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grounds”?
    Wormholes are after all merely one example of time machines that have been proposed in the
     context of general relativity. Given our previous discussion about the nature of the
     theory, it is perhaps not so surprising that time travel becomes a possibility. Let's
     recall the heuristic description of Einstein's equations which I gave earlier:
    The left-hand side of this equation fixes the geometry of spacetime. The right-hand side
     fixes the matter and energy distribution. Generally we would ask: For a given distribution
     of matter and energy, what will be the resulting curvature of space? But we can also work
     backward: For any given geometry of space, including one with “closed timelike curves”that
     is, the “causality loops,” which allow you to return to where you began in space and time,
     like the loop the
    
    
     Enterprise
    
    
     was caught in before, during, and after crashing into the
    
    
     Boze-man
    
    
     Einstein's equations tell you exactly what distribution of matter and energy must be
     present. So in principle you can design any kind of time-travel universe you want;
     Einstein's equations will tell you what matter and energy distribution is necessary. The
     key question then simply becomes: Is such a matter and energy distribution physically
     possible?
    We have already seen how this question arises in the context of wormholes. Stable
     wormholes require exotic matter with negative energy. Kurt Gšdel's time-machine solution
     in genera! relativity involves a universe with constant uniform energy density and zero
     pressure which spins but does not expand. More recently, a proposed time machine involving
     “cosmic strings” was shown to require a negative-energy configuration. In fact, it was
     recently proved that any configuration of matter in general relativity which might allow
     time travel must involve
    exotic types of matter with negative energy as viewed by at least one observer.
    It is interesting that almost all the episodes in Star Trek involving time travel or
     temporal distortions also involve some catastrophic form of energy release, usually
     associated with a warp core breach. For example, the temporal causality loop in which the
    
    
     Enterprise
    
    
     was trapped resulted only after (although the concepts of “before” and “after” lose their
     meaning in a causality loop) a collision with the
    
    
     Bozeman,
    
    
     which caused the warp core to breach and thereby caused the destruction of the
    
    
     Enterprise,
    
    
     a series of events that kept repeating over and over, until finally in one cycle the crew
     managed to avoid the collision. The momentary freezing of time aboard the
    
    
     Enterprise,
    
    
     discovered by Picard, Data, Troi, and LaForge in the episode “Timescape,” also appears to
     have been produced by a nascent warp core breach combined with a failure of the engine
     core aboard a nearby Romulan vessel. In “Time Squared,” a vast “energy vortex” propelled
     Picard back in time. In the original example of Star Trek time travel, “The Naked Time,”
     the
    
    
     Enterprise
    
    
     was thrown back three days following a warp core implosion. And the mammoth spacetime
     distortion in the final episode of
    
    
     The Next Generation,
    
    
     which travels backward in time and threatens to engulf the entire universe, was caused by
     the simultaneous explosion of three different temporal versions of the
    
    
     Enterprise,
    
    
     which converged at the same point in space.
    So, time travel in the real universe, as in the Star Trek universe, seems to hinge on the
     possibility of exotic configurations of matter. Could some sufficiently advanced alien
     civilization construct a stable wormhole? Or can we characterize
    
    
     all
    
    
     mass distributions that might lead to time travel and then exclude them, as a set, “on
     physical grounds,” as Einstein might have