, then simplify it further to light going round a square path. The horizontals bend in the gravitational gradient, and the electron falls down. It's similar if you accelerate the electron in gravity-free space. Sadly you never seem to see any texts which combine relativity with the wave nature of matter.

Of course it has to be a function of g. If you are stationary on the earth you are experiencing time dilation. You are not accelerating away from it. The velocity involved in escaping the field is due to kinetic energy. If the gravitational field were absent the time dilation would be due to the velocity. At a stationary position on the earth there is still a potential for acceleration. However there is NO potential for escape unless there is an impetus away from the surface. This will INDUCE a time dilation. Without the impetus there is no dilation due to Ve. Can't you see that? You only have the value of g operating.

jeffreyH

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Re: Can we relate time dilation to escape velocity?
« Reply #15 on: 10/03/2015 23:34:14 »
What are the consequences of reformulating the Ve equation this way?

c = SQRT(2gr)

This is then simplified to:

g = c^2/2r

EDIT: Of course this should be:

g = c^2/2rs

This is likely the reason for the survival of the G2 gas cloud.
« Last Edit: 11/03/2015 00:08:02 by jeffreyH »

jeffreyH

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Re: Can we relate time dilation to escape velocity?
« Reply #16 on: 11/03/2015 00:58:28 »
In the proper Schwarzschild form we have:

a = (GM)/(r^2*SQRT[1-rs/r])

Where the above is:

g = c^2/2rs

In the Schwarzschild form the acceleration goes infinite at the event horizon.
« Last Edit: 11/03/2015 01:03:30 by jeffreyH »

jeffreyH

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Re: Can we relate time dilation to escape velocity?
« Reply #17 on: 11/03/2015 01:53:06 »
Considering a point an infinitesimal distance from rs denoted by n we can then produce the following,

g = c^2/(2[rs+n]*SQRT(1-rs/[rs+n]))

The value of n can then be increased as needed to plot the gradient of g away from the event horizon.

JohnDuffield

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Re: Can we relate time dilation to escape velocity?
« Reply #18 on: 11/03/2015 14:11:00 »
Of course it has to be a function of g.
No it isn't. See this depiction of gravitational potential:

CCASA image by AllenMcC, see Wikipedia.

The time dilation is represented by the depth of potential, how low in the plot you are, whilst g is the slope of the plot. Note that there's an inflection, so g at one elevation is the same as g at another. You can find two places on the plot where you can draw the same tangent.

If you are stationary on the earth you are experiencing time dilation.
Yes.

You are not accelerating away from it.
Yes.

The velocity involved in escaping the field is due to kinetic energy.
Yes, and in your ascending rocket going faster and faster you are swapping gravitational time dilation for special-relativistic time dilation.

If the gravitational field were absent the time dilation would be due to the velocity.
Yes.

At a stationary position on the earth there is still a potential for acceleration. However there is NO potential for escape unless there is an impetus away from the surface. This will INDUCE a time dilation.
Yes. And as you ascend away from the surface you reduce the gravitational time dilation.

Without the impetus there is no dilation due to Ve. Can't you see that?
Yes.

You only have the value of g operating.
« Last Edit: 11/03/2015 14:16:32 by JohnDuffield »

jeffreyH

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Re: Can we relate time dilation to escape velocity?
« Reply #19 on: 11/03/2015 14:56:45 »
Of course it has to be a function of g.
No it isn't. See this depiction of gravitational potential:

CCASA image by AllenMcC, see Wikipedia.

The time dilation is represented by the depth of potential, how low in the plot you are, whilst g is the slope of the plot. Note that there's an inflection, so g at one elevation is the same as g at another. You can find two places on the plot where you can draw the same tangent.

If you are stationary on the earth you are experiencing time dilation.
Yes.

You are not accelerating away from it.
Yes.

The velocity involved in escaping the field is due to kinetic energy.
Yes, and in your ascending rocket going faster and faster you are swapping gravitational time dilation for special-relativistic time dilation.

If the gravitational field were absent the time dilation would be due to the velocity.
Yes.

At a stationary position on the earth there is still a potential for acceleration. However there is NO potential for escape unless there is an impetus away from the surface. This will INDUCE a time dilation.
Yes. And as you ascend away from the surface you reduce the gravitational time dilation.

Without the impetus there is no dilation due to Ve. Can't you see that?
Yes.

You only have the value of g operating.

You have just contradicted yourself and described time dilation as a function of g.

David Cooper

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Re: Can we relate time dilation to escape velocity?
« Reply #20 on: 11/03/2015 18:41:23 »
It's no coincidence. The thing is, GR doesn't actually explain why matter falls down. It doesn't actually say why an object acquires some given speed, which is escape velocity when you flip things round. However it's very easy to understand if you think about the wave nature of matter and stuff electron diffraction and spin. Just simplify the electron to light going round and round, then simplify it further to light going round a square path. The horizontals bend in the gravitational gradient, and the electron falls down. It's similar if you accelerate the electron in gravity-free space. Sadly you never seem to see any texts which combine relativity with the wave nature of matter.

That's a useful way of looking at things. Where do you get your knowledge from, because I'd like to explore the source.

JohnDuffield

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Re: Can we relate time dilation to escape velocity?
« Reply #21 on: 12/03/2015 17:18:43 »
That's a useful way of looking at things. Where do you get your knowledge from, because I'd like to explore the source.
A whole rack of different places, ranging from old papers by the likes of Maxwell and Einstein, to newer papers and articles  that tend not to get much publicity. See for example http://arxiv.org/abs/physics/0512265 and S A Goudsmit talking about the discovery of electron spin: "When the day came I had to tell Uhlenbeck about the Pauli principle - of course using my own quantum numbers - then he said to me: "But don't you see what this implies? It means that there is a fourth degree of freedom for the electron. It means that the electron has a spin, that it rotates". Also see magnetic moment and the Einstein-de Haas effect which "demonstrates that spin angular momentum is indeed of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics". However if you were to ask a contemporary particle physicist about this, he might say the electron is a point particle, that its spin is not a real rotation, and that gravity is all down to gravitons flitting back and forth.

NB: note that only the horizontals bend downwards, which is why  light is deflected twice as much as matter.
« Last Edit: 12/03/2015 17:32:06 by JohnDuffield »

David Cooper

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Re: Can we relate time dilation to escape velocity?
« Reply #22 on: 12/03/2015 19:43:58 »
Thanks John - I'll follow all of those up.

JohnDuffield

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Re: Can we relate time dilation to escape velocity?
« Reply #23 on: 12/03/2015 22:21:21 »
There's loads more interesting material out there. IMHO an interesting read is On Vortex Particles by David St John. He's a recent physics PhD. It isn't peer reviewed or anything, but it gives a wealth of information going back to Thomson and Tait, which arguably goes back to Maxwell's theory of molecular vortices. The gist of it is that the electron is a spinor which is akin to a cyclone. The positron is a similar spinor with the opposite chirality which is akin to an anticyclone, and counter-rotating vortices attract. I really ought to write a paper on it. I was talking to the SPIE  guys about this, but I can't go to San Diego in August.

jeffreyH

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Re: Can we relate time dilation to escape velocity?
« Reply #24 on: 12/03/2015 22:37:25 »
On the Hubius Helix.

Read, enjoy and maybe provide all the proofs required.

http://www.quora.com/What-do-physicists-think-of-the-hubius-helix-model-of-electron-structure

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Re: Can we relate time dilation to escape velocity?
« Reply #24 on: 12/03/2015 22:37:25 »