Writing my final Gaucho’s Guide column fills me with musings on the passage of time. As such, it’s only fitting that today we talk about time travel.

Time travel logic is notoriously difficult to get right, and in fact, most writers don’t even try. The movie “Looper,” for instance, has a scene in which Bruce Willis tells Joseph Gordon-Levitt (and the audience) not to worry about the plentiful logical errors that the plot contains.

Most writers prefer to use time travel as a magical plot-resolving device that creates and fixes problems as the story demands, regardless of internal consistency. I rarely see a time travel movie that completely obeys its own logic.

Movies typically take one of three approaches to time travel:

1. You can go back in time, but you can’t change anything (“Harry Potter and the Prisoner of Askaban,” “The Terminator”). This version of time travel avoids the logical paradoxes created by changing the past by allowing characters only to go back and cause events that already happened. In a sense, the universe is deterministic and every event is predestined. Disturbing philosophical implications aside, this one is the most logically sound of the three, though the field of quantum physics isn’t quite sure on pure determinism just yet.

2. You can change history, but not in a way that creates a paradox. In “The Time Machine,” the protagonist could go back and change how his girlfriend died, but not the fact that she died, which was what motivated him to go back in the first place. This presents the universe as both changing and deterministic in different ways. It seems to make sense, but it does raise the question of just how much change won’t affect the future.

3. Anything goes (“Back to the Future,” “Terminator 2,” “Looper,” “Star Trek”). In these movies, it doesn’t matter whether something is self-contradictory or not — anything that’s fun to watch happens. The only issue is that these movies tend to break their own rules constantly; in “Back to the Future Part II,” for instance, Marty is cautioned against creating a paradox for fear that it would destroy the universe, even though most of what he does throughout the trilogy causes paradoxes.

Of course, all of this is pure fantasy. What are the real rules of time travel? Well, there are several ways a very advanced civilization might make it happen.

One symptom of space and time being constructed of the same material is that gravity, the warping of space, affects time. Any gravity well slows the effects of time the closer one gets to its center. This means that the center of a black hole, which has extreme gravity, is almost frozen in time relative to the world outside it.

The effects of this kind of distortion can be measured even on our relatively miniscule Earth; there is a practically negligible, but nonetheless measurable, difference between the speed at which a clock ticks when placed on the ground and the speed at which it ticks on a table several feet above the ground.

Thus, if a civilization could generate a powerful enough gravity — well, a person could approach its center (assuming measures were taken to prevent getting crushed inward by the gravity) and experience a slowing of time while the world outside carried on as normal, facilitating a form of rapid travel to the future.

As for going the other way, there are no viable theories as of yet.

One hypothesis posits the existence of particles, called tachyons, that move so rapidly that they go beyond instantaneous travel and actually move backward in time. If a message could be sent via tachyon pulses, it would arrive at its destination before it was sent. Unfortunately for those of us who would love to call ourselves in the past and prevent a mistake, the existence of tachyons is unsupported by evidence, and if they did exist, nobody is exactly sure how they would affect the laws of causality as we know them.

As for now, we can continue to live out our time-traveling fantasies in the contradictory and oh-so-entertaining inconsistencies of our favorite blockbusters.




A version of this article appeared on page 10 of May 21st’s print edition of the Daily Nexus.