Settle in for another episode of the critically acclaimed (OK, emphasis on critical) Skeptical Science Theater 3000!
Will we ever invent it? What consequences might it have for the basic framework of cause and effect, the scientific method as we know it, and life itself?
Today’s topic: time travel!
Mechanical Engineer Husband (MEH): That was a bad joke. Really bad.
Me: Look, I don’t need your fictional heckling. I get it enough in real life.
MEH: Fine. So it’s time travel today, huh? I don’t see what the big deal is. I mean, I’ve been time traveling for years!
Me: Oh no.
MEH: At the leisurely rate of—
MEH & Me (unison): —one second per second.
MEH: Ah, you’ve heard that one before. Seriously, though, time travel?
This is kind of different from the weird physics stuff I usually scoff at. I mean, you don’t have to convince me that time travel would make a huge impact on human life. The only questions are whether it’s possible, and whether the consequences are too terrifying or unethical to justify using it if we knew how.
It seems almost meaningless to talk scientifically about the consequences without even knowing if time travel can be done or how it would work, so how about we skip the part where we dither about whether going to the past would mess up our own timeline or take us to a parallel universe or complete a deterministic framework where we caused historical events in the first place? There’s plenty of fiction exploring all those options.
Me: Yeah, let’s not get into that, it’d take up too much time. …What?
MEH: I’ll ignore that. All right, so fictional time travel is heavy on the philosophy and ethics and light on actual science or well-defined rules, which is totally understandable. It does make me wonder how serious a possibility it is in scientific terms, though, since it pretty much seems like fantastic wish-fulfillment. Philosophers are one thing, but why would physicists today actually think it might be possible?
Me: Well, originally the theoretical possibility of time travel had a lot to do with the concept of symmetry in particles. Symmetries in charge, parity, and time mean that we expect to see the laws of physics work the same way on particles if the positive or negative signs for their charges, physical coordinates, or time coordinate are reversed. That means mirroring a particle or actually reversing the direction of time itself, which might indicate that time travel isn’t forbidden by the laws of physics.
However, CP (charge-parity) violations have been recorded, with one of the latest cases being the discovery of the pear-shaped Ba-144 nucleus. This development could make things go, well, pear-shaped for our hopes of symmetry in time as well as space, since the symmetries are theoretically linked. Many physicists don’t see the violations we’ve found so far as strong enough evidence to discount overall symmetry, but they can’t be ignored.
MEH: All right, so CPT symmetry may not be a completely reliable reason to hope for time travel. What else do physicists have?
Me: One of the things you’ll see people getting excited about online is totally non-crackpot physicists talking about time travel being totally possible…
MEH: Oh really?
Me: …in the forward direction. This should be possible at a faster rate than one second per second, thanks to the time dilation we expect from special relativity.
It’s what Einstein introduced as the “twin paradox“—one twin hops onto a really fast spaceship and goes on a trip at relativistic speeds approaching the speed of light, and when she comes back, she’s experienced much less time than her twin.
(This is called a paradox because a partial understanding of time dilation leads to the conclusion that both twins see time moving more slowly for the other one, so they should both age the same amount. Some explanations of why this isn’t the case focus on the acceleration of the twin who turns around to come back, but it can be explained entirely by the relativistic Doppler shift.)
MEH: OK, I see how you could use this for accelerated forward time travel pretty straightforwardly, aside from, you know, the trivial challenges of traveling at relativistic speeds. Head out and experience a fun little three-hour tour, and come back in a century or so!
Me: Exactly. Unfortunately, this method has the disadvantage of being one-way, since you’re literally traveling while the time passes instead of hopping from one point in time to another.
There are also theories on how we could take advantage of general relativity to travel to the past. These include the possible existence of closed time-like curves, in which you could repeat your own past. Or closed time-like curves, in which you could repeat your own past. Or—
Me: You’re no fun. Closed time-like curves seem like a long shot, since we don’t know if our universe has the right characteristics to allow them. Another possibly more promising idea is to take two ends of a wormhole and move one away from the other and back again at relativistic speed.
MEH: Oh, yeah, that should be easy. We just have to find evidence of wormholes as a physical reality instead of a mathematical prediction, and then develop the technology to wrangle them so we can bring them along on our simple relativistic jaunts!
Me: Yes. Anyway, the idea is that time at both ends of the wormhole passes at the same rate for a traveler inside the wormhole. That means that theoretically, if the relativistic end has only experienced, say, 20 years while the fixed end has experienced 100, a traveler could pop in at the relativistic end and pop out at the stationary end at the point in time when that end had also experienced only 20 years. That is, it would travel 80 years into the past compared to how time has passed for the fixed end.
MEH: So…we could travel into the past, but only to that one point. That’s really limited and frankly pretty disappointing, since we wouldn’t be able to use time travel for its most awesome purpose, which is obviously going to various historical events and picking up passengers. Or, you know, just observing for science or whatever.
Me: I agree it’s kind of a bummer, but those seem to be our most reasonable options right now. Of course, it’s possible future advancements will allow us to invent cooler forms of time travel, but that raises the question of why we haven’t hosted any future time travelers that we know of.
There are theories that any actual time travelers would be dealing with some of the issues that we currently can only explore in fiction, like what might happen if they inadvertently (or very advertently) changed anything in their past. Between that and the cynical observation that they may prefer to avoid this period in history, there are some good reasons why we wouldn’t notice them. At this point, of course, we can’t say anything scientifically, so it’s all philosophical speculation.
MEH: I know there have been time traveler conventions to give time travelers a common place and time to meet, but if I were a time traveler, I think I’d go disguised as a regular old present-day time travel nerd. It’d be the perfect camouflage!
Me: Definitely! I bet they win all the costume contests.
So…we don’t actually disagree on this one, right? Time travel doesn’t seem like it can possibly be as powerful or fun as our imaginary version of it, and we’ve got a long way to go before we can handle even the most basic relativity-based time differences. We can’t write it off, but we shouldn’t be too surprised if it turns out to be beyond us.
MEH: It seems we are in accord. Don’t get used to it.