If a new theory is valid, it must account for the verified results of the old theory. This is the correspondence principle, first articulated by Bohr. New theory and old must correspond, that is they must overlap and agree in the region where the results of the old theory have been fully verified.

            When the techniques of quantum mechanics are applied to macroscopic systems rather than atomic systems, the results are essentially identical to those of classical mechanics. For a large system such as the solar system, where classical physics is successful, the Schrodinger equation leads to results that differ from classical theory only by infinitesimal amounts.