Physicists disagree on what the real possibilities might be, for time travel. Each of the theories has been explored several times in novels and movies alike.
  • Coherent History: Events can't be changed.
    If a traveler goes back, and adjusts something, he finds out later that his origin time hasn't changed at all; the adjustment was fated--nee required!--to happen.
  • Bi-modal History: Two histories cause each other.
    Terminator and Terminator 2: Judgement Day create the two histories that invent themselves. SkyNet sends a Terminator back to destroy John Connor and fails. John Connor grows up, leads the resistance against SkyNet, and sends a Terminator back to protect his child-self. The technology that leads to SkyNet comes solely from the parts of the first Terminator.
  • Free History: You can change anything.
    Usually, these plots also include the convenient resolution; if you change something important, all its ramifications show up. Timecop revolved around such manipulation. The young Senator is wounded; the old Senator grows an immediate scar. The Senator is killed, nobody but the time-traveler knew that History changed.
Here's the way I've always heard time travel theorized:

Plastic Time: It's very easy to alter the timeline, but you run the risk of drastically changing the world--or even yourself--by accident. Go back in time and kill Hitler as a baby? Fine, World War II never happens, but now your grandparents never met in that London hospital...

High-Resistance Plastic Time: It's very hard to alter the timeline. Any changes you make will probably not be noticed. Go back in time and shoot Lincoln in Ford's Theater? Fine, but John Wilkes Booth still gets blamed...

Fixed Time: You can't alter the timeline, no matter what. Go back in time and kill Hitler as a baby? Whoops, you picked the wrong house. Try to nuke ancient Egypt? Too bad the bomb's a dud...

Chaotic Time: The timeline is extremely easy to alter, and any changes you make will propagate wildly. Go back to the 1950s and buy a newspaper? Now France rules the world. Go back to the Permian Age and step on an ant? The human race no longer exists... (See Ray Bradbury's short story "A Sound of Thunder" or the "Time and Punishment" episode of "The Simpsons" for some good examples of this)

Paradox-Proof Time: Very similar to Fixed Time, but if you start to do anything that would alter the timeline, you immediately get bounced back to your own time where you can't do any damage. Alternately, you can change history, but it's always the history of a parallel universe--never your own. So you can kill Hitler in his crib, but WWII still takes place in your universe.

The Chronomancer AD&D supplement contained a model based on "historical inertia". The idea was that things that happened were going to happen pretty much regardless of what time travelers did. So if you were to go back in time and kill Hitler, the course of events would elevate another genocidal leader and the result would be almost indistinguishable. However, on a personal level, great changes could be made, you could rescue your sister from the concentration camp with few consequences, or get back at that asshole Hitler for having her killed in the first place.

The only model which makes sense (apart from the fact that time travel doesn't make sense) is having a single time. Travel back in time and change something? Well, it was already changed (by You Know Who) before you left! Attempt to change some outcome? Before you bought that overpriced ticket for the 2020 model ChronoGo H2, you'd already did (done?) what you'd do (did?), and you already know the results.

Or, in other words, Nothing Happens.

I beg to differ, ariels: the only model that makes sense (If you allow time travel at all) is one with an infinite number of parallel universes.

In that model, whatever a time traveller does just creates a new universe, but doesn't change the old one that the time traveller came from - thus, time travel paradoxes are not a problem (I'm not absolutely sure if this holds true with multiple time travellers and crossover manipulations).

Another one that makes a bit of sense is one in which time travellers are freak events that create loops which perpetuate themselves: in Terminator, the time traveller doesn't actually change anything.

Any other model has at least one major logical flaw: if there is a single timeline which a time traveller can change (the by far most frequent model, because it allows for cheap let's go back in time and prevent <insert negative event> from happening plots), it is a total contradiction, because change and causality is a function of time - but you just fixed time inside the timeline you are observing! Time progresses along that line, any kind of change happens along that line!

This is the root of time travel paradoxes. It might be resolvable by using some kind of "meta-time", but certainly not by using conventional temporal and causal thinking when talking about "sequences" of evens that don't follow the kind of order which we live in. That way of thinking results in glaring logical flaws like an alteration caused by a time traveller manifesting as a gradual change decades later that leaves people enough "time" to react in some way (see Back to the Future or Tenchi Muyo in Love for examples).

I have a different theory on time travel and what happens if you travel back in time, based on the "time is a just another dimension" argument. Imagine a universe of only two perceivable dimensions, (two dimensional) people could move about freely in either of these dimensions and interact with anything at the same point in time. Now imagine that this plane is moving through a third dimension which cannot be seen by the beings living in this universe. The entire universe would move at a constant speed and they would have no way of altering that speed. They could only interact with other objects that existed at the same point in space-time (all three dimensions, effectively) but they had no way of altering their movement through the third dimension, which would be equivalent to time travel to them.

Project this into our Universe, everything we can perceive is moving through the Universe at a constant speed (objects moving through time at a different speed would appear to blink in and out of existance instantly as our paths crossed). We cannot alter the rate at which we move through time but we have seen that our movement in the "normal" three dimensions can affect the speed at which time passes relative to a third party.

I know that this is probably full of flawed physics, but it does provide a way of explaining time travel paradoxes. If we could move through time, we would not find the past or the future as we would see it, we would see different Universes that simply moved through time at different positions (for example, if you drive back down on a road, you cannot see yourself drive along it again, you simply see other cars moving at different speeds along the road). So there is in effect, no past. Simply a present that is constantly changing. It could be that we experience time passing because it is impossible to move in space without also moving in time, so if time were to "stop" all movement would also stop. Likewise, we as we approach the speed of light, time passes more slowly relative to our viewpoint because we are moving faster in relation to time.

I realise this is all pretty far fetched, and I don't consider it to be true for a second, but I feel it is an interesting idea (hence its presence in fictional models of time travel).

Time travel is a subject of great interest to me - not the actual, you know, nuts and bolts physics of it, but the various possible models that can be imagined. I've written a variety of fiction based around these. The most important thing, I find, when coming up with a new theory, is to try to trip it up in every way imaginable and make sure it still works.

Here I go with every model of time travel I can think of.

Time travel is impossible

By FAR the most scientifically likely of all the various possibilities. There are no time travellers because it's impossible to travel through time. Time is an illusion, perhaps, not even a proper dimension. Spacetime is a rigid crystal with worms (us) stuck inside, never changing.

This is also by far the dullest time travel model, and is implicitly used in almost every piece of fiction ever written.

Time is a loop

The postulation that time is a loop - that is, after the universe ends in a Big Crunch, it goes back to the original Big Bang and starts all over again exactly the same - essentially doesn't change the fact that time travel is impossible. As nothing coherent can actually survive the Crunch and Bang, nothing can actually go all the way around the loop and be present in its own past.

Fixed History model

This states that:

  • Time travel IS possible.
  • There is only one timeline.
  • History CANNOT be changed. Any time you try to change history, it will turn out you were supposed to make that change all along.

This model, assuming it allows an individual to travel back in time to their own past light cone, allows individuals to interfere with their own past and thus necessarily includes some level of predestination paradox. For example: somebody comes from the future and tells you to get into the time machine. You get into the time machine. You go back to the past. You tell your past self to get into the time machine. It is also entirely possible for universes employing this model to include causal loops: ideas and objects which have no origin point.

The majority of stories involving time travel make use of this very simple model. The major difference within stories is in how the universe resists changes to the timeline. For example, if you go back in time and try to kill Hitler or your own grandfather, something, somehow, will stop you. But how? There are four ways I can think of, four sub-models of time travel:

The timeline is preserved through dumb luck

This is a favourite; it's used with varying degrees of seriousness in 12 Monkeys, Futurama and The Hitch-Hiker's Guide To The Galaxy.

The timeline is actively protected by an intelligence of some kind

The degree of interaction can vary here. We can have anything from human intelligences who send things or people back in time because historical records SAY that was what happened (Bill & Ted's Excellent Adventure, The Terminator) to (occasionally cosmic) police forces which actually have to go back in time after rogue time travellers to stop them from wrecking the timeline (DC comic book universe, Doctor Who).

But in some cases it's dangerous to put certain works of fiction under the heading of "history can never be changed" just because history never IS changed. Just because a timeline is always preserved doesn't mean that it's impossible for the timeline to be altered.

Indeed, if it really was impossible to change history, such active preservation would be completely unnecessary!

The timeline is preserved by quantum

Another possibility is detailed in Stephen Baxter's book "Time", in which it seems that time travel involves the sending back and forth through time of quantum packets of information, which alter history multiple times in a kind of feedback loop until the universe settles into a stable structure in which the instance of time travel is completely internally consistent.

Time travel isn't always possible

Alternatively, one may have a universe which only allows time travel at certain instants in time and space and only allows a single certain object to travel through time at each instant. This means a time machine will only work at predetermined points in time and will be completely non-functional the rest of the time, or if the wrong thing is in the machine.

In this case the universe is kind of like an office building, with time being height, and a finite number elevator shafts leading between certain floors symbolising individual instances of time travel. You can't go back in time unless it's predetermined that you will, and everything will work out fine: if you try, you'll fail.

This model is approximately used in Time Bandits.

Malleable History model

This is the model which is used (which slight customisations) in the Back To The Future movies and in Star Trek.

  • There is only one timeline.
  • History is a resilient beast. You can travel back in time, and alter history slightly, but generally you are free to wander around and as long as you don't do anything hugely anachronistic you can get away with it, and return to the future safely.
  • However, there are certain points in history with relatively large "historical imperative", events which are almost fated to happen. If you alter these, it can radically affect the future. The death of Edith Keeler. The point where Marty's dad first meets his future wife. First contact. Mess these up, and future history will jump onto a very different track. But - here's the important thing - you can restore history to the right track if you set everything approximately straight.

This doesn't actually make a whole lot of logical sense. Scientific impartiality necessitates that every point in history be as significant as every other point, and chaos theory dictates that small changes to a highly complex system such as the Earth's weather system can result in massive changes in the long term. Over the course of a generation, even if you reset everything to approximately back how they're supposed to be (Marty's future parents meet five days later than usual, hey ho), the changes will add up again. Sooner or later, not to put too fine a point on it, different children will be born to the same parents, and after only a generation or two, the whole world will be being run by different people, and thus the whole world will run differently.

Of course, intuitively this is a difficult concept for the non-scientifically-minded to grasp so it's easy to see why this isn't the case in these two important continuities.

Important sub-models:

Back To The Future

BTTF contains elements which make relatively little scientific sense. For one thing: alterations to the timeline take time to propagate. For example, Old Biff Tannen goes back in time from 2015 to 1955 and changes history so that 1985 becomes dark and nasty, but then he returns to the good 2015, which somehow has not been erased yet. And changes don't propagate fully, either: Marty partially disappears, but there's no way that he could somehow be partially born. And changes don't change things consistently. Marty's photo of Doc's gravestone changes to a photo of a blank gravestone, then a photo of just random grass - so why doesn't the photo disappear completely? Why would past Marty have taken a photo of nothing, and why, for that matter, does he remember what the gravestone used to say when the photo doesn't? Shouldn't his mind also change?

Most of these inconsistencies are dramatic elements more than anything else. A consistent model to explain BTTF in its entirety can probably be constructed, but not easily.

Star Trek

Star Trek has run long enough to include examples of all kinds of other models of time travel, as well as plenty of crazy rubbish which makes no sense. The model given here is approximately correct, but there are technicalities.

Sensitive History model

This is the version of the Malleable Future model which incorporates the chaos theory elements I mentioned above.

  • There's only one timeline.
  • Any time you go back in time, you change history.
  • You can NEVER set everything perfectly back the way it was.

How big a problem this is depends on how far back in time you go. If you go back five minutes, who cares? But if you go back to before you were born and change history enough that you were never born, or worse, somebody else was born in your place (your sibling, by most definitions), you're in trouble.

Of course, there are bigger issues here. What if you go back in time and stop yourself getting into the time machine? It's a common misconception that this would cause a paradox. There is NO SUCH THING as a paradox. They can't exist, by definition. And since you, just before getting into your time machine, didn't meet anybody right beforehand, that means the person you're looking at CANNOT be your past self. So you can stop him getting into the time machine, and there will be no problem, except that there's now two of you! One of whom just appeared out of thin air. This violates the law of conservation of mass-energy, but hell, we're screwing science over something rotten already, aye?

This model is used in Schlock Mercenary.

Pause for thought: Why single-modifiable-history models don't work

The major problem with both the Malleable and Sensitive History models is: where are all the time travellers? Suppose you get in a time machine and go back in time. The universe alters itself, starting from your re-entry point with you appearing instead of, say, nothing happening. But what makes you and your perceptions special? What makes this new universe one in which you appear in mid-air out of nowhere, but nobody else? Theoretically, anybody could appear out of space at any time, having travelled back from some hypothetical previous timeline. What stops them? The answer: nothing, really, unless time travel is REALLY, REALLY hard.

This means that basic history in any fictional universe which utilises these models SHOULD be absolutely crammed to the gills with timelines. The entire universe is horrifyingly unstable.

Also: how do the changes occur? For a timeline to initially be like this but then change to be like that, something - something very like time - must have passed. Somewhere along this secondary time axis there's an initial timeline and further along there's the timeline we're in now. But we just said there's only one timeline, when clearly there are at least two!

So from a standpoint of a fiction writer, these models are problematic, or at least contradictory. But we can solve these issues with the following:

Multiple History model

This is the simplest consistent model of time travel I could devise which allowed history to change, but also explained the lack of time travellers.

  • You can go back in time whenever you like.
  • Every time you go back in time, you create an entirely new timeline.
  • If you go forwards in time, you stay in your current timeline.

This means you can go back in time and change history. Easily. But that act creates a new and different timeline. You can never return to the timeline where you originally started, because going forwards in time just leaves you in the timeline two and going backwards in time for a second attempt would leave you in a third timeline. The net effect in your home timeline is that you have simply disappeared forever. If you go back in time to kill Hitler, you don't undo the Second World War, it stays happened, you just give everybody involved a chance to die all over again.

This is the model of time travel first described in my story Be Here Now and explored very thoroughly in later Ed stories.

Nominally this results in a finite (but large) number of timelines running parallel with each other, as people go back in time over and over and create more timelines. However, it could also be that the timelines simply run sequentially, one after the other. After the original universe ends, the first universe that was created through time travel starts over right away, and so on until they're all done.

Obviously this model comes in the Malleable and Sensitive varieties described above. The Malleable version COULD be the version used in Star Trek and does seem to be the version used in Terry Pratchett's novel "Night Watch" - this would mean, when you go back in time, you DO create a new timeline and alter the universe, just not significantly; possibly insignificantly enough that the two slightly disparate timelines could even be recombined at a later date! (Something very much along these lines happens during DC's Crisis On Infinite Earths.)

A variation on this model could have timelines adjusting slowly into one another across this "secondary time" axis, instead of just instantly changing. Adding in the condition that every time traveller remains "tied" to their "original era", this would enable the scenario seen in the DC crossover "Zero Hour", in which people born thousands of years ago disappear in increasing order of birthdate, and people from the future also disappearing, as history is "eaten up" from both ends.

More imaginative structures

Consider a timeline as a line on a page, with a definite beginning (the Big Bang), but possibly no end. Now let that timeline twist...

Two histories create each other

This is a variation on the Fixed History model. When you go back in time, you also switch tracks into a different timeline entirely. You're free to change history because it's not YOUR history. Somehow, somewhere, somebody goes back in time and makes changes to THEIR history to thus create YOUR timeline. It's less of a causal loop than a causal figure-of-eight. This model becomes much, much more complicated once more than two instances of time travel are introduced.

This model is used in my story The Four-And-A-Halfth Planet. I realise this node is sounding more and more like a promotional article for my fiction. It's not - as I say, I like exploring different models of time travel, and I like writing fiction. And clearly, people aren't imaginative enough!

Helix

Time travel is possible, but you can only jump backwards or forwards by a certain fixed amount of time - every other era is inaccessible to you. This kind of model seems to be implicitly used in many time travel stories when some important character is accidentally blasted backwards or forwards through time, and other people hurry to rescue them. You'll get, say, two hours passing in both eras before the stranded person is brought home again.

The Sherri S. Tepper novel "Family Tree" explicitly uses this model with a period of 3,000 years.

Two timelines going past each other in opposite directions

You can't go back in time - all you can do is jump tracks. But if you want to go back in time a year, you just jump tracks, wait a year, and jump tracks again! Each track-jump sort of "knits" the two universes together a little. Of course, this means that local time in the two universes will be different. You would have to calculate the point where the two clocks meet to be somewhere contemporary to get a decent story out of this.

One timeline, which suddenly reverses direction

Maybe it reverses many times. Who knows?

Keep going along these lines. You can get CRAZY stuff out. There's more I haven't said, and, naturally, the dramatic possibilities are limited only by your imagination (apologies, those of you with poor imaginations).

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