This book arose from a series of lectures which I gave at the University of Heidelberg during the summer terms of 1979,1982 and 1986. They led in 1984 the publication in German of Die Physik der Zeitrichtung, which appeared to as Vol. 200 of the Springer Lecture Notes in Physics. The present English version is not merely a translation of these notes, but has been widely revised and extended. The number of changes and additions roughly increases with chapter number. Chaps. 5 and 6 have been completely rewritten (except for Sect. 5. 1, which is a revised version of the former § 5. 2). The new title is intended to express the somewhat more ambitious program of this book as compared to its German predecessor. My interest in this subject stemmed originally from an attempt to place the quantum mechanical measurement process in its proper relation to other irreversible phenomena. It soon became evident that statistical thermody namics is too limited for the search for the common roots of the obviously related arrows of time. It is precisely the interconnectedness of many areas of physics, and not least their relation to some fundamental concepts (or per haps prejudices) of epistemology, which sustained my fascination with the subject of this book over many years. Thus it was not my intention to de scribe technicalities or mathematical problems, but to point out the essential physical ideas (which are often overlooked).
