Ferdinand F. Liu

Time travel is the concept of moving backwards and/or forwards to different points in time. Some interpretations of time travel suggest the possibility of travel between parallel realities or universes. Although time travel has been a common plot device in fiction since the 19th century, and one-way travel into the future is arguably possible given the phenomenon of time dilation in the theory of relativity, it is currently unknown whether the laws of physics would allow backwards time travel. Any technological device, whether fictional or hypothetical, that is used to achieve two-way time travel is known as a time machine.

The idea of traveling across time  has long been a mainstay of science fiction and fantasy, including such recently popular films as Back to the Future and its sequels and the television series Quantum Leap and many chapter of Star Trek series. From the physicist’s point of view, however, time travel is effectively ruled out by the principle of causality, which plausibly argues that an effect cannot occur before its cause. Nonetheless, there have been some serious attempts by physicists to explore, within the framework of Einstein’s relativity theories, under what conditions time travel might possibly occur.

Some theories, most notably special and general relativity, suggest that suitable geometries of space-time universe, or specific types of motion in space, might allow time travel into the past and future if these geometries or motions are possible.  In technical papers physicists generally avoid the commonplace language of “moving” or “traveling” through time (’movement’ normally refers only to a change in spatial position as the time coordinate is varied), and instead discuss the possibility of closed time-like curves, which are worldlines that form closed loops in space-time, allowing objects to return to their own past. There are known to be solutions to the equations of general relativity that describe space-times which contain closed time-like curves, but the physical plausibility of these solutions is uncertain.

Some scientist take for granted that if one were to move away from the Earth at relativistic velocities and return, more time would have passed on Earth than for the traveler, so in this sense it is accepted that relativity allows “travel into the future” (although according to relativity there is no single objective answer to how much time has ‘really’ passed between the departure and the return).

In 1991, for example, J. Richard Gott of Princeton University published a speculative model involving objects called cosmic strings. There is no experimental evidence that cosmic strings actually exist, but they are plausible ingredients of some astrophysical theories. Gott’s model requires two cosmic strings, each infinitely long and each moving at nearly the speed of light past one another. The key characteristic of cosmic strings is that they are unimaginably dense, cramming 40 million billion tons into each inch of string. In relativity theory large masses are represented as distortions in space-time, the usual analogy being a heavy ball resting on a rubber sheet, where the ball produces a depression in the sheet. Some kinds of distortions allow one to effectively go faster than the speed of light, which is forbidden in relativity theory but is also the key to time travel. Gott showed that by looping around the pair of cosmic strings one could return to the starting point before one left — a description of time travel.

Predictably, Gott’s idea stimulated counterarguments. Many in the scientific community believe that backwards time travel is highly unlikely. Any theory which would allow time travel would require that issues of causality be resolved. For example, what if one were to go back in time and kill one’s own grandfather before one’s father was conceived? This famous hypothetical problem is known as ‘Grandfather paradox’.

The most well-known defender of orthodoxy has been the celebrated theorist Stephen Hawking of Cambridge University in England, the author of the best-seller A Brief History of Time and the subject of a 1992 film of the same name. Hawking argued for a ‘chronology protection principle’ that prevents time travel. In the case of Gott’s model the very act of looping around the pair of cosmic strings would result in an energy buildup that would disrupt the process.

In any case, whether time travel is or is not theoretically possible (actually constructing a time machine is another matter), physicists view such speculation as a helpful tool in refining their understanding of space and time.

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