The mass of a gas of massless photons, H. Kolbenstvedt, Am. J. Phys. 63(1), January 1995
Using minimal formalism, we demonstrate that the massless photons, constituting the radiation in a cavity, contribute to the mass of the mass of the cavity in agreement with Einstein's mass-energy formula. We assume that a photon has energy and momentum related to frequency by E = hv, p = hv/c. Restricting velocities to nonrelativeistic values, we impart a uniform acceleration to the cavity. Reflection from a moving mirror produces a Doppler shift, and thus the momentum delivered to the front and rear walls can easily be calculated in the laboratory frame. A simple application of F = ma leads to the usual conclusion that the mass of a gas of total energy U is U/c2. An alternative argument based on calculations