All the really great discoveries in theoretical physics – with a few exceptions that stand out because of their oddity – have been made by men under thirty.” Bernstein 1973, 89, emphasis in the original. Einstein finished his work on general relativity when he was 36, but his initial step, what he called his “happiest thought” about the equivalence of gravity and acceleration, came when he was 28. Max Planck was 42 when, in Dec. 1900, he gave his lecture on the quantum.