This is the craziest thing I’ve ever come across. I’m about to bend your brain, and – be warned – it may snap. Or catch fire. Or something.
You know how you think of negative numbers as just maths? They couldn’t possibly correlate to something in the real world: you can have two particles, or three, or four…but you can’t have minus two particles, or minus three particles….? Think again. A paper came out in Physical Review Letters this month showing you can have exactly that. As you might have guessed, it’s a quantum trick. But it’s real, nonetheless.
It resolves a fascinating and long-standing paradox of quantum physics. It's called Hardy's paradox, after Lucien Hardy, a quantum researcher who works at the Perimeter Institute in Ontario. Basically, the paradox is this. First you set up a quantum system of mirrors, particle sources and particle detectors. Then you fire in a particle and its antiparticle in a way that means they meet. The thing is, they will fail to annihilate each other.
The situation is to do with the fact that, according to quantum rules, the particles can simultaneously be in two places at once.
OK, I hear you - meeting but not annihilating is weird, but quantum stuff is weird, and I’ll buy that two places at once thing as a get-out. But here’s where it starts to get crazy. The results you get (when you set it out as Hardy suggested) suggest that there are actually two pairs of particles in the apparatus at the same time, not one.
Back in 2003, someone came up with a solution to this. They did a “thought experiment”, where they basically did lots of quantum calculations (but not a real experiment). Here’s their answer: you can make everything add up if there is minus one (yes, -1) pair of particles sitting in another part of the interferometer.
You could dismiss this as “not a real solution” if it were just a thought experiment, but now it's been done for real, and that -1 pair of particles REALLY is there.
Don’t ask me what it means. I don’t know. It's truly bizarre, but it is also an enormously important validation of the true weirdness of quantum physics – we really don’t know the half of it yet.
Anyway, if you want to know more, I wrote about this in a New Scientist cover feature in 2003, back when the thought experiment was published. Just don’t blame me if your head explodes while trying to make sense of the fact that this is real.
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