Ian Mallett wrote:
On Dec 5, 2007 4:03 PM, Greg Ewing <greg.ewing@xxxxxxxxxxxxxxxx>
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/cλ. 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 λ,
thats: E = hc/λ, where h
is Planck's constant
We also have E = mc^2, so dividing both sides by c^2,
we get mass: m = h/cλ
Momentum is p = mv = hv/cλ = h/λ (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
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