system
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human
In quantum entanglement, how do we know that the entangled particle pair do in fact "interact" over a distance, and wasn't just aligned when they first interacted?

I don't know anything about this really, except for what I've learned from watching clips on youtube and reading wikipedia. But I'm sort of stuck with this question, and can't find an answer. It says everywhere that the attribute of the second particle measured is always opposite to the first one, almost no matter how close together in time the measurements are made, and how far apart they are in distance. 

Sounds to me like they were aligned oppositely the whole time, after interacting with each other the first time. How do we prove this is not the case?
gpt
This is addressed by a result known as Bell's theorem. 

In short, Bell's theorem proves the following. Suppose you build a theory in which particles' observable properties or attributes really do have definite values at all times (call this assumption R, for "realism"), and that nothing that takes place at one region of space can instantaneously affect the results of a measurement at some distant region of space (call this assumption L, for "locality"). 

Bell showed that, assuming these two results, the degree of correlation between the outcomes of two distant measurements of systems whose states were prepared in the same place is bounded by a certain number.  So if your suspicion is true, and one can say the spins were aligned oppositely the whole time (that is if R is a good assumption), and assumption L holds, then when we actually carry out this experiment we should see correlations that do not exceed this certain bound.

However, a quantum-mechanical calculation of the results of this experiment predict a degree of correlation that exceeds that for the above case, when R and L hold. So if we do the experiment, and find that the correlation exceeds Bell's bound, then one of the assumptions (R,L) must be false, favouring the view of quantum mechanics, where things just don't have definite properties before measurement.

Finally, many such measurements have been carried out, and it is found that Bell's bound is exceeded, favouring the standard quantum mechanical view. However, two things should be noted:

1) many of these experiments have minor loopholes, and so don't technically show completely that the assumptions (R,L) taken together fail

2) even if the violation of Bell's bound is experimentally shown without any loopholes, we may still drop ONE of the assumptions (R,L) and have a theory consistent with quantum mechanics. For example, we can drop the locality assumption, L, and build a theory that satisfies realism R, which means that things do really have objective attributes before measurement, and still agree with the predictions of quantum mechanics. Such theories exist, the Bohm-de Broglie "causal interpretation" of quantum mechanics being one such notable example. In this interpretation of quantum mechanics, causal influences really do travel instantaneously. At this point, it is a matter of taste which interpretation one chooses.