Number 62 (Story #1), 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.)