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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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    $\begingroup$ Hi geturownwifi. Welcome to Physics. It's worth to check if there are questions related to your topic already asked. It'd be good if you don't ask questions all at once. Please split non-related questions and ask them as a different one ;-) $\endgroup$
    – Waffle's Crazy Peanut
    Commented Apr 21, 2013 at 17:35
  • $\begingroup$ possible duplicates: physics.stackexchange.com/q/2229/11062 and physics.stackexchange.com/q/3541/11062 $\endgroup$
    – Waffle's Crazy Peanut
    Commented Apr 21, 2013 at 17:36
  • $\begingroup$ It has energy, not mass. Energy and mass are equivalent, but not the same. $\endgroup$
    – Yukterez
    Commented Apr 2, 2019 at 3:24

2 Answers 2

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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
    Commented Jan 9, 2016 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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