# What is the difference between Lamb shift and the Darwin term?

I am currently learning for an exam and I wonder what the difference between the Lamb shift and the Darwin term is.

What I know is that the Darwin term affects only the s-states of the hydrogen atom and shifts its energy level to that of the p-state, while the Lamb shift affects the p-state and shifts its energy level, so that s- and p-states have different energies after all.

However, what I do not understand is that both are explained in terms of Zitterbewegung. The electron, as it moves around the nucleus, interacts with the electromagnetic field by emitting and absorbing virtual particles and experiences a recoil with each interaction. Thus, its distance from the nucleus varies by a certain $\delta r\approx \lambda_C$, which is accounted for in perturbation theory.

But, in this picture, what is here the actual cause for the Lamb shift and and what is the cause for the Darwin term?

• You do not cite this : Fine Structure Constant but from your question, you have possibly read it, or something pretty much like it, and it provides an answer, as far as I can see. – user163104 Jul 27 '17 at 23:46
• Yes, I read it and also the article about the Lamb shift but both are described in terms of zitterbewegung but there has to be a difference because the effect of Lamb shift and the Darwin term are different. – Thomas Wening Jul 28 '17 at 0:22
• My best help to you here is to +1 nnd hope that you get a better answer than I can give you, I apologize, but my answer would not be good enough and might mislead you. – user163104 Jul 28 '17 at 2:35