Light have mass or not ?

If not then prove it !

If yes then prove it!

Light have mass or not ?

If not then prove it !

If yes then prove it!

Tutors, please sign in to answer this question.

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.

I refer you to: http://www.desy.de/user/projects/Physics/Relativity/SR/light_mass.html

The short answer is "no"

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## Comments

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 = ?mc

^{2}(3) E

^{2}= (pc)^{2}+ (mc^{2})^{2}where E is energy, ?=(1-u

^{2}/c^{2})^{-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.

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

-HH-

Final revision---

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

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

-HH-