" Wheeler's brilliant student Richard Feynman had developed a unique approach to quantum mechanics, called 'sum over histories', that generalized Hamilton's least-action principle to the study of how photons transfer between electrons and other charged particles to generate the electromagnetic force. In creating a force, the photon acts as what is called an 'exchange particle'. (Its existence is required through Weyl's gauge theory of electromagnetism.) Unlike in classical mechanics, in which particles travel along unique paths, Feynman showed how in quantum interactions all possible paths are taken, weighted by their probabilities to create a net result. "

― Paul Halpern , Einstein's Dice and Schrödinger's Cat: How Two Great Minds Battled Quantum Randomness to Create a Unified Theory of Physics