Postulate 1. The state of a quantum mechanical system is completely specified by a function that depends on the coordinates of the particle(s) and on time. This function, called the wave function or state function, has the important property that is the probability that the particle lies in the volume element located at at timet.

The wavefunction must satisfy certain mathematical conditions because of
this probabilistic interpretation. For the case of a single particle,
the probability of finding it *somewhere* is 1, so that we have
the normalization condition

(110) |

It is customary to also normalize many-particle wavefunctions to 1.

Postulate 2. In any measurement of the observable associated with operator , the only values that will ever be observed are the eigenvaluesa, which satisfy the eigenvalue equation

(111) |

This postulate captures the central point of quantum mechanics--the values of dynamical variables can be quantized (although it is still possible to have a continuum of eigenvalues in the case of unbound states). If the system is in an eigenstate of with eigenvalue

Although measurements must always yield an eigenvalue, the state does
not have to be an eigenstate of .
An arbitrary state can be
expanded in the complete set of eigenvectors of
(
as

(112) |

where

Postulate 3. If a system is in a state described by a normalized wave function , then the average value of the observable corresponding to is given by

(113) |

This postulate comes about because of the considerations raised in section 3.1.5: if we require that the expectation value of an operator is real, then must be a Hermitian operator. Some common operators occuring in quantum mechanics are collected in Table 1.Postulate 4. To every observable in classical mechanics there corresponds a linear, Hermitian operator in quantum mechanics.

Postulate 5. The wavefunction or state function of a system evolves in time according to the time-dependent Schrödinger equation

(114) |

The central equation of quantum mechanics must be accepted as a postulate, as discussed in section 2.2.

The Pauli exclusion principle is a direct result of thisPostulate 6. The total wavefunction must be antisymmetric with respect to the interchange of all coordinates of one fermion with those of another. Electronic spin must be included in this set of coordinates.