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 of matter taking a single path but a processing wave spreading over many paths.
How then do photons travel in our world? Feynman’s sum over histories method predicts how light travels 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 origin, became a method that works but not a theory that happens, because physical particles can’t do what it describes.
However, now suppose that a photon really is a processing wave. Feynman’s method then works because photon instances really do take all possible paths, and the first to trigger a physical event is where we see it arrive. The photon doesn’t need to know the fastest path to a detector in advance if it takes every path, and the instance that happens to arrive first reincarnates it in a physical event. This event makes its path the one the photon took, and its restart makes all other instances disappear, like a clever magician removing the evidence of how a trick is done after it happens.
This explains the law of least action that has puzzled science for centuries, as after all, how else could it happen? A particle can’t know the best path to an unknown destination before it leaves, so to do what it does, the photon takes every path and lets the physical result choose the fastest.
To recap, knowing nothing in advance, the photon spreads down every path, and when it reaches a detector, restarts, 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.
It follows that every physical event 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 tried, it may be the best it can be. Just as a photon tries every path before picking the best, perhaps our universe did the same, given what was possible.
The physical law of least action can then be seen to derive 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 is chosen. This law of all action, that whatever can happen does happen in the quantum world, is universal, so it applies not only to how light travels, but also to quantum spin.