Many of you stated how ad hoc is QFT as the field is supposed to be non-interacting yet how could they get an incredibly accurate value of calculated magnetic moment of the electron of value 1.0011596522 compared to measured 1.00115965219 with accuracy to better than one part in 10^10, or about three parts in 100 billion! How does Perturbation theory really work (I don't know how exactly the power series work but let's use QED as example to illustrate the concept)? I know the Fine Structure Constant or coupling constant is 1/137 which is acquired from actual measurement and can't be calculated. Now without Perturbation, the fields are not supposed to interact, so what value do you get? Would it be zero? After adding the first series. You get the initial value of 1.00 (how do you get this from 1/137?) After adding the fourth series. You get the value of 1.0011596522 How did they do the series expanding to get the fourth series amount? Are the above steps correct? Then it is really ad hoc since you just do power series expanding and don't really solve for the interacting fields. By the way.. in the magnetic moment of the electron. What is interacting there since there is no external field? I'd like to understand how ad hoc is perturbation theory as it is used in almost all aspects of physics. Thanks a lot!