In 1955, he became Professor in Princeton University. Although he has carried out many studies in the fields of physics, he concentrated on statistic mechanics and symmetry principle.
It is to be found that each physics law, which based on experiment, is not mathematical perfect and accurate, but statistic and proximate in the extreme situation.
A statistic interpretation of action principle is proposed, in which the action of physics field is equal to the free energy of a statistic system hidden in background.