Gravitational Waves, Photons & the Uncertainty Principle

[cragar]

If photons emit gravitons, then where does this energy come from? It can't pull it from its kinetic energy or maybe it just redshifts it but this seems weird. In order to emit gravitational waves do i haft to accelerate energy . I am trying to draw an analogy between gravitational waves and an accelerating electron emitting light but this may be too simple. Or maybe the uncertainty principle has something to say about it . Any input will be much appreciated.

 

[zhermes]

To emit gravitational waves you have to 'accelerate energy' in a certain way; but presumably yes, photons could end up emitting gravitons, and the energy would come from the photons---redshifting (in effect). It might be better to think about it in terms of a decay: i.e. a photon decays into a graviton and a lower energy photon, or something like that (there may need to be multiple photons produced, or other details, I have no idea).

 

[Bill_K]

This can't happen on the mass shell.

 

[cosmik debris]

I cannot see how a photon can emit a graviton. If there is a theory of gravity in which the graviton is the "charge" carrier for gravitational interactions then a photon would have to have a least a quadrupole moment. As Bill points out this could happen off mass shell and a virtual graviton emitted, after all anti-parallel photons do gravitationally attract one another.

EDIT: I should have said anti-parallel light beams attract one another, I'm not sure if this extends to single photons.

 

[cragar]

What do you mean you say a photon has to have a quadrupole moment. The only time I have seen that is in a multipole expansion in electrodynamics.

 

[cosmik debris]

What do you mean you say a photon has to have a quadrupole moment. The only time I have seen that is in a multipole expansion in electrodynamics.

If there were a quantum theory of gravity then the graviton would be gauge particle that carries the gravitational "charge". It would probably have to be spin 2 (only one charge type) and massless (long range). Similarly in Electrodynamics we have a photon which carries the EM charge, spin 1 (two charge types), and massless (long range). Real photons are not exchanged in a static electric field, here we employ the use of virtual photons which live a fairly like existence. Real photons only appear in propagating EM waves which are generated by an accelerating electric charge (dipole moment).

Gravitons are emitted when the centre of mass/energy experiences a change in acceleration (quadrupole moment). A photon cannot accelerate let alone change its acceleration.

 

[Naty1]

This discussion may be of interest:
https://www.physicsforums.com/showthread.php?t=473684&highlight=photon+gravity

The original question would be a good one to post in the quantum mechanics section if you don't get a complete answer that satisfies. Surely light couples to gravity, but I don't know exactly what QM proposes about that.

 

[cosmik debris]

This discussion may be of interest:
https://www.physicsforums.com/showthread.php?t=473684&highlight=photon+gravity

The original question would be a good one to post in the quantum mechanics section if you don't get a complete answer that satisfies. Surely light couples to gravity, but I don't know exactly what QM proposes about that.

Yes light beams couple to gravity but since there is no quantum theory of gravity one cannot talk about photons with any certainty. If photons do interact with each other gravitationally they would not use gravitons, they would be virtual gravitons in analogy with the EM case.

 

[kcajrenreb]

Yes light beams couple to gravity but since there is no quantum theory of gravity one cannot talk about photons with any certainty. If photons do interact with each other gravitationally they would not use gravitons, they would be virtual gravitons in analogy with the EM case.

Do you mean that one could not talk about gravitons with any certainty?

Also, does it necessarily take energy to emit a graviton?

 

[cosmik debris]

Do you mean that one could not talk about gravitons with any certainty?

Also, does it necessarily take energy to emit a graviton?

One cannot talk about photons in General Relativity with any certainty and one cannot talk about gravitons in QFT with any certainty as there is no theory which connects them. There are effective field theories of gravity in which the field is comprised of spin 2 particles which are gravitons. String theories do contain both photons and gravitons but are probably not considered main stream.

Yes, if there were gravitons in GR they would carry energy away as gravitational waves, and this has been calculated by Taylor and Hulse.

 

[Milt]

If photons emit gravity, could microwaves also? If they do, and you think about the microwave map of the universe, it may explain dark matter. I don't think their looking for missing matter as much as where is the missing gravity coming from. And being this energy permeates the universe, and I dare say it is the largest single common structure in it, wouldn't it make sense that this could be the missing source of gravity?

 

[cragar]

microwaves are photons, and they gravitate

 

[cragar]

A photon cannot accelerate let alone change its acceleration.

It can have centripetal acceleration.

 

[cosmik debris]

It can have centripetal acceleration.

Sort of, the photon is following a geodesic of spacetime and this type of motion has no quadrupole moment. It is 4-acceleration in the case of a photon that is important not 3-acceleration. In this case the photon's 4-acceleration is zero.

 

[cragar]

so if gravitons existed and we bent one along a geodesic, it would not be able to emit more gravitons because its not quite accelerating from what you said above.

 

[bcrowell]

Sort of, the photon is following a geodesic of spacetime and this type of motion has no quadrupole moment. It is 4-acceleration in the case of a photon that is important not 3-acceleration. In this case the photon's 4-acceleration is zero.

No, this is incorrect. Both its 3-acceleration and its 4-acceleration can be nonzero.

so if gravitons existed and we bent one along a geodesic, it would not be able to emit more gravitons because its not quite accelerating from what you said above.

I don't understand why you say "bent one along a geodesic." A geodesic is the definition of straight.

 

[cragar]

could gravitons emit other gravitons if a graviton followed a geodesic.