@annav In the question I linked it was asked if they can curve spacetime. But why asking this if classical electromagnetic fields can curve spacetime?

I did read it finally, I am with John Renne too. Anything that has energy momentum enters the general relativity equation, and effective quantization of gravity works with this too. (If at sometime definitive quantization of gravity would treat what we now call photons differently , is a future question.)

Assume that inside a container is filled with radiation (a box of photon gas). Then, all you need is the photon gas stress-energy tensor which has been already obtained in several papers (as far as I remember it matches with the zero-mass limit of the stress-energy density of a molecular gas having ultra-relativistic motion). Having that, according to GR, photons are basically able to curve spacetime as well.

But a single photon can have any energy density you want by boosting yourself, so it cannot form a black hole

@Wolphramjonny But it can cause an infinite frame drag.

I am not an expert, I just remembered that

@Wolphramjonny Well it can't be infinite, but a light beam (classical) can make you accelerate in the direction of motion of the beam. So if the photon has an energy that goes to infinity, you will be sucked along, as if a black hole passed you.

I see, thanks..

I have read this question. You didn’t just read it, you answered it.

@G.Smith I only see that now! An answer as could be expected.

It is about INTERactions. While photons move slower in a higher gravitational potential (as seen from a third observer), they interact with the gravitational field. This is a good argument that they in turn change the curvature of space-time.

There are no photons or gravitons in General Relativity, remember? ;)

@m4r35n357 Aha! Is that it? The assumption that they are there is wrong in the first place? But what if you assume a (non-quantum) massless particle? Can't it exist?

You can set the mass of a "particle" to zero in various equations (if appropriate!), or you can just talk about a "pulse of light" or something like that. I prefer the latter.

@m4r35n357 But light isn't seen as a particle in c(classical) GR. Can't massless particles exist in GR?

I think you know that am near to the limits of my knowledge here, but if forced I would say that if there were an operational need for specifically massless particles (as opposed to light) I would have probably encountered some reference to it by now ;) I have not. I am a huge fan of Occam's Razor! en.wikipedia.org/wiki/Occam%27s_razor

@m4r35n357 Once in a while I think that Occam's Razor cuts a little too much...:) Thanks for the link!

@DescheleSchilder Because photons can’t emit gravitons. Only charged particles emitting or absorbing can bend space time . Therefore , photons not by themselves can bend space time. Does this answer you question mate ?

@SrijanM.T Do you mean that only fermions can emit force particles (bosons)?

@SrijanM.T Force particles are bound to charge. But charge in the case of gravity is mass/anergy. Photons contain energy.

Does this answer your question? Do photons bend spacetime or not?