|MadSci Network: Physics|
Excellent question for which there is no common-sense answer. As you advance in your studies in physics, you will find many more of these occurrences, but please don't allow this to discourage you; it's fun. To answer your question, consider the analogy of "how does a raindrop know that it's going to strike you on the head?" It doesn't, of course, any more than an electron "knows" when/whether a photon is going to strike it. Further, it does not matter to the electron whether the photon came from a positron or another electron or from some other source. It only knows that a photon has struck it. The way you approach this problem quantitively is to invoke the laws of probability in an arcane, but fascinating discipline known as "statistical mechanics", usually approached in the last years of undergraduate study and certainly in grad school. What you do is calculate the probability that one of a group of photons will strike a given electron and kick its energy level up. BTW, the same thing applies to particle attraction/repulsion, where the attraction occurs thru the exchange of a particle called a 'graviton'. To help (and also complicate matters) there are the uncertainty principles of quantum mechanics, which I surmise you are studying right now. They are contrasensical, but have never been disproven. I don't want to get into the mathematics now, but a couple of good references are: 1. BH Mahan, Quantum Mechanics, ~1972. Paperback with very good english explanations along with careful, reasonable mathematics. 2. More advanced: SM Blinder, Advanced Physical Chemistry. Lots of math, but excellent english explanations. Excellent coverage of quantum and statistical mechanics, along with some thermodynamics if you like. Good Luck!
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