Richard FeynmanQuantum? It’s Complicated.

The sticker says Quantum? It's Complicated. Feynman's own view was that the rule is simple and you will not like it.

In 1948 Richard Feynman published a new way to do quantum mechanics. To find the chance that a particle gets from a to b, do not follow one path. Follow all of them, even those looping past the Moon. Give each path a little arrow, a complex number of fixed length, turned through an angle equal to the path's action S divided by ħ. Add every arrow. The first formula on this page is that sum. Square the length of the total, the second formula, and that is the probability.

Why, then, does a ball follow one curve? ħ is tiny. For paths far from the classical one, the angle swings wildly between neighbouring paths, their arrows point every which way and cancel. Only near the path of stationary action do neighbours share nearly the same angle, so their arrows line up and add. Classical mechanics is what remains after everything else cancels.

Applied to electrons and light, this thinking gave quantum electrodynamics and the diagrams named for him. He shared the 1965 Nobel Prize with Sin-Itiro Tomonaga and Julian Schwinger, who had reached the same theory by other routes.

The line on the live page comes from his lectures published as The Character of Physical Law: "I think I can safely say that nobody understands quantum mechanics." He meant the rules work perfectly and explain nothing about themselves.

On the commission investigating the 1986 Challenger disaster, at a televised hearing, he dropped a piece of the booster's rubber seal into a glass of ice water to show how badly it behaved in the cold.

The rule is one line. The complicated part is that every path counts.