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Time's Arrows Today: Recent Physical and Philosophical Work on the Direction of Time

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The nature of time is one of the oldest and most basic questions in Western philosophy; since we are all destined to grow older and watch irreversible changes in our bodies, and we are all aware that we will eventually die, it is probably a basic and inherent question in all philosophical systems. Whoever tries to understand the nature of time must explain two of its apparent characteristics: it seems to be unidirectional and unchanging. This perhaps explains the name of this book, which is concerned not only with the direction of time but also with time's essential nature. This book is one of a large number on the subject which have been published recently. I haven't read the others, being more familiar with earlier discussions of the question, but I have two remarks to make about this one: Anyone who doesn't read it is missing something, and the general level of academic philosophy has greatly improved in the last few years, which I've noticed in other books as well.

Most of the articles in this collection are good, but I particularly enjoyed four of them. The first is Savitt's historical overview of modern discussions of the direction of time, which is just that, and excellent.

The second is Unruh's discusssion of the new role of time in general relativity and quantum mechanics. Unruh begins by by saying the Newton "tells us that it is unnecessary to define time, but then proceeds to do just that". Unruh misunderstands Newton, who indeed leaves time undefined; what Unruh thinks is a definition is a warning not to confuse physical time, which needs no substrate, with common concepts of time based on recurrent phenomena in concrete objects. It's when Unruh talks about relativistic time that he really comes into his own. He explains that gravitation is an inherent consequence, an epiphenomenon, of the concept of spacetime. Few of his colleagues try to explain what gravity is, one of the really basic questions; they delude themselves into thinking they have done so when they have really only described anew how is behaves quantitatively. He also has a section on time in quantum mechanics which can serve as an ultrashort introduction to some of the basic concepts of that entire subject.

The second of Sklar's articles here is mainly a critique of attempts to explain perceived time in terms of entropy. Most of his objections can be countered by replacing the concept of entropy in systems external to the person by that of neurochemical systems which are inherently asymmetrical under physiological constraints, but his discussion is deep and enlightening.

Barrett and Sober point out that entropy, the most popular word in modern explanations of time, "is well-defined [only] for chambers of gases", and proceed to construct an abstract mathematical concept of entropy which will also be useful in discussions of time.

In addition to everything else, most of the articles in this book, including these four, are beautifully literate, written by and for educated people. Anybody with an interest in time, or metaphysics generally, will profit from and enjoy reading it. Only elemantary mathematics, if any, is really necessary