Isn't the point that photons have kinetic energy, but no rest mass. i.e.,
 if a photon could possibly be at rest (which I don't think it can), then
it would have no rest mass.

We've got a lot of real estate up there on the roof; how much power could
we get by lining our Vanagon roofs with solar cells?

YauMan Chan wrote:

>And the question is, if in fact photon really don't have mass, then what
>make this thing spin? Or does this proof photon have mass?