Why does gravity propagate at the speed of light?

[greatfool66]

I've never heard a reason given for the speed of gravity.

Newton believed that changes in gravity would happen instantaneously over the universe and that faster than light speed communication was thus possible.

I guess something in relativity disproved this. Any ideas?

 

[Biftheunderstudy]

There are a couple of answers to this. The first is that you have to look at newtonian gravity, its an action at a distance force. This means that the theory is a static theory and cannot describe how the force changes when something is moving. This was one of Einstein's motivations behind his formulation of GR. GR on the other hand is a dynamic theory with fields much like maxwell's equations. So the short of it is, to comply with special relativity, gravity must propagate at the speed of light.

The second answer involves a lot more math, but follows the exact same derivation for the speed of a electromagnetic wave. First Einstein's equations must be linearized since the full non-linear equations are a bit tricky to solve analytically. Next you can do the same thing as in an EM wave and make a gravitational wave(there are some subtleties here that I'm omitting) and you find a wave number k. When you plug in the right constants, the speed of the wave comes out to be c just like in the EM case.

In light of this there are actually a couple of theories that use different values for the propagation of gravity. I don't know any details, but I have heard arguments that GR still holds true under these situations.

 

[Nathelion]

A more simplistic answer (which is, i admit, all that I'm capable of) would be that gravity is (believed to be, the graviton has yet to be experimentally detected, although the Large Hadron Collider at Cern may have a shot at it in the coming decade or two) mediated by gravitons - virtual massless particles. This is analogous to how electromagnetic forces are mediated by virtual photons. Now particles with mass can only travel at speeds less than c, but particles without mass - such as photons and gravitons - have no choice BUT to travel AT the speed of light. So one way of saying it is that c is more accurately described as the speed of all massless particles, of which photons and gravitons are a subset. As to why that is so... umm... maybe I can tell you in two years after I finish my physics major

 

[PooBeetle]

you have to refine your question,
there is body in motion gravity waves, which proagate at the speed of light, and then instantaneous action at a distance via gravity, a paradox ie WHY can't you push a particle faster than the speed of light? obviously structural (space-time continuum) gravity is communicating information to the lone little particle waaay out in space that you're trying to get to exceed the speed limit.

the slow speed gravity you seem to be asking about is the very simple, newtownian sort. do some reading on the substrate/instantaneous/space=time to be far more confused.

 

[PooBeetle]

oh, also faster than light communication is of course possible, many universities and compnaies have quantum computers. quantum tunneling is also faster (instantaneous). um, shadows can travel much faster than light. basically information can travel faster.

also, generally in maths and physics, there are generally just higher powered mathematical theories that come out. like getting a new microscope every year. this doesn't make your old microscope WRONG at all. newtonian mathematics are perfect. relativity just allowed you to add about 27 decimal places to the accuracy is all. string theory adds another 20, many worlds adds on top of that.

 

[f95toli]

Originally posted by: PooBeetle
you have to refine your question,
there is body in motion gravity waves, which proagate at the speed of light, and then instantaneous action at a distance via gravity, a paradox ie WHY can't you push a particle faster than the speed of light? obviously structural (space-time continuum) gravity is communicating information to the lone little particle waaay out in space that you're trying to get to exceed the speed limit.

the slow speed gravity you seem to be asking about is the very simple, newtownian sort. do some reading on the substrate/instantaneous/space=time to be far more confused.

I am not sure I understand what yo are getting at. However, as has already been pointed out, according to GR "gravitaional interaction" (which more or less means "ripples" in space-time created by e.g. a moving mass) propagates at the speed of light.
In Newtonian mechanics the interaction is instantenous. Also, this also means there is e.g. no way to use gravity to communicate FTL:

oh, also faster than light communication is of course possible, many universities and compnaies have quantum computers. quantum tunneling is also faster (instantaneous). um, shadows can travel much faster than light. basically information can travel faster.

No, quantum computing has nothing to do with faster-than-light interactions. Relativistic(SR) quantum measurement theory is (not surprissingly) messy but suffice to say that it is possible to show that even in an "idealized" quantum entanglement experiment the maximum interaction speed is set by c. This is well known in quantum optics.

The "speed of tunnelling" is a controversional subject. There are a few papers where the authors claim to have measured a tunnelling time which is shorter than the thickness of the barrier/c. However, all of these experiments are very controversional and all measurements are obviously "indirect" meaning you have to make a number of assumptions in order to interpret the data. At least some of these assumptions have been shown to be false.

 

[Nathelion]

Yeah FTL communication is very probably not possible, and has most certainly not been demonstrated yet. As to quantum tunneling being FTL, look up the ER paradox.

 

[1prophet]

Light takes about 8.3 minutes to travel from the sun to the earth.

If the sun was to somehow be removed instantaneously would the earth experience the gravitational effects before or 8.3 minutes later at the same time the first visible evidence of the sun's disappearance is observed?

 

[lyssword]

Originally posted by: 1prophet
Light takes about 8.3 minutes to travel from the sun to the earth.

If the sun was to somehow be removed instantaneously would the earth experience the gravitational effects before or 8.3 minutes later at the same time the first visible evidence of the sun's disappearance is observed?

At the same time

 

[Unitary]

Originally posted by: lyssword

Originally posted by: 1prophet
Light takes about 8.3 minutes to travel from the sun to the earth.

If the sun was to somehow be removed instantaneously would the earth experience the gravitational effects before or 8.3 minutes later at the same time the first visible evidence of the sun's disappearance is observed?

At the same time

Certainly, from the (linearlized) GR point of view gravity waves should propagate at the speed of light like Biftheunderstudy mentioned. Does anyone know if this has this been verified experimentally?

 

[greatfool66]

Originally posted by: 1prophet
Light takes about 8.3 minutes to travel from the sun to the earth.

If the sun was to somehow be removed instantaneously would the earth experience the gravitational effects before or 8.3 minutes later at the same time the first visible evidence of the sun's disappearance is observed?

This was exactly the question that I was wondering about with the speed of gravity, except I was thinking about the galactic center about 26000 light years away I think. It seems that if that huge mass disappeared, the earth would not notice for 26,000 years?

The speed of gravity has been experimentally verified recently but I can't find the link anywhere. It involved observing light and how stars affect it I think.

BTW thanks for the answers, I think I understand a little better, but maybe a unified theory is needed to explain more? I'm going to read about relativity

 

[f95toli]

Originally posted by: lyssword

Originally posted by: 1prophet
Light takes about 8.3 minutes to travel from the sun to the earth.

If the sun was to somehow be removed instantaneously would the earth experience the gravitational effects before or 8.3 minutes later at the same time the first visible evidence of the sun's disappearance is observed?

At the same time

No, 8.3 minuters later since gravity "travels" at the speed of light. You should be able to find a longer discussion about this is e.g. one of Greens books.

It seems that if that huge mass disappeared, the earth would not notice for 26,000 years

Correct.

 

[silverpig]

Originally posted by: PooBeetle
oh, also faster than light communication is of course possible, many universities and compnaies have quantum computers. quantum tunneling is also faster (instantaneous). um, shadows can travel much faster than light. basically information can travel faster.

also, generally in maths and physics, there are generally just higher powered mathematical theories that come out. like getting a new microscope every year. this doesn't make your old microscope WRONG at all. newtonian mathematics are perfect. relativity just allowed you to add about 27 decimal places to the accuracy is all. string theory adds another 20, many worlds adds on top of that.

While it is true that signals can travel faster than light, they cannot carry information if they do.

The only way I can think of having gravity travel faster than light is if string theory is correct and the gravitons travel along a straight path in some higher dimensional space which is shorter than the path that light must take in our 3-space. However, as the curvature around bodies even as massive as the sun is still rather slight, we would be hard pressed to notice until much better experiments were designed.

 

[greatfool66]

While it is true that signals can travel faster than light, they cannot carry information if they do.

The only way I can think of having gravity travel faster than light is if string theory is correct and the gravitons travel along a straight path in some higher dimensional space which is shorter than the path that light must take in our 3-space. However, as the curvature around bodies even as massive as the sun is still rather slight, we would be hard pressed to notice until much better experiments were designed.[/quote]

Doesn't a signal carry information by definition? If it doesn't carry information then how can it be a signal?

 

[silverpig]

Originally posted by: greatfool66

While it is true that signals can travel faster than light, they cannot carry information if they do.

The only way I can think of having gravity travel faster than light is if string theory is correct and the gravitons travel along a straight path in some higher dimensional space which is shorter than the path that light must take in our 3-space. However, as the curvature around bodies even as massive as the sun is still rather slight, we would be hard pressed to notice until much better experiments were designed.

Doesn't a signal carry information by definition? If it doesn't carry information then how can it be a signal?
[/quote]

That's just semantics really. The point is there are phenomena (signal/whatever you want to call it) that can travel at velocities greater than c, so long as they don't carry any information.

 

[taaresu]

Actually there is a new study that completely disproves any such thing as the speed of light after trying to get information across cpu's using a beam of light which has been made almost impossible due to the fact that they found out light can exist in two places at once. Therefor completely disproving the "Speed Of Light" theory.

 

[f95toli]

Originally posted by: silverpig

Originally posted by: greatfool66

While it is true that signals can travel faster than light, they cannot carry information if they do.

The only way I can think of having gravity travel faster than light is if string theory is correct and the gravitons travel along a straight path in some higher dimensional space which is shorter than the path that light must take in our 3-space. However, as the curvature around bodies even as massive as the sun is still rather slight, we would be hard pressed to notice until much better experiments were designed.

Doesn't a signal carry information by definition? If it doesn't carry information then how can it be a signal?

That's just semantics really. The point is there are phenomena (signal/whatever you want to call it) that can travel at velocities greater than c, so long as they don't carry any information.[/quote]

To be more specific: The PHASE velocity of light can exeed c, the GROUP velocity (the greatest speed at which you can transfer information) can not.

There is nothing "weird" about this, the fact that phase velocity can be larger than the speed at which the wave travels through a medium is just a mathematical property of all waves (not only light) and has probably been known for 150 years or so.

The first link google came upo with
http://www.mathpages.com/home/kmath210/kmath210.htm

 

[silverpig]

Originally posted by: f95toli

Originally posted by: silverpig

Originally posted by: greatfool66

While it is true that signals can travel faster than light, they cannot carry information if they do.

The only way I can think of having gravity travel faster than light is if string theory is correct and the gravitons travel along a straight path in some higher dimensional space which is shorter than the path that light must take in our 3-space. However, as the curvature around bodies even as massive as the sun is still rather slight, we would be hard pressed to notice until much better experiments were designed.

Doesn't a signal carry information by definition? If it doesn't carry information then how can it be a signal?

That's just semantics really. The point is there are phenomena (signal/whatever you want to call it) that can travel at velocities greater than c, so long as they don't carry any information.

To be more specific: The PHASE velocity of light can exeed c, the GROUP velocity (the greatest speed at which you can transfer information) can not.

There is nothing "weird" about this, the fact that phase velocity can be larger than the speed at which the wave travels through a medium is just a mathematical property of all waves (not only light) and has probably been known for 150 years or so.

The first link google came upo with
http://www.mathpages.com/home/kmath210/kmath210.htm


[/quote]

It doesn't even have to be light though. In a well timed string of people clapping at a predetermined time the wave of claps can move faster than c.

 

[lyssword]

Originally posted by: f95toli

Originally posted by: lyssword

Originally posted by: 1prophet
Light takes about 8.3 minutes to travel from the sun to the earth.

If the sun was to somehow be removed instantaneously would the earth experience the gravitational effects before or 8.3 minutes later at the same time the first visible evidence of the sun's disappearance is observed?

At the same time

No, 8.3 minuters later since gravity "travels" at the speed of light. You should be able to find a longer discussion about this is e.g. one of Greens books.

It seems that if that huge mass disappeared, the earth would not notice for 26,000 years

Correct.

I was comparing with point of view for those on the planet. Not as an observer outside the limits of speed of light. A person sees the sun gone (everything dark) and at same time he will feel/measure the effect of gravity (not sure exactly what that would be)

Ok, so you're saying that somehow you can notice the change in light information (sun gone for the viewer) faster than the event happening? Doesn't make sense.

 

[f95toli]

Sorry, I missunderstood you comment.
I guess we both mean the same thing: People on earth would notice the disapperance of gravity and light at the same time.

 

[manowar821]

God I love discussions like these.

I thought of a funny way to communicate with a distant world, just make a solid rod that goes from here to there, and push it back and forth. There, faster than light communication!

 

[f95toli]

Originally posted by: manowar821
God I love discussions like these.

I thought of a funny way to communicate with a distant world, just make a solid rod that goes from here to there, and push it back and forth. There, faster than light communication!

Doesn't work. And you don't even need general relativity to show that; the plain old special version is enough.
Again, there is no way to transmitt information faster than light; and there are no known loopholes (not counting wormholes since they actually reduce the distance between two point)

 

[manowar821]

Originally posted by: f95toli

Originally posted by: manowar821
God I love discussions like these.

I thought of a funny way to communicate with a distant world, just make a solid rod that goes from here to there, and push it back and forth. There, faster than light communication!

Doesn't work. And you don't even need general relativity to show that; the plain old special version is enough.
Again, there is no way to transmitt information faster than light; and there are no known loopholes (not counting wormholes since they actually reduce the distance between two point)

But, the other end of the solid rod is going to move in the amount, and the same time that I move my end. Even if the rod is 200 light years long. Am I wrong?

 

[f95toli]

Yes, but the problem is that there is no such thing as a solid rod.
When you push one end of the stick you are only transmitting "information" to the atoms AT THAT END, in order for that information to reach the OTHER end of the rod these atoms have to interact with their neighbors etc.
Hitting sticks is therefore a very slow way of communicating; essentially the effect only travels at the speed of sound in the material.

 

[beansbaxter]

Is the gravity proportional to the object's mass that are attracted to one another?