Do Photons Have Mass?

Are you sure that there are particles with no mass? Photons have mass don't they?
- Bill Thomas (age 68)
Waterville, Ohio

As far as we know, photons have no mass.  The upper limit (from the behavior of galactic magnetic fields) is 10^-27 (a billionth-billionth-billionth) of the hydrogen atom's mass.

There is a lot of confusion about the relation between mass and energy.  Photons do have energy, even though they have no (or very little) mass.  In the age of special relativity, the modern definition of mass is not the Newtonian one, in which E = 1/2 mv².  This formula does not work for objects (even massive ones) that move near the speed of light.  The relativistic definition is E² -(pc)² = (mc²)², where p is the momentum.  For an object at rest (p = 0), this reduces to the famous E = mc² formula.  Massless particles always move at the speed of light. (I won't prove it here.)  It was long thought that neutrinos were also massless, but this has been shown to be incorrect.

(published on 01/25/2018)