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I am confused about what gravitons exactly are.

On the one hand it is said that gravitons are presumed to represent gravity (see Wikipedia "Graviton" : "In theories of quantum gravity, the graviton is the hypothetical quantum of gravity, an elementary particle that mediates the force of gravitational interaction.")

On the other hand, gravitons are compared to photons which are representing electromagnetic radiation (see Wikipedia "Electromagnetic radiation" : "In quantum mechanics, an alternate way of viewing EMR is that it consists of photons".)

The point is that electromagnetic waves are generated by accelerating charges and gravitational waves are supposed to be generated by accelerating masses, and as far as I know, gravitational waves are not a direct effect of gravity but an effect of accelerating masses.

So, if gravitational waves are not a direct effect of gravity itself, are gravitons an "alternate way of viewing" gravitational waves or are they a direct effect of gravity?

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    $\begingroup$ I don't understand this question. Do you think photons "refer" to electromagnetism or electromagnetic waves? What is the actual physical meaning of this distinction? Whatever your answer for the photon is, it's the same for the graviton. $\endgroup$
    – ACuriousMind ♦
    Commented Jul 15, 2023 at 16:45
  • $\begingroup$ @AcuriousMind: As I understand, electromagnetic force is generated by accelerating charges. In contrast, for gravity, no acceleration of the mass is required. $\endgroup$
    – Moonraker
    Commented Jul 15, 2023 at 16:50
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    $\begingroup$ But electromagnetic forces do NOT require accelerating charges. $\vec{F} = q(\vec{E} + \vec{v}\times \vec{B})$. $\endgroup$
    – ProfRob
    Commented Jul 15, 2023 at 16:54
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    $\begingroup$ What do you mean, "electromagnetic force is generated by accelerating charges"? There a whole subset of electromagnetism we call electrostatics because the charges don't move there! $\endgroup$
    – ACuriousMind ♦
    Commented Jul 15, 2023 at 16:54
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    $\begingroup$ There is a difference: EM is linear, GR not. So the question still makes sense! Do gravitons allow to recover linearzed GR in the classical limit or the full GR? $\endgroup$
    – Quillo
    Commented Jul 15, 2023 at 18:52

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Photons with zero rest mass, travelling at the speed of light, can be used to represent electromagnetic waves caused by accelerating charged particles. Virtual photons are also used to mediate the electromagnetic forces between charged particles.

In analogy, gravitons with zero rest mass, travelling at the speed of light, might be used to represent gravitational waves (produced by certain types of accelerating mass) and virtual gravitons would mediate gravitational forces between masses in a quantum theory of gravity.

See also Are GWs made of gravitons (are gravitons the quanta of GWs) or not?

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    $\begingroup$ Thank you very much! I did not know the notion of virtual photons yet. $\endgroup$
    – Moonraker
    Commented Jul 15, 2023 at 17:29
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    $\begingroup$ +1. Just a curiosity: EM is linear, GR not. Do gravitons allow to recover linearzed GR in the classical limit or the full GR? (exactly like "photons" allow us to recover full vacuum-EM in the classical limit). $\endgroup$
    – Quillo
    Commented Jul 15, 2023 at 18:55
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    $\begingroup$ @Quillo You recover full, non-linear GR using spin-2 interacting particles (physics.stackexchange.com/a/762264/133418). $\endgroup$
    – Avantgarde
    Commented Jul 15, 2023 at 21:16
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    $\begingroup$ en.wikipedia.org/wiki/… $\endgroup$
    – ProfRob
    Commented Jul 15, 2023 at 22:45
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    $\begingroup$ @Avantgarde That's a philosophical interpretation of quantum field theory, not a physically inarguable fact. Which is fine - it's a totally defensible interpretation - but you should clarify that it essentially a subjective belief that many equally expert physicists disagree with. $\endgroup$
    – tparker
    Commented Jul 16, 2023 at 4:23

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