The Thermodynamic Arrow: Puzzles and Pseudo-puzzles

For more than a century, physics has known of a puzzling conflict between the T-asymmetry of thermodynamic phenomena and the T-symmetry of the underlying microphysics on which these phenomena depend. This paper provides a guide to the current status of this puzzle, distinguishing the central issue from various issues with which it may be confused. It is shown that there are two competing conceptions of what is needed to resolve the puzzle of the thermodynamic asymmetry, which differ with respect to the number of distinct T-asymmetries they take to be manifest in the physical world. On the preferable one-asymmetry conception, the remaining puzzle concerns the ordered distribution of matter in the early universe. The puzzle of the thermodynamic arrow thus becomes a puzzle for cosmology. 1 The puzzle of temporal bias Late in the nineteenth century, on the shoulders of Maxwell, Boltzmann and many lesser giants, physicists saw that there is a deep puzzle behind the familiar phenomena described by the new science of thermodynamics. On the one hand, many such phenomena show a striking temporal bias. They are common in one temporal orientation, but rare or non-existent in reverse. On the other hand, the underlying laws of mechanics show no such temporal preference. If they allow a process in one direction, they also allow its temporal mirror image. Hence the puzzle: if the laws are so even-handed, why are the phenomema themselves so one-sided? What has happened to this puzzle since the 1890s? I suspect that many contemporary physicists regard it as a dead issue, long since laid to rest. Didn't it turn out to be just a matter of statistics, after all? However, while there are certainly would-be solutions on offer—if anything, as we'll see, too many of them—it is far from clear that the puzzle has actually been solved. Late in the twentieth century, in fact, one of the most authoritative writers on the conceptual foundations of statistical mechanics could still refer to an understanding of the time-asymmetry of thermodynamics as 'that obscure object of desire'. 1 One of the obstacles to declaring the problem solved is that there are several distinct approaches, not obviously compatible with one another. Which of these, if any, is supposed to be the solution, now in our grasp? Even more interestingly, it turns out that not all these would-be solutions are answers to the same question. There are different and incompatible …