Time Travel

Insofar as one of the themes of this issue is travel, I thought I’d write a column about time travel


By Steve Humphrey


We are all quite familiar with stories about time travel. Doctor Who, 12 Monkeys, Back to the Future, The Time Machine, et al. So far as I know, the very first person to even think about traveling through time was H. G. Wells, in his story written in 1895. This kind of travel involves some machine that enables people to travel from one public time to another, in a short amount of personal time. These all seem to depend upon the Newtonian notion of Absolute Time, which exists independently of anything else and is like space in that you can move through it. I have argued in previous columns that this sort of time does not exist, but time travel stories still seem plausible.

But is it possible? That naïve question is unanswerable, as asked. Before we can even begin to address it, we have to ask several preliminary questions, having to do what, exactly, we mean by ‘time travel’ and then figuring out what ‘possible’ means. 

Time travel must involve an abrupt change of temporal position, either into the future or into the past. Simply living in the world and being at one time and then another is not what we’re interested in. Nor is sleeping. It has been said that a bed is a time machine. We go to sleep at one time and wake up at another without being conscious of eight hours passing. No, time travel must involve something more significant than this.

According to the special theory of relativity, travel into the future is quite possible. If I take off in a high-speed rocket ship and return, I will have aged less than those I left behind, so I will have traveled into their future. But this isn’t quite what we usually mean by time travel. Just because I age more slowly than others doesn’t mean I have traveled through time. How about a journey into the past? Is that possible? There are many senses of ‘possible,’ but let us look at ‘logically possible.’ Arguments to show that time travel is not logically possible usually involve ‘self-defeating loops.’ 

The Grandfather Paradox is the most famous of these arguments. It attempts to show that time travel is logically impossible by showing that the assumption of time travel leads to a contradiction. The argument goes, suppose time travel into the past was possible. Then the time traveler could go back to when her grandfather was a young man and kill him, thus preventing him from having children, and thus avoiding the birth of the time traveler, thus preventing the time traveler from killing granddad. The contradiction is that grandfather is both killed and not killed, thus implying that the initial assumption is false.

How do we avoid this paradox? Most attempts focus on the second step. Could the time traveler kill granddad? Prima facie, there is no reason that she couldn’t. She could have means, motive and opportunity. There is no reason to think that she couldn’t kill any number of other people. Why not granddad? But her presence in her grandfather’s past entails that she didn’t kill him. Why not? This is the source of the tension in this argument. How are we to explain her failure, given her ability? 

A series of misfortunes might befall the would-be grand-patricidal time traveler. Though she tries and tries, something always prevents her from succeeding. The gun jams, the dynamite gets wet, she loses her nerve, etc. This might solve the problem but at the cost of introducing a new mystery. Why are all these things happening? Each one, individually, might be explicable, but how do you explain the continued failures? One way around this might be to say that there is a law preventing self-defeating loops in a world where time travel occurs. Such a law has been referred to as ‘Novikov’s self-consistency principle’. Even though no local laws prevent murder, one might argue that global laws do. That is, if we look at the local environment immediately surrounding the attempts upon granddad’s life, we can find no reason for her failure to accomplish her mission. But if we look at the universe, as a whole, including past and future, then we see that there are compelling reasons why she doesn’t.

As an aside, the philosopher David Lewis points out an ambiguity in the word ‘can,’ as in ‘she can kill her grandfather.’ A gorilla cannot speak Finnish. It doesn’t have the necessary speech apparatus that humans do, but unlike a gorilla, I do have that apparatus, so I can speak Finnish. But don’t take me to Helsinki as a translator because I can’t speak Finnish. So, in what sense ‘can’ she kill her grandfather?

Another potential paradox arises in the so-called discovery or ontological paradox cases. Someone finds instructions for building a time machine in a dusty book in a forgotten drawer in the attic. He uses the instructions to build the time machine and then, after having several adventures, takes them back to see where they came from. He finds they are not in the drawer, but hears his earlier self coming up the stairs, so he quickly places the instructions into the drawer and departs, leaving his earlier self to find them. This is a consistent loop, where a previous step explains each step, but the entire loop as a whole seems like it needs explanation. Where did the information on how to build a time machine come from in the first place? And if there is no explanation, is that important? Several features of the universe lack explanation, and that doesn’t seem to bother us—Big Bang, inflation, dark energy, quantum entanglement, etc.

As I hope you can see, this is a very tricky issue. I could write 10,000 words about it, but, unfortunately, I only get 1,000. I’m going to have to talk to my publisher about that.

Steve Humphrey has a Ph.D. in the history and philosophy of science, with a specialty in the philosophy of physics.
Questions? Comments? Suggestions? 
Email him at Steve@thevoicelouisville.com