However, further studies by Felix Bloch with Arnold Nordsieck, and Victor Weisskopf, in 1937 and 1939, revealed that such computations were reliable only at a first order of perturbation theory, a problem already pointed out by Robert Oppenheimer.
Imidlertid viste yderligere studier af Felix Bloch med Arnold Nordsiek og Victor Weisskopf i 1937 og 1939, at sådanne beregninger kun var tilforladelige til første orden af en perturbationsteori, et problem der allerede var påpeget af Robert Oppenheimer.