Since H.G. Wells' Time Machine (1895), time travel has been a recurring theme in fiction. Michael Crichton's Timeline is the latest offering, and given this author's track record, we can expect a blockbuster film to follow in a year or so. All his books read like movie scripts, and this one is no exception.
Time travel has also received considerable attention in serious scientific literature, as in Kip Thorne's 1994 bestseller Black Holes & Time Warps and in several articles in Physical Review and other respectable journals. Since spacetime is curved in general relativity, we can imagine a "wormhole" looping back to join the time axis at an earlier time, just like a walk around the block. However, Thorne did not make it seem very promising that humans, or any information-carrying system, would ever be able to travel though such a wormhole, a quantum fluctuations would likely destroy the system. The best he could do was leave the door open slightly by saying that we still lacked a quantum theory of gravity and so anything is still possible.
None of the laws of fundamental physics, classical or quantum, forbid travel back in time. In fact they do not even distinguish backward from forward. Time irreversibility is implied by the second law of thermodynamics, but as Boltzmann showed over a century ago, this is a statistical effect of the large amount of randomness present in the many body systems that constitute macroscopic objects. Nothing prevents a broken glass from reassembling by chance; it is just very unlikely.
On the quantum scale, however, reverse causality actually seems to be taking place. Experimental results depend on future conditions as well as the past. Indeed, many of the so-called paradoxes of quantum mechanics result when people insist on interpreting quantum events in terms of the one-way time of their common experience. When that restriction is relaxed, most of the paradoxes disappear.
Physicists have known for some time that time symmetry provides a ready solution to a great number of the puzzles of quantum mechanics. John Bell, for example, recognized the possibility but rejected it. He, and many others, resisted the time symmetric solution because of another paradox that then raises its head: Time travel would appear to allow you to go back in time and kill your grandfather when he was a child, thereby eliminating the possibility of your existence. This is the famous grandfather paradox. However, it is not commonly known, even among physicists, that the time-travel paradox does not occur for pure quantum states.
Let me explain this metaphorically. Imagine yourself to be the time traveler and your grandfather to be represented by a pure quantum state vector or wave function. Pure quantum states are characterized by their "entanglement" with each other. Thus your grandfather's state is entangled with the states of all the other men his age. Remember, this is a metaphor for pure quantum states, so all these men are indistinguishable--like electrons or photons.
Now picture yourself going back and trying to find your grandfather and kill him. All the men of his age would be indistinguishable and you would not know who to shoot! The best you can do was shoot one at random. To accomplish this, you do not have to be from the future. Thus no causal paradox occurs. You cannot use information from the future to make the hit.
You might try to identify your grandfather among all these identical men. But this would be equivalent to making a measurement, or observation, which reduces (or "collapses") the entangled state. You could of course do this, but again it would not require any information from the future and so nothing paradoxical takes place.
On the hand, if your grandfather is not a pure (entangled) quantum state, then you could recognize and kill him. This is the situation in familiar, macroscopic life; grandfathers and most other objects are composed of many elementary particles that exist in incoherent, mixed states rather than the coherent superpositions that constitute pure quantum systems. Thus the causal paradox seems to rule out the possibility of human time travel, consistent with Thorne's observation that any information will be destroyed traveling through a worm hole.
In the clever Back to the Future films, the young time traveler Marty (played by Michael J. Fox) had to take certain actions to make sure his father became his father. He had to watch over him, protect him from bullies, and make sure that he dated Marty's future mother (who found Marty strangely attractive). The time travel paradox was solved in these films by having parallel universes. Crichton also utilizes this expedient in Timeline, but Doc in Back to the Future II explains it better.
Unless parallel universes exist, time travel appears to be impossible in the classical world of mixed states while remaining logically possible in the quantum world of pure, entangled states. Furthermore, time travel seems to be taking place deep down in the recesses of our atoms and nuclei. With quantum time travel, we can see how a body can be two or more places at once. It simply starts "here and now," goes back in time and then reappears "there and now." This is fundamentally no more absurd than being at the same place at two or more times, like my sitting in front of the computer terminal, day after day, year after year, decade after decade.
This article is based on the author's latest book: Timeless Reality: Symmetry, Simplicity, and Multiple Universes, to be published this Fall by Prometheus Books.