Quantum Field Theory

The classical picture, as we have discussed, holds that particles take paths that minimize the action. Quantum field theory presents a more complicated view in which a particle doesn’t just take one path; it takes all possible paths, and a phase (a complex number of unit length) is assigned to each path. The probability of a particle going from any initial point to any final point is proportional to the sum of the phases. We will now put this into more technical language (that some readers might choose to skip over).