Mark Kac, probabilist

Mark Kac had a great influence on mathematical physics for his work on the Wiener process, where he obtained an expression for the perturbed semigroup of a heat equation in terms of an integral over path space. This was soon recognised to be the rigorous form of the Feynman integral.

Kac has also contributed significantly to equilibrium and non-equilibrium statistical mechanics. In particular, the Kac ring model shows how it is possible for a fully time-reversible dynamics to exhibit an increasing entropy.