A serious discussion of time and time travel requires verb tenses that don’t exist in English, but I will do the best I can. BTW, the term “time travel” has the implied meaning of any experience of time other than going from past to future at one second per second (normal life). Even hibernation and cryogenic suspension can be viewed as very limited sorts of time travel. Inventions and extreme time dilation in space travel are other time-slowing mechanisms for one-way travel to the future. Alternate history stories describe worlds with history different from ours, but don’t have to include travel between time lines.
To explore concepts of time travel, start with a model of the nature of time, decide what exceptions to or violations of normal sequentiality and causality might be created by time travel, decide on a mechanism for time travel, determine the limitations of the mechanism, then explore repercussions.
Time is a perceptual illusion of the human mind. Everything from the beginning of the universe to the farthest future already exists and can be seen/visited if you can get beyond the limited human perspective.
There is one and only one stream of history and everything a time traveler does is already incorporated into that history, so there is no risk or chance of changing the past. Time travelers simply fill in pieces of the past that they are participants in. This is close to the previous, but accepts time as real.
There is only one stream of history, but it is subject to change by the actions of time travelers.
There is only one stream of history and it is mostly self-repairing. Changes tend to merge back into the main flow.
There is only one stream of history, and it has considerable inertia or self-repair, but it can be changed with sufficient effort.
There is only one stream of history, and time is quantized so the arrival of a time traveler in a past time quantum has no effect on any later time quantum.
There are many streams of history, and every event in every history produces all possible outcomes, producing an infinite number of branching time streams. In this model, time travel simply produces more branches in the river (delta) of time. If you assume branching occurs with quantum-scale events, not just with macroscopic events, you get larger infinities.
There are many streams of history, but new branches are only produced by major events.
There are time dimensions other than the one we experience, either parallel to ours, perpendicular to ours, or meeting ours at some other angle. Time in these other dimensions can pass at the same rate as ours, or faster or slower or opposite.
Travel to the past is possible, but not to the traveler’s “real” past.
Physical travel to the past is not possible, but the mind / spirit / soul can travel to or look at other times. Precognition and postcognition are both look-but-don’t-touch viewing of other times without leaving the present. Psychometry is also sometimes treated as allowing history linked to an object to be viewed.
There is only one living present moment, which moves only forward. No future exists, and the past is dead. (This severely curtails time travel.)
Time can pass at different rates for different individuals.
Most discussions of time travel limit their scope to a single planet. Add interplanetary, interstellar or intergalactic travel to the branching time of any planet and you get even larger infinities.
“The grandfather paradox” is the name for a standard objection to the concept of time travel capable of changing the past. If you can change the past you could kill your grandfather before he sires your father. This would prevent your birth, which would prevent you from changing the past.
A couple points that have occurred to me: it should be called the grandmother paradox, since killing the man recorded in your genealogy as your grandfather may have no effect if he was not, in fact, your grandmother’s sperm donor. The standard presentation of the paradox also ignores the possibility of secret adoption. Basically, the standard paradox relies on the would-be history changer having true knowledge of the history to be changed.
Various arguments have been invented over the years to deal with this paradox.
Larry Niven wrote an essay years ago in which he argued that time travel will never be developed in any universe in which it is physically possible because developing it would always lead to eventual changes that would prevent the development.
Another line of thought basically allows action in the past that changes the present without affecting the time traveler because they already left the present “before” they acted in the past (causation can operate out of historical sequence because a time traveler has a timeline partially separated from the main stream of time).
Another line of thought says time travelers’ actions are constrained because any travelers who prevent their own birth erase their trips. In this view, a time traveler can change some things but not things that would erase their trip.
Another variation assumes that changes to history take (meta)time to propagate up and down the stream of time from the point of change. This allows a time traveler to make a change that will prevent their birth or their trip. They are OK until the effects of the change reach their moment of birth or trip departure, then they vanish. Depending on the model of time, the changes they made remain or they also vanish (revert to the pre-change version).
Branching time has no problem with time travel because the arrival of a time traveler will always start a new branch of history and time travelers can never change their own past. I don’t recall many F&SF stories that deal with both past/future travel and branching time travel--most stories deal with only one of the two.
One special class of time paradox is a causal loop. A time-traveler is his/her own parent.
Exploring paradoxes is one of the more interesting features of time-travel fiction. There are many books worth of explorations beyond what I have covered here.
The classic mechanism is literally that--someone invents a time machine. How the machine works can be explained with ideas from the fringes of current (at the time of writing) science or left completely unexplained.
The next most common mechanism is magic. A spell or curse or geis or action by fairies moves someone through time or opens a hole in time that anyone can stumble through. Magical objects are also widely used.
Another mechanism is the human mind. Some people are born with or develop the ability to physically travel through time by mental effort.
Some authors mix time travel and space travel, using one to achieve the other.
Gabaldon has natural time-slips marked with megaliths by ancient people who knew the spots were trouble.
One book (1632, now the start of a series with many books) that I like has time travel as an accidental byproduct of reckless alien art.
Other mechanisms include lightning, hypnosis, psychic links, psychometry, etc.
Some stories set up permanent doorways or portals between different times rather than having one-shot trips, using either scientific or magical methods.
Most time travel mechanisms take the whole person, but people have explored other ideas.
Some mechanisms only let the mind / spirit / soul travel.
A few stories have explored travel that moves the body but harms the mind.
Some mechanisms don’t allow group travel--each person through the gate/portal/whatever ends up at a different destination.
Gabaldon’s Outlander has a time-slip that works only for some people.
One large field for questions linked to time and time travel is the connection of body and soul. Some of these questions relate to the ideas of transmigration and reincarnation, and fiction exploring these ideas sometimes blends with fiction exploring time travel.
The materialist/physicalist world-view denies that there is a soul or reduces it to an epiphenomenon of the body or brain. In this view, there are no issues about souls separate from bodies.
Is the soul tied to one body and can it only go forward in time without a special mechanism for time travel?
Are new souls created for each body or can old souls join a body?
Can a soul sequentially inhabit many bodies, but only move forward in time?
Can a soul detach from a body and from the time stream and reenter the time stream at any moment in the past or future and reattach to the same body or enter a different body?
If a soul can sequentially inhabit many bodies, is the soul’s joining with a body voluntary or involuntary? Can a soul pick its next body or is there a tropism the soul can’t resist?
Are souls completely outside the time stream of the physical world of mainstream Western science?
Do souls not so much vitalize bodies as play them like puppets or characters in a game?
As one example, the Piper story “Last Enemy” explores several concepts. It is set in his Paratime universe with a race that travels laterally between branches of time. In the time line of the story, reincarnation is a proven fact with good techniques available to boost memory, but there are heated political battles about whether a disembodied soul can hang around and pick its next body or whether a soul is immediately drawn to the next available body. The Paratimers decide they must avoid that time line until they figure out a way to move disembodied souls across time, because any of their people killed there might reincarnate and blab their big secret.
Any F&SF stories that use a scan-based transport mechanism also need to deal with questions about bodies and souls. If the mechanism is described as destructively scanning the original at the sending location and reconstructing it at the receiving location or if it is described as including an option to record, either complete materialism (there are only bodies) must be assumed or there must be a mechanism to get the soul from the destroyed original body to the recreated body. They must also deal with the question of whether they are creating doppelgängers rather than transporting originals. Stories that describe the original as moving from place to place don’t have the same problem.
Examples where this kind of technology is described:
The Star Trek series
Brunner, John: The World Swappers (1959)
McCarthy, Wil: The Collapsium (2000), The Wellstone (2003), Lost in Transmission (2004), To Crush the Moon (2005)
Simak, Clifford D.: Way Station (1963)
Smith, George O.: Venus Equilateral (1947)
Here is a very incomplete list of F&SF titles that I think I remember well enough to categorize, though I probably got a few wrong anyway. Time travel is a very popular subject in F&SF, so there are many more time travel books and stories that I haven’t read, that I don’t remember, or that I didn’t think were worth listing. Since I’ve read a lot more romance than F&SF for the last decade, this list is biased toward older works, many of which will be OOP (though a steadily increasing number of older books are becoming available as ebooks).
Branching time:
Brunner, John: Times Without Number (1962)
De Camp, L. Sprague: “The Wheels of If” (10/1940, 1948,1986,1988,1990)
Laumer, Keith: Worlds of the Imperium (1962) (and its sequels)
Piper, H. Beam: Paratime/paracop stories; Lord Kalvan of Otherwhen (1965)
History changes (with or without inertia)
Anderson, Poul: The Corridors of Time (1965)
Anderson, Poul: The Guardians of Time (1981)
Anderson, Poul: Time Patrolmen (1983)
De Camp, L. Sprague: Lest Darkness Fall (1941)
Frankowski, Leo: The Cross-Time Engineer (1986) (and its sequels) (series has both past/future and eventually time branches)
Lieber, Fritz: The Big Time (1957)
Norton, Andre: Operation Time Search (1967)
Mobile souls:
Grimwood, Ken: Replay (1987) (this involves a time loop, but not a causal loop)
Piper, H. Beam: “Time and Time Again” (4/1947; anth. 1948,1952,1983,1983,1985)
Other dimensions of time:
Busby, F. M.: All These Earths (1978)
Heinlein, Robert A.: The Number of the Beast (1980) (including fictional worlds)
High, Philip E.: Twin Planets (1967)
Hubbard, L. Ron: Slaves of Sleep (1948)
Simak, Clifford D.: Ring Around the Sun (1953)
Quantized time:
Kessel, John: Corrupting Dr. Nice (1997) (this explores moral questions about invasion and exploitation by a time-traveling culture)
Reality as playing ground for souls:
Dalmas, John: The Reality Matrix (1986)
Dalmas, John: The Scroll of Man (1985)
Reckless alien art:
Flint, Eric: 1632 (1999) (and its many sequels)
Reincarnation & voluntary vs. involuntary embodiment/ensoulment:
Dalmas, John: The Regiment (1987) (and its sequels)
Piper, H. Beam: “Last Enemy” (8/1950; anth. 1952,1965,1981,1987)
Slowing time
Anderson, Poul: Tau Zero (1970) (time dilation)
Haldeman, Joe: The Forever War (1974) (time dilation)
Heinlein, Robert A.: Beyond This Horizon (1948) (machine)
Heinlein, Robert A.: Tunnel in the Sky (1955) (machine)
Niven, Larry: Known Space stories (stasis boxes): World of Ptavvs (1966), others
Niven, Larry: A World Out of Time (1976) (time dilation)
Richmond, Walt & Leigh: Shock Wave (1967) (machine)
Time diverging and reconverging:
Frankowski, Leo: The Cross-Time Engineer (1986) (and its sequels)
Niven, Larry: “For a Foggy Night” (7/1968; anth. 1971,1987,1990)
Time (causal) loops:
Gerrold, David: The Man Who Folded Himself (1973)
Heinlein, Robert A.: “All You Zombies--” (3/1959; anth. many)
Time running at different rates:
Brunner, John: The Wrong End of Time (1971)
Lisle, Holly: Bones of the Past (1993)
MacDonald, John D.: The Girl, the Gold Watch, & Everything (1962)
Russell, Eric Frank: “The Waitabits” (7/1955; anth. 1958,1961,1966,1973)
Simak, Clifford D.: Ring Around the Sun (1953)
Simak, Clifford D.: Time Is the Simplest Thing (1971)
Many fairy tales with a land of fairy where one night is years in our world.
Some variations of the Arthurian legends have Merlin living time reversed from everyone else (his future is their past).
Time travel as damaging:
White, James: Tomorrow is Too Far (1971)
Time travel by soul:
Lisle, Holly: Fire in the Mist (1992)
Time travel completing (not changing) history:
Heinlein, Robert A.: The Door Into Summer (1957)
McCaffrey, Anne: Pern series: Dragonflight (1968) (and its many sequels)
Time travel creating new branches of history:
Flint, Eric: 1632 (1999) (and its many sequels)
Weber, David: The Apocalypse Troll (1999)
Time travel through mental effort:
Anderson, Poul: There Will be Time (1972)
Burroughs, Edgar Rice: A Princess of Mars (1917) (and its sequels)
Modesitt, L. E. Jr.: Timediver’s Dawn (1992) (and its sequels)
Time travel to imaginary pasts:
Niven, Larry: essay: “The Theory and Practice of Time Travel” (1971)
Niven, Larry: Svetz stories: The Flight of the Horse (1973)
Permanent door between times:
Leinster, Murray: Time Tunnel (1967)
Single present, dead past:
Simak, Clifford D.: Time Is the Simplest Thing (1971)
Other time travel or related stories
Laumer, Keith: The Great Time Machine Hoax (1963)
Laumer, Keith: The Time Bender (1971) (and its sequels)
Norton, Andre: The Crossroads of Time (1956) (and its sequels)
Norton, Andre: The Time Traders (1958) (and its sequels)
Robinson, Spider: Callahan’s Crosstime Saloon (1977) (and its sequels)
Schenck, Hilbert, Jr.: “The Morphology of the Kirkham Wreck” (9/1978; anth. 1979,1980,1994)
Van Vogt, A. E.: The Weapon Shops of Isher (1951)
Van Vogt, A. E.: The Weapon Makers (1947)
Having said earlier that time travel requires verb tenses that don’t exist in current English, I decided to expand on that point. The basic indicative tenses are past, present and future. There are numerous other verb forms, such as past perfect, present perfect, future perfect, imperfect, conditional, subjunctive, imperative, etc., but I won’t complicate this further by trying to cover them. I will just propose conjugations for indicative verbs in a time-traveling context. (I will use heesh here for he/she. I hope any culture advanced enough to have time travel won’t still be sexist.)
See
Past Heesh saw
Present Heesh sees
Future Heesh will see
The simplest form of time travel cosmology is one in which the whole timeline of the universe already exists and any actions of time travelers are part of the complete picture. The conjugation I suggest keeps the existing tenses for any verb where time travel is not involved or the action is in the speaker’s present, and adds tenses for time travel. Past is shown by “did”, present is shown by “do”, and future is shown by “will”. The first part of the tense applies to the time-placement of the action in the universe relative to the speaker (“the” past/present/future), the second part of the tense applies to the personal timeline of the speaker (“my” past/present/future), and the last part of the tense applies to the entity performing the action of the verb and uses the current English form (“heesh’s” personal timeline past/present/future).
Keep the current English conjugation above. Add:
See
Past Past Past Heesh did did saw
Past Past Present Heesh did did sees
Past Past Future Heesh did did will see
Past Present Past Heesh did do saw
Past Present Present Heesh did do sees
Past Present Future Heesh did do will see
Past Future Past Heesh did will saw
Past Future Present Heesh did will sees
Past Future Future Heesh did will will see
Present Past Past Heesh do did saw
Present Past Present Heesh do did sees
Present Past Future Heesh do did will see
Present Present Past Heesh do do saw
Present Present Present Heesh do do sees
Present Present Future Heesh do do will see
Present Future Past Heesh do will saw
Present Future Present Heesh do will sees
Present Future Future Heesh do will will see
Future Past Past Heesh will did saw
Future Past Present Heesh will did sees
Future Past Future Heesh will did will see
Future Present Past Heesh will do saw
Future Present Present Heesh will do sees
Future Present Future Heesh will do will see
Future Future Past Heesh will will saw
Future Future Present Heesh will will sees
Future Future Future Heesh will will will see
Examples:
Before this moment, in my personal past, John saw a dinosaur: John did did saw a dinosaur.
Later than this moment, in my personal past, John will see a starship: John will did will see a starship.
Any cosmology that allows time travel to produce changes will require even more complex conjugations. The language must be able to describe past and future events that happened but won’t have happened after a change, changes that are propagating in meta-time, past events that have not happened but will have happened after a change, etc.