Thirty-one years ago [1949], Richard Feynman told me about his "sum
over histories" version of quantum mechanics. "The electron does
anything it likes," he said. "It just goes in any direction at any
speed, forward or backward in time, however it likes, and then you add
up the amplitudes and it gives you the wave-function." I said to him,
"You're crazy." But he wasn't.
* Freeman J. Dyson, 1983