Black Holes and Time Machines

Other theoretical examples of systems that seem to obey the laws of physics but which allow closed loops in time have been proposed. For example, Princeton theorist Richard Gott showed that a time machine could be constructed from two so called cosmic strings - long microscopically thin tubes of hyperdense material, heavy enough to distort space. Gott and his colleague Li-Xin Li also devised a cosmological model even stranger than Godel's in which an entire universe, with a finite life cycle, traces out a loop in time so that its end is also its beginning.

One much-discussed design for a time machine involves a "wormhole": two black holes linked together by a tunnel or "spacewarp". The tunnel could exist only if it were made of a substance that has very large negative pressure (or tension). Theorists speculate that exotic stuff of this kind did exist in the early universe, but even if such material still existed, the mass needed in order to make a wormhole wide enough to be comfortably traversed by a human would be 10,000 times that of the Sun!

Wormhole Travel?

Godel's discovery and its aftermath opened up a debate. Is there a future law of physics, more restrictive than Einstein's equations that rule out such effects? One might call it a "chronology protection law". Or could a time machine in principle exist? Such an artefact plainly still lies in the hypothetical reaches of science fiction, but we can still ask whether the barriers to constructing a time machine are merely technological, or whether there is a fundamental physical law that prohibit them. (To clarify the distinction, most physicists would say that a large spaceship travelling at 99.99 percent of the speed of light is in the first category, but one that travels faster than light is in the second.)

The events on the time loop must close up self-consistently, as in a movie whose last scene recapitulates its first. Paradoxes arise if you come back into the past and undo something that was a precondition of your existence: for instance, murdering your grandmother in her cradle would raise issues of logical consistency, not just of ethics. Time travel makes sense only if some law of nature precludes inconsistency of this kind. The implication that there must be "time police" to constrain our free will might seem paradoxical. But I am convinced by the robust retort of Igor Novikov, a leading physicist who has explored these ideas, that physical laws already constrain our choices: we cannot, for instance, exercise our free will by walking on the ceiling. The prohibition on violating the consistency of a time loop is, in a sense, analogous.