
The Question
(Submitted July 31, 1996)
Do photons have mass?
Because the equations E=mc^{2}, and E=hf, imply that m=hf/c^{2
}.
Is it so?
The Answer
No, photons do not have mass, but they do have momentum. The proper,
general equation to use is
E^{2} = m^{2}c^{4} + p^{2}c^{2}
So in the case of a photon, m=0 so E = pc or p = E/c. On the other hand,
for a particle with mass m at rest (i.e., p = 0), you get back the famous
E = mc^{2}.
This equation often enters theoretical work in Xray and Gammaray
astrophysics, for example in Compton scattering where photons are treated as
particles colliding with electrons.
Andy Ptak
