Ian Mallett wrote:
On Dec 5, 2007 4:03 PM, Greg Ewing <[EMAIL PROTECTED]
<mailto:[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?
When a physicists say that a photon has zero mass, what they generally
mean is that photons have zero /rest mass/ (a rather hypothetical notion
since a photon can't be at rest). A photon that is moving (the only
kind of photon there is) has a mass of *h/c**?*. Since particles have
ever increasing mass as their velocity increases, approaching infinity
as velocity approaches *c*. You can loosely imagine a photon as having
it's rest mass multiplied by infinity, which would be m = 0*?, which
doesn't help us much. The actual equations are:
Energy of a photon is inversely proportional to wavelength* **?*, thats:
*E = hc/?*, where *h* is Planck's constant
We also have *E = mc^2*, so dividing both sides by *c^2*, we get mass:
*m = h/c**?*
Momentum is *p = mv = hv/**c**? = h/****? *(because *v=c* since it is a
photon)
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
