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Why does light and gravity travel at the same speed?

by Abidal Sala
Tags: gravity, light, speed, travel
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ynot1
#19
Feb11-12, 11:26 PM
P: 90
Quote Quote by JonDE View Post
This has always been something that bothers me that I have never fully understood.
If gravity is bound by the same rules as light and cannot travel faster then light, that suggests that gravity should be effected by the curvature of space. If that is so, then how can gravity escape a black hole? The two seem contradictory to me.
Per Einstein gravity is curvature of spacetime. The gravitational field of the black hole is the sum of the individual contributions from each particle in the black hole. If I understand Hawking radiation it annihilates particles in the black hole, and so the contribution of their gravitational field to that of the black hole will be lost, reducing the gravitational field of the black hole itself and reducing the curvature of spacetime.
Chronos
#20
Feb12-12, 11:05 PM
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You don't need 'gravitons' if gravity is a scalar field. See also http://imagine.gsfc.nasa.gov/docs/as...s/980601a.html
universe21!
#21
Feb17-12, 06:48 PM
P: 6
Gravitons don't exist because a theory out there says space time effects everything even where it doesn't exist. This means space-time doesn't exist where a planet is but does effect it. the effect is space-time pushes towards the (physical) Centre of the planet, as gravity.

and we know space doesn't have force carriers to meditate its range so the same applies to gravity. Since it (gravity) is Space time pushing against a Object it doesn't need the Graviton Boson hence why we have never found one. but this doesn't violate QM.

I find this a very interesting and simple theory because we know space-time and gravity is around. and we have never seen a graviton during an experiment. sounds like a very plausible theory.

What i can say about your question is gravity requires 0 mass to operate and light operates at 0 mass. so the laws of physics consern their speed should be identical.
Drakkith
#22
Feb17-12, 07:50 PM
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Universe21, spacetime most certainly exists everywhere in the universe. And we don't know that gravity DOESN'T have a force carrier, we simply haven't been able to find one yet.
ynot1
#23
Feb17-12, 09:00 PM
P: 90
Quote Quote by Drakkith View Post
Universe21, spacetime most certainly exists everywhere in the universe. And we don't know that gravity DOESN'T have a force carrier, we simply haven't been able to find one yet.
looks like whatever force carrier gravity has would be the same as that for light if they both propagate at the same speed.
ynot1
#24
Feb17-12, 09:14 PM
P: 90
Quote Quote by universe21! View Post
What i can say about your question is gravity requires 0 mass to operate and light operates at 0 mass. so the laws of physics consern their speed should be identical.
So the propagator (for both), i guess, has no mass, so must be spacetime, unless there is something strange going on.
alphachapmtl
#25
Feb17-12, 10:10 PM
P: 81
Quote Quote by Abidal Sala View Post
Why does gravity effect objects as fast as light. For instance, if the sun suddenly disappeared, earth would continue orbiting the place the sun left for 8 minutes, which is the same time it takes light from the sun to reach us.. i think this might be some sort of standard speed in the universe in which information can travel.. but also why when exceeding that speed would you go back in time, why specifically THAT speed (299m/s).. it has to be a result of something else, doesn't it?
The photon are massless, so (it is believe) are the gravitons (which have never been observed). So they both travel at the speed of light.
coelho
#26
Feb18-12, 11:38 PM
P: 52
I have a question, and it seems this is the most apropriate place and time to ask it.

From what i remember, black holes are created when a very massive star shrinks itself to a radius smaller than the so-called Schwarzschild radius. But, from a observer far away from it (like us, for example), as closer the radius of the star gets to the Schwarzschild radius, slower the shrinking gets, so from a observer far away, it takes an infinite time to a star became a black hole, and so, from a observer far away (like us), there are still no actual black holes in the universe, but only stars in the process of becoming one.

It is right?
ynot1
#27
Feb19-12, 12:35 AM
P: 90
Quote Quote by coelho View Post
as closer the radius of the star gets to the Schwarzschild radius, slower the shrinking gets
It seems as long as there is gravity the mass falling in will accelerate until the compressive force of matter inside the black hole balances the force of gravity. Of course the force of impact of infalling matter with matter inside the black hole is going to create heat and of turbulence depending on the compressibility and rebound effects of the mixture of matter inside. The spin of the star also effects the dynamics inside the black hole. For example if the radius of the star collapses by a factor of 1000 the spin of the black hole would increase by about 1000. So stars with spin will be spinning off all kinds of matter as it collapses. If we had data on the high pressure impacts we could do some very interesting theory about what's really going on inside for various spin rates.
ynot1
#28
Feb21-12, 08:29 AM
P: 90
Quote Quote by coelho View Post
I have a question, and it seems this is the most apropriate place and time to ask it.

From what i remember, black holes are created when a very massive star shrinks itself to a radius smaller than the so-called Schwarzschild radius.
True. But note this depends not only on the mass of the star but also its spin. That is, the higher the spin, the more heavy matter will be thrown off the star before it completely collapses, reducing the probability of the star forming a black hole.
zhermes
#29
Feb21-12, 10:06 AM
P: 1,261
Quote Quote by coelho View Post
But, from a observer far away from it (like us, for example), as closer the radius of the star gets to the Schwarzschild radius, slower the shrinking gets, so from a observer far away, it takes an infinite time to a star became a black hole
I think that that is correct. An observer will never see an infalling shell cross an event horizon.
At the same time, however, infalling material becomes increasingly redshifted (quite rapidly), and thus an observer will stop 'seeing' any of that outside material. I guess the infalling matter would appear to dim/fade as it asymptotically approaches the event horizon (which also grows as additional matter falls in).

See: http://adsabs.harvard.edu/abs/2012PhLB..707..233P
ynot1
#30
Feb21-12, 10:51 AM
P: 90
Quote Quote by coelho View Post
I have a question, and it seems this is the most apropriate place and time to ask it.

From what i remember, black holes are created when a very massive star shrinks itself to a radius smaller than the so-called Schwarzschild radius.
Quote Quote by zhermes View Post
I think that that is correct. An observer will never see an infalling shell cross an event horizon.
At the same time, however, infalling material becomes increasingly redshifted (quite rapidly), and thus an observer will stop 'seeing' any of that outside material. I guess the infalling matter would appear to dim/fade as it asymptotically approaches the event horizon (which also grows as additional matter falls in).

See: http://adsabs.harvard.edu/abs/2012PhLB..707..233P
This reference applies to a pre-existing black hole but the question is creation of a black hole from a massive star, not collapse of a shell onto a pre-existing black hole.
zhermes
#31
Feb21-12, 12:49 PM
P: 1,261
Quote Quote by ynot1 View Post
This reference applies to a pre-existing black hole but the question is creation of a black hole from a massive star, not collapse of a shell onto a pre-existing black hole.
It seems like it should apply as soon as an event horizon forms---i.e. as soon as you pass critical density.
Naty1
#32
Feb21-12, 01:59 PM
P: 5,632
The gravitational field of the black hole is the sum of the individual contributions from each particle in the black hole. If I understand Hawking radiation it annihilates particles in the black hole, and so the contribution of their gravitational field to that of the black hole will be lost, reducing the gravitational field of the black hole itself and reducing the curvature of spacetime.
This seems preposterous....but then, again, some things are so!

For example, there are few if any 'particles' within a black hole.
Except for recent infalls, all are destroyed at the singularity.

Hawking radiation is formed outside the horizon; Perhaps you mean infalling particles with negative energy combine with recent particles of positive energy...even that doesn't seem
likely ....how would one catch up with the other, or slow down, to affect annihilation...?

Have you any references that describe black holes as you posted??
Naty1
#33
Feb21-12, 02:03 PM
P: 5,632
But, from a observer far away from it (like us, for example), as closer the radius of the star gets to the Schwarzschild radius, slower the shrinking gets, so from a observer far away, it takes an infinite time to a star became a black hole

I think that that is correct. An observer will never see an infalling shell cross an event horizon.
This has been discussed before and I am not positive, but what I concluded from conflicting posts is that such an 'infinite' time is an 'ideal' perspective from infinity in flat spacetime....in our real universe, such a frame does not exist...so dilation IS extended but not infinite.
If it were, how could we observe ANY black hole??
Naty1
#34
Feb21-12, 02:11 PM
P: 5,632
ok I found the source which I have on the preceding issue:

From Kip Thorne in BLACK HOLES AND TIME WARPS

when the star forms a black hole:

....Finkelstein's reference frame was large enough to describe the star's implosion ...simultaneously from the viewpoint of far away static observers and from the viewpoint of observers who ride inward with the imploding star. The resulting description reconciled...the freezing of the implosion as observed from far away with (in contrast to) the continued implosion as observed from the stars surface....an imploding star really does shrink through the critical circumference without hesitation....That it appears to freeze as seen from far away is an illusion....General relativity insists that the star's matter will be crunched out of existence in the singularity at the center of the black...
I think the guy who 'accidently' discovered this is David Finkelstein...

I just checked Wikipedia...looks like THIS is what he discovered with Misner:

The simplest kink exhibited an easily understood event horizon that led him to recognize the one in the Schwarzschild metric and eliminate its coordinate singularity. This work influenced the decisions of Roger Penrose and John Archibald Wheeler to accept the physical existence of event horizons and black holes.
But what does it mean??
ynot1
#35
Feb21-12, 03:29 PM
P: 90
Quote Quote by zhermes View Post
I think that that is correct. An observer will never see an infalling shell cross an event horizon.
At the same time, however, infalling material becomes increasingly redshifted (quite rapidly), and thus an observer will stop 'seeing' any of that outside material. I guess the infalling matter would appear to dim/fade as it asymptotically approaches the event horizon (which also grows as additional matter falls in).

See: http://adsabs.harvard.edu/abs/2012PhLB..707..233P
Quote Quote by ynot1 View Post
This reference applies to a pre-existing black hole but the question is creation of a black hole from a massive star, not collapse of a shell onto a pre-existing black hole.
Quote Quote by zhermes View Post
It seems like it should apply as soon as an event horizon forms---i.e. as soon as you pass critical density.
The critical density has already been passed for pre-existing black holes.

Quote Quote by Naty1 View Post
there are few if any 'particles' within a black hole.
Except for recent infalls, all are destroyed at the singularity.
Really? Check out http://www.theregister.co.uk/2010/11..._lead_results/

Quote Quote by Naty1 View Post
Hawking radiation is formed outside the horizon; Perhaps you mean infalling particles with negative energy combine with recent particles of positive energy...even that doesn't seem
likely ....how would one catch up with the other, or slow down, to affect annihilation...?
Incoming particles would collide with the quark-gluon plasma.
ynot1
#36
Feb21-12, 04:25 PM
P: 90
The simplest kink exhibited an easily understood event horizon that led him to recognize the one in the Schwarzschild metric and...

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