Superheros Round 7.0; Fantasy League Round 9.0; SciFi Round 16.0: Time Travel
Time travel is a popular theme in all three of our main categories here, so this will be a cross-category post. This time, we’re only discussing time travel that preserves a single timeline; the many-worlds interpretation version will be discussed in a later post.
In all currently-verified models of physics, backwards time travel is impossible. There are many reasons why this is true. Thanks to relativity, traveling backwards in time in any reference frame is equivalent to going faster than the speed of light, which is impossible for any particle with positive or zero mass. The laws of physics are also not time symmetric (although they are charge-parity-time symmetric). Time has a direction. And statistical mechanics lets us derive the second law of thermodynamics, which states that the overall entropy of the universe must increase monotonically with time in all reference frames.
That last point is important. If backwards time travel occurs at any point, then there are a set of world-lines that have infinite duration – closed loops looping back through the time travel device. But overall entropy has to increase monotonically with time along those paths, which is a contradiction since they are closed curves. The only way for this to work is for the universe to have already reached a state of maximum possible entropy. That’s called heat death – which means the universe has run down to thermodynamic equilibrium and time has lost all meaning because nothing can change, which means nobody exists to build time machines.
General and special relativity produce interesting effects on how fast time advances in difference reference frames, but there is no way to make time go backwards. But that hasn’t stopped people at various times from writing time-travel stories.
Closed Time Loops
Although they are already breaking the laws of physics, a lot of time travel stories make an effort to ensure that the time travel that does occur does not produce/has not produced/is not producing a causal paradox (yes, there will be tense trouble – I direct you to Streetmentioner’s book). This gets invoked to make prophecy work. In Babylon 5, the Minbari leadership know that the Babylon 4 station will be was their main starbase 1000 years ago, and so are the only ones unsurprised when it vanishes – with a momentary pause a few years in the future to drop off the human crew. It is sent back in time with one person, Jeffrey Sinclair, who received a letter he had will have written to himself 1000 years ago explaining everything that was going to have happen.
The problem is that it is very hard to make a closed loop consistent in terms of the characters. In Babylon 5, it takes a planet-sized machine several days of output to send a few-kilometer-long space station back 1000 years. But why couldn’t the Great Machine also have been used to send back equal or smaller masses back shorter times and win the Shadow War before it started or stop EarthGov from going sideways, or be used in the future for the same purpose? The in-universe answer is “nobody did that back then, so nobody can do that now”, but that doesn’t really make sense. The real reason is that closed-loop time travel makes some plots impossible to do consisently.
For another example: in Harry Potter, Hermione is given a Time Turner so that she can attend more classes than there are hours in the day (and be chronically sleep-deprived, subtracting from her expected subjective and external lifespan, and perpetually running into her past/future self). That is a relatively trivial use of a time machine, so they must be fairly common in the wizarding world and we see a vault full of them at the Ministry of Magic. But the Ministry isn’t using them for anything useful. Fans of the Potterverse have already dissected this from every possible angle, so I will just copy this photo:
When authors do think through all of the possibilities for closed-loop time travel, things get strange. Heinlein wrote a story called By His Bootstraps, where a 20th-century college student transports himself into a 20,000-year-or-so distant future. He is abducted by his subjective-future self, who sends him through the time machine’s portal to his subjective-further-future self. Further-future guy sends the guy back to pick up now-past guy and various other 20th century items. When he gets back from shopping, he tries to double-cross further-future guy by taking control of the time machine and sending himself back from 20,000 years in the future to 19,990 years in the future. But that only means that he spends ten years in the future until he becomes further-future guy.
Setting Right What Once Went Wrong
Since closed loops are hard to set up correctly, and pose annoying questions about freedom of action and if random events are truly random, many authors have the past be malleable and just conveniently ignore the paradoxes that that would imply.
Terminator starts out with an apparent closed loop, but in the second and later movies the future is specifically malleable. And even in the first movie, Skynet is trying to change the past by sending back its assassin (although it does not succeed). In the original Superman movie, Superman acquires new powers as the plot demands and flies faster than the speed of light, sending himself back in time to save Lois Lane. The Back to the Future movies took this sort of plot further, with multiple changes to the timeline from 1885 to 2015.
But there a lot of problems with this sort of plot. The first is the causal paradox again. Marty McFly goes back in time by accident, and spends a lot of time in 1955 ensuring that his parents actually do get together. Until he fixes it, he sees his siblings and himself slowly vanishing from a picture he brought back from 1985. But that doesn’t make sense – if the timeline had already been changed when the picture was taken, why does it take subjective time for it to change?
A second problem is ethics. In Schlock Mercenary, Kevyn Andreyasn goes back in time to save the entire Milky Way from being completely destroyed. Ethically, that’s pretty good. But consider Superman. He goes back in time and saves Lois Lane. But couldn’t he have gone back in time a little further and prevented the whole two-missiles-can’t-catch-them-both dilemma in the first place? And if he can go that fast, why didn’t he just do that in the first place and avoid the missile hitting New York at all? Back to the Future is even worse in that respect – all Marty changes is making his parents happier and more successful in life, making Biff less successful, and saving Doc Brown from getting shot. But there are more important things that could be changed. Even if Marty was unable to get things right on his first trip, he could go back again and relieve a huge amount of human suffering.
There is a countering ethical consideration, of course: history is complex and chaotic. Go back in time and step on a butterfly, and humanity may not exist anymore. You just killed everyone. Oops. But you can go back and fix it again, as long as there is any possible solution. If there isn’t, you go back and reset everything and it all reverts back to the prior timeline.
We’ll talk more about the complex nature of history in the next post. But for now, be grateful that backwards time travel is impossible.