Time Machines

Last Book Read: Time Machines, Paul J. Nahin

Despite being received with heavy criticism, the 2002 remake of the film The Time Machine seemed to make poignant points that the simpler, earlier versions missed. The time traveler finally comes to learn the answer to his quest: he cannot go back in time and change the past to save his girlfriend, because if she had lived he never would have built the time machine to go back. But, the film holds out hope - we can make choices and change the future.

Nahin's Time Machine is the most comprehensive book I have found on the subject. It covers the topic from the fields of physics, philosophy (metaphysics) and literature. Despite being a mammoth undertaking to cover the scope of time travel, the book does suffer from a singular viewpoint. Nahin's theory is straight forward: time travel is possible and there are no paradoxes involved because the past cannot be changed - whatever will happen in the course of time travel has already happened in our past -if it didn't then it won't. Causality does not require temporal order, backward causality is built in to the nature of time travel. All that is required is logical consistency. I accept his arguments as the most coherent and likely explanation of time travel (in a single one-dimensional timeline), but he finds major faults with any ideas that don't conform to this one. He is overly critical of philosophers and their "thought experiments" of time travel because they don't always agree with the known laws of physics. He doesn't seem to notice that physicists sometimes contemplate what would happen to the universe if natural laws were different, in their own "though experiments." Secondly, he heavily criticizes most fictional accounts of time travel for using "illogical" scenarios where the past can be changed. What he fails to realize though, its that even if its true that past can't be changed, these works present us with the idea "what if" it could. I think its far more likely that in fact time travel is not possible and then it really doesn't matter what form the paradoxes would take, it would all be a futile thought experiment.

The basis of his cosmology is the four-dimensional Minkowski spacetime. The universe consists of the three dimensions of space and extends off into the fourth dimension of time. Just as any place in the universe exists "somewhere," any time in the universe exists "sometime," it is already present within spacetime. If we can travel in time, say into the future by steping though a time portal, that means the future must exist "now." And, if we can step in and go to the past right now, that means the past exists "now." So the universe is one solid, frozen "block" Everything that has ever happened or ever will happen already has. Apparently from the very first instant of existence. This certainly presents a fatalistic viewpoint, nothing can ever be different from what it is. Nahin argues that this is fatalistic, not deterministic. I'm not sure his distinctions really matter. I tend to think this view is overstated. Just as the universe is expanding in space and there are places which will eventually exists which do not at the present state, I think time is expanding into the future and there are "times" which do not yet exist. Perhaps the past is already frozen into spacetime, but it doesn't seem clear to me that the future is. A trip to the future while seeming instant to the traveler may in fact take the full length in universal time while the traveler is suspended in some kind of fifth-dimensional no-where.

Along the way Nahin touches on many alternative theories while dismissing them. One such theory is that time is not one dimension of spacetime, but two or more. In this case we can go back to a familiar starting point, but take a different direction while proceeding in a parallel direction. A similar idea is the "many worlds hypothesis". A trip through time would result in the universe splitting into two worlds, and the one we left would still continue on but the one we enter can be changed. This seems like the most compelling explanation for many of the time travel science-fiction stories. I agree that what has already happened cannot be undone, in fact God cannot even make that the case. So, in a show like Seven Days, when the chrononaut goes back to prevent a nuclear war that destroyed the planet, it would seem that he is in a parallel world and is able to change the outcome for them, but the people he left behind are still in their predicament, unable to change it. Star Trek (notably absent from Nahin's thorough readings) seems to vacillate between the two ideas. Certainly there are many parallel universes such as the ones seen in "Mirror, Mirror" and "All Good Things", as well as a time where they encountered a nearly infinite number of Enterprises. But, at other points such, as Star Trek IV, they seemed to imply that the past was changeable, unless they advocated the same view that what they were going to do had in fact been what had already happened. I won't even try to decipher what happened in the last few series, where time travel became a more common theme, yet utterly inexplicable and not nearly as interesting as The Next Generation.

This is the best overall, comprehensive explanation of time travel, although its really about the nature of time more than machines. The main problem with the book is simply one of organization. The 400+ pages are divided into 4 chapters, making it very difficult to focus on one aspect at a time. Secondly, Nahin attempts to explain things while it seems he may not have a complete understading of them, or at least he is unable to articulate and communicate a complete understanding of them. While sometimes obtuse, it does present a good framework for studying the history of time travel concepts.