Quantum strangeness: a reminder

It doesn't hurt to remind oneself every now and then about quantum strangeness, and I quite like the way this article in Aeon (which seems a pretty good on line magazine, incidentally) explains it. 

Here's the key part:

Here’s the basic problem. While the mathematics of quantum theory works very well in telling us what to expect at the end of an experiment, it seems peculiarly conceptually confusing when we try to understand what was happening during the experiment. To calculate what outcomes we might expect when we fire protons at one another in the Large Hadron Collider, we need to analyse what – at first sight – look like many different stories. The same final set of particles detected after a collision might have been generated by lots of different possible sequences of energy exchanges involving lots of different possible collections of particles. We can’t tell which particles were involved from the final set of detected particles.

Now, if the trouble was only that we have a list of possible ways that things could have gone in a given experiment and we can’t tell which way they actually went just by looking at the results, that wouldn’t be so puzzling. If you find some flowers at your front door and you’re not sure which of your friends left them there, you don’t start worrying that there are inconsistencies in your understanding of physical reality. You just reason that, of all the people who could have brought them, one of them presumably did. You don’t have a logical or conceptual problem, just a patchy record of events.

Quantum theory isn’t like this, as far as we presently understand it. We don’t get a list of possible explanations for what happened, of which one (although we don’t know which) must be the correct one. We get a mathematical recipe that tells us to combine, in an elegant but conceptually mysterious way, numbers attached to each possible explanation. Then we use the result of this calculation to work out the likelihood of any given final result. But here’s the twist. Unlike the mathematical theory of probability, this quantum recipe requires us to make different possible stories cancel each other out, or fully or partially reinforce each other. This means that the net chance of an outcome arising from several possible stories can be more or less than the sum of the chances associated with each.

To get a sense of the conceptual mystery we face here, imagine you have three friends, John, Mary and Jo, who absolutely never talk to each other or interact in any other way. If any one of them is in town, there’s a one-in-four chance that this person will bring you flowers on any given day. (They’re generous and affectionate friends. They’re also entirely random and spontaneous – nothing about the particular choice of day affects the chance they might bring you flowers.) But if John and Mary are both in town, you know there’s no chance you’ll get any flowers that day – even though they never interact, so neither of them should have any idea whether the other one is around. And if Mary and Jo are both in town, you’ll certainly get exactly one bunch of flowers – again, even though Mary and Jo never interact either, and you’d have thought that if they’re acting independently, your chance of getting any flowers is a bit less than a half, while once in a while you should get two bunches.

If you think this doesn’t make any sense, that there has to be something missing from this flower delivery fable, well, that’s how many thoughtful physicists feel about quantum theory and our understanding of nature. Pretty precisely analogous things happen in quantum experiments.

You should read the whole thing...