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Name: Unknown
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Date: Around 1993


Question:
When gravity acts to bend light, is it because a photon does have mass when it has velocity, or does gravity act on the energy of the photon, which I understand has no rest mass?



Replies:
Well, gravity acts on everything since it really acts on space itself! Yes, a photon has no rest mass, and that means that it is always traveling at the speed of light. The energy of a photon is actually proportional to its frequency, y, which does not really have much to do with the way gravity acts on it. However, since bending involves a change of momentum, the force that gravity exerts on the photon is proportional to its momentum (which is in turn proportional to its energy). And in fact, it is generally the total energy (rest energy plus kinetic energy etc.) that is important for gravity, whether the particle in question has a rest mass or not.

Arthur Smith



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