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Q & A: photon mass again

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Most recent answer: 10/06/2009
Q:
U said photons r massless ,move with the speed of light(c)but it does not suit the eqm=m0c/sqrt(c2-v2).please explain. and by E=pc ,for p=0 as mass of photon is 0, how can it have the energy to move at the speed of light? thanx
- xxx
A:
The equation m=m0c/sqrt(c2-v2) is not generally valid. It breaks down when m0=0 and v=c. The more general form is

E2=p2c2-(m0c2)2, or (mc)2=(mv)2- (m0c)2. These latter forms work fine for massless (m0=0) particles, giving E=pc.

Mike W.

(published on 10/06/2009)

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