(D1)General Relativity and Cosmology
The relation between the energy E and the momentum p of the particle with the mass m is given as
(D1)
The photon satisfies relations 
,
and 
where h is Planck's constant.
By substituting these relations into equation (D1),
the mass m=0 is obtained.
That is to say, the photon mass m equals zero (0).
Question: Photons don't have mass. Do photons have energy?
Answer: Yes. You realize that, when you look at an X-ray photograph.
Question: Why does a photon diffract?
Answer: Because it propagates as a wave. An electron also propagates as a wave.
General Relativity and Cosmology
Takao Suda (2001/1/16) updated(2001/11/26)