Do photons have mass?

[Faulkner]

This has been puzzling me for a while. I know it's more a physics question, but I'd like to thrash this out a bit. I haven't been able to find much satisfying answers on the net.

If photons are massless, how can they "push" a solar sail?

I read that it's due to the fact that although a photon is massless, it has "momentum". And yet "momentum" is mass times velocity. So what's the deal? Are physicists (as usual) having their cake & eating it too?

It would seem to me that if photons have any mass at all, it would have to be an infinite mass - because they are travelling at the speed of light. This seems impossible.

So where does the photon's "momentum" come from?

[Jet3050]

Hello,

Like photons from the sun pushing a Solar away? Is it possible that the Photons or any heat from the sun can propel the sail? Like Heat rising up out of a lamp shade can spin a sheet of paper if it cut into a saircase like shape. Perhaps it is the heat that propels the Sail?

-Kyle

[Jet3050]

Kind of like The Yarkovsky Effect, A heated object emits heat. So since one side of the sail is warmer than the opposite this imbalance in heat can act like a very weak rocket engine. Propelling the Sail slowly until it gathers more momentum and speeds up more and more.

-Kyle

[Faulkner]

Perhaps!? I guess you could say photons carry "heat energy" - eg X-rays are "hotter" than, say, radio waves.

Another question: How come light is "bent" by gravity? Doesn't gravity only affect objects with "mass"?

EDIT: OK, scrub that, light ISN'T affected by gravity, it's just following a straight line through curved spacetime... oh man, I need another drink!

[Jet3050]

Going out on a shot here and a guess. I guess that the light that is bent by gravity is only being bent becuase the space around it is being bent? Like wind going around a jet in motion, the wind bends around the shape of the jet. Perhaps here the light in motion bends around with the shape of the Space it is tavelling in. And if the space itself is bent, this Light would appear to be bent as it travels along through the bent space due to high gravity?

-Kyle


EDIT: Yes, that is what I was talking about :P

[Jet3050]

And here about the Photon Mass

I don't really understand it but I am sure someone here does:

http://imagine.gsfc.nasa.gov/docs/ask_astr...ers/960731.html


-Kyle

[TheThorn]

That's a good link. I like the explanation there.

But let me throw out another way to look at it.

Photons have 0 rest mass. But they have energy, which is inversely related to their wavelength - the shorter the wavelength the higher the energy.

We all know that energy and mass are equivalent (the famous E=mc^2), and usually we think of it as mass becoming energy, but in this case you can think of it as energy acting as mass. E= Mc^2 implies that M = E/c^2.

The energy in the photon has the effect of mass in the momentum equation you quoted.

Like you said, Momentum (P) is mass times velocity, or P=MV.

For a photon velocity is c, and we already know mass equivalent of the energy in the photon is E/c^2, so substituting in the equation above, we get P = (E/c^2)c or P = E/c, which is the equation you can find on that site Jet posted.