COMPETING ARROWS OF TIME. Lawrence S. Schulman of Clarkson University has found that time might actually flow backwards in certain regions of space. This time reversal has nothing to do with quantum fluctuations or the spacetime-warping effects of a black hole. It's just ordinary matter obeying the ordinary and mostly time-symmetric laws of physics.

The difference lies in its statistics. If the laws of physics have no preferred direction then why do we never see a shattered wineglass jump back up on the table and reassemble itself? The "arrow of time" concept enshrines this domestic disaster in the form of a law, the second law of thermodynamics. The arrow describes the tendency for macroscopic systems consisting of many particles (the falling wineglass) to evolve in time in such a way that disorder grows and information decreases. This tendency is statistical and does not prevail at the microscopic level, where a movie of two atoms colliding would seem credible if run in the forward or reverse direction. The wineglass, however, consists of zillions of atoms. The reason we never see the glass re-assemble and lift itself (courtesy of the heat released by the original breakage returning from the floor and air) back onto the table is that this highly specialized (and, as we would say, unlikely) scenario is but one of a myriad of possible configurations, in most of which the glass shards stay on the floor.

This statistical explanation leads to two puzzles. First, why does this arrow point the way it does? Why not the other way? And second, why should it point at all? On the first question, Schulman subscribes to the view that the "thermodynamic" arrow of time is a consequence of the "cosmological" arrow reflected in the one-way expansion of the universe, a theory advanced some years ago by Thomas Gold of Cornell.

As to the second question, that's exactly where Schulman's (schulman@clarkson.edu) new results have their impact. The prevailing view holds that if opposite-arrow systems came into even the mildest of contact, the order in at least one of them would be destroyed. Not so, says Schulman who, in his computer modeling of the universe, specifies not one boundary condition in time (the big bang) but two, the other being a supposed "big crunch" when the universe would contract (or so it would seem to us; from the perspective of the opposite arrow, the universe would be expanding). In his model the two arrows of time (one growing out of either end of the "timeline"; see the figure at Physics News Graphics) can be mildly in contact and nevertheless each have its wineglasses break and its rain fall appropriately. Observers associated with either arrow might even watch the other grow young -- from a distance. Some relatively isolated relics of matter subject to the opposite arrow might be found in our vicinity.

By its own clock such a region would be very old and no longer luminous, although it would exert a conventional form of gravity; this is exactly the hallmark of dark matter. Or we might see an opposite-arrow black hole giving matter back to an accretion disk, which in turn would feed it back to a companion star which would seem (to us) to be coming into existence. Schulman concedes that recent observations may rule out a final crunch in our actual universe but argues that there is still a lot we don't understand about our thermodynamic arrow, and that a competing time arrow might arise from another, as yet unknown, cause. (Physical Review Letters, 27 Dec.