# What is the mass of a photon moving at the speed of light? [duplicate]

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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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 ;-) –  Waffle's Crazy Peanut Apr 21 at 17:35

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

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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