The Emperor's New Mind: Concerning Computers, Minds, and the Laws of Physics (Oxford Landmark Science)

by Roger Penrose

(The ‘Pythagorean theorem’, |ψ|2 = |ψY|2 + |ψN|2, asserts that these probabilities sum to unity, as they should!) Note that the probability that |ψ〉 jumps to |ψY〉 is given by the ratio by which its squared length is reduced upon this projection.