Can the Graviton be faster than light?

[marcgrissz]

Hello everyone,
I was thinking of the 4 forces and in particular of the gravity force.
We know that black holes and their gravity force can bend light and light gets lost on black holes.
How can the ipothetic particle of the graviton interact with the photon?
I thought that if the interaction is real, the graviton must be not only faster than light to perceive the mass, but it also has to be smaller and photons should be able to interchange gravitons.
I must be wrong, I know, but could anyone explain this to me?

 

[mfb]

I thought that if the interaction is real, the graviton must be not only faster than light to perceive the mass, but it also has to be smaller

Particles do not have a size in the classical sense. No, hypothetical gravitons would not be faster than light, and I don't see why you would expect that.

and photons should be able to interchange gravitons.

Right.

 

[marcgrissz]

Particles do not have a size in the classical sense. No, hypothetical gravitons would not be faster than light, and I don't see why you would expect that.
Right.

The idea that some peace of rock in the universe is interacting with my shoe, for example, makes me thinking of some velocity faster than light.

 

[bcrowell]

This question doesn't need to be posed in terms of gravitons. Whatever speed the classical wave travels at, that's got to be the speed of the (real) quanta. That speed is c:

https://www.physicsforums.com/insights/how-fast-do-changes-in-the-gravitational-field-propagate/

 

[mfb]

The idea that some peace of rock in the universe is interacting with my shoe, for example, makes me thinking of some velocity faster than light.

Your shoe won't react to the current position of that rock. It will react according to the position where the rock was at some point in the past (which depends on its distance).

 

[marcgrissz]

This question doesn't need to be posed in terms of gravitons. Whatever speed the classical wave travels at, that's got to be the speed of the (real) quanta. That speed is c:

https://www.physicsforums.com/insights/how-fast-do-changes-in-the-gravitational-field-propagate/

That's awesome, thank you.

 

[bcrowell]

Your shoe won't react to the current position of that rock. It will react according to the position where the rock was at some point in the past (which depends on its distance).

Actually this is not quite right. The shoe will react to the field that has propagated to it from the rock at previous times. However, the field does not actually point toward the retarded position of the rock. It points toward the quadratically extrapolated current position of the rock. A similar but easier example comes from E&M; the electric field of a moving charge points toward the charge's current linearly extrapolated position. For the gravitational case, see Carlip, http://arxiv.org/abs/gr-qc/9909087v2 . I have a description of the E&M version in section 10.4 of my SR book: http://www.lightandmatter.com/sr/ .

 

[mfb]

I didn't include velocity as this makes it more complicated. My point was the non-instantaneous action.

 

[bcrowell]

I didn't include velocity as this makes it more complicated. My point was the non-instantaneous action.

I don't see how the non-instantaneous action can make any difference in a frame where the source is at rest.

 

[mfb]

Well, you can leave an object 1 light year away at position A for a year, then move it one light-hour in some direction during roughly an hour, then leave it at position B for a year. For one year we'll see no change. After that year, we'll see a short transition period, and then the effect of the object at rest at B.

 

[bcrowell]

Well, you can leave an object 1 light year away at position A for a year, then move it one light-hour in some direction during roughly an hour, then leave it at position B for a year. For one year we'll see no change. After that year, we'll see a short transition period, and then the effect of the object at rest at B.

OK, but during the period when the information about the transition is propagating past B, the field will not point to the retarded position. In the E&M case, I believe it will point to the current linearly extrapolated position, and in the gravitational case to the quadratically extrapolated position. (I could be getting some details wrong.)

 

[mfb]

Could be, but that's still a short transition period where the pointing is off in some way.