General Relativity predicts the bending of light due to gravity. But, does this explanation require light to be corpuscular? Can the EM waves of classical electromagnetism be bend in Einstein's gravity? Or does the fact that light bends due to gravity alone prove that it is photons(corpuscles) in General Relativity?

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    $\begingroup$ I think both answers are selling the question short. The question is not about quantum mechanics. The point is that the geodesic equation used to derive light bending applies to pointlike particles -- it is not immediately obvious that it also applies to waves. You can make a heuristic argument by saying you have a wavepacket, and the successive wave crests bend by invoking Huygens' principle (as Feynman does in his quantum mechanics lectures in a slightly different context), but you have to do some real work here. $\endgroup$ – knzhou Feb 1 '18 at 23:41
  • $\begingroup$ @knzhou The quality of answers doesn't matter when a bunch of experts keep up-voting each other. John's answer is OK, but over a dozen up-votes for the other one? Wow. $\endgroup$ – safesphere Feb 2 '18 at 9:29
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    $\begingroup$ @safesphere You’re wrong there — the upvotes are because this got on the Hot Network Queue. Actually, 75% of the upvotes are probably from people who haven’t taken a physics course beyond freshman mechanics. $\endgroup$ – knzhou Feb 2 '18 at 9:37
  • $\begingroup$ @knzhou What's "Hot Network Queue"? $\endgroup$ – safesphere Feb 2 '18 at 9:39
  • $\begingroup$ Side trivia: The gravitational bending of light passing close by a star according to GR is about twice as large as Newton's theory would predict for a particle with miniscule, positive mass moving at a speed of $c$. $\endgroup$ – Arthur Feb 2 '18 at 15:05

In relativity (both flavours) light rays follow null geodesics. That is when you calculate the proper length of any part of the light's trajectory it comes out zero. More precisely the trajectory of a light ray is described by the null geodesic equation.

So to calculate the bending of light you simply have to solve the null geodesic equation. No resort to quantum mechanics or the particulate nature of the light is required.

Light is not unique in this respect. In the weak field limit the trajectory of gravitational waves is also described by the null geodesic equation. In fact massless particles also follow the same trajectory, though the key property here is not that they are particles but that they are massless.


Does GR prove light is photons?

No, it does not. General relativity, like special relativity, is a classical (non-quantum) theory of physics.

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    $\begingroup$ Sorry, but that's not an answer. Not a "real" one. $\endgroup$ – PhyEnthusiast Feb 1 '18 at 14:53
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    $\begingroup$ @PhyEnthusiast Yes, it is a perfectly fine answer. GR is indeed a classical theory and photons play no part in it. GR predicts that electromagnetic waves, classical solutions to Maxwell's equations, would be influenced by the curvature of spacetime. $\endgroup$ – tfb Feb 1 '18 at 14:59
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    $\begingroup$ @tfb But shouldn't a answer contain some more detail and a little intuition?? $\endgroup$ – PhyEnthusiast Feb 1 '18 at 15:03
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    $\begingroup$ This is a complete answer as it stands. As you presumably know, photons are a quantum-mechanical construct, so the fact that GR is not a quantum theory means that it is silent on the subject. $\endgroup$ – Emilio Pisanty Feb 1 '18 at 15:14
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    $\begingroup$ Sir. Isaac Newton would disagree with the choir of experts here. The question does not imply the quantum nature of photons. They could be little classical corpuscles flying in space. The answer still is no, same result for particles or waves, but the OP is right that this answer is incomplete. $\endgroup$ – safesphere Feb 1 '18 at 16:28

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