# Photon Inertial Mass

*Most recent answer: 11/24/2015*

Q:

Given that photons have zero rest mass, can the mass of a photon be calculated (with a non-zero result) given its speed of 'c'?

- Tom Gillis (age 72)

Bangor, Maine, USA

- Tom Gillis (age 72)

Bangor, Maine, USA

A:

You need to know the frequency, f, in your reference frame. The photon energy is given by E=hf, where h is Planck's constant. (This relation is universal, not just for photons.) Since E=mc^{2}, where m is inertial mass, we have m=hf/c^{2}. The momentum p=mc or hf/c.

You might notice that since E is just a universal constant times inertial m, m and E are really just the same thing measured in different units. Likewise E is just a universal constant times f, so f is also the same thing in another set of units!

We have some old discussions of these issues, e.g. :

Mike W.

*(published on 11/24/2015)*