It can also be defined in terms of unit ray space; i.e., with no other changes. In this case it is sometimes called a Wigner automorphism. It can then be extended to by means of multiplication by reals as described earlier. In particular, unit rays are taken to unit rays. The significance of this definition is that transition probabilities are preserved. In particular the Born rule, another postulate of quantum mechanics, will predict the same probabilities in the transformed and untransformed systems,