Text: Some years later, in 1940 or 1941, I came up with yet another way to look at the positron (also without changing the mathematics of Dirac's theory). Sitting at home in Princeton one evening, it occurred to me that a positron could be interpreted as an electron moving backward in time. I was excited enough about the idea to phone my graduate student Richard Feynman at once at the Graduate College, the on-campus residence where he lived. "Dick," I said, "I know why all electrons and positrons have the same mass and the same charge. They are all the same particle!" What had sprung into my head was a vision of a single electron, tracing its world line‹its path in spacetime‹first forward in time, then reversing to go backward in time, then reversing again to go forward in time, and so on. At any given moment of time‹that is to say, on a single slice of spacetime‹one sees numerous electrons and numerous positrons, not knowing that at various future times and past times, their world lines are joined into a single tangled thread. I knew, of course, that, at least in our corner of the universe, there are lots more electrons than positrons, but I still found it an exciting idea to think of trajectories in spacetime that could go unrestricted in any direction‹forward in time, backward in time, up, down, left, or right. Feynman later incorporated this idea into the diagrams that now bear his name and that have become powerful tools of calculation in quantum electrodynamics and elementary-particle physics. The lines in a Feynman diagram are pieces of world-lines that one can take hold of and twist into any direction in spacetime. ‹ John Archibald Wheeeler, Geons, Black Holes & Quantum Foam

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