If time is one-dimensional, then one of two things will happen.  Either the timeline is immutable and all actions you take in your travels are the same actions other versions of you took, or a the timeline is changed/overwritten and the universe will overwrite the previous timeline, get caught in an infinite loop, correct itself to the best of its abilities, or explode/implode in paradox.

If the universe is two-dimensional, then alternate realities exist as the timeline splits from various choices made.  The differences in realities depend on sensitivity to initial conditions, but that’s beside the point.  The mere presence of the time traveler sends him down an altered path in the multiverse.  Given this second theory (which seems to fit the Narbonic model much better), there is a small jump from traveling to alternate pasts to creating a dimensional gate to jump between alternate presents and parallel realities.

Finally, the fate of the time traveler depends on the nature of the time travel.  If time cannot change, then the traveler goes on his or her merry way, except that it is shown that free choice is a crock because it’s what he or she has already done.  If the universe is self-correcting or overwrites itself, then the time traveler will either fade out of existence, certain details change in each iteration until the loop is broken, or the time traveler becomes a rogue without a home reality.  If the universe explodes, then goodbye everyone.  If there are alternate timelines, then the time traveler also goes on his or her merry way, but free choice is still maintained.

Like is said, don’t get me started.  This was without diagrams.

“In an infinity of worlds, anything is not only possible; it’s mandatory.” ~Neil Gaiman

I think this strip strayed into Fantasy a little too much, but, hey, that makes it more fun!

(And we’ll ignore Terminator 3 entirely, because it was just nonsense.)

And I only wish that the Terminator stories were the worst offender.

Any time travel story (and remember that prophesy is a form of time travel, just information instead of people) has to deal with the issue of causality, the basic logical premise that things that happen effect things that happen next, which in turn effect things that happen later.  Lazy writers assume that only the events that we want to change will be changed, but consider how unlikely that is – go back in time, buy a gun and kill a dictator, and you have changed not only the dictator-related timeline, but the timeline relating to the gun shop and that gun.  If you waited in line, you slightly delayed dozens of people – maybe someone misses a bus, or doesn’t step in front of a car.  It goes back to the Butterfly Effect – small changes have significant unintended consequences.

There are thee ways to deal with causality: the first, and tragically most common, is to simply ignore it.  Time travel is magic, or the Universe is being guided back to a ‘proper’ timeline by some godlike force.  When you return from the past, your friends will still be the same people, with the same personalities and wardrobes, despite you having altered the course of history.  You might suddenly be the Commandant of a police state, but you will still hang around with the girl who won’t date you, your goofball college buddy, and the snide jackass nobody likes.  Why? Why is your group of friends the only immutable facet of the universe?  These can be fun stories, but you can’t think too hard about them, which makes them poor Science Fiction.

The second way to deal with causality is the Closed Loop timeline.  Douglas Adams actually used this in his Hitchhiker’s Guide to the Galaxy – you can go back in time, sure, and try to change things, but remember that whatever you are *going* to do has *already happened*.  This comes up often in prophecy stories – by trying to avert the prophecy, the hero causes it to occur, because it was always going to happen.  This turns the whole time-travel story into a tragedy about destiny – you can’t save her, you can’t stop the bomb, and you won’t ever, ever, get there in time, because it has always happened that way.  The satisfaction I get from the good logic in these stories never seems to overcome the hopelessness of the basic premise.

The third way is the Branching Timeline.  This is the most useful form of time-travel universe to be in, because knowing the future changes the future – as soon as you start to change things, you take your memories of your original timeline and become a member of a second timeline.  This is what happened to Dave in Narbonic.  Although the Butterfly Effect says it is difficult to predict how the future will unfold, you can change things.

My pet peeve regarding time travel stories is when the author tries to include a Branching Timeline and a Closed Loop in the same story.  Logically, if you can change *anything*, you can change *everything*; conversely, if you can’t change something, you shouldn’t be able to change anything.  The worst offender in my mind here is the Terminator series – we can change the date of Judgement Day, we can kill the primary developer, but Skynet will always become sentient and Judgement Day will always occur.  The only reason for that combination of free will and destiny is some kind of magic, and I don’t need magic in my time-travelling robot assassin movies. 

First consider if there is an inertia to history.  If history is altered, does it try to push itself back into its original shape?  If you save the Archduke Ferdinand from being shot, someone else will do it a week later, and World War One still happens.  If you kill your own grandfather, you might end up taking his place (after all, you still have his genes, and Grandma was a hottie in her day).

On the other hand, if your changes in history are permanent, then one person will go back in time to alter history to their liking, another person will try to undo the changes the first person did, then someone else will try to make further changes … with enough time travellers, the laws of probability state that eventually, someone will alter time in such a way that time machines can’t be invented.

Niven’s Law:  If the laws of physics permit time travel, then no time machine will be invented in that universe.

The thing about today’s strip is that Dave is going to/has already created a new history while still remembering the old one, and we, the viewers, are going to meet someone whose mind blocks the irrational.

Not sure about paradoxes triggering a universe-wide segmentation fault, though. It hasn’t happened yet, but various intimations on quantum immortality such as Mr. Shiga’s “Meanwhile” suggest that even if universal destruction was possible, we, the readers, would only see the storyline where it never happened.