For a non-relativistic quantum mechanical system, its time evolution is described by a one-parameter group of automorphisms {α t } t of Q. This can be narrowed to unitary transformations: under certain weak technical conditions (see the article on quantum logic and the Varadarajan reference), there is a strongly continuous one-parameter group { U t } t of unitary transformations of the underlying Hilbert space such that the elements E of Q evolve according to the formula