In a somewhat whimsical podcast episode, Sean Carroll explores the physics and “rules” of time travel. Probably the first two thirds explore the physics. Carroll notes that if time travel under general relativity is at all possible, it would more likely involve a spaceship attempting to navigate some kind of closed timelike curve than stepping into a teleporter, phone booth, or something else along those lines.
The discussion is an interesting one, but it can be summed up that general relativity seems to dangle the possibility of time travel in front of us, but there always seems to be something that gets in the way. Carroll’s final summation is that it doesn’t appear possible, at least as things stand with our current understanding of physics.
He doesn’t really look at FTL (faster than light) proposals, such as the Alcubierre warp drive, although he does note that traveling faster than the speed of light is always moving backward in time in someone’s frame of reference. As I noted in a post a while back, having FTL without time travel requires adding special assumptions to remove the possibility.
This seems to accord with what I consider the Fermi paradox of time travel: if it’s possible, where are all the tourists from the future? That said, Carroll does point out that this observation only pertains to travel to arbitrary points in the past. If wormholes, or something similar, can be used to travel to the past, then we shouldn’t expect to see future visitors until the first such wormhole is built.
Anyway, the last 50 minutes or so of the discussion focus on what the rules of time travel might be. Carroll notes that he was a scientific advisor on Avengers: Endgame, a movie I criticized for its handling of time travel. But Carroll argues that he thought it got it right. I went back and watched some of the key moments in the movie, and can see a narrative for how it might be consistent. Interesting. Carroll notes that the role of the science advisor on movies and TV shows is basically to find a plausible scientific account for what happens in the story, not to sit around and tell the scriptwriters what they can’t do.
Anyway, time travel in most science fiction seems to take place within three broad paradigms.
- Whatever happened, happened
- The past can be changed, with consequences
- Changing the past creates new timelines
Under option 1, you can’t really change the past. If you try to do so, something always intervenes to stop you, or it inevitably works out that your actions were always part of what happened. Carroll’s example (with appropriate ethical caveats) is trying to back in time and kill baby-Hitler. Under this paradigm, anything you might try do only becomes part of Hitler’s biography. Maybe you end up traumatizing baby-Hitler in a way that sets him down his destructive path.
This paradigm shows up a lot in fiction. Notable examples are the first Terminator movie, or the original 12 Monkeys. It also tends to show up in fiction which has time travel in it, but the time travel dynamics, in and of themselves, aren’t the focus. So Doctor Who usually operates under this paradigm. (Of course, like all long running franchises, it isn’t consistent, flouting the rule whenever convenient to the plot.)
The biggest issue with this option is that it becomes hard to maintain if someone really wants to make a change. Suppose we form a league determined to kill baby-Hitler. Undeterred by initial failures, we keep sending expeditions to the past. All these expeditions must fail, in perhaps increasingly improbable ways. And they all must merge together in the historical timeline, all while not being noticed by any historians.
But whenever sci-fi does focus on the time travel element, it’s usually under the second option: the past can be changed, also changing the future. But this brings up the infamous grandfather paradox. If I go back in time and kill my grandfather before he met my grandmother, then I was never born. But if so, then who went back in time and killed my grandfather? Using the baby-Hitler example, I go back in time and kill baby-Hitler, but then I grow up never having known anything about Hitler, so what motivates me to go back in time and kill him?
Most movies and shows ignore the grandfather paradox and simply allow changes to exist despite the consequences. All the Terminator sequels fall into this category. But now we appear to have a situation where effects (the changes to the past) can continue to exist orphaned from their cause (someone deciding to go back in time to change something). This appears to break causality, violating a central understanding of how reality is supposed to work.
A way to perhaps avoid that is the third option: any change to the past creates a new timeline. If I go back in time and kill baby-Hitler, I change history, but only in a new timeline. The old timeline still exists. So killing Hitler doesn’t really save anyone, since World War II still happens in the other timeline. Causal orphans still appear to exist in any one timeline, but across all timelines causality is preserved.
In most fiction, it’s not possible to travel between timelines. This has the interesting effect of making Option 3 look like Option 2 for any traveler. It raises the question of how anyone could even know they exist in an Option 3 universe. At least unless the time traveling technology also allows travel between timelines. Although we might imagine a traveler going back to before a divergence and traveling forward again, but then, which timeline do they end up traveling on?
Option 3 also bears some resemblance to the many-worlds interpretation (MWI) of quantum mechanics, but the resemblance might be somewhat superficial. Certainly if the MWI is true, events diverge in other worlds. However, under the MWI, any travel to the past might pretty much make it impossible to return to your original world, even if you were careful not to change anything, since traveling forward again, the nature of quantum interactions means there’s no way to guarantee you would go down the exact same branches. Some minority portion of the different versions of you might make it back to a world that at least resembled your original, but most would be forever exiled from the world you left.
The multiple timelines thing does offer another possible explanation for the Fermi paradox of time. Maybe we’re just in the timeline that hasn’t yet been polluted with time travel. Although the probability of us just happening to be in that particular timeline seems infinitesimally remote. Still, we might imagine there is at least one timeline like that, and the people in it would consider it extremely unlikely they’re in it.
That said, a far simpler, and therefore more probable explanation remains: time travel isn’t possible. Still, it’s fun to think through what it would mean if it were.
What do you think? Is there an explanation for the absence of tourists from the future that I overlooked?