Time Travel If You Must

In my last post, I demonstrated why faster-than-light Travel means having to deal with time travel. You now see why this series is called “… Space and Time …” and not just “… Faster-Than-Light …”. I’d rather have done time travel in a different series, and just focus on how my hero to gets from Earth to Cygnus and save the princess. But, science won’t let me, and now, I have to do some posts dedicated time travel. Let’s start with what the arguments are about time travel.

In Space Travel (Science Fiction Writing Series) by Ben Bova, Bova starts by saying he is not going to talk about faster-than-light travel. He argues, quite understandably, that time travel is fantasy, and he is writing about science fiction. Unfortunately, soon after the physicists, Kip Thorne, Mike Morris, and Ulvi Yurtsever published a paper on closed time-like curves. (Closed time-like curves is what physicist call time travel when they don’t want us regular folk to know they’re talking about time travel.)

When physicist publish a paper about time travel (sorry, I meant closed time-like curves) they’re usually not trying to defend it. They’re saying to other physicists, “Hey, our current theories suggest time travel. What did we get wrong here?”. A lot of science is discovered like this. However, soon after the physicist, Joseph Polchinski, made a convincing argument for the Novikov self-consistency principle. (The writer, Larry Nivens, also deftly described this as the Conservation of Events in The Theory and Practice of Time Travel.) Dr. Polchinski relieved scientists concerns about time travel enough for Dr. Thorne’s paper to be taken seriously.

Dr. Thorne’s paper is another variation of a Warp Drive as I talked about in post 3, but I’ll deal with that in another post where I can go into more detail. For now, I need to focus on the arguments for and against time travel, which honestly is more philosophy than science.

So, why is this such a headache for physicists? Simple, it shakes up science at its core. We don’t think of science as a philosophy because it’s been so successful at giving us things like the internet, vaccines, and microwave ovens. In other words, for most of us, science is about the technology it gives, and as long we keep getting, we’ll tolerate scientist and their silly theories. (I’ve never met a creationist that gave up their microwave oven.) But, it is based on a philosophy, in particular, empiricism.

Empiricism states that we get the truth from experience and our experiences are real (as oppose to “we are all butterfly’s dreaming we are people“). And from that, there are cause and effect relationships we can observe, causality. In the last post, I already showed how this reverse it, effect proceeds cause.

But maybe, that is not as big a deal as it sounds. Quantum mechanics introduces a similar problem. Another tenant of science is the objective observer, that we can make observations without affecting them. The observer effect contradicts that. However, the expected effect is predictable and negligible over enough distance, so scientists have learned to work with it. If time travel occurred with the same predictability, scientists could work with that as well.

The bigger issue is the well-known grandfather paradox, where you go back in time and kill your grandfather. Essentially, you create an effect with no cause and that just spoils everything. maintaining why the Novikov self-consistency principle spares science the headache. It says that if you time traveled, you could not change the past. Dr. Polchinski then calmed everybody with a thought experiment. In the thought experiment, a billiard ball is shot through a wormhole, back in time, and on a collision that knocks it out of the way. He showed that no matter how you set the experiment up the billiard ball COULD always miss. The thought experiment did not prove the self-consistency principle, but it did mean that allowing time travel did not automatically disprove it.

You’ve probably read a bunch of stories using the self-consistency principle. Remembering that communicating back in time is scientifically the same as time travel, the greeks were fond of such stories and called it fatalism. Even the Bible dabbled in time travel. Saint Peter was going to deny Christ three times no matter what.

But what about our free will? What about it? Physicist and chemists don’t worry about free will, and social scientists have to at least pretend it does not exist. (This is known as determinism.) Though in their off-time many scientists are just as uncomfortable with these thoughts as you and I. They’d much rather not talk about it. (Nobody likes determinists, not that they think we have a choice.)

Of course, us creative types don’t have to accept this. We can just let the protagonist kill his grandfather. No causality, who cares. Well, then you just made reality unreal. You have to tread carefully here. You have a deus ex machina waiting to happen.

Another way to deal with this is with parallel universes. Either the hero creates another universe when the paradox occurs or he travels to a parallel universe from the beginning. This works really great if you just want the hero to fight dinosaurs. Most of the support for this kind of time travel comes from the many-worlds interpretation of quantum physics (also known as the Everett interpretation). When done faithfully this idea has its own philosophical problems.

We can think of this as having three choices when us creative types use time travel. Either self-consistent time travel (you can’t kill your grandfather), paradoxical time travel (you can kill your grandfather), or parallel time travel (you both kill and don’t kill your grandfather).

Much of this discussion is reminiscent of the same weirdness we see in quantum mechanics. Perhaps that is where the resolution lies, and there has been recent experimentation along these lines.

Be warned, time travel stories are easy to do badly. Once you start with it, whatever choice you make, you end up taking positions on free will and metaphysics. And yes, some of us notice when you ignore it. Worse, since stories are about people, you will be interweaving physics with history. In other words, history will have physical laws. Think about that for a bit.

There’s a whole lot of strangeness, more than can go into with one post. Which I again must do. However, I’ll leave you with this. Larry Niven pointed out that a time machine could not exist when paradoxes are possible. Many would not want a time machine to exist and would go back in time to stop it. Eventually, someone would succeed, most likely by killing the inventor. So, if you know how to build a time machine, you might want to keep it to yourself.