Is time travel really plausible? Any discussion of time travel should begin with the observation that Kurt Gödel found a solution to the equations of Einstein's General Theory of Relativity that permits any observer to travel into either the past or the future. While this confirms that time travel is possible at least in principle, Gödel's solution involves an intrinsic rotation of space itself, and we know from direct observation that our universe has no such thing. Gödel's solution is important and quite interesting, but it doesn't apply to our universe.
Much has been made of the so-called "grandfather paradox." If you travel back in time and kill your own grandfather-- before he had an opportunity to father one of your natural parents-- then you would not exist and would not be able to travel back into the past to kill your grandfather (or anyone else.) Some have suggested that the laws of physics must be such as to prevent any such possibility. Alec begets Bill; Bill begets Charles; Charles goes back in time and kills Alec-- and now the link from Alec-to-Bill-to-Charles is broken. But if Charles is really capable of going back in time, what physical law could possibly prevent him from killing his own grandfather? Physical law might prevent travel backward through time, perhaps, or it might prevent interaction with people or events in the past. But once the door is open to traveling back in time and interacting with the past it is difficult to imagine what kind of physical law could prevent a time traveler from killing his own grandfather. Imagine that the time traveler has a gun and is pointing it at his own grandfather. Would physical law prevent the gun from firing? Would it prevent him from pulling the trigger? Would it prevent the bullet from reaching its target? It just doesn't make sense.
So let's go back to the beginning. Is it really possible for someone to travel back in time-- assuming as we may that our universe does not satisfy the conditions of Gödel's solution? Well, quantum mechanics allows one to think of an electron traveling forward in time as equivalent to a positron traveling backwards in time. So one might be tempted to say that quantum mechanics is agnostic on this issue-- but there are complications. For this idea to be applicable to a macroscopic body such as a person, one would have to convert the entire object into an antimatter image of itself-- in an instant-- without letting it come in contact with normal matter; and then would have to convert it back to normal matter when it had reached its destination. There is no known way to do that. More than that, there isn't even a hypothetical way to do it-- at least, not within the realm of known physics.
In the Special Theory of Relativity, any object traveling faster than the speed of light would appear to be moving backward in time to an independent stationary observer. So all that is necessary to travel backwards in time is to travel faster than the speed of light. But that, according to the same Special Theory of Relativity, is physically impossible as it would require an infinite amount of energy to accelerate an object to the speed of light.
Despite these constraints there is a small possibility for physical objects to travel backwards in time. Quantum mechanics allows objects to transition to states that would be forbidden by classical physics through a process known as "tunneling." Relativity says that a particle can only travel faster than light through the investment of infinite energy. That is precisely the sort of barrier that quantum tunneling is capable of violating. It therefore seems possible that particles-- and even entire macroscopic bodies-- may be able to travel faster than light through quantum tunneling and to thereby travel backwards in time.
So one could imagine a quantum time traveler accelerating to a speed very close to that of light and then invoking a quantum mechanical trick-- yet to be devised-- that would permit him to tunnel through the speed limit of Special Relativity, rather like a jet smashing through the sound "barrier." Physically this seems at least plausible. But from a practical point of view no one has yet demonstrated that this slim possibility could be applied to a delicate macroscopic body like a living person without ripping it to shreds.
Physics does, however, allow for travel into the distant future. One need only travel at a speed very close to the speed of light. Relativity says that the clock of a moving traveler slows down as compared to that of a stationary observer. So by simply traveling very fast one can travel into the future of observers who are at rest. For example, a traveler who circumnavigates the galaxy's habitable region at 99.99999999999999% the speed of light would return to his point of origin in about 170,000 years but would only have aged about 24 years. So he would in effect have traveled 170,000 years into the future at the expense of a mere 24 years of life. Unfortunately our time traveler would not be able to return to his own time.
And that's the real problem of time travel. Yes, it makes a great story element. You can send your hero back to the library of Alexandria to retrieve a scroll written by Archimedes, and then send him forward in time to save the Earth from alien invaders whose approach only the ancient Greeks could have anticipated (thanks, no doubt, to their mastery of the high art of astrology.) But there is no known physical process by which a person could travel back in time without being torn to pieces; and the forward-only option comes only at the expense of immense quantities of energy. It therefore has little or no practical possibility, based on our present understanding of the laws of physics. Contemplation of the possibilities of time travel can be provocative and interesting, but there is little possibility that humans will ever experience it.
Copyright (c) 2012 by David S. Moore. All rights reserved.