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If photons don't have mass, how come gravity can bend light? Because if photons have no weight, wouldn't they not be affected by the fabric of spacetime?

Qmechanic
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Kellan Heerdegen
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1 Answers1

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Any massless particle, like a photon or gluon, can only travel at c. It can never have zero speed or exist at zero speed. So there is no acceleration and it is not appropriate to think about acceleration in the normal sense.

foolishmuse
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  • Thanks for clearing that up, but what is c? – Kellan Heerdegen Mar 04 '24 at 19:20
  • c is the speed of light. You'll find it on your calculator. – foolishmuse Mar 04 '24 at 19:23
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    @foolishmuse not all calculators have physical constants of course....and very likely few do unless they were for some reason made to have it (I know some have it programmed in or written on the inside of the jacket). More likely any key with some association to the letter C on a calculator likely has conversions to/from hexadecimal values instead of physical constants. – Triatticus Mar 04 '24 at 19:57
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    No calculator I’ve owned has ever had a speed-of-light key. – Ghoster Mar 04 '24 at 20:27
  • @Ghoster you're just not in the right space to see it. You're just not in the right frame of mind to see it. You're just not fast enough to see it. Choose one of those :-) – foolishmuse Mar 04 '24 at 21:49
  • No offense, but questions are for answers, go to a chat to talk. – Kellan Heerdegen Mar 04 '24 at 21:56
  • Acceleration is a change in velocity and massless particles can certainly do that. Depends on who's measuring the velocity. – ProfRob Mar 05 '24 at 12:59
  • @ProfRob a photon cannot exist at anything other than the speed of light. It cannot exist at zero kilometers per hour. Since there can be no change in velocity, there can be no acceleration. It does not go from 0 to c. It is just always at c. – foolishmuse Mar 05 '24 at 14:04
  • @foolishmuse Something going around in a circle doesn't change its speed either. But its velocity is changing and it is accelerating. – ProfRob Mar 05 '24 at 14:29
  • @ProfRob yes you are correct in that which is I guess the reason why we refer to the speed of light rather than the velocity of light. Questions also arise when the speed of light changes as light exits a piece of glass. Where does that energy come from to allow for the acceleration. This is why I say that we cannot speak of acceleration in the normal sense when dealing with light. I think my response is what the original question was looking for. – foolishmuse Mar 05 '24 at 15:42
  • No, I don't think it was. The correct answers are in the duplicate. If you say that a geodesic is an unaccelerated path, then the path of a photon is by definition unaccelerated. But then so is the trajectory of an object wth mass. If we measure the "acceleration" as the second time derivative of a position in some arbitrary coordinate system then light is accelerated. – ProfRob Mar 05 '24 at 16:08