In quantum mechanics, configuration space can be used (see for example the Mott problem), but the classical mechanics extension to phase space cannot. Instead, a rather different set of formalisms and notation are used in the analogous concept called quantum state space. The analog of a "point particle" becomes a single point in C P 1 , the complex projective line, also known as the Bloch sphere. It is complex, because a quantum-mechanical wave function has a complex phase; it is projective because the wave-function is normalized to unit probability. That is, given a wave-function ψ ψ one is free to normalize it by the total probability ∫ ∫ ψ ψ ∗ ∗ ψ ψ , thus making it projective.