Number 62, January 10, 1992 by Phillip F. Schewe and Ben Stein A TIME MACHINE may be difficult to build from cosmic strings. J. Richard Gott of Princeton proposed (Phys. Rev. Lett., 4 March 1991) that two parallel cosmic strings, passing each other in opposite directions, would so warp the fabric of spacetime as to create closed timelike curves (CTC's), trajectories along which physical particles, although moving at less-than-light speeds, could go backwards in time. Now, a pair of articles in the 20 January 1992 issue of Physical Review Letters raise reservations to Gott's scheme. In one article, Sean M. Carroll, Edward Farhi (617-253-4871), and Alan H. Guth of MIT and Harvard assert that in an open universe, one in which expansion continues indefinitely, there is not enough mass to build such a time machine. They note that it may be possible if the universe were closed. In the other paper, Stanley Deser (Brandeis), Roman W. Jackiw (MIT), and Gerard 't Hooft (Utrecht) argue that CTC's cannot be generated by physical, timelike sources. (NOTE: Advance information about articles accepted for publication, but not yet published, in Physical Review Letters, is under no embargo constraint; reporters may use the information as they wish.) MORE PLANETS AROUND PULSARS have been found by radio astronomers. Alexander Wolszczan of the Arecibo Observatory (809-878-2612) in Puerto Rico and Dale Frail of the National Radio Astronomy Observatory in New Mexico report evidence for two and possibly three planets around the pulsar PSR 1257+12. The two more substantially inferred planets have orbits about the size of Mercury's around our Sun, whereas the third planet, if it exists, has an orbit about the size of Earth's. Support for the planet hypothesis comes in the form of the measured delay in the expected arrival of radio waves from PSR 1257+12. This was the case also with the report of the first such pulsar/planet system, PSR 1829-10, announced last summer by astronomers in Britain. (Nature, 9 January 1992.) FEMTOCHEMISTRY is the study of chemical reactions with a time resolution of femtoseconds. Ahmed H. Zewail and his colleagues at Caltech use two lasers to both excite and observe the reaction of xenon atoms with iodine molecules. The timing of the laser pulses can be adjusted to modulate the yield of reaction products. (Nature, 2 Jan. 1992.) INFRARED ANTENNAS are the result of adapting existing microwave technology to shorter wavelengths. Donald G. McDonald and his co-workers at NIST in Boulder have developed a device which consists of a tiny spiral-shaped film of gold deposited on a niobium surface; this microantenna is sensitive to radiation in the 3-30 micron wavelength range (Applied Physics Letters, 16 Dec. 1991). The use of antennas, rather than more conventional infrared detectors, means that the size of the detector can be much smaller than the wavelength of the incident infrared radiation. This in turn would facilitate infrared imaging with better resolution. (Science News, 4 Jan. 1992.)