If such a powerful technology as time travel possible, it would have implications for reality itself, and could change everything; or, it would make such profound changes, before the things that ended up changing ever existed, that we couldn't tell it happened. Thus, it is a potentially all-powerful force that could both be responsible for everything, and could be undetectable, and is thus a PEP, a pointless epistemological problem. And we should expect to be visited not only by humans from our own future, but aliens who have their own purposes in their interactions with us. The recent time travelers party held by Stephen Hawking was quite sparsely attended, so either there were none around, or they didn't want to reveal themselves. So we should ask the same question about time travelers that Fermi asked about aliens: where is everybody?
Above: Horst Wessel, an early member of the SA under Goebbels who was at one point kicked out of a pre-Nazi poltical party for being (get this) too radical. Goebbels is known to have been particularly impressed by the cut of Wessel's jib and had placed him in charge of many of his men in Berlin. Imagine if this ambitious fellow had risen in the ranks of the NSDAP, perhaps even assassinating Hitler to command an even darker incarnation of the Reich than the one we knew. He would be worthy of going back in time to kill. Actually, Wessel was assassinated in 1930 by a supposed group of communists although it remains unclear if the police actually found the responsible party. Are we living in a timeline which is actually better than the one Wessel would have led us down?
In fiction time travel has been imagined in certain ways. It's interesting to think of how we could explain the universe as we currently observe it in terms of those depictions, and quite fun to look for the effects of possible time travelers. (Don't worry, I'm not going to send you to a stupid listicle of pictures from the 1920s that look like someone has a cell phone.)
Time travel model #1: there is no such thing as time travel. Or, there is no way for us ever to detect it; or, it is only trivially real, but useless; or, it creates "branching timepoints" so you kill Hitler and end up in a Hitler-less universe, but your friends back in the "normal" history timeline stay in that timeline, and don't see any benefit from your action.
Time travel model #2: there is time travel and you can change things. However, we certainly aren't aware of big changes, so either they happen and Back to the Future-style, our brains change too, and we don't know (in which case you can't tell the difference between this and one of the versions of #1, and it actually is #1). Also it gets a little silly when there are time travel stories and alternate histories that presuppose this but somehow the same people end up being born, hundreds of years down the line. Stephen Barnes's Lion's Blood series has the branch point occurring in the early 4th century BCE in the Near East, and then somehow in the 7th century CE, Mohammed is still born. What, history is malleable but which sperm meet which eggs is not? I mean come on. I complain about it again in a later post about alternate history.) The One, which is a Jet Li movie that's an interesting combination of The Highlander, The Terminator and several other movies, pretty well-done despite the obviousness of these tropes, has essentially the same problem.
Article: "Psychiatric hospitals filling up with time travellers sent back to kill Donald Trump"
If history-altering time travel is possible, there's another problem. It sets up an inevitable arms race for who can go back before the other guy and cut off their moves pre-emptively (remember how Bill and Ted got out of a plot problem by "remembering" to go back in time from the future and drop a trash can on the head of the guy who was about to shoot them? I don't understand why the bad guy wouldn't have done the same thing to stop them. And Bill and Ted do something to stop him...and so on.) In fact, if history-changing time travel is possible, then (for example) if gold is your thing, piddling around with the California Gold Rush of 1849 is stupid - why limit yourself to the deposits that exist on Earth in your (current) timeline when you can influence the supernova that created the Solar System? Or the amount and distribution of various nuclei at the Big Bang? Efforts would then focus on being able to influence earlier and earlier instants in ways that produce desired outcomes, and the whole universe becomes a game of temporal oneupmanship, where everyone wants to squeeze closer to the causal high ground at the earliest possible instant.
Even if we could somehow "sense" that history had shifted, a la Marty McFly or the returning safari hunter fromBradbury's A Sound of Thunder, there's no reason to be fooling around with dinosaurs, and the universe we're looking at is likely already the final outcome of various struggles that crammed the turns of their game into a handful of Planck times after the Big Bang. (This could explain the strange stringy/foamy distribution of matter in the universe. It's from the entity who captured the first move and canceled all the later ones, and proclaimed Fiat lux! Or Fiat aurum, or whatever it was maximizing. Anyway, it's more likely light or gold than the happiness of conscious beings. Clearly the universe we're in is not optimizing for happiness in a Parfitian or Neil-DeGrasse-Tysonian or any other sense.)
Time travel model #3: there is time travel, but nothing can change anyway. Time travel and fate are both true. This is the model used by Douglas Adams in the Hitchhikers Guide series, in Twelve Monkeys, and to some degree in Terminator 3, also known as "everything has already happened, in order." Note that models #1 and 2 are agnostic on the question of whether the future is as set in stone as the past, or the more provincial question of whether certain entities in that universe (humans) can make non-predetermined choices. BUt if you think consciousness in an epiphenomenon as some people suggest the Libet button-pushing experiment does, then you believe we're "locked in" exactly like coma patients, except we're looking out through the eyes of meat robots and we're deceived into thinking we're making the robots move, when in fact we're just watching and along for the ride. Consequently, when there's a shift, all we can do is watch, as in Vonnegut's Timequake. (Of note, Vonnegut's meditations on time, evident elsewhere including Slaughterhouse Five, inevitably shade into discussion of morality and meaning.
A variant on this is the idea of temporal homeostasis - maybe you can make a few changes that persist for a while, but there's an equilibrium principle that will try to return the universe to baseline. The series 11/22/63 employs this heavily. Although not explicitly about time travel, Final Destination shows a universe trying to restore equilibrium, and Pohl's Coming of the Quantum Cats shows how material moving between timelines causes physical imbalances (spoiler: in that case, they were rectified by super-advanced humans or unseen aliens that stepped in after they saw the damage we were unknowingly doing.)
In this model, even once people know they can't change anything - i.e. they want to stop the A-bomb from being developed, they even see pictures of themselves in the declassified Manhattan Project materials (maybe that's where they got the idea! because everything already happened, in order) but they still can't help themselves. Hoping against hope they say dammit, I'm still going to try. Of course, by bizarre coincidences, it's going to happen exactly the way it already happened. (This was suggested to have happened with scientists from the future interfering with the large hadron collider, but of course it still came online.)
It's this last model that interests me the most, because what it would produce is a handful of out-of-place people throughout our history; people who know too much, who get involved in historically important events; who disappear or seem to be swallowed by time. Having some fun, I've compiled four good examples.
Jesus Salas Barraza, Pancho Villa's killer. I've written before about this event, and the person who caused it. Underappreciated outside his country, Villa was planning a run for president of Mexico at the time of his assassination. And imagine the kind and gentle government he would have established had he won! Of course, a successful time-traveling assassin would not obviously be stopping a dictator, from the perspective of people in that time, and would have lived out their lives in prison (or in some other obscurity). For example, if he had been successful, Bruce Willis in Looper would've just seemed to be a child killer. Hence the curiosity of Barraza's dying words: "I'm not a murderer. I rid humanity of a monster..."
Above, Jesus Salas Barraza, clearly a guy from the alternate Villista dystopian future. From Escrito Sangre.
Thomas Conway was an Irishman who fought as an officer leading the colonial rebels in the American Revolution, but in 1778, he tried to have Washington removed as head of the Continental Army - and presumably to take the role for himself. Conway's maneuvering was reported and he ended up resigning, first going to France, from which "he was compelled to flee [to Ireland] for his life. After that Conway disappeared from history. He is supposed to have died about 1800 in poverty and exile." Cue Twilight Zone music!
Lastly, Benjamin Franklin, because I mean come on. To name a few of his inventions and accomplishments: fire companies, electricity, discovering the Gulf Stream, paper-based goal-setting software, and (seriously) depositing interest-accumulating money for the future people of Philadelphia. Since his childhood is pretty well-documented, what may have happened is thta some British time traveler realized they bore a resemblance to him, and went back to the date when he "ran away" from his printing apprenticeship knowing that he didn't come back to see anyone who knew him for many years, throwing the real Benjamin Franklin into Boston Harbor one night. Then, he went to London for some time, gradually leaking small innovations here and there and generally having the better of his contemporaries, until coming back to the colonies and marrying. After the Seven Years War Franklin went back to London, leaving his wife to mind the house in Philadelphia. The man we remember as a patriot showed a curious early loyalty to the crown, taking the English side in the Stamp Tax controversy (as any anti-American time traveler would), but this incited Philadelphians to take up arms against his household and wife, perceiving them as in league with the British rule-makers. Franklin then changed his position and returned to Philadelphia, eventually drafting into the role of architect of the revolution and the government which followed. (One is reminded of the ending of Terminator 3, when John Connor realized his position and reluctantly announced over the radio that he was in charge.) Thus, our anti-American British time traveler tried, but failed, to steer the Americans away from revolt, only to take a softer position out of concern for the woman from the past he ended up falling in love with in the past - just as he always had...
Below: I think I've found him. Maybe he wanted to keep the British and American markets combined so he could insult more people.