The Universe may contain regions where milk would stir itself out of coffee and eggs would un-break, according to a physicist in New York state. The idea that there may be regions where time runs backwards could explain invisible dark matter. This kind of matter, invoked by cosmologists to explain the movement of galaxies, could originate in a future contracting phase of the Universe.
For coffee to unstir or an egg to un-break requires monumental coordination-all the pieces of egg shell would need to follow the exact trajectories to re-form into the egg, for instance. This is why theorists thought that the slightest interaction with a region having normal time would destroy the opposite "arrow of time".
But calculations by Lawrence Schulman of Clarkson University in Potsdam, New York, suggest that this might not be the case. "The problem in thinking about opposite arrows of time is to define the question sensibly," he says. "Once you have the right context, statistical physics shows that opposing arrows are actually compatible."
So how would a reverse-time region arise? Schulman suggests that they may be relics from the far future. This possibility requires that the expanding Universe eventually starts to contract into a "big crunch".
In such a situation, the so-called "thermodynamic arrow of time" may reverse during the contraction, creating order out of chaos-an idea first pro-posed by Thomas Gold of Cornell University. "Because of the opposite-running time, anyone around in this phase would actually see the contraction as an expansion," says Schulman.
A bizarre consequence of this is that reverse-time regions from the contracting phase could be around us today, perhaps as close as a few tens of light years. Since they are from the far future, any stars would have long burnt out, but not yet reignited under the reverse-time effect. "But we would still feel their gravity," says Schulman. "Such matter would have all the attributes of dark matter."
Alternatively, dark matter could be normal-time matter that has already collided with reverse-time matter from the future, resulting in matter that has no time direction. "Once again, it would appear exactly like dark matter," says Schulman.
Although Schulman has shown that a reverse-time region is not destroyed by interactions with a region of normal time, there are still paradoxes to grapple with. For instance, normal-time Alice could see rain wetting a carpet through reverse-time Bob's window. She could then wait until before the rain started and shout to Bob to close his window.
"So did Bob's floor get wet or not?" says Schulman. "My guess is that the paradox goes away if the problem is well posed-but I haven't yet shown it."
Schulman's calculations will appear in a forthcoming issue of Physical Review Letters. Except, of course, in a reverse-time region, where, having been published, they are now being unwritten.
Author: Marcus Chown
NEW SCIENTIST issue 27th November 99
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