- #1
Silviu
- 624
- 11
Hello! I am reading about the S matrix, and I see that one of the assumption that the derivations are based on is the fact that interacting particles are free at ##t=\pm \infty## and I am not sure I understand why. One of the given examples is the ##\phi^4## theory which contains an interaction term of the form ##\frac{\lambda}{4!}\phi(x)^4##. Why would this term vanish far away from our experiment. More concretely, if we would have an electron Dirac spinor field (assuming we collide electron and positrons), why would we assume that far from the experiment the electron is free? Isn't it still interacting with the vacuum QED i.e. the vacuum would be ##|\Omega>## and not ##|0>## no matter at which point we are (even at big distances from our collision point). Can someone explain this to me please? Thank you!