Where is the frontier of physics? Some would say 10−33 cm, some 10−15 cm and some 10+28 cm. My vote is for 10−6 cm. Two of the greatest puzzles of our age have their origins at this interface between the macroscopic and microscopic worlds. The older mystery is the thermodynamic arrow of time, the way that (mostly) time-symmetric microscopic laws acquire a manifest asymmetry at larger scales. And then there's the superposition principle of quantum mechanics, a profound revolution of the twentieth century. When this principle is extrapolated to macroscopic scales, its predictions seem wildly at odds with ordinary experience.

This book deals with both these ‘mysteries,’ the foundations of statistical mechanics and the foundations of quantum mechanics. It is my thesis that they are related. Moreover, I have teased the reader with the word ‘foundations,’ a term that many of our hardheaded colleagues view with disdain. I think that new experimental techniques will soon subject these ‘foundations’ to the usual scrutiny, provided the right questions and tests can be formulated. Historically, it is controlled observation that transforms philosophy into science, and I am optimistic that the time has come for speculations on these two important issues to undergo that transformation.