Time travel: Being a bit more thoughtful than most fiction

By Joel K. Pettersson. Added 2021-11-20. Updated 2021-11-28.

This is not mainly about the usual time travel paradoxes, nor about the simplest way to avoid them in fiction involving travel into the past (the idea of such travel forming part of a closed loop with rigidly determined events, nothing changing because the travel led to itself). Instead, it's speculation about a broader picture of change, chaos, and things beyond people.

The following may be technically wrong; it simply tries to take more basic known things into account than much speculation does. Fiction in which time travelers change the world they came from in a neat and simple way by changing its past essentially depends on deliberately not thinking too hard, because that would destroy the narrative.

Timing is important

What would it take to prevent a person from being born, something which often plays a part in time travel paradoxes? If someone were to travel back to and appear in the past before you were conceived, and approach one of your parents and simply talk about the weather a little, and then go away and disappear, it would almost certainly erase your existence, simply due to the small difference in timing introduced.

The timing of the influenced parent's life would be a little different, leading to cascading differences in events and their timing (and timing and more in the lives of others). As for you, a small difference in the timing of events would almost certainly be enough to lead to a different genetic combination at the moment of conception, if events are still that similar, meaning that someone else may be born, but not you.

That same type of change would of course extend to all the other persons whose life proceeds with altered timing and future generations arising from them, spreading across the world until most future lives are erased and replaced. The question becomes, how is it possible to travel to and interact at all with anyone in the past without bringing that type of change about? In turn, can you as much as sneeze at the past without erasing and replacing yourself along with everything else to come from that past?

What about free will in the present?

If free will is really free, i.e. the choices made by people are not merely the results of prior causes but something more enters into them, then choice becomes a mechanism of timeline alteration. After all, at each moment in a timeline, each free choice then adds something to the chains of cause and effect which wasn't there before, a causal factor which is part of shaping what follows.

The effects of each choice compared to if it had been otherwise may expand to a very large scale in a chaotic way, as in the earlier thought on time travel and timing. Potentially, any trivial everyday choice, by any of the billions of persons on the planet, could easily erase and replace all but the nearest future of humanity, including the existence of most belonging to future generations and their experiences. If this is considered to happen all of the time – as the general idea would suggest – it makes for a view of "the future" as being something constantly in flux, a little like what's in a stream of noise, each possible future like a snapshot of static. Except that there are going to be larger patterns more resistant to fluctuations, and also patterns to the chaotic fluctuations.

Considering time travel, the nature of free will would complicate matters further, as each timeline is then no longer really a simple line, but something far more complex and messy. From the perspective of the future – or perhaps a place outside the line of time – what is then the definite history, as possible choices branch out and freely change independently of prior causes across the whole of history? What's then the difference in the nature of versions if they are compared, if any?

Branching timelines

As the above thinking goes, it seems the past can barely be changed at all without doing so ending up thoroughly reshuffling the deck from which the future is laid out. And if free will makes the future open, that deck may be constantly reshuffled just as dramatically by the smallest everyday choices people make. Continuing these lines of thought, then in light of such a chaos, is there a larger order? At this point, it becomes tempting to take the plunge into considering time as something multi-dimensional.

It's common in fiction and metaphysical speculation to deal with ranges of timelines, or "parallel dimensions". Sometimes a two-dimensional time is considered in which past-to-future is one dimension and a range of possibilities for what may be is the other dimension, also allowing time travelers to travel in both dimensions and end up in different histories which all continue existing. This can be a solution to paradoxes caused by e.g. erasing oneself by changing the past; there's no paradox because there's not one past and future, but two. (Maybe timelines branch out, time travel creating a new history co-existing with the old, or maybe all kinds of strange possibilities are real in one or another "parallel dimension" and all kinds of time travel scenarios are simply part of that.)

Traveling to the past from timeline A and introducing change may bring one from timeline A to timeline B; the old past still exists unchanged in A, giving rise to the time-traveling A-self, even as the A-self becomes a defining part of the alternative history of B, which then also exists. (What if a future B-self then travels to the past in C, and a future C-self then travels to the past in D, etc.? Maybe that makes for another problem, if it is a problem to have a potentially infinite branching. Intuitively, it only seems like a problem if there's not infinite parallel timelines anyway.)

But are timelines other than the current one really real? Some variations on the idea instead make for a no. For example, the idea of a two-dimensional time in which forward-movement occurs in both dimensions and what's current in one dimension is the present and what's current in the other is manifest reality (as in the timeline having a physical existence). Time travel then also involves movement "upward" in the direction perpendicular to linear time, leaving the old history and replacing it with a new. A person or thing moving up still has its causes for being further below, and thus can't be erased by the past changing in the new history.

In the thoughtful but unrealistic story of Chrono Cross, part of its explanation for how time works is basically that. Furthermore, it contains the idea of places where and beings for which the experience of time flows along the second dimension of time instead of the first, so that they can possibly observe timelines changing and even take part in bringing it about. (The story also combines this idea with the more common idea of "parallel dimension" histories that exist side by side, for a third-dimensional time; the main protagonist moves between two similar parallel worlds, but a large-scale destructive rewriting of history involving both also takes place, mostly as part of the complex backstory.) But why are two worlds, or timelines, arranged one way rather than the other (existing side by side vs. only one being real)? The explanations for the structure of it all can be a little too ad-hoc.

Chaos takes less work and energy

A main idea remains that very small changes to the past may have chaotic effects and give rise to a far future completely different in its details, however similar or dissimilar larger patterns may be. If changes were somehow made from a place outside the target timeline, from which results can be simulated and the changes created with precision, how would it be if such history-shaping projects were underway? (Let's grant sci-fi supercomputer capabilities and some means of producing changes.)

In understanding that changing any one thing may give rise to many consequences, mostly unwanted, in order to limit those and get a more "pure" result, there may be the need to create many changes which interact and mostly cancel out in their consequences, instead of just naively creating a main change. However, the stability of consequences produced by such an effort may be limited to a range of time, no matter how competent and powerful the effort, depending on what's aimed for.

Perhaps a larger number of well-calculated and precisely made changes could add up to eliminating noise and making a timeline show only controlled, acceptable differences in events – for a year, or a decade, etc. Some patterns may be easier to maintain, and/or more long-lived, than others – maybe some subset could actually be shown to be stable. But generally, as time goes on, the compensations added would eventually add up less and less. A little like a high order polynomial eventually shoots off towards positive or negative infinity, chaotic effects would emerge over time as differences not compensated for would lead to more of the same.

There's also the possibility of sabotage, or a simple mistake, destroying everything worked for; anyone introducing the most simple, naive, low-effort change to the timeline may possibly wreck a more orderly and effortful change striven for.