The Unexpected and the Unexplainable

(This image, which is not directly related to the post, is by Josef Spalenka at

Niels Bohr, the quantum theorist, once said “If quantum theory hasn’t shocked you, that just means you haven’t understood it yet”. People love this quote because it gets at how counterintuitive quantum mechanics can be. But it’s also discouraging, making it sound like quantum mechanics is something only a few select people will ever get. Like rocket science or brain surgery. And if there is no way you’re going to understand something, why even bother trying?

But non-specialists shouldn’t give up on quantum mechanics. The same goes for rocket science and brain surgery – just as long as we remember to be realistic about what we don’t know rather than just jumping into trepanning after looking up “the brain” on wikipedia.

Quantum reality is never far away, from laser scanners at the store, to the transistors that make your computer work, to, for some reason, this Czech real estate company. It makes sense to find out about it. And to learn enough to be surprised isn’t very hard. That’s the difference we need to be clear about: being shocked by something unexpected – which is what the Bohr quote says will happen – on one end, and being confused by something difficult on the other.

One thing that really shocked me when I was first learning about quantum mechanics is something that’s pretty well known: the Einstein-Podolsky-Rosen (EPR) paradox. It is the “spooky action at a distance” you may have heard of, in, for example, your favourite vampire movie. What the EPR  paradox says is that, according to one interpretation of quantum mechanics, measuring something in one part of the universe could have a strong effect on another part of the universe. A part of the universe that could be located light years away and should have no way to find out whether you’re doing quantum mechanics or eating a sandwich is still affected. Podolsky, Rosen and their funny-haired friend weren’t the first to claim that this is what happens, but the first to be troubled by how spooky this seems. Too spooky, they said, for it to be true. Still, as far as we can tell, that is an interpretation consistent with how the world works.

But how is it that we can tell? That always should be the first and most important question in science. If someone can’t answer that, then they aren’t talking about surprising results anymore – they are just confusing people.

In quantum mechanics, we can create a situation (called an entangled state) where two particles (say photon A and photon B) have to have opposite values in some measurable quantity (say “spin up” and “spin down”). But which particle has which value is not set unless you measure them. They both exist in a superposition of “up” and “down”. (Confused about the meaning of entanglement and superposition? Check them on wikipedia above, and also, I talk more about what each means in the Schroedinger’s Cat post). The EPR paradox says: let’s wait and not measure these two photons for a while, until they are really far away from one another. Then, when we check A and see that it’s “up”, does B become “down” only then?

Experimental physicists came up with a way to check. What they do is randomly choose what direction to measure “up” and “down” in. If they pick that direction right before measuring particle A, there is no way that information could have gotten to particle B. When they pick different measurement directions for the two particles, they find that the results for A and B are correlated in a way that only makes sense if the measurement of one had affected the other. That is spooky, sure enough, and people disagree about what it means. But even if we can’t explain why the universe works this way, we can explain why we believe that it does.

But not if we give up before we even start. Not if a specialist says, oh quantum mechanics is too hard for you, you won’t get it. Not if a non-specialist says, oh, why bother trying to understand something I’ll never understand. If we do that, we’re ceding the territory to people who don’t care if they can be understood, or if what they say is even true.

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