Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

This question already has an answer here:

Almost everybody knows that light is massless. But where this come from and how it can be proven (experimentally or theoretically)? I actually found this article which explains and calculates the mass of light at rest (which is not 0). So how do we know that light is massless?

share|cite|improve this question

marked as duplicate by anna v, Qmechanic Apr 14 '13 at 18:13

This question was marked as an exact duplicate of an existing question.

The 'article' confuses relativistic mass and rest mass. Possible duplicates: and links therein. – Qmechanic Apr 14 '13 at 17:00

The duplicate link has good answers.

The experimental proof comes from fitting decays where photons are involved. For example the pi_0 decays into two photons. If these had a mass the energies would not balance in the fits.

Pair production would also be problematic, because a "massive photon" disappears into an electron positron pair in the field of a nucleus or an electron. Photons have not been seen to decay in vacuum.

If the photon had a mass,it would have to be twice the mass of the electron in the photon center of mass, not a negligible value , and energy and momentum balances would never work.

All these arguments are encapsulated in the theoretical framework described in the answers to the duplicate question.

share|cite|improve this answer

Not the answer you're looking for? Browse other questions tagged or ask your own question.