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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/c
2. That's the m that appears in the momentum-velocity relation
p=m
v and as a source term for gravity. It doesn't have "invariant mass", i.e. rest-mass sqrt(E
2/c
4-p
2/c
2), because, since it travels at c, E=pc.
Mike W.
(published on 12/18/2011)
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