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Light's mass

Light have mass or not ?

If not then prove it !

If yes then prove it!

3 Answers by Expert Tutors

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Grigori S. | Certified Physics and Math Teacher G.S.Certified Physics and Math Teacher G.S.

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.

Stuart R. | Math Tutor - Online and In Home -20 Years ExperienceMath Tutor - Online and In Home -20 Year...



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".


Benjamin K. | Tutoring in Social Studies, Reading, or basic subjectsTutoring in Social Studies, Reading, or ...

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.