Grigori S. answered • 08/04/13
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Photon doesn't have a mass, othervise it wouldn't travel with the speed of light. The fact that light path is deflected in a gravitational field doesn't mean that the photon has a mass attracted by the center of gravity.
It just shows that light travels along geodesic lines in space-time, which are straight lines in free space and curved lines in the presence of gravitational field.
Hassan H.
Prasenjeet,
It looks like Stuart beat me to it (sorry to piggyback yet again, Stuart!), but as I was just about to post, I will nevertheless add my comment.
If you accept the relativistic equations
(1) p = ?mu
(2) E = ?mc2
(3) E2 = (pc)2 + (mc2)2
where E is energy, ?=(1-u2/c2)-1/2, u is velocity, p=?mu is momentum, m is mass, and c is the speed of light in a vacuum, then, taking as given that photons travel at the speed of light (u=c), we get from (1) that p=mc, and subsequently from (2) that E=pc. Substituting E=pc into (3) then implies that m=0.
Hope this helps.
Regards,
Hassan H.
08/04/13