February 20, 2010

Contradictory vs preservative time travel paradoxes

Time travel paradoxes are a very exciting phenomenon. Regardless of whether time traveling is possible or not, one can imagine time traveling and the paradoxes that occur inevitably. In order to have a base for future posts, I want to explain two kinds of time travel paradoxes I am distinguishing. Both of them can only appear in backwards time travels, i.e. when traveling back in time. Furthermore, the travel is ought to be a travel into the past of the own time line, i.e. it is not a travel to an alternative universe.

Self-contradictory time travel paradoxes

A self-contradictory time travel paradox is an event that prevents itself. An example is a time traveler who travels back in time and prevents the construction of his time machine. Without the time machine he cannot travel back and prevent the construction of his time machine. Sounds contradictory? Yes it is, hence the name. A more classic example would be killing ones own ancestor in the past.

Such paradoxes are paradox because the effect of a cause is preventing that very same cause. Obviously, such paradoxes cannot occur in nature, but this does not prevent me from writing about them.

In literature, self-contradictory time travel paradoxes are sometimes resolved by defining time travels to be travels to an alternative universe. Actions in that alternative universe do not have an effect to the original universe of the time traveler. In the 2002 movie adaption of H.G.Wells' novel The Time Machine the protagonist, a time traveler, is unable to prevent the murder of his fiancé because that would lead to a self-contradictory paradox. In that movie, time travels are indeed travels in the own universe, but one cannot change the past in a way that it would prevent itself.

Self-preservative time travel paradoxes

Self-preservative time travel paradoxes are harder to explain. Unlike the other kind, this paradox is not self-contradicting. So why is it a paradox then? Because it forms a circular dependency in its cause-and-effect chain. This is contradicting to our observation that every event in the world has a cause that happened before the event. An example is a time traveler who travels back in time to enable the construction of his time machine, e.g. by giving the construction plans to himself. A classic chicken-and-egg problem.

There is also an analogy to the ancestor example above, but it is to perverse to explain here. A more moderated version of that analogy is used in the movie Terminator I. In that movie John Connor sends his own father back in time to meet John's mother and conceive John.

The television series Lost was full of self-preserving time travel paradoxes until the end of season 5. In the series, this was refered to as "what happened, happened". [Spoiler alert: do not read further if you are interested in Lost and have not yet seen the first episode of season 6]. Beginning with the last episode of season 5, this seems to have changed. A self-contradictory paradox occured and an alternative time line spawned. This seems to be contradicting with the statement "what happened, happened" postulated until then. In a future post I will explain why this does not have to be contradictory in Lost.


  1. If time travelling to the past would be allowed, either timelines may split or results of all actions lead to the same environment. Since the latter does imply fate I can't beleave that. Since the first can't be proved there's no need to investigate.
    Anyway, time-travelling to the past is only based on entering an undefined state caused by a devision by zero and we all hope The Big Programmer unit tested this before, don't we? :)

  2. Whether we will ever be able to prove one way or another is not important here. The topic of time travel paradoxes yields great stories and discussions anyway.

    I believe that self-preserving time travels could indeed be possible. If all actions of the time traveler are self-preserving actions, then there is no need to split time lines. I don't see why such situations can only be explained using fate. What happened, may have happened coincidentally. It will always happen in the same way, because the preconditions are the same.

    Although I am not a big fan of self-contradicting paradoxes, I believe that they, too, may be possible in certain situations. More about that in a later post. ;-)