A major talking point at the conference was the discovery by Willis Lamb and his graduate student, Robert Retherford, shortly before the conference began that one of the two possible quantum states of hydrogen atoms had slightly more energy than that predicted by the theory of Paul Dirac ; this became known as the Lamb shift. Oppenheimer and Weisskopf suggested that this was a result of quantum fluctuations of the electromagnetic field, which gave the electron more energy. According to pre-war quantum electrodynamics (QED), the energy of the electron consisted of the bare energy it had when uncoupled from an electromagnetic field, and the self-energy resulting from the electromagnetic coupling, but both were unobservable, since the electromagnetic field cannot be switched off. QED gave infinite values for the self-energies; but the Lamb shift showed that they were both real and finite. Hans Kramers proposed renormalization as a solution, but no one knew how to do the calculation.