Yes, it is due to isotropic averaging, the final version of the calculations being given in Note 395(7), using a Taylor series expansion. To first order in <delta r dot delta r> the result is Eq. (10) of Note 395(7). This gives the Lamb shift using mode theory.
Date: Sat, Dec 30, 2017 at 6:34 PM
Subject: note 395(5)
To: Myron Evans <myronevans123>
Where does the correction of 1/3 in eq. (5) come from? Is it from the isotropic averaving in note (4)?
Horst