Do Massless Particles Cause Gravitation?

[Constantin]

I know physical bodies with mass cause gravitation.
But do massless particles, like photons, also cause gravitation ? I know they're affected by gravity, but would a cloud of photons bend space by itself?

Thank you

 

[PeterDonis]

Yes, photons have energy, and anything with energy causes gravitation.

More precisely, photons have a non-zero stress-energy tensor, which is what acts as the "source" of gravitation. See, for example, this Wikipedia page:

http://en.wikipedia.org/wiki/Stress–energy_tensor

The stress-energy tensor of a source-free electromagnetic field, down the page, would apply to photons.

I add this qualification because there are also cases where an "energy" can be assigned to the gravitational field itself, but this "energy" is not part of the stress-energy tensor and so it does not act as a "source" of gravitation in the above sense.

 

[jedishrfu]

JA Wheeler once investigated the idea of a geon. Light traveling in a self perpetuating circle due to its own gravity.

http://en.wikipedia.org/wiki/Geon_(physics )