Abstract
Using minimal formalism, we demonstrate that the massless photons, constituting the radiation in a cavity, contribute to 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=hν, p=hν/c. Restricting velocities to nonrelativistic 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 photons of total energy U is U/c2. An alternative argument based on calculations in the co-accelerating frame is also given.
Published Version
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