Super-computers running a million-million cycles a second take millions of seconds (months) to simulate not just what a photon does in a million-millionth of a second, but in a million-millionth of that (Wilczek, 2008), (p113). How can these tiniest bits of the universe with no known structures make such complex choices? The answer now proposed is that a photon isn’t a particle following a fixed path but a cloud of instances that take many paths.
Feynman’s sum over histories method predicts how light travels from A to B by calculating all the 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 in practice, but like its parent, it was accepted as a method that works but not as a theory to believe because physical particles can’t do what it describes.
If a photon is a processing wave, Feynman’s method works because it describes real events, so its instances really do take all available 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 to a detector in advance if it takes every path, and the instance that happens to find that path then reincarnates it in a physical event. This makes its path the one the photon took, and that event is a server restart, so all the other instances disappear, like a clever magician removing the evidence of how a trick is done after it happens.
Indeed, if we consider the law of least action logically, how else could it arise? A photon particle can’t know in advance the best path to an unknown destination before it leaves, so the only way it can do what it does is to take every path and let the first to arrive restart it in a physical event.
To recap, knowing nothing in advance, the photon spreads itself down every path, and when it happens to hit a detector, restarts as only a processing wave can. What arrives at 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 comes from a myriad of quantum events. The quantum world tries every option and the physical world takes the best and drops the rest, so if this isn’t the best of all possible worlds, it isn’t for lack of trying. The physical law of least action then derives from the quantum law of all action, which is 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“, and also to Gellman’s quantum totalitarian principle that “Whatever isn’t explicitly forbidden must happen“. Both imply that a photon takes every possible path to a detector, and the first instance to trigger a physical event becomes the path it took. As will be seen, this law of all action, that quantum events explore all the possibilities before every physical event, is universal, so it applies not only to how light travels but also to quantum spin, as the next section explores.