ABSTRACT
Albert Einstein was the first and for his whole life the only scientist to be aware of this problem with respect to quantum mechanics. The main goal has been to show that there is a fairly systematic approach to understanding the relations between classical and quantum mechanics, even when the classical system is ergodic. The movement of a billiard ball on a surface of constant negative curvature can be understood mathematically thanks to its high degree of symmetry which leads to A. Selberg's trace formula. A physicist or a chemist would like to see an example of ergodic classical motion which is realized in the laboratory if not in nature. The small imaginary part in the value of the approximate energy level seems to be the prices which have to pay for wanting to approximate the energy levels of a system with classically ergodic behavior.
