Chapter V Interacting Fields Lecture 2 Books Recommended: Lectures on Quantum Field Theory by Ashok Das Advanced Quantum Mechanics by Schwabl

Perturbation Theory Eq. of motion for the time evolution operator in Interaction picture ----(1) To solve above, use initial condition -----(2) Thus, from (1), we write ----(3)

Using iterative method, we write ---(4) Or ----(5)

Using time ordering product we can write ---(6) Above equation is known as Dyson series.

If at t0, system is in state |i> then the probability of finding it in state |f> at later time t will be ----(7) Transition rate ----(8)