In a recent post, I joked that I could understand quantum mechanics because it is so abstract that it’s similar to the metaphysics I studied, Lo, these many years ago.
(Hawking’s hidden dice, January 12th)
I spoke too soon, because there are two characteristics of the quantum wave function that are so counter-intuitive, so downright weird, that I don’t know what to make of them.
I don’t think that anybody else really knows what to do with them, either. Richard Feynman apparently agreed: “I think I can safely say that nobody understands quantum mechanics.” Still, Feynman is the “author” of the first weird characteristic of the wave function. With the inimical clarity so typical of modern physics, Wikipedia puts it this way:
A Feynman diagram [below] is a tool used by physicists to calculate probabilities of reactions between elementary particles. …
More precisely, a Feynman diagram is a graphical representation of a perturbative contribution to the transition amplitude or correlation function of a quantum mechanical or statistical field theory.
Well, that was clear, wasn’t it? It was Feynman who also said, in an interview with People magazine in 1985: “If I could explain it to the average person, I wouldn’t have been worth the Nobel Prize.” So I guess we shouldn’t feel so bad.
Stephen Hawking puts the underlying principle, known as “sum over histories,” a bit more accessibly in The Grand Design. Writing of the classic “double-slit” experiment:
According to Newtonian physics — and to the way the experiment would work if we did it with soccer balls instead of molecules — each particle follows a single well-defined route from its source to the screen. There is no room in this picture for a detour in which the particle visits the neighborhood of each slit along the way. …
So far, so good. Throw soccer balls at a barrier with two holes, and any ball that makes it through to the back screen travels through one hole or the other. No surprise there. That’s the way the world in which we live works.
Well, it seems that the world doesn’t work that way at the extremely small scale of quantum mechanics:
… According to the quantum model, however, the particle is said to have no definite position during the time it is between the starting point and the endpoint. Feynman realized one does not have to interpret that to mean that particles take no path as they travel between source and screen. It could mean instead that particles take every possible path connecting those points.
Uh-oh. We just lost contact with the world in which we live. Did he say that during the time it travels the particle not only doesn’t have a position, it has every position? That can’t be right. Can it?
This, Feynman asserted, is what makes quantum physics different from Newtonian physics. The situation at both slits matters because, rather than following a single definite path, particles take every path, and they take them all simultaneously!
Now, how are we supposed to deal with this kind of idea? The future is composed of the probability function of all possible pasts? Come on, they can’t be serious, can they?
In the double-slit experiment Feynman’s ideas mean the particles take paths that go through only one slit or only the other; paths that thread through the first slit, back out through the second slit, and then through the first again; paths that visit the restaurant that serves that great curried shrimp, and then circle Jupiter a few times before heading home; even paths that go across the universe and back.
That’s quite enough of that! Maybe we’ll have more luck with the second strange idea — quantum entanglement. Or maybe we won’t, since it’s also known as “spooky action at a distance.” Doesn’t sound very promising, does it?
“Entanglement” refers to the way that two paired particles (how they are paired is beyond the scope of this article — way beyond!) act in certain ways as a single particle. No matter how far apart they are, if one is + the other is -. If one spins up, the other spins down. The “spooky” part is that neither particle is + or -, or up or down, until we measure the other one. At the instant that the value of one particle is established, the value of the other becomes established, without communication over distance or even the passage of time. Instantaneous, uncommunicated conservation of matter and energy.
Sure. We all got that, right? Ironically, entanglement was first proposed in a thought experiment by Einstein, as part of his “God doesn’t play dice” attack on quantum mechanics. It was Einstein who called it “spooky action at a distance.” He thought that he had proved quantum mechanics logically impossible by demonstrating that according to quantum theory entangled particles could exist. Decades later, experiments proved entanglement right. So much for the “real” world!
Brian Clegg, author of The God Effect, a book on entanglement, puts the implications of “spooky action” this way:
Entanglement doesn’t throw away the concept of cause and effect. But it does underline the fact that quantum particles really do only have a range of probabilities on the values of their properties rather than fixed values. And while it seems to contradict Einstein’s special relativity, which says nothing can travel faster than light, it’s more likely that entanglement challenges our ideas of what distance and time really mean.
Great. It just keeps getting worse! Now time and distance don’t really mean what they seem to mean. What’s left? At least I know that I exist. Good old Descartes comes to my rescue, just when I’m losing my last grip on reality.
Or does he? Next time, how our selves don’t exist, either: “Drafting our narratives”
will be posted here January 24th.