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鈥攁ll 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 鈥渁rrow of time鈥�.
But calculations by Lawrence Schulman of Clarkson University in Potsdam, New
York, suggest that this might not be the case. 鈥淭he problem in thinking about
opposite arrows of time is to define the question sensibly,鈥� he says. 鈥淥nce you
have the right context, statistical physics shows that opposing arrows are
actually compatible.鈥�
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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 鈥渂ig crunch鈥�.
In such a situation, the so-called 鈥渢hermodynamic arrow of time鈥� may reverse
during the contraction, creating order out of chaos鈥攁n idea first
proposed by Thomas Gold of Cornell University. 鈥淏ecause of the opposite-running
time, anyone around in this phase would actually see the contraction as an
expansion,鈥� 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. 鈥淏ut we would still
feel their gravity,鈥� says Schulman. 鈥淪uch matter would have all the attributes
of dark matter.鈥�
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. 鈥淥nce again, it would appear exactly like dark matter,鈥� 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鈥檚 window. She could then wait until before the rain started and
shout to Bob to close his window.
鈥淪o did Bob鈥檚 floor get wet or not?鈥� says Schulman. 鈥淢y guess is that the
paradox goes away if the problem is well posed鈥攂ut I haven鈥檛 yet shown
颈迟.鈥�
Schulman鈥檚 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.