So, I\'m only a high school student researching quantum physics, and I find it v

Question

So, I'm only a high school student researching quantum physics, and I find it very interesting. However, there's one question that keeps nagging at me in the back of my head. How exactly do odd behaviors like quantum parallelism that occur on the atomic level lead to the behaviors that we consider normal at everyday sizes and scales? That is, what is it about having so many atoms together (classical physics) that makes them behave so very differently from the way a single atom behaves (quantum physics)?

Sorry if it seems like I don't know what I'm talking about... because I may not! So, if there are any misconceptions on my behalf, please tell me so I can actually learn something... :)

Thanks in advance!

Explanation / Answer

There is a phenomenon called decoherence in quantum mechanics which is largely responsible for this. Basically (the following is a simplification), all the strange behavior that occurs in QM tends to happen when the wavefunctions of different particles are in phase. Decoherence occurs when the phases are randomized, so there's no special correlation between different particles. In that case, the properties of the different particles tend to just combine the way we'd expect them to classically.

A decent (but very basic) analogy for this would be like having a bunch of identical cars whose drivers all turn their turn signals on at the very same time. The turn signals would be blinking together, so we'd say they are in phase. But on a real road, that's not the case at all; different drivers turn their turn signals on at different, pretty much random times. And besides that, there are many different models of car whose turn signals blink at different rates. For both those reasons, the turn signals on a real road are not in phase. That's kind of like decoherence.

The reason I bring this up is that I've posted an answer about it which you might be interested to read. The gist of that answer is that when you have a small system like a single particle, any interaction makes a big difference to the system's momentum. But the same interaction will make only a little difference to a system which contains a large number of particles with partially uncorrelated momenta, like a measuring device. Now, in the paragraphs above, I talked about phase, whereas my other answer talks about momentum, but the idea involved is similar in both cases.

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