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  1. What is the mass of a photon moving at the speed of light?

  2. And if it does not have mass, how is it affected by gravity?

  3. Also why does Einstein's general relativity support that a gravitational wave must travel at the speed of light?

I'm just an A-level student. So, I would appreciate it if you could explain it as idiotically as possible.

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marked as duplicate by Waffle's Crazy Peanut, akhmeteli, Manishearth Apr 23 '13 at 7:29

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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Zero. Mass is defined as the constant specific to each type of quantum, satisfying $$ E^2 = p^2c^2 + m^2c^4 $$

For a photon, $E=pc$ or in other words is 4-momentum is a null vector, meaning its timelike component and spacelike components have equal magnitude (after applying the spacetime conversion constant $c$ appropriately). Thus we put $m=0$.

In quantum field theory, electromagnetic forces are described in terms of virtual photons, for which $m$ does not have to be zero. These virtual photons are sometimes said to be "off the mass shell".

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  • $\begingroup$ In that formula $m$ is the rest mass. I think the OP is asking about effective mass. $\endgroup$ – kotozna Jan 9 '16 at 12:09
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A photon has no rest mass, that is to say if you could stop it, it would have zero mass. As it is always moving at c (unless in a dielectric material) it has some effective mass which would experience acceleration in a gravitational field.

Gravitation waves contain infomation and infomation can only travel at a speed up to c, usally infomation travels slower in fact. Speed of gravity link: http://en.wikipedia.org/wiki/Speed_of_gravity

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