Light tries
every route at once
Feynman's account of why light appears to travel in straight lines: it does not choose. Every path from source to detector contributes a little arrow, turned by the time that path takes. Near the quickest route the times barely differ, so those arrows line up and add. Everywhere else they curl into a circle and cancel.
lawEach path carries a phase proportional to its action, here the travel time in units of the period. The amplitude is the vector sum of all of them, laid end to end.
tryDrag the detector sideways, then shorten the wavelength.
noticeThe spiral's two ends stop moving once the paths get long: only the middle stretch, where time is stationary, contributes anything. Shorten the wavelength and that stretch narrows into the straight line of ordinary optics.
in lifeDiffraction gratings work by removing the cancelling paths, leaving the rest to add — which is why a CD throws colours.
found byFeynman's 1948 path-integral formulation, building on Dirac; the arrows-and-spiral picture is the one he used in his 1979 Auckland lectures on QED.