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Q & A: photon mass?

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Most recent answer: 12/18/2011
Q:
According to the equation E=hv, does a photon have mass? This because it has energy, not because it is moving at the speed of light,
- Richard Ong (age 14)
Winston-Salem, NC, USA
A:
As we've noted many times here, a photon has effective mass m= E/c2. That's the m that appears in the momentum-velocity relation p=mv and as a source term for gravity. It doesn't have "invariant mass", i.e. rest-mass sqrt(E2/c4-p2/c2), because, since it travels at c, E=pc.

Mike W.

(published on 12/18/2011)

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