ZitterbewegungDirac's equation for a free electron says it never actually travels in a straight line. Riding along with its average drift, the electron is really corkscrewing around that path in a tiny, impossibly fast circle — a "trembling motion" too small and too fast to ever see directly, yet sitting right there in the exact solution.Schrödinger noticed it in 1930: interference between the positive- and negative-energy pieces of a Dirac wave packet makes the position oscillate at ω = 2E/ħ — about 10²&sup9; times a second at rest — in a loop of radius ħc/2E, the reduced Compton wavelength. The loop's own speed works out to exactly c no matter how fast the electron drifts, which is why the path always curls back on itself: nothing with mass is ever really moving in a straight line at less than lightspeed.