Physics News Update, Number 365

Number 365, April 2, 1998 by Phillip F. Schewe and Ben Stein

MOVING FORWARD INTO THE PAST. Many novelists and not a few scientists have pondered the possibility of returning to a point in one's past. In principle physics does allow time travel via closed timelike curves (CTCs). Along such a warped spacetime trajectory the traveler is always moving into the future (locally) but eventually finds himself back where he started from. (Think of sailing around the world; heading west you return from the east.) J. Richard Gott and Li-Xin Li at Princeton have speculated on whether spacetime could both allow travel into the past and insure a consistent chronology (the traveler must remember shaking hands with his older self as he sets off). They have determined that in part of space no time travel would be allowable, but in another part (separated from the first by a surface called a Cauchy horizon) a time machine could be built, subject to this restraint: if you build a time machine in the year 3000, you might be able to use it to go from 3002 to the year 3000, but not back to the year 1998 because that would have been before the machine was built. Of course even this limited sort of time travel would be very difficult because you would need a lot spacetime warp for it to work, and this could only be provided through the agency of a black hole or a decaying cosmic string loop. For example, to move even one microsecond back into the past would require the presence of a space-warping mass equal to one tenth the mass of the sun! And then there's no guarantee the black hole wouldn't swallow you and the space around you. Then, as Gott says, you'd be able to circle around and meet your earlier self, but you wouldn't be able to escape to boast about it. (Physical Review Letters, 6 April; as usual journalists can obtain a copy by contacting AIP Public Information.)

TWO FORMS OF LIQUID WATER MIGHT COEXIST at very low temperatures, possibly shedding light on why water has such unusual properties compared to other liquids. For example, water shrinks when warmed from 0 to 4 degrees Celsius, while most other liquids expand whenever they're heated. At the recent APS March Meeting in Los Angeles, Gene Stanley of Boston University (hes@miranda.bu.edu) and Osamu Mishima of the National Institute for Research in Inorganic Materials in Japan (mishima@nirim.go.jp) presented a new study of supercooled water, subzero-degree H₂O that exists as a liquid because it is particularly pure or under high pressure. (Mishima & Stanley, Nature, 12 March 1998) Working with ice IV, one of the 14 known solid forms of H₂0, they discovered that melting the sample required drastically different pressures below a temperature of 220 K, consistent with the possibility that the supercooled water may undergo a transition between two liquid structures or "phases." In this scenario, one phase is hypothesized to contain large clusterings of water molecules at short distance scales while the other phase would have smaller congregations of the molecules at these scales. For two phases of supercooled water to coexist would suggest the existence of a "critical point" which could consequently influence the properties of water all the way up to room temperatures and pressures. Alternative explanations exist; for example, perhaps water stays in one form but dramatically changes its density and its entropy (amount of disorder) at decreasing temperature and increasing pressure. Still, recent neutron scattering experiments in supercooled water (M.-C.Bellissent-Funel, mcbel@llb.saclay.cea.fr, Europhysics Letters, 15 April 1998) support the two-phase picture for the liquid.