In Quantum Electrodynamics (QED) everything is calculated through probability. That is the square of the amplitude of the reaction.
If the photon is resonant with the electron, the probability that the photon will be absorbed is high. If it is not resonant, the probability of coupling is low but not impossible. If the photon is not absorbed by one electron, it will encounter many more until it is absorbed. >From QED, this type of ordinary electron has a probability of absorbing the photon and then reemitting it. This happens all the time when plain old electrons orbit the nucleus when an electron jumps between orbital shells of an atom. The electron will jump up to the next energy level when it finds a photon with the proper quanta. This is how things are colored. Grass absorbs red light and blue light but reflects green light. These ordinary electrons do gain and lose energy in quanta. Cheers: Axil On Thu, Apr 18, 2013 at 1:18 PM, David Roberson <dlrober...@aol.com> wrote: > Does anyone know how a free electron in space can absorb a photon of > moderate energy? I was thinking of the interaction between a light photon > and a free electron when it occurred to me that it might be impossible for > the photon to deliver all of its energy and momentum to a single particle. > I chose the frame of reference as being where the electron is stationary > for this thought experiment although it might be better to choose a > different one. > > From the electron view point an incoming photon has both energy and > momentum. If we assume that the photon is completely absorbed by the > electron then the total energy and momentum must remain in balance. The > electron was initially at rest so it had zero momentum and the energy > associated with its mass. After the collision the electron would be in > relative motion from our reference frame at which time it would have both > momentum and extra energy. I can go through the math in detail if needed, > but it is apparent that these calculations would not hold up to keep the > system in balance except for perhaps one special energy photon(.511 MeV). > In that case a pair of .511 MeV gammas is emitted when an electron and > positron annihilate each other. Also, the two gammas are emitted in > opposite directions to conserve momentum. > > In light of the above argument, it appears as though a free space > electron can not absorb a photon in total and that it must instead interact > to a lesser degree. Does that mean that these collisions virtually always > result in the scattering of incoming electromagnetic radiation? Does a > photon keep getting bounced around as it looses energy in a non ending > series of collisions? If this is true then it would not be too big of a > stretch to imagine that the cosmic background radiation might be correlated > with the eventual fate of high energy photons that have been beaten to > death on their way through the free charged particles of space. > > The other situation to examine is that since a single photon apparently > can not be totally absorbed by a free electron, then the reverse should be > true. A free electron should not be able to spawn a single photon. The > conservation laws should get in the way of that activity and I would expect > a pair of photons most likely is the consequence. It might happen that > these are entangled due to their close relationship at birth. > > Can anyone add support to these conclusions? It is always interesting > to expand ones knowledge by thought experiments. > > Dave > > >
