We show that when two quantum-mechanical particles of the same mass, interacting through a hard-core repulsive potential, collide in one dimension, they exchange their wavefunctions, making them look as if they had passed through each other. We give a simple explanation of this phenomenon, and compare to its classical analog, which can be demonstrated using the "Newton's cradle" toy. We also consider briefly what happens when the masses are unequal, showing how entanglement arises in this case. (C) 2015 American Association of Physics Teachers.
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