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As is proven, light can be bent by a heavy mass in the context of classical General Relativity. In a quantum theory of gravity, the gravity field around a heavy mass consists of a condensate of (virtual) gravitons, contrary to the GR case, where a static field is just a non-changing distortion of spacetime.
Because photons don't experience the passage of time they can't emit or absorb gravitons (real or virtual). So how can this process be described by quantum gravity and the gravitons that come along with it? I'm not asking if photons can bend spacetime (see this question).

Deschele Schilder
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You are saying that "Because photons don't experience the passage of time they can't emit or absorb gravitons (real or virtual).", but this is wrong, the source of gravity is stress-energy, which massless particles, like the photon do have too. Thus, photons are able to emit/absorb gravitons (theoretically) too.

in fact all particles would emit and absorb gravitons, not just those with rest mass. The source of gravitational fields is the stress-energy tensor, not rest mass. Roughly speaking, mass and energy and equivalent (E=mc2), so energy participates in gravitational forces as well.

You are saying "In a quantum theory of gravity, the gravity field around a heavy mass consists of a condensate of (virtual) gravitons.", but in reality, it is a gravity field (static gravitational field) around any object with stress-energy. A photon, having stress-energy is able to emit/absorb gravitons, thus in your description, the static gravitational field around the photon is described by a condensate of virtual gravitons.

Anything with electric charge can emit photons. That includes quarks, Z bosons, any chemical ion, and so on. Similarly, anything with "gravitational charge" can emit gravitons but that really means anything with energy.

Which particles emit and absorb gravitons?

Deschele Schilder
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Árpád Szendrei
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  • The gravitational charge (mass) of a photon is zero, so a photon can't interact by means of a virtual graviton with a particle with a charge (mass) that is non-zero. In a quantum theory of gravity, the static gravitational field is described by a condensate of gravitons, contrary to the non-quantum version (classical GR). I think you made a nice salad of the both in your answer. – Deschele Schilder Aug 13 '20 at 16:46
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    @descheleschilder "The gravitational charge (mass) of a photon is zero" This is completely wrong. Gravitational charge is based on stress-energy, not mass. Please retract the downvote, as I do not deserve it. – Árpád Szendrei Aug 13 '20 at 21:54
  • @descheleschilder "In a quantum theory of gravity, the static gravitational field is described by a condensate of gravitons," This is wrong too, since the static field is describe by virtual gravitons. Again, your misconception is what I am trying to help with, and I do not deserve the downvote I believe. – Árpád Szendrei Aug 13 '20 at 21:56
  • I returned the favor, by minor edits (mainly a little bit spelling and grammar). – Deschele Schilder Aug 14 '20 at 04:10
  • @descheleschilder thank you so much! – Árpád Szendrei Aug 14 '20 at 04:56
  • You're always welcome! – Deschele Schilder Aug 14 '20 at 04:59
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Consider photon-photon scattering, which is forbidden at tree level because the photon is an uncharged object. However, a real photon spends part of its time as virtual particle-antiparticle pairs, which do have charge and which can interact with other photons. This is one way that photon-photon scattering enters the Standard Model as a higher-order, perturbative effect.

It's fine to say that any quantum model of gravity which doesn't predict bending of light in a gravitational field is a bad theory. But I don't think you can use a pop-science tidbit like "photons don't experience time" to rule out such a theory decisively.

rob
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