Super-computers running a million-million cycles a second take millions of seconds (months) to emulate not just what a photon does in a million-millionth of a second, but what it does in a million-millionth of that (Wilczek, 2008), (p113). Why do these tiniest bits of the universe, with no known internal structure, need so much computing to emulate? The answer proposed is that a photon isn’t a tiny particle taking a single path but a processing wave spreading over many paths.
How then do photons travel? Feynman’s sum over histories method predicts how light goes from A to B by calculating all possible paths, then choosing the one with the least action integral (Feynman et al., 1977) p26-7. It is based on quantum theory, so it predicts perfectly, but like its source, became a method that works not a theory that happens, because it describes what can’t physically happen.
But if a photon is a processing wave, then Feynman’s method works because its instances really do take all possible paths, and the first to trigger a physical event is where we see it arrive. A photon doesn’t need to know the fastest path in advance if it takes every path, as the instance that happens to arrive first reincarnates it in a physical event. This makes its path the one the photon took, and the restart removes all the other instances, like a magician removing the evidence of how a trick is done after it happens.
The physical law of least action that has puzzled science for centuries is then explained by a quantum of all action. A particle can’t know the fastest path to an unknown destination in advance, but the photon just takes every path and lets the first physical result decide its path.
To recap, knowing nothing in advance, the photon spreads down every path, and restarts when it reaches a detector, as only a processing wave can. What reaches a detector by the fastest route isn’t a solitary particle that magically knows the best path in advance, but a quantum ensemble that explores every path and disbands when the job is done.
Every physical event then arises from a myriad of quantum events. The quantum world tries every option and the physical world takes the best and ignores the rest, so if this isn’t the best of all possible worlds as Leibniz said, it isn’t for lack of trying. The world we live in isn’t the only possible world but of all possibilities, it could be the best it can be. Just as a photon tries every path before picking the best, if our universe did the same, it couldn’t be any better.
The physical law of least action then derives from the quantum law of all action that:
Everything that can happen as a physical event, does happen as a quantum event.
This is equivalent to Feynman’s everything that can happen does happen (Cox & Forshaw, 2011), as well as Gellman’s quantum totalitarian principle that everything not forbidden is compulsory. Both imply that quantum events explore every possibility before a physical event occurs. The law of all action is universal, so it applies not only to how light travels but also to quantum spin.