# 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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## 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.

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 '13 at 17:35

## 2 Answers

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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In that formula $m$ is the rest mass. I think the OP is asking about effective mass. – kotozna Jan 9 at 12:09

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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