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.

Contents

Time travel is impossible

By FAR the most 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 Twelve Monkeys, Futurama, Timeline 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. This is a recognised possible variant on every valid time travel model; for example, it may be that there are no time travellers around us because nobody has built a "receiving station" to accept an incoming time traveller from the future yet, or because there is a blockade across spacetime at some future point in time, across which time travellers are simply unable to pass.

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 a little 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?

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

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 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. But if you go back five minutes, who cares?

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 that's a small price to pay for working time travel.

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 a model of time travel which I described in my story Be Here Now and explored very thoroughly in later Ed stories. Several other people have invented it independently and several other stories can fit alongside it, which is logical, since it is the second-simplest wholly consistent model.

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. Equally, it could be that the original timeline simply ceases to exist when the time travellers depart backwards in time. In this case, it is more as if the time travellers are "unwinding" the universe back to some earlier state and then resuming normal movement through space afterwards -- again, as if the multiple timelines are running sequentially.

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.

Primer

After being encouraged to do so by several people, I watched the movie Primer, which I would describe as the Mount Everest of time travel movies. I could expend a great deal of effort reconstructing the precise network of timelines which the movie follows but it appears that this has already been done.

Primer does indeed obey this Multiple History model, with a few subtle elaborations. Primer's time machines do not operate instantaneously. They are enclosed boxes in which - loosely speaking - time flows backwards instead of forwards while the box is turned on. To travel back in time you actually have to sit inside the box and wait for time to pass (backwards). In addition, you cannot travel back to any point in time you like, only to the point when you originally switched the box on. Leaving earlier than this point is impossible because the box is not turned on. Leaving later (e.g. minutes or hours AFTER the box was turned on) results in illness, coma or (presumably) death. Therefore, time travel in the Primer universe involves, more than anything else, preparation and forward thinking.

Time loops

This is an extremely important subgenre in the time travel genre. If you have watched Groundhog Day or Day Break then you know precisely what I'm about to describe.

The idea is that a single individual is uncontrollably bumped backwards in time to the same moment in time, over and over again. In the two examples described above it is the beginning of the day. The protagonist wakes up, makes whatever actions he wishes, and then at the end of the day he suddenly wakes up in bed again at the exact same time, retaining his memories from all the previous loops... until he "gets it right" and time continues on as normal. In these two specific cases, an actual mechanism or explanation for the time looping is never provided. This is a relatively explicit acknowledgement of a fundamental tenet of science fiction, which is that, very frequently, the science has nothing to do with the true story.

The actual appearance of the timeline as all of this occurs is debatable. Does each loop create a real additional timeline, as in the Multiple History model? Do events continue to play out as normal in those additional timelines? Does the protagonist appear to physically vanish from those timelines when he is warped back to the beginning of the day, or does some kind of "split" occur?

It's clear from the presentation of such stories (the abandoned, "imperfect" timelines are invariably completely ignored) that this is not actually the case. They are personal stories about the looped person, nothing else. Thus we can adapt the Fixed History model to suit this instead. Here, there is only a single timeline, and the protagonist is the only one who continues to experience time in the normal fashion while everything else in the universe repeatedly reverts to the same initial state all around him. From "above", the timeline appears to repeat over and over (all except for our one guy), until the "skipping" is "fixed".

Day Break

In the TV series Day Break, the system is very internally consistent. At the end of the day, the hero's body is actually, literally reinserted at the beginning of the day, preserving his overall mental state as well as his physical state. For this reason, a bullet wound he receives early on the story has to be dressed every morning as he gradually heals up over the course of more days. (Only his physical body goes back in time; his clothes and bandages and the bullet itself do not. This proves problematic if he is badly wounded just before time loops.) In addition, we could logically conclude that if he stayed in the time loop for long enough, he would start to show physical signs of ageing.

In Day Break it is explicitly acknowledged by the hero that the additional, "bad" timelines may still go on existing "somewhere out there", and so his repeated failures continue to haunt him even though everything appears to revert to normal from his point of view.

Groundhog Day

In Groundhog Day the hero is driven to desperation by his inability to escape the loop and attempts (and succeeds in) suicide, multiple times. Nevertheless he still wakes up intact at the beginning of the day every time. Groundhog Day is much more of a "spiritual" kind of movie, though; the story is not a tangled crime mystery but a journey of personal growth for Bill Murray's character, so it would make sense to say that it is his "immortal soul" which gets sent back in time each day, not the rest of him.

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 article 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! I am told that the book The Time Ships also uses this model, though with more than two timelines.

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...

Helix

Time travel is possible, but you can only jump backwards or forwards by precisely a hundred years - every other era is inaccessible to you. The Sherri S. Tepper novel "Family Tree" 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.

Timequake

Kurt Vonnegut's book Timequake is very difficult to classify. In the book, in 2001, everybody is suddenly plunged back in time 10 years but forced to repeat all their actions of those ten years a second time. They watch helplessly while their past selves make the same mistakes all over again. In other words, history goes completely unchanged, unless you count what goes on inside of people's heads. Actually, if people's mental patterns did indeed change, then their actions (which are directly affected by these patterns) would also change, so this can't actually be what happens either. The phenomenon is better explained as simply having your memories of those ten years replayed, at maximum clarity, at high speed, all in that single moment in 2001 - in other words, no actual time travel occurs, just a perverse trick of memory.

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).