Quantum mechanical

Mathematical formulation

In the mathematically rigorous formulation of quantum mechanics, the state of a quantum mechanical system is a vector ${\displaystyle \psi }$ belonging to a (separable) complex Hilbert space${\displaystyle {\mathcal {H}}}$. This vector is postulated to be normalized under the Hilbert space inner product, that is, it obeys ${\displaystyle \langle \psi ,\psi \rangle =1}$, and it is well-defined up to a complex number of modulus 1 (the global phase), that is, ${\displaystyle \psi }$ and ${\displaystyle e^{i\alpha }\psi }$ represent the same physical system. In other words, the possible states are points in the projective space of a Hilbert space, usually called the complex projective space. The exact nature of this Hilbert space is dependent on the system – for example, for describing position and momentum the Hilbert space is the space of complex square-integrable functions ${\displaystyle L^{2}(\mathbb {C} )}$, while the Hilbert space for the spin of a single proton is simply the space of two-dimensional complex vectors ${\displaystyle \mathbb {C} ^{2}}$ with the usual inner product.