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3

The standard treatment of identical particles starts with one-particle states and then imposes symmetry conditions. This is kind of backwards. If you look at a formula like $$|x_1,x_2\rangle = \frac{1}{\sqrt 2}(|x_1\rangle | x_2\rangle \pm |x_1\rangle| x_2\rangle)$$ you are saying that the state is a linear combination of states where particle A is at $x_1$ ...

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It's a force like no other. It is fundamentally a quantum property and there is no classical way to think of it (at least to my knowledge). That's just how the universe is, and we haven't understood any deep reason "why" it should be that way. Mathematical consistency seems to dictate it. It comes down to the observation that there can be some objects such ...

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If the protons are in a spin singlet, the neutron spin determines the spin of the $^3\text{He}$ nucleus. Without loss of generality you can define the direction of the $^3\text{He}$ polarization as $\uparrow$. Under isospin symmetry, the proton and the neutron are two states of the same particle. If your problem includes this symmetry (and if you can assume ...

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