A.K.A. How to Recognize Your Chronoverse

You know, there’s not just one way to time travel.¹ There’s no best kind of time travel. Yes, some time travel might have more of a scientific basis than others, but others might work better for the narrative at hand. There’s really just time travel that works best for a given story.

I’ll start by looking at some of the different ways the timeline itself could react to time travel and how to recognize them. What I’m going to talk about here is what I call chronotopology, the shape of the timeline. It’s the most important facet when analyzing time travel.

Single Static Timeline: You can’t change the past

A single static timeline (SST) is the simplest form the chronoverse can take—there is one timeline and it cannot change.

How to recognize SST:

The past cannot be changed.
At all.
Right down to the smallest detail.
Also the future cannot be changed.²

If a chrononaut traverses a single static timeline, it creates a stable time loop. “Time loop” should be a pretty intuitive term–time moves forward, then our time traveler loops back to earlier, time continues to the point where they went back in time, and the cycle continues. The “stable” part refers to the fact that the events don’t change. They exist without contradiction. There is no force that can break the loop, so it is stable. In physics, it is referred to as a causal loop. In other contexts it may be referred to as Jigsaw Time Travel, Block Time, Eternalism, or Novikov Time Travel. I prefer the term “stable time loop” because it lends itself to other types of time loops, like unstable, metastable, elastic, and inescapable.³

Most modern physics conceptions of time travel are, in one way or another, based on the SST model. As such, if you want to be as scientifically accurate as possible, you might lean toward SST. You can read about the scientific concepts in Black Holes & Time Warps, by Nobel laureate Kip Thorne, or From Eternity to Here by Sean Carroll (both of whom work in the physics department at Caltech.⁴ I guess Caltech just attracts time travel research).

Perhaps the most famous example of a single static timeline and stable time loop is The Terminator (at least if we only consider the original movie). As the tale goes, there is a robot uprising, and a man named John Connor saves humanity. In a last ditch effort to save themselves, the machines send a robot⁵ assassin back in time to kill John’s mother Sarah before she has John. In response, John sends his friend Kyle back in time to protect his mother. Kyle and Sarah fall in love, Kyle and the Terminator die, and Kyle ends up being John’s father. Also, the company (Cyberdyne) that invented the robots in the future recovers the Terminator’s wreckage, which leads them to developing the robots in the first place.⁶

All the events fit together without contradiction. Past events cause future events, which go back in time and cause the past events. That might sound a bit contradictory. In fact, there’s a term for events that essentially cause themselves: the bootstrap paradox. Breaking the line of causality is an interesting prospect for a number of reasons, but let’s skip ahead to the crazy conclusion: there isn’t actually any reason that causation has to be a one-way street.⁷ So the bootstrap paradox doesn’t really matter!

So, what happens if someone does try to change the past on an SST? Well…they don’t succeed, for one thing. If something didn’t happen, then it can’t happen. If something did happen, it can’t unhappen. The timeline is immutable and the past cannot be changed, therefore causality itself must arrange the series of events so that it forces the timeline to be correct.

Let’s take the classic grandfather paradox: your goal is to go back in time and assassinate your grandfather (let’s say with a gun) before he can father your father. As we’ve established, you can’t successfully go back and kill your grandfather, but that’s not the interesting part. The interesting part is why you can’t kill your grandfather; or rather, that there is no constraint on why:

The gun jams.
You miss.
You only injure your grandfather. Maybe him recalling this is why he treats you so poorly that you want to go back in time and kill him.
Your grandfather thwarts your attempt and counter-murders you.
Someone else thwarts your attempt and you are arrested.
You get lost and can’t find your grandfather.
You successfully kill the man that you thought was your grandfather, but it turns out someone else was your grandfather. In the worst scenarios, this someone else might have to be you (blech).
You step on a land mine and explode before you get to your grandfather.
Your time machine doesn’t go to the right place.
Your time machine doesn’t go to the right time.
Your time machine just straight up explodes.
You have a heart attack before you can kill him.
You think better of it and decide not to kill him.

You’ll notice a non-trivial number of these involve some terrible misfortune befalling you in order to prevent you from succeeding. You can try your best to prevent any of these, but if the timeline is immutable, you already know that your assassination attempt is a failure. Something will go wrong, so you are most likely better off choosing that last option of your own free (ish) will.

Multiverse: You can change the past

That’s great and all, but half the point of going back in time is to change the past, so let’s assume you can. Well, by definition, you can’t change history. If something happened, it can’t unhappen. That means that if something different does happen, it must be on a different timeline, a different universe. So when someone goes back in time and changes something, they don’t change their own native timeline, they change a wholly different one. We can call the collection of all the relevant timelines the chronoverse. If there’s only one timeline, that timeline makes up the chronoverse. If there’s 25 timelines, they all comprise the chronoverse.

By contrast, a multiverse just means there are multiple universes, but not necessarily multiple timelines. Since I’m talking about time travel, though, generally I will be referring to multiple timelines when I talk about the multiverse.

There are two basic ways to have a multiversal chronoverse: we start with one timeline and it branches out into more, or all the timelines are already there side-by-side.

Branching Timeline: The timeline splits

The most obvious form of multiversal time travel is branching. When time travel occurs, everything before the traveler’s arrival stays the same, but everything after they arrive is different.

Funnily enough, branching timelines aren’t as technically complicated as static timelines. That’s because we don’t have to fit time travel into existing chronology like a jigsaw puzzle; anytime something changes, we just split it off into a new timeline.

Our standard conception of causation returns now, too. It’s just that everything from one timeline happens before the next, and then time travelers arriving on the new timeline is something of a causal anomaly on the timeline. Beyond that, it’s normal linear causation. For example:

Here we have a timeline where some guy named Matt’s parents die in an accident. Then he goes back in time and rescues them. The timeline thus branches. In this new timeline, everything is perfectly normal until this older version of Matt arrives from the previous timeline, causing it to branch off of the old one. We can point out that the timelines don’t branch when Matt saves his parents; they branch right when he arrives from the future, because he didn’t arrive from the future in the original timeline.

Branching timelines in time travel are often mixed up with the many-worlds interpretation (MWI) of quantum mechanics. MWI is the idea that any time a truly random event occurs, the universe splits. If you entangle a pair of electrons so they have opposite z-spins, the universe will split into one universe where electron A is spin up and B is spin down and a different universe where A is spin down and B is spin up. The reason I bring this up is because a coin flip is not a truly random event. Making the choice to go to college instead of technical school is not a truly random event. These are deterministic events; the randomness is just from us not being able to measure all the variables adequately. In sciencey terms, this is called chaos. MWI only applies to quantum mechanical events, because only quantum mechanics experiences randomness that can’t be explained by causation. By the same token, time travel wouldn’t split using quantum mechanics because time travel is also not truly random.

Preexisting Timelines: An infinite multiverse is already there

What happens if going back in time doesn’t create a new timeline? What if that new timeline was there to begin with? What if every timeline already exists since the beginning of time?

Preexisting timelines (or multiple static timelines) are entire universes separate from the universe a traveler leaves. They are simply offset in time from each other. In one universe it might be 1998 and in the other it could be 1999. So going back in time is really a matter of travelling through dimensions to a different universe rather than travelling through time. The timeline doesn’t need to branch, because it’s already a different one from the one the traveler left.

There is a possibility that multiple travelers could go to the same timeline, so it could already be changed by the time you get there.

Differences between Branching and Preexisting

So if the past is mutable, how do you know if the timelines are branching or preexisting? The most obvious difference is that preexisting timelines are easier for multiple travelers to get to independently, so you could easily end up on a timeline that doesn’t share your past. Thanks to quantum mechanics, chaos theory, and a looong timescale, small changes might develop even without time travelers. On branching timelines, every time someone leaves, they will split off into a different timeline.

That aside, there aren’t a lot of noticeable differences, unless you can observe the timelines from the outside. Obviously, if you see a bunch of timelines that are all the same size, it’s preexisting.

Elastic: You can change the past…a bit

What if the past can change, but only a little? You can change the details, but not the overall picture. The idea that the past is resistant to change, but not immovable, could be referred to as historical inertia. The particular events that can’t change have different names in different stories, including convergences or absolute points, but I will use the term node, since that’s usually what we called unmoving points back when I did science.

The most common sort of node you’ll see in stories with elastic timelines is someone’s death. Using time travel to stop someone from dying can apparently be a very difficult prospect.

Historical inertia is not an altogether scientifically sound concept. There’s no reason that events need to happen. If an event is prevented, it shouldn’t happen. For it to happen anyways, it would mean there’s some form of higher-order causality permeating the multiverse, forcing things to happen.

Elasticity is not really recognizable in a single static timeline, since nothing can be changed anyway. Elasticity as a property could be applied to any type of timeline except static.

How to Recognize elastic timelines:

Some events in the past can change.
Some events in the past can’t change.

Dynamic Timeline: The timeline is malleable

Saved for last is perhaps the most complicated and least scientifically consistent. Ironically though, it is the first one people usually talk about—the dynamic timeline, a single timeline where the past and future can change.

When we think about the grandfather paradox, it’s description is like that of a dynamic timeline: If you go back in time to kill your grandfather, you are never born, so you don’t go back in time, so your grandfather doesn’t die, so you are born, so you go back in time to kill your grandfather. The timeline itself changes back and forth as a reaction to time travel.

These changes propagate through the timeline, as timewaves. So time itself changes as it progresses. It gets mathematically complicated, but we have to measure time in the universe, universal time, differently from the time it takes timewaves to propagate, causal time.

This is most clearly seen in Back to the Future. Marty goes back in time, prevents his parents from getting together, then he starts to fade away because he is not going to be born. Whoops. If he was in a multiverse, he wouldn’t fade out because he’s just from a different universe. The fact that he fades out means that he must be in the same timeline.

The problem, scientifically, is that these timewaves make the timeline discontinuous. They make the whole universe discontinuous. In a multiverse, matter can just move from one timeline to another, but in a dynamic timeline, things just pop in and out of existence. Events don’t fit together. I mentioned earlier that causality isn’t important, but consistency is. If spacetime doesn’t fit properly, physics can’t work.

How to recognize:

Past can be changed.
Timeline is overwritten around you. Things fade in/out of existence

Conclusion

These are just basic categories, but there are many more. You might have one universe that repeats itself entirely, or maybe a preexisting multiverse with a finite number of timelines, or combinations of different kinds. However, the vast majority of time travel narratives can be slotted into some variation of the above 5 topologies. We will look at many more, and in greater detail in The Chronoversal Compendium.⁸

Footnotes

I mean, there’s generally only one way to do it in a given chronoverse.
Because the present is the future’s past and the butterfly effect says that changing the present changes the future.
We’ll talk about these later. They’re fun.
As much as I’d like to say I work with them and leverage that into evidence of my own authority, the physics department is actually pretty big and I don’t see Sean and Kip much. Kip is actually emeritus, so I don’t think he spends a lot of time on campus nowadays anyways.
Despite the T-800’s insistence that it is a cybernetic organism, a cyborg generally has organic parts as its base (hence cybernetic organism, not organic robot), augmented by technology, rather than vice-versa. A Terminator’s organic parts do not contribute to its abilities at all beyond camouflage, so I feel it is more accurate to call it a robot or android than a cyborg.
This was part of a deleted scene, but it was revealed to have happened for the sequel, so I am counting it as part of the first movie for this discussion.
Except maybe entropy.
Real subtle self-promotion, Zach.

The Basics of Time Travel series:

Basics of Time Travel: Chronotopology