I said I would not write about Paul Dirac until my review of his new biography appeared in the Globe and Mail. The review has now long since appeared. So let me touch on a subject not mentioned there: age and theoretical physics.
Dirac made most of his epoch-making contributions to physics while still in his twenties. At thirty-one, he was the youngest theoretician ever to win the Nobel Prize for physics. At thirty-three, he wondered if his career was over.
It has long been an axiom of theoretical physics and mathematics that if you haven't made your mark by thirty, you might as well fold your tent and go away. There are exceptions, of course, but in general the exceptions seem to prove the rule. Abstract thought is a young person's game.
In physics and mathematics, there is a necessary period of coming up to speed, absorbing what is already known and mastering the current theoretical tools. This is often accomplished by the early twenties. The mark of a truly great scientist is the ability to identify a fundamental research problem that is waiting to be cracked. Then comes a burst of creative activity that might exhaust itself sooner than the theoretician might like.
Dirac lived on into his eighties, revered like Einstein but frustrated by his inability to repeat his youthful triumphs. When, in the last months of his life, he was invited to give a talk at the University of Florida, he responded: "No! I have nothing to talk about. My life has been a failure." This from a man who was universally recognized as one of the great geniuses of the century -- but prone by his nature to disillusion.