CollapseaQuantum Measurement / Wave Function Collapse

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The first rule on this page is Max Born's, from 1926. Working out how particles scatter, he wrote that the wavefunction gives the probability of an outcome; in a note added while the paper was in proof, he corrected it to the square. The squared magnitude of the amplitude, \(\lvert\langle a\vert\psi\rangle\rvert^{2}\), is the Born rule, and his 1954 Nobel Prize cites his statistical interpretation of the wavefunction.

The second rule says what happens to the state once you have an answer. Measure and find a, and the state is now \(\lvert a\rangle\). John von Neumann made this a formal postulate in his 1932 book. Its test is simple: repeat the same measurement immediately and you get a again, every time.

Here is where the character comes from. Send spin-½ particles through a magnet that measures spin along the vertical and keep those that come out up. Measure vertical again: all up. Now measure along a horizontal axis instead: half go one way, half the other. Take one of those groups and measure vertical once more: half up, half down. The first answer did not survive the incompatible question. Each measurement leaves a state that is definite for what was asked and spread out for what was not.

What collapse is, physically, is open. Some interpretations say the state really jumps; others, that every outcome occurs in its own branch. Decoherence explains why interference disappears once a measuring device is involved, but on most readings it does not choose which outcome you see. All of them agree on the two rules printed here, which is why the predictions are not in dispute.

Collapsea keeps her word. She only refuses to answer questions she was not asked.