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The Question

(Submitted July 31, 1996)

Do photons have mass? Because the equations E=mc2, and E=hf, imply that m=hf/c2 . Is it so?

The Answer

No, photons do not have mass, but they do have momentum. The proper, general equation to use is E2 = m2c4 + p2c2 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 = mc2.

This equation often enters theoretical work in X-ray and Gamma-ray astrophysics, for example in Compton scattering where photons are treated as particles colliding with electrons.

Andy Ptak

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