But if two particles of the same species are in the same quantum state—except, possibly, for one particle having a high likelihood of being here while the other particle has a high likelihood of being over there—the laws of quantum mechanics ensure that they are indistinguishable, not just in practice but in principle. They are perfect twins. If someone were to exchange the particles’ positions (more precisely, exchange the two particles’ probabilities of being located at any given position), there’d be absolutely no way to tell.
