Why does light and gravity travel at the same speed?

[Abidal Sala]

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?

 

[HallsofIvy]

Essentially, that is the speed of waves in the "fabric" of space. And your statement that "when exceeding that speed would you go back in time" is not correct- you cannot go faster than the speed of light. It happens that, in the formula for time dilation, if you simply put in a speed greater than the speed of light, you get an imaginary number solution. Some people interpret that as "going back in time" but it really just means you have done a meaningless operation.

 

[Abidal Sala]

Some people interpret that as "going back in time" but it really just means you have done a meaningless operation.

but see this: http://www.telegraph.co.uk/science/...of-light-broken-at-CERN-scientists-claim.html
also see the video there.. they tried everything, and their calculations insist to show that the particle exceeded the speed of light

 

[Ryan_m_b]

but see this: http://www.telegraph.co.uk/science/...of-light-broken-at-CERN-scientists-claim.html
also see the video there.. they tried everything, and their calculations insist to show that the particle exceeded the speed of light

Actually the jury is still out on that one. See discussions here: https://www.physicsforums.com/showthread.php?t=532620

 

[salvestrom]

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 most direct answer to your question from a quantum mechanics viewpoint is that the force carrier for gravity, the as yet undiscovered 'graviton', would be massless and is therefore able to travel at the maximum possible speed for a massless object. I'm not sure what the explanation is in terms of General Relativity because gravity isn't a force in that model, and I've no idea by what maths and reasoning its propagation are constrained to c.

The notion of exceeding the speed of light and traveling back in time is derived from the idea that any object at the speed of light experiences no movement in time, therefore going beyond that speed implies time reversing. As has been pointed out, exceeding that speed is currently considered impossible - particularly if the object has any actual mass.

As for "why 299m/s?"... Well, I'd like to see what others say. The speed can be derived from (I think) Maxwell's equations using the eletric and magnetic constant, but when I checked into that it seemed at least one of them was itself defined using c, which struck me as circular.

 

[cristo]

I'm not sure what the explanation is in terms of General Relativity because gravity isn't a force in that model, and I've no idea by what maths and reasoning its propagation are constrained to c.

Isn't it just by considering small deviations away from a flat spacetime, so your metric takes the form g_{ab}=\eta_{ab}+h_{ab}. Then, in a vacuum, the field equations imply that the perturbations propagate according to a wave equation,

<br /> \Bigg(-\frac{\partial^2}{\partial t^2}+c^2\nabla^2\Bigg)h_{ab}=0<br />

with speed of propagation, c.

 

[ynot1]

In my view anything which propagates through spacetime does so at the speed of light. Gravitational waves propagate like radiation but with zero frequency. That is only the amplitude of the wave propagates, leaving spacetime in a state of curvature.

 

[Irishwake]

That is only the amplitude of the wave propagates, leaving spacetime in a state of curvature.

Would you mind going into a little more detail here on what causes a lack of frequency?

I guess I've just never imagined gravity to "propagate in waves" since it doesn't display any of the classical properties like EM radiation does.

 

[ynot1]

Would you mind going into a little more detail here on what causes a lack of frequency?

I guess I've just never imagined gravity to "propagate in waves" since it doesn't display any of the classical properties like EM radiation does.

Yes I certainly wouldn't say propagates in waves. More like a disturbance (Einstein would say curvature) which spreads out throughout spacetime. My take on it is that particles are forms of compressed spacetime causing a stretching effect on neighboring elements of spacetime, like a quantum theory of gravity. The stretching effect of course diminishes farther from objects as the effect is shared by more elements of spacetime. I like to think spacetime has memory: it tries to restore itself to its original configuration. This memory might also be the source of other forces. From this viewpoint the universe is really a re-configuration of spacetime. The internal energy of matter is balanced by negative gravitational potential energy. Of course eventually spacetime overcomes this imbalance as quark matter decays or evaporates through black holes. The leptons, however, seem to be subject only to annihilation, as I understand.

 

[JonDE]

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.

 

[ynot1]

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.

Except for Hawing radiation I don't believe gravity does escape from a black hole. Note if too much gravity escapes the black hole it seems it wouldn't be a black hole anymore.

 

[DaveC426913]

Except for Hawing radiation I don't believe gravity does escape from a black hole. Note if too much gravity escapes the black hole it seems it wouldn't be a black hole anymore.

Gravity does not have to "escape" a black hole. Gravity is a field. That means it has a value everywhere at all times. Before the black hole formed, the mass that curved space was there. After the black hole formed, the value of the field does not change.

 

[JonDE]

Gravity does not have to "escape" a black hole. Gravity is a field. That means it has a value everywhere at all times. Before the black hole formed, the mass that curved space was there. After the black hole formed, the value of the field does not change.

Yeap I'm still confused. I'm afraid you may not be able to dumb this down enough for me. Let's try this example in reverse and maybe then you can point out where my thinking goes wrong.
If a black hole losses mass, you would expect that it would have less gravitational effect on a star orbiting outside of its event horizon. This effect should travel at the speed of light to the areas surrounding the black hole. I still just don't see how it can relay the information out of the black hole. Maybe my confusion is because I think of it as a wave or particle (graviton), and I don't see how a graviton can escape when other particles cannot.

 

[DaveC426913]

Maybe my confusion is because I think of it as a wave or particle (graviton), and I don't see how a graviton can escape when other particles cannot.

Yep. That's the problem. We do not yet have a quantum mechanical description of gravity. Currently, relativity is the going theory. And it describes gravity as a field.

 

[lukesfn]

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.

This one confuses me too, but I was wondering if the answer has something to do with things that fall into a black hole appearing as frozen on the horizon, and redshifted so far they are invisible, but maybe a hypothetical graviton particle or similar could still be observed.

Same for the event horizon of the universe.

I don't know if this makes any sense or not though, but maybe it is one way to look at it.

 

[ynot1]

Gravity does not have to "escape" a black hole. Gravity is a field. That means it has a value everywhere at all times. Before the black hole formed, the mass that curved space was there. After the black hole formed, the value of the field does not change.

Good points.

 

[Tanelorn]

Originally Posted by JonDE
Maybe my confusion is because I think of it as a wave or particle (graviton), and I don't see how a graviton can escape when other particles cannot.

Yep. That's the problem. We do not yet have a quantum mechanical description of gravity. Currently, relativity is the going theory. And it describes gravity as a field.
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These statements are very profound. Seems we need new Physics? Electromagnetic waves / particles cannot escape an extremely high gravitational field, yet hypothetical gravitons can?

 

[ynot1]

Originally Posted by JonDE
Maybe my confusion is because I think of it as a wave or particle (graviton), and I don't see how a graviton can escape when other particles cannot.

Yep. That's the problem. We do not yet have a quantum mechanical description of gravity. Currently, relativity is the going theory. And it describes gravity as a field.
__________________
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These statements are very profound. Seems we need new Physics? Electromagnetic waves / particles cannot escape an extremely high gravitational field, yet hypothetical gravitons can?

Gravitons are the gravitational equivalent to photons for the electromagnetic fields. Gravitons spread the gravitational field of a particle at its creation. The gravitational field of the particle then becomes attached to the particle as long as it's not acceerated. If that happens then the gravitons come into play again to update the gravitational field. But gravitons play no part in a static gravitational field for a particle moving with no acceleration.

Radiation escapes black holes when its wavelength is greater than the diameter of the event horizon. I don't think gravitons are part of a black hole. They only update the gravitational field, as when the black holes lose mass by Hawing radiation.

 

[ynot1]

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]

You don't need 'gravitons' if gravity is a scalar field. See also http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980601a.html

 

[universe21!]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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.

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.