Quantum weirdness

Physicists themselves often say that their heads swim when they think too hard about quantum mechanics. As I have stated earlier in this book, quantum mechanics is eerie not just because it violates common sense. It is strange for deeper reasons: it deals with unobservable quantities; it shows that nature’s fundamental laws are probabilistic; it permits particles to be in two or more states of motion at the same time; it allows a particle to interfere with itself; it says that two widely separated particles can be entangled.

` Kenneth Ford, The Quantum World

…And we don’t know why.

But take the last one. Consider first that a particle like a photon is not in one determinate state until it interacts with something else. There are certain probabilities for each of the states it could possibly be in, but it would be incorrect to say that it is in one of those states and we just don’t know which. It follows all paths until it interacts with something at some point in space and time. Second, note that particles at higher energies (including the energy contained as mass – think E=mc2) decay into lower energy particles, while certain quantities like energy and spin must be conserved.

So, take a particle that, in decaying, results in two photons (and something else) with a certain kinetic energy (since photons are massless, they must have kinetic energy after the decay in order for total energy to be conserved). The photons are actually a photon and an ‘anti-photon’ (their spins must be opposite). It could be either that photon 1 has positive spin and photon 2 negative, or the other way around. But here’s the thing: both possibilities are true until they interact with something, at which point their spin is pinned down. So our two photons can fly off in somewhat different directions, until they’re a long ways apart, and a scientist on earth observes one of them. Snap. The spin of that one is determined — and the spin of the other, maybe light-years away, is instantly determined as well. Observing one photon affects another over a vast distance, instantaneously.

I think it has something to do with the wave nature of particles. It’s kind of creepy. We shouldn’t be able to affect something light-years away without it taking, well, years; that seems to defy the universal speed limit.

At any rate, having brought up the subject of Schrodinger’s cat (albeit without mentioning the cat ’till now), I will leave you with a parodist’s alternative to that thought experiment:

In fact, the mere act of opening the box will determine the state of the cat, although in this case there were three determinate states the cat could be in: these being Alive, Dead, and Bloody Furious.

`Pratchett, Lords and Ladies



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