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From my general understanding, it is widely accepted that light will bend around a massive object, as it will experience gravity, is predicted by GR, and was experimentally verified.

Also, the energy in EM fields, like the EM fields inside an atom, also contribute to the overall gravitational effect of the atom, as it will contribute to the total amount of energy in the atom (and thus its mass).

However, 'free' or 'un-bounded' photos, the kind of photons that travel far, are not bound to a single atom or area.

They are affected by gravity, but do they exert gravity?

  1. according to GR?
  2. was this verified experimentally in any way?

The most basic experiment I could imagine is shining two photons in parallel in opposing directions and see whether they bend closer to each other, however I realize this experiment is impractical due to the weakness of the force.

Shining a continuous beam of light might not constitute as 'unbound', because you could say that while the photons are moving, the volume of space occupied by the photons is constant, and it itself causes gravity. I am interesting in specific sources of gravity that move at the speed of light.

This question could broaden up: Do any form of energy that is not locally bound and travels in the speed of light exert gravity?

Another instance I could think about are gravitational waves. They carry energy, thus could they, themselves, exert gravity?

What does GR say about that, and have there been any relevant experiments for it?

Qmechanic
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Mark Segal
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    I think this thread answers your question: https://physics.stackexchange.com/q/481557/, I am assuming that by "exert gravity" you mean cause space-time curvature? – tau1777 Apr 22 '22 at 23:55

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Do photons have gravity according to GR?

A (classical) electromagnetic field has an associated stress-energy tensor all by itself. See wikipedia. The question of whether and how individual photons have gravity is a question for a theory of quantum gravity (of which we don't have a successful one yet); GR is not equipped to deal with it formally.

Has it been directly measured?

As far as I know, no. However, if we assume that light does have gravity and make predictions, the predictions match reality; and if we assume that light does not have gravity and make predictions, the predictions don't match reality.

Also, the fact that light (and an individual photon) has momentum has been directly measured, and so has the fact that interactions with gravity changes the direction of light (and individual photons), and the fact that momentum is conserved is possibly the best-attested fact in the history of science, so it's extremely unlikely that there's not an equal and opposite reaction.

g s
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