With Compton scattering, why is it impossible for a a free electron to absorb all the energy of a photon with which it collides?
Any explanation that uses smaller words would be appreciated. (longer words are cool too, but i don’t know if i’ll understand them.)
Thanks!
You have to conserve not only energy, but momentum as well. That’s why Free Electron Lasers aren’t, really – you need a magnetic field or something so you can meet the conservation laws.
Another way of thinking about it is that (most) physics is invariant under time-reversal, and in all reference frames with constant velocity. That means, essentially, that if you shot a movie of some physical process, and ran the film backwards, then the new film should also lok like a possible physical event. However, if you took a film of an electron absorbing a photon and coming to rest, and ran it backwards, you’d end up with a film in which an electron is just sitting there for a while, and then suddenly emits a photon in one direction and flies off in the opposite direction. Doesn’t seem too plausible, does it?
One also must remember that all frames of reference are equally valid. XWalrus2 is probably not thinking of cases where the electron comes to rest after the collision, but remember, all cases can be considered to have the electron coming to rest after the collision. You just have to choose the right reference frame, which (like every other reference frame) is perfectly valid.