Time Travel
This article stems from a lifelong interest in science and in science fiction, also a long correspondence with Poul Anderson, who wrote a series of Time Patrol novels, and finally a May 2022 discussion panel at the ConQuesT science fiction and fantasy convention in Kansas City.
Types
A) One class of time travel subgenre science fiction involves time viewers, a one-way method of seeing past events. Great mysteries could be solved, and awesome discoveries made. Also, considering the past includes five minutes ago, privacy and secrecy might be utterly obliviated. (As depicted in Baxter & Clarke's novel The Light of Other Days.)
B) A second class of this science fiction subgenre focuses on the slowing of time. In particular, a character who can greatly slow, or essentially freeze, time itself. The world around them ceases to move, including all the people. Much mischief could be done, as shown in several stories, not all science fiction. It is usually presumed that the protagonist can move items, or touch people, without damaging them. (How they, same as with an invisible person, can still see things, is generally ignored.)
C) A third subgenre is time loops. An endless repeat of a certain day, or other time period. This notion was made famous by Bill Murray in the movie Groundhog Day, and then by various imitators. Here the author's views begin to tell. In several Star Trek episodes, there is repetition, but at least early on, the characters do not realize this. In contrast, Murray knows what is happening, and eventually he takes best advantage of the unique situation. Further, in each reset, he retains his memories but does not physically age.
D) The fourth subgenre, that of time slippage, is by far the oldest. This long predates formal science and science fiction, a mythic literature category of its own.
There is a very old story, in the Mahabarata, in which King Kakudmi of India and his daughter slip forward in time.
There's another time-slip story, much like The Legend of Sleepy Hollow, but in the Quran! In it, a man and his companions enter a cave, then sleep 300 years.
In a very similar tale from Japan, a fisherman named Taro Urashima slips 300 years while visiting an undersea palace. There's also an Irish tale, in which the hero Oisin slips forwards while across the sea. Yet another, La niña de las peras, comes from old Spain. A young girl slips forward while in a cave.
E) A fifth subgenre is time signaling. It's easy enough to send a message to the future, and burying a time capsule in special places is now common. However, while sending data to the past might be impossible, some great science fiction stories have been written about the potential consequences.
Modern
F) But that's not the modern notion of time travel, with its back-and-forth physical ability. This opens up many potential phenomena, including strange paradoxes. Many people have dreamed of going into the past, and directly experiencing some historical events and eras -- or at least, their imagined versions thereof. This was popularized in fiction by Mark Twain's 1889 novel A Connecticut Yankee in King Arthur's Court, although the means of travel was essentially magic.
Overall, such stories are surprisingly modern, the very first being "The Clock that Went Backward" by Edward Page Mitchell, which appeared in the New York Sun in 1881. Similar to Twain's later work, the mechanism utilized borders on fantasy.
The description of fictional scientific discoveries, specifically new 'temporal' technologies enabling travel throughout time, appeared almost overnight. H. G. Wells' novel The Time Machine, published in 1895, popularized the concept of time travel by precise mechanical means. Movie versions followed, as soon as that actual film technology allowed. Even since, it's been redone in further books and movies, countless . . . times.
Books such as Diana Gabaldon's The Outlander, written as science fiction, but lucratively published in 1991 as romance, hark back to magical methods. (More recently, it's been filmed as a television series.) It shows the protagonist, then later her daughter, following a somewhat fixed route, 200 years back into an earlier Scotland.
Technologically enabled time travel has become a favorite of 'hard science fiction,' with the idea that time can be visited, both eras and places, past and future, then the traveler can return to their exact origin point.
Much has been made of the fact that Earth moves, as does the Sun, so that past and future locations are actually far distant. A handful of stories, by Jay Werkheiser and others (especially in Analog magazine) account for this motion. In some, the protagonist doesn't know, and gets stranded in open space. In others, the shift in location is accounted for. But almost all time travel stories "hand wave" the issue, simply assuming it's not going to be a problem. The fixed stones of The Outlander hint at this, because the time journeys only occur at those specific distinct locations.
Paradoxes and Butterflies
Everyone has heard of the Grandfather Paradox. What if you go back in time, and kill your own grandfather, when he was young and had no children? Then he would never sire your own parent, so that you yourself would never have existed. So, who killed the grandfather? Nobody, so the man lives, and you are born. Once again, you're ready to get into the time machine.
There are several other, less known, paradoxes. What if I, as an author, want to produce a novel? And I'm going to work very hard at polishing it, then getting in on the market. I can go forward in time ten years, and obtain a copy of my book. Then go back eleven years, and leave it for me to read. Thus, all I need to do is copy it into manuscript form, then submit it to the same publisher who I know will purchase it.
Science fiction has been expounding on these themes for decades. Novels such as Paul Levinson's The Plot to Save Socrates develop these exotic paradoxes into some mind-bogglingly complicated situations. In popular culture, the Ashton Kutcher movie The Butterfly Effect depicts many heartbreaking attempts, by the protagonist, to get things right.
The butterfly effect refers to a scientific notion, in meteorology, that a butterfly flapping its wings in Africa sets off a chain of air motions which eventually produce a hurricane in the Atlantic Ocean. Adding in time travel, would the smallest actions of someone who's visiting the past have some effect, back then, with major consequences for the future? Science does not know, however, authors have speculated on a vast range of possibilities
The older time-slip stories, and time-freezing or viewing tales, would not seem to cause butterfly effect phenomena, or allow for paradoxes. But is that strictly correct? One obvious possibility would be to make special investments, or hide unusual items, then retrieve them in the far future, when they'd (presumably) be a lot more valuable. Beyond this, your friendly web author is not certain.
Parameters
Let's take a look at the various categories of modern time travel story. What are the methods, parameters, and effects? As with vampire stories, regarding those key character's strengths and weakness and lifestyle and more, each author gets to choose!
1) Is the time travel one way, two way, multiple, or otherwise? Can the protagonist go into the past (or future) and come back? More than once? Or is there, for one reason or another, some kind of strict limit?
2) Is time travel easy to accomplish, or difficult? Must gigantic power inputs be involved, to achieve the effect? If so, are those inputs reliably available? Perhaps, as with Marty and Doc in the movie Back to the Future, a convenient lightning strike must be harnessed.
Or, is it easy? Can it be done via some learned magic, or subtle inborn ability? Would there be technologies like Poul Anderson's time motorcycles, that any person can ride through time and space? Or even a small device, that whisks a protagonist on a whim?
3) Is time travel secret? Has only one brilliant scientist or laboratory, or visiting alien, presented the capability? Is it kept secret by some government agency, and/or temporal police force?
Or, is it a well-known thing? Are there time roving scholars or treasure hunters? Temporal tourist companies? Ruinous paradox-invoking terrorists? Yearning lovers across the ages?
4) Is time travel effortless, breezy, undertaken countless times? Or might there be some analog to seasickness, jet lag, or worse personal effects? Would a time traveler fall prey to unfamiliar germs and endemic violence; or could luck, skill, and technology minimize such factors?
5) What about those paradoxes? This consideration most widely distinguishes the various scholarly queries, and multitudes of fantastic fiction, surrounding this topic. In more detail:
5a) Is time immutable? By some means, would the timeline resist, or forbid, any changes? No matter what a visitor to the past might attempt, they find themselves unable to make any changes that will consequentially affect the future. Murder the infant Hitler, and the desperate nanny snatches a nearby Jewish baby, managing to pass it off as the dead Hitler. (As depicted in at least one short story.) Unseen willful forces foil any action by the time traveler. Even if the deed is done, then by some twist, that's actually what was "needed" for time to continue as familiar. [This is known to scholars as the Novikov Self-Consistency Principle.]
5b) Is time somewhat flexible? Would minor changes not really matter? Any number to butterflies flapping their wings, or friendships made, might alter a few things, but the main events of the future continue as always.
5c) Would timelines split? Aside from extreme theories about every single quantum event (or personal choice made) splitting the whole cosmos in two, only time travel (or signaling) could accomplish such a thing. There could be another America, in which the South won the US Civil War. Until a time traveler comes along, and inadvertently reverses key events at Gettysburg. (As in Bring the Jubilee by Ward Moore.) Might another Earth exist, in which the Roman Empire never fell, aided in its survival by a lone scholar from the future? (Shown in the novel Lest Darkness Fall by L. Sprague deCamp.)
Perhaps these timelines could be repaired, or rejoined, or perhaps they would continue to split, so long as temporal intrusions continue. In many stories there's a dedicated Time Police force, working continually (and sometimes humorously) to keep the standard timeline intact. Whether it's our own, or something recognizably similar.
5d) Would the time traveler be able to return to their own "home" time and place? Or would they be stuck in the changed-by-them (or by others) past? Otherwise, upon returning, would they find the future altered: in minor and subtle, or in dramatic and major, ways?
5e) If the timeline changed, would those shifts be serious and multiple, yet unknown to the traveler? Would the time traveler, much less the general public, realize the world has changed? Or would they go on, oblivious to the fact that the whole world, along with them in it (or not!), just radically changed? What if such changes were known, at least by the protagonist? Then taken advantage of by the traveler? Might they loop around, and gather multiple versions of their own living self? Might a time traveling individual take advantage of timeline changes, to become their own father, and mentor, and maybe even their actual mother? (As in The Man Who Folded Himself by David Gerrold.)
5f) Might there be Fated Events, such as immutable personal destinies? As with Star Trek's Captain Pike, or the Marvel character Black Widow (in the movie Avengers: Endgame), can someone be fated, actually required, to die? To end their existence, despite any time traveling or other powerful abilities? That could be dismissed as a silly concept, invoking a nonexistent "temporal deity," or something that violates basic free will.
5g) Would time (and, necessarily, place) travel be done exclusively by humans? Might there be far future travelers that are very different? Or aliens, from outer space, or parallel timelines, or some other part of the cosmos, also traveling in time? Then, whether by accident or routinely, encountered by regular humans.
Summary
Fermi's Paradox asks, if life and intelligence and technology are common in the universe, then where are all those aliens? Even more obviously, if time travel is possible, how come the present world isn't full of such visitors now? (Many stories include period-matching disguises, but that could only go so far.) Even if rare, over a very long future, quite a few ought to have shown up.
Actually, there have been individuals who openly claimed to be from the future. They had, or at least shared, no definitive proof, yet in some cases, they actually did vanish. Not, poof! and blinked out, but were completely lost to those who knew them.
In one case a man was found in 1948 on an Australian beach, dry yet dead, with cryptic notes but no documents. He was never identified, and his strange appearance never explained.
Science knows of no actual way to travel backwards in time, although a spacecraft moving at relativistic speeds really would bring a (well shielded and carefully nourished) traveler rapidly into the distant future.
There might be wormholes, with each end apart in time, technologically constructed. If so, the farthest back one could communicate, if not travel, would be the time of the wormhole's origination. If so, in the future, humanity might begin to face such conundrums!
(c) 2023 by Paul Carlson