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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?

PhyEnthusiast
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    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. – knzhou Feb 01 '18 at 23:41
  • @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. – safesphere Feb 02 '18 at 09:29
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    @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. – knzhou Feb 02 '18 at 09:37
  • @knzhou What's "Hot Network Queue"? – safesphere Feb 02 '18 at 09:39
  • 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 $c$. – Arthur Feb 02 '18 at 15:05

2 Answers2

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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.

John Rennie
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Does GR prove light is photons?

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

David Hammen
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    Sorry, but that's not an answer. Not a "real" one. – PhyEnthusiast Feb 01 '18 at 14:53
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    @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. –  Feb 01 '18 at 14:59
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    @tfb But shouldn't a answer contain some more detail and a little intuition?? – PhyEnthusiast Feb 01 '18 at 15:03
  • @PhyEnthusiast: It is all included in the keywords used: Photons are quantum field excitations, general relativity is classical (that is non-quantum) theory, so general relativity does not describe quantum field excitations, photons. – Photon Feb 01 '18 at 15:07
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    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. – Emilio Pisanty Feb 01 '18 at 15:14
  • @PhyEnthusiast: I don't really know what more can be said than 'photons are not part of a classical theory'. –  Feb 01 '18 at 15:59
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    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. – safesphere Feb 01 '18 at 16:28
  • I think I get the confusion here. When I said "photons", I meant particles of some type, not necessarily quantum type. I didn't really get the difference until safesphere explicitly pointed it out. Sorry, I guess it was entirely my fault – PhyEnthusiast Feb 01 '18 at 16:53
  • @safesphere - Newton's corpuscular theory of light had been proven false long before Einstein, who worked in the context of Maxwell's electrodynamics. – David Hammen Feb 02 '18 at 09:00
  • @DavidHammen It doesn't matter. This question is not about Newton or Maxwell or whether light's nature is classical or quantum. The question is if in GR light rays would bend equally or not if light consisted of particles or waves. And it's not self evident that waves would follow the null geodesics. – safesphere Feb 02 '18 at 09:14
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    Actually, this is a great answer. Why write several pages, when a single sentence does the trick? – Dawood ibn Kareem Feb 02 '18 at 09:27
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    Now that the question has been edited, this answer is not that great, although it does perfectly answer the original version of the question. – Ruslan Feb 02 '18 at 12:01