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Light's mass

Light have mass or not ?

If not then prove it !

If yes then prove it!

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3 Answers

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.



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.


Hassan H.

The ? in my comment above is supposed to be Greek lowercase gamma, but the formatting apparently doesn't allow this character in comments.


Final revision---

In mt first comment, the line "we get from (1)..." should read:

"we get from (1) that p=?mc".


Light is both a particle and a wave length.  While in all practical matters light has no mass, we find that as a particle is affected by strong gravitational fields.  If light had no mass then its course would not be affected by gravity.