On Dec 5, 2007 4:03 PM, Greg Ewing <[EMAIL PROTECTED]> wrote: > Actually, it does -- a photon is an example of an object > with no mass. Such an object always travels at the speed > of light -- it doesn't even need a push to get it going. > It's fundamentally incapable of standing still.
Heh heh. Try hitting that with a paddle. > While it has no mass, it does have both energy and > momentum, both of which are proportional to its frequency. Momentum is defined as mass*velocity. If mass is zero, how does a photon have momentum? > These are conserved in any collision, so when it > bounces off a wall, the wall gains some momentum, just > as it would if a massive particle with the same > momentum bounced off it. And if the wall starts to > move as a result, then it has also gained some energy, > which must have come from the photon, so the reflected > photon must be red-shifted slightly (longer wavelength > = lower frequency = less energy). All this is true, but *how* exactly does a massless particle have momentum? > -- > Greg > Ian
