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.
We’ve talked about how many different fictional universes would be dramatically different if whatever characters or technology they have in them were to be publicly known in real life, with a particular focus on economics and sociology. This obviously poses a problem for an author: how can you write a straight-forward detective story featuring a vampire detective when the readers will be focused on the incongruity of nobody using magic to make a million dollars?
One way to do this is to establish a convention that while the world may look like the real world, that is just the outward appearance. There is a Masquerade – an organized effort to hide the flying saucers / vampires / wizards / witches / parallel dimensions / alien space bats from the muggles. But this has its own problems.
Why is there a Masquerade?
Why are characters with fantastic skills hiding from the world? There have been many different in-story reasons for this, ranging from the all-too-realistic to the entirely absurd.
The reason many mutants have secret identities in the X-Men comics is all-too-realistic. Society sets up the Masquerade, forcing the mutants to either hide their skills or be discriminated against for not being ‘normal’. In real life, society has forced masquerades into existence for no good reason at all. This analogy is not lost on the Marvel comics staff. But this form of masquerade only goes so far. It only works as long as the population has some idea, however distorted and mistaken, of who they are forcing into hiding: the world has already been partially unmasqued. In the Marvel comic universe, everyone knows that mutants exist, even if they don’t know who the mutants are. Why would a group hide from the world so well that nobody believes or even knows to not believe that they exist? And why would someone discovering something extraordinary not tell others about it?
Perhaps the default version is “but they wouldn’t believe me if I told them”. That one might work for the first time Lucy Pevensie goes to Narnia, with the portal to the other world opening and closing at the whim of a not-particularly-kind deity. But it’s absurd when the group with the supernatural skill can prove what they have. We need another motivation for people to stay silent.
The term Masquerade as we’re using it here comes from Heinlein’s “Methuselah’s Children”, where it is used by the Howard Families, a small group of people selectively bred for longer lifespans, to refer to their hiding from the ephemerals around them. There the motivation was to avoid attention and people trying to steal the secrets of their immortality. In that story, the Masquerade is eventually broken and society is angry at the immortals, not believing that they were simply selected for long lifespans – although there is no good reason for that, since the Howards’ lives are well documented. Here the Masquerade came from fear. But if someone had been public about living longer in the past, then the Masquerade would not have been necessary. So the fear was misplaced, and doesn’t work as a motivation.
In Harry Potter, we’re told the wizards hide because otherwise “people would want magical solutions to their problems”. That’s not a good reason either. People wanting magical solutions to their problems is a great opportunity to make a lot of money. In order for the Masquerade to make sense, it has to be in the self-interest of the groups that are hiding to stay hidden.
In Buffy The Vampire Slayer, Vampire: The Masquerade and some other vampire settings, the Masquerade is justified by the various groups of vampires agreeing to hide from humans because otherwise we’d all gang up on them and kill them until they died from it. That’s a pretty good reason, and it’s enforced in Vampire by a group of vampire enforcers taking down anyone who does try to go public and in Buffy by stakes, scythes, and rocket launchers. A similar reason is invoked in The Dresden Files. And in Mage: The Ascension, there are non-sympathetic Men In Black hunting down anyone who is doing unsanctioned magic.
But even these self-interested Masquerades tend not to work out, because while it may be in the interest of the group to remain unknown to the world, there is always a million-dollar incentive for each individual to be the one who goes public. We’ll come back to this at the end, after we show that even the best-maintained Masquerade couldn’t survive anyway.
How is there a Masquerade?
How do you prevent Muggles from walking into Hogwarts? Sure, it may be impossible to label on a map and loaded up with nothing-to-see-here, but why doesn’t it and all the other wizard enclaves show up on airplane and satellite imagery? A Wizard Did It only works so well. If high concentrations of unspecified “magical energy” fry electronics, then people will notice the bizarre dead spot on the train platform where their cell phones suddenly crash and start tapping on the wall. Dragons flying around will set off air-traffic control radar, to say nothing of people’s reaction to a flying car. Actually, Harry Potter’s masquerade has more holes than the Menger sponge. It’s as bad as how almost everybody misses that Superman is obviously Clark Kent and Batman is almost as obviously Bruce Wayne. Let’s try something else.
In The Dresden Files, the masquerade is maintained by the “Unseelie Accords”, a set of agreements between various magical groups to not reveal their presence to the world. The different groups all hate each others’ guts, but aren’t willing to risk total destruction by angry normals (or by vampires or faerie who are angry at being revealed). This is an okay setup, but somehow it survives vampire bodies showing up in a Chicago morgue, a wizard fighting horror-movie monsters in a hotel convention center, and a horde of zombies being hunted through a city by a resurrected T. Rex. That’s bad enough. Then Jim Butcher turns it up to eleven.
Harry Dresden (listed in the Chicago yellow pages as “wizard”) and a group of his friends are fighting an army of vampires at Chichen Itza. Near the beginning of the fight, Dresden briefly negates gravity over a large area and focuses the force into a very small area, smashing a field of vampires into the ground. It produces a minor earthquake. Cool story, isn’t it? The problem for the Masquerade is that there are several different global seismic monitoring networks specifically looking for small near-surface earthquakes emanating from point sources well away from established faults – because that’s what a nuclear detonation looks like. Everyone with a seismometer now thinks that a nuke went off in the Yucatan.
From there, the Masquerade would fail quickly. 30 minutes or less will put blurry spy-satellite imagery of Dresden’s team fighting vampires on a large number of computer monitors. If I read the US-Mexico airspace and joint defense agreements and approximate aircraft base positions correctly, within an hour there would be high-speed jets from US Air Force bases along the Gulf over Chichen Itza doing close recon, closely followed by Mexican aircraft and probably some curious private planes and helicopters from Cancun. And then neither the vampires nor the wizards could hide from the world, even if they all teleported away immediately. The Masquerade may have been set up by the Wizard Who Did It, but Muggles Do It Better.
Why Isn’t The Masquerade Brought Down?
If Masquerades are so easily broken and there are such large rewards for being a whistleblower, how can a Masquerade last in a story for any length of time? This one is actually easy to answer: if the Masquerade fails, the story changes too much and the author(s) can’t write a sequel.
There may be in-story reasons why nobody has brought down the Masquerade, especially if there is an organization dedicated to maintaining it. If showing the world that aliens exist will summon the MIB and lead to you and everybody else in the world being immediately mind-wiped then perhaps fame and wealth isn’t as much of an incentive. But where is the supernatural equivalent of Wikileaks or radical open-source? And if there isn’t a penalty for going public, the Masquerade doesn’t make any sense at all.
Even if there is a penalty for going public, you just need to do so untraceably. Gather compelling evidence of whatever The Masquerade is hiding. Use dummy email accounts and free wifi to get copies of it printed and mailed to hundreds of different groups at once. University science departments may be deluged with crackpot manuscripts, but most get looked at for a few minutes, so you have a chance of being taken seriously. Once again, Wikileaks is the model. For physical evidence, shuffle it through mail forwarding services a couple of times and not even you will know exactly where it has been before it gets where it needs to go. A few pebbles from Narnia would be enough for the geochemists; a dragon egg makes the biologists go wild; vampire skulls sent to an anthropologist shows that they exist.
Granted, it’s not quite that simple. It would be a huge leap from “you found a place with isotope ratios that don’t match anything in the solar system” to “you were in a land of talking animals ruled over by C.S. Lewis’ version of Jesus personified as a lion”. And “there are humanoids with large retractable canines and strange bones that disintegrate in sunlight” isn’t the same as “we need to find the vampire leaders and convince them that they and their followers should drink pig’s blood rather than human”. But as soon as the basic supernatural element of whatever Masquerade you are bringing down is illustrated, everyone would be willing to listen and to invest considerable effort in what you say.
So it is very hard to have a convincing Masquerade in a story. I suppose you could specify some sort of universal law that the Masquerade cannot be broken, but that would be an arbitrary law enforced by a trickster god. It’s probably best to simply go with authorial fiat and acknowledge the inconsistency.