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Re: [pygame] Why does my ball vibrate?



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/c
λ
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