hahaha

[blike]

Someone's proof that photons have mass:

"Light can be distorted by gravitation.
Gravitation is mass based.
Therefore light has mass."

thought i'd share.

[Radical Edward]

similar to a proof that I heard from my physics teacher at school when I told him about the momentum of photons, althugh in his case he thought that light doesn't have mass.

if photons have momentum, then p=mv, and so they must have mass, which they don't.
so photons can't have momentum.

silly man. I knew more about physics than he did when I was doing my A-Levels.

[fafalone]

Momentum is a property of matter, not a property of light. It's one of Newton's laws. Any competent physicist realizes Newton's equations don't apply to anything besides matter.

Gravitation is NOT mass based for affected entities, it is space-time curvature based. It makes perfect sense for massless objects to be effected, as they still follow the same course in space-time.

[Tom Mattson]

Originally posted by fafalone
Momentum is a property of matter, not a property of light.

Actually, momentum is very much a property of light. If it weren't, you wouldn't have the Compton or photoelectric effects, for example.

The relativistic energy-momentum relation is:
E2=(pc)2+(mc2)2

So, even for m=0, we still have E=pc.

Quantum mechanics tells us that the momentum of a photon (or of any wave) is p=h/(lambda)

[fafalone]

But there's a different between the momentums. You talking about quantum mechanics equations to find the momentum, it would be like comparing quantum gravity to classical gravity. Doesn't work.

[Tom Mattson]

Whatever gave you that idea?

The momenta of photons and of particles are indeed of the same nature. This should come as no surprise, because momentum is a defined concept, and the photon momentum is defined in such a way as to make it usable in conservation of momentum problems. Look at the derivation of the Compton shift and you'll see what I'm talking about.

[aman]

Thank you Tom,
I read up on the Compton shift and it does show that photons striking electrons does suggest a measurable transfer of momentum. Makes sense to me.
Just aman

[JoeDaWolf]

Originally posted by aman
Thank you Tom,
I read up on the Compton shift and it does show that photons striking electrons does suggest a measurable transfer of momentum. Makes sense to me.
Just aman

Didn't einstein win a nobel prize for explaining what happens when photons hit electrons?

E = (1/2)mv^2= hf - Fc*f

The photons give all their energy to both releasing the electron (Fc*f) and transfering the energy to the electron (hf). I don't believe he ever mentioned momentum :)

~Wolf

[JaKiri]

Einstein won the nobel prize for his work on the photoelectric effect, which whilst being about electrons and photons, (as implies the name), is more about the wave/particulate nature of an EM wavepacket.