XPost: sci.physics.relativity
From: ram@zedat.fu-berlin.de
john larkin wrote or quoted:
>So when two gammas collide and create a particle pair, mass appears
>where there was none.
>Right so far?
While a single photon does not have mass, a pair of photons can
have mass. When two photons collide with opposite momenta, all their
energy is mass, and this mass is conserved during the process.
So it is /not/ correct to say, "mass appears where there was none".
Possible causes of why many people do not seem to get this
(I already proved it before [1] in a recent post of mine):
Some may confuse mass with /matter/. Matter usually is taken
to be fermions. In the process, non-matter (bosons) is in
fact converted into matter (fermions). So you /can/ say,
"matter appears where there was none".
While energy and momentum is additive, mass is /not additive/.
You can add two photons, both of which have /no/ mass, and get a
compound system /with/ mass. However, in the everyday world, mass
/is/ additive because we do not deal with relativistic systems
there. So, some people might transfer their everday concept of
mass to relativistic situations where it does not apply.
[1] For your convenience:
|Let's call the momenta of the two photons p0 and p1.
|
| We may assume p1 = -p0 as the two photons are moving towards
| each other from opposite directions. Let's call the momentum
| of the system of these two photons "p", then we have:
|
|p = p0 + p1 = p0 +( -p0 )= p0 - p0 = 0
|
| . Let's call the energy of this pair "E" and its mass "m". From
|
|E^2 = m^2 + p^2
|
| (in units with c=1) and
|
|p = 0
|
| , we get,
|
|E^2 = m^2
|
| for the pair. I.e., all its energy is mass. And this is the
| mass the particle pair has after the collision.
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
|
