Clearly I’m in the minority in this opinion. Time travel stories continue to delight, generation after generation. Why should this be, if my objections are so irrefutable? Well, one reason is some people just like a good story and don’t care how unbelievable it is. As I pointed out in my last blog, Superman’s “impossible” ability to fly doesn’t deter my enjoyment of his adventures (although I prefer Batman). However, there's more to it than that.
The Objections to Time Travel
I raised four basic objections to time travel as portrayed in fiction: Physical: As one approaches the speed of light, required for going back in time, mass becomes infinite, effectively destroying the time traveler; Metaphysical: Specifically the Grandfather Paradox (you go back in time and kill your grandfather, therefore you are never born and cannot go back in time to kill your grandfather, etc.) and the Ontological Paradox (Future Craig hands me an envelope which he tells me to hold on to for five years. Five years later I go back in time and hand myself the envelope. Where did the envelope come from?). Logical: Specifically, the Tourist Objection; If time travel is possible, how come we don’t know about it yet? We should be being visited by an effectively infinite number of time travelers all the time, where are they? And Aesthetic: How can any story have internal consistency if someone can always go back in time and change what happened?
I mentioned that there are rebuttals to all of these objections (I excepted the aesthetic but I will stipulate that for most people, if the other objections are satisfied the aesthetic one should be as well, even if I personally am not comfortable with it), and hence, some time travel stories that work. So here goes:
Let’s start by addressing the Metaphysical objections. The famous grandfather paradox and the ontological paradox. There are two main ways that philosophers approach the problem of time travel paradoxes: The Many Worlds Theory and the Novikov Self-Consistency Principle.
The Novikov Self-Consistency Principle
I do not find this theory particularly compelling. There is nothing in the universe to suggest that it actively attempts to sort out paradoxes at a macro level. If you shoot someone, they will die, and to suggest that the universe somehow “knows” to protect them seems to be so fanciful as to make the theory meaningless. This also does not explain the ontological paradox.
The Many Worlds Theory
The many worlds theory, popularized in D.C. Comics with their “multiverse,” and based on the Heisenberg Uncertainty Principle which says that the states of certain particles at the quantum level are not fixed until observed, and that until they are observed they exist in multiple states at once, is much more compelling. It is referenced in Back to the Future and used in comic book literature and other works of fiction, including the television show Sliders and the Michael Crichton book “Timeline.”
The idea of the multiverse is that the universe as we know it is just one of many possible universes that exist in different dimensions, and that each choice we make represents the universe that we fix in time out of the infinite universes that are possible. As it pertains to time travel, the theory goes that when we go back in time and change something, we create an alternate timeline. Our original timeline, the one that spawned us, continues on into infinity while we exist in the new, altered timeline.
This idea seems to work pretty well. It addresses the grandfather paradox: when you kill your grandfather, you create a universe where you never existed, however you, the killer, came from a parallel universe where you did exist. It addresses the ontological paradox too. In Terminator, Kyle Reese is sent back in time to protect Sarah Connor so that John Connor will be born. He ends up becoming John Connor’s father. According to the theory, there was a “prime” universe where John Connor had some other father. In that universe, Kyle Reese was sent back in time, creating an alternate universe where Kyle Reese is John Connor’s father, as we see in the movie.
How this is handled can get complicated. In Back to the Future, Marty and his siblings start to disappear when it seems that Marty may have prevented his parents from ever falling in love. If this is an alternate universe, that shouldn’t be an issue. In “Timeline” Crichton addresses this problem by suggesting that events in an alternate timeline have a “ripple effect.” That over the course of time, minor changes happen in the original timeline so that it will “catch up,” and become consistent with that timeline. In other words, events in one timeline can have an effect on others.
I feel that the many worlds theory is pretty effective in addressing the metaphysical objection, and as I stated in my first post, the metaphysical objection doesn't trouble me that much. The many worlds theory does also address the logical objection to some extent. If travelers going back in time are always creating alternate timelines, we shouldn’t expect to ever meet one in our “prime” universe. However I don’t find this entirely compelling. Somehow it seems that we still might receive a visitor from an alternate future, so that our prime timeline is their alternate one, or that the ripple effect should still produce premature time travel in our universe. I feel that there are stronger rebuttals to the “Time Tourist” objection, which is tied in with the rebuttal to the physical objection (hint: Wormholes), and I’ll discuss those in Part II.