  This article is mostly based on a talk I gave at the March 2021 meeting
(virtual) of the American Physical Society on the occasion of receiving the
Dannie Heineman prize for Mathematical Physics from the American Institute of
Physics and the American Physical Society. I am greatly indebted to many
colleagues for the results leading to this award. To name them all would take
up all the space allotted to this article. (I have had more than 200
collaborators so far), I will therefore mention just a few: Michael Aizenman,
Bernard Derrida, Shelly Goldstein, Elliott Lieb, Oliver Penrose, Errico
Presutti, Gene Speer and Herbert Spohn. I am grateful to all of my
collaborators, listed and unlisted. I would also like to acknowledge here long
time support form the AFOSR and the NSF.
