That's not how it works in QM. It comes out as squared amplitudes because they're orthogonal.
Isn't that just a tautological consequence of how orthogonality is defined in Hilbert space? One way to develop quantum mechanics is to start with the pure states for some observable, which is all that you ever "actually experience", and then define an inner product on Hilbert space such that these states are orthonormal.
Today's post, Where Physics Meets Experience was originally published on 25 April 2008. A summary (taken from the LW wiki):
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