Alice in Quantum Land

I’ve joked that when I was a physicist, sometimes I’ve believed as many as six impossible things before breakfast. It’s “impossible” for an electron, for example, to pass though a barrier that is stronger than the electron’s energy. Except they do, and if they didn’t, the flash memory in your iPod wouldn’t work.

The “impossibility”, though, is entirely in the eye of the beholder. Electrons do what electrons do, and if it seems contrary to common sense, that’s because common sense is tuned to work at the scale of elephants, not electrons.

For a physicist, I had the distinct blessing of having no common sense whatsoever, so I was quite happy to believe the results of quantum mechanics without worrying how things could possibly be that way. If the interference pattern means that the atom simultaneously took two different routes through space, well, why not?

Just once, was my mind boggled slightly by something in the quantum world, because there is absolutely no parallel or analogue for it in the elephant-scale world. It’s something that happens, and is predicted perfectly by the mathematics, but doesn’t make any sense.

I’m talking about the phenomenon of “exchange interaction“. When you are using quantum mechanics to describe a quantum situation, you need to take account of the fact that objects may be indistinguishable. In the quantum world, entities can be described entirely by just a handful of parameters — charge, spin and so on — which can only have a limited number of values.

What this means in the mathematics is that you have to make an allowance for, say, two identical particles being swapped. Quantum mechanics being an affair of probabilities, when you work it out you find that the two particles behave as though they are being acted on by a force. (Either pushing them together or apart, depending on their type.)

So this “force” appears out of nowhere simply because two quantum objects are identical. Makes you think, doesn’t it?


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