What Is Time Travel?

Lately, I’ve been on a time-travel kick, enough so that I decided to make a list of time travel stories that I’ve read or watched. Of course, that brings up the question of exactly what is time travel? After all, I have to have some criteria for including or excluding a story in my list!

With that quetion in mind, I read the excellent comprehensive treatise on the fiction and the theory of time travel: Paul J. Nahin’s Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction (second edition). I give it five stars for sure! I don’t think that Nahin gives a concise definition of what he considers to be time travel, although he does give some negative examples and he rejects a quote from H. Bruce Franklin that “when one says time travel, what one really means is an extraordinary dislocation of someone’s consciousness in time.”

The Definition

So, here’s my attempt at a definition:
Time travel is any backwards or discontinuous occurrence of the usual situation of cause preceding effect.
That’s my definition, and I’ sticking to it. In order to be considered time travel, moving to the future must be discontinuous: I am here and now, then without occupying the intervening time, I am at some future time. Anything going backwards is time travel.

Categories of Stories That Are Not Time Travel:

  1. Reconstructing the Past:
    I used to be rather a hardnose on this: “Viewing the past without being able to affect it is not time travel, not even if you are a holiday ghost,” I said. In my older and wiser self, I am now more lenient. Unless the past is being reconstructed from items that never time traveled, then I’ll assume that viewing the past generally involves some information or noncorporeal being undergoing time travel. See “A Christmas Carol”. This same philosophy applies to:

    1. Sending a Vision to the Future
    2. Ancestral Memory
    3. Viewing Alternate Pasts
    4. Continually Viewing the Past

  2. Calculating the Future:
    In the same way as viewing the past, viewing a possible future (or observing it in some other way such as a computer generated image) will generally be considered time travel unless we are simply viewing a possible future that is computed based on items readily available in the present. In addition, I'll assume that stories involving the following psychic abilities are on the same plane as indigestion unless the story explicitly states that information is being passed from the future to the present; although as keeper of the Big List, I reserve the right to include such stories on a whim, especially if there are interesting questions about predestination afoot.

    1. Precognition
    2. Predictions
    3. Premonitions
    4. Prophecies

  3. Differing Time Rates:
    Clocks may run at different forward time rates under different circumstances, but such differences are not time travel. Includes:

    1. Time Dilation (relativistic or gravitational)
    2. Personal Time Rate Differences
    3. Stopping Time

  4. Civilization Regresses:
    Which may seem like time travel, but in fact, it’s just something that will happen sooner or later.

  5. Just a Dream:
    Yes, they still write stories like this. No, they are still not time travel. However, if the characters of the story believe that the events in the story actually happened in their own world, then I shall trust the story is not a dream or travel to a separate world—so, yay for Connecticut Yankee in King Arthur’s Court!

    1. Just a Dream
    2. Just an Hallucination
    3. Just a Wish

  6. Supernatural Story:
    Supernatural stories, in and of themselves, are not time travel. Not even Ben Franklin’s ghost appears. Includes:

    1. Ghost Story
    2. Flying Dutchman

  7. 4D Spacial Topology;
    The fourth dimension does not always refer to time. Many stories consider the consequences of a fourth spacial dimension that we are not usually aware of, in much the same way that Flatlanders are not aware of the third spacial dimension. For example, these stories allow shortcuts by folding our 3-D world in four dimensions or flipping a left hand into a right hand. Klein bottles may also apply to this category.

  8. Bizarre Aging:
    Bodies or objects aging quickly or decaying or growing young again or renewing—none of that is time travel when time is otherwise experienced ina normal fashion. Includes:

    1. Backward Aging
    2. Memory Tricks
    3. Rapid Aging
    4. Fountain of Youth

  9. Odd Entropy:
    I am reluctant to use the phrase “time travel” for a world in which the entropy of a closed system always decreases (or other odd entropy behavior). Of course it is bizarre to us to read about a world of unscrambling eggs, increting poop, exgesting food, and perhaps (as in Knight’s “The Way to the Regress”) the history shelves are full of predictions—but that’s just the way they live.

  10. Sped Up Evolution:
    This may allow us to imagine what humankind’s future will be, but it’s not time travel.

  11. Long Sleep:
    As did Rip Van Winkle, someone falls into a long sleep and awakens in the future. The sleep could be described in many ways, such as cryogenic sleep or suspended animation, but it amounts to the same thing: no time travel.

  12. Other Universes:
    Traveling between different universes is not usually time travel (although it could be if you both move into that universe and move through time). Many possibilities:

    1. Parallel Universes (worlds similar to but not derived from ours)
    2. Alternate Timelines (timelines diverge from ours)
    3. Alternate History (a single alternate timeline with historically significant change from ours)
    4. Many-Worlds Quantum Mechanics (universe branches at quantum wave collapse)
    5. Secondary Worlds (e.g., fantasy world)
    6. Monster-Filled Universes
    7. People-Trapping Dimensions

  13. FTL:
    Oddly enough, faster-than-light travel or communication is always equivalent to backwards time travel from an appropriate relativistic frame of reference, but unless the story makes the time travel explicit, I won’t include it in the list.

  14. Long Life and Immortality:
    Neither living for centuries nor living forever is not time travel.

  15. Reincarnation:
    ...is not time travel. Although I suppose it could be if you were not living your lives in chronological order.

  16. Legendary Figures:
    Greek gods, Paul Bunyan, and their kin may pop up anywhere in history without invoking time travel (unless time travel is explicitly used as an explanation).

  17. Calendar Tomfoolery:
    Crossing time zones and date lines and otherwise messing with the calendar is not time travel, not even if the characters in the story believe that it is.

  18. Despite Appearances, No Time Travel:
    A hoax or ruse or misunderstanding or other circumstances (including misdirection by the author) may make it seem that time travel has occurred, but that doesn’t mean that it has. Includes:

    1. Despite Title, No Time Travel
    2. Just Teleportation
    3. Time Phenomena without Time Travel
    4. Simulacrums
    5. Clones
    6. Virtual Reality

  19. Post-1969 Comic Books:
    After 1969, everything in comic books became time travel, which is logically equivalent to nothing being time travel.

  20. Romance Travel:
    If at any point there is a whole bodice, and at some subsequent (or previous) point there is a bodice that is less than whole, then the story is not time travel.

  21. No Definite Time Travel:
    Sometimes it’s just plain hard to tell whether time travel occurred or not. In this case, I will generally say not time travel. Includes:

    1. No Time Travel . . . Yet!: Characters traveled in later story.
    2. Sequel: Characters traveled in earlier storys.
    3. Surreal
    4. Parallel Stories from Different Times

Please enjoy! —Michael Main (main@colorado.edu)