... … but we know what the heuristic answer is: the photon takes every possible distinct path, and so we have to sum over all possible combinations of slits from A to B. Here’s a nice picture: Indeed, ... a continuous sum is called an integral, so this is also called the path integral formulation of quantum mechanics. It was pioneered by Richard Feynman and is, in fact, the topic of his PhD ...

... ;level" can emerge when Bob reads Alice's equations. (For example, take Alice = Dirac and Bob = Feynman, with the new meaning of the old story giving us the path-integral formulation of quantum mechanics. Or, take Alice = Feynman and Bob = everybody who's read The Feynman Lectures on Physics. Someone who has studied the subject for several years ...

... to the post office and picked up a book I ordered from Amazon.com called “Perfectly Reasonable Deviations from the Beaten Path” by Michelle Feynman that she wrote about her father Richard Feynman. Then over to the Health Food Store to get two pounds of yeast that I use in my oatmeal for breakfast. My last stop was to the library to look ...