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The New Time Travel

By Jeremy Colangelo

What then is time? Provided that no one asks me, I know. If I want to explain it to an inquirer, I do not know.

- St. Augustine, Confessions

Is it possible to travel to the past without inevitably changing the future such that the act of travelling through time is itself annulled by the result of that travel? This question, known as the “Grandfather Paradox” – “what would happen if you went back in time and killed your own grandfather before he met your grandmother?” – has been at play in one way or another in every time travel story which involves visiting the past, and it relies on the assumption that a change to the past changes the present. Sometimes, in stories, these changes are desired by the time travellers themselves (as in The Terminator), or perhaps the plot revolves around avoiding them (as in Back to the Future), or perhaps in showing how the changes are always-already there such that the time travel doesn’t lead to paradox at all (as in Star Trek IV, where Scotty delivers the formula for transparent aluminum to the man who will go on to invent it). In each of these cases, travel to the past is assumed to either affect or to have already affected the present, and this assumption guides and creates the conflict of the plot. Conversely, time travel stories about visiting the future (H.G. Wells’s The Time Machine being the inescapable example) are far more free to structure their plots on the immediacy of the time travel situation: in Wells, the conflict about the Morlocks and the Eloi is able to simply play out, simply be a story, without constant questions about the danger of changing the future to come. Nothing that the future-travelling protagonist does once they reach the future has the potential effect of eliminating their act of time travel, and so there is no danger or question of paradox which the story must address, which changes the way that such plots work. 

Time travel has not (yet?) been achieved, and so the question of what would actually happen if one went to the past remains a scientific mystery. But if speculative fiction writers were required to always adhere to the strict limits of what is known and knowable, there would be no Star Wars, no Lord of the Rings, indeed many classic and important works going all the way back to Frankenstein would be unthinkable, and the world and the genre would be far worse off. What is needed is not a really-existing scientific theory of time travel, but rather a way of imagining the nature of time such that the grandfather paradox does not exist. Before science fiction, then, there must be metaphysical meta-fiction (or perhaps one could say, metaphysical fan-fiction). A framework must exist for imagining how time travel could, hypothetically, work, in order to enable new kinds of time travel plots. That is the task of this essay. 

But, to paraphrase Carl Sagan, in order to bake a time-travel story from scratch, you must first invent spacetime. So, before we begin imagining what it would be like to travel through time, we have to begin developing a way to imagine time itself – one which, though perhaps not true, is coherent enough and grounded enough in science as it currently exists and in reason as we are able to conduct it that it could form the basis of a plot which achieves plausibility. What is space? What is time? And how much of spacetime is there? 

I start with the question of “how much?” because it is fundamental to the act of travelling as it is understood on an everyday human level. Before beginning any serious journey through space (which always involves travelling through time) we quite commonly ask how far the distance is, how long we will be travelling: “Are we there yet?” If I build a time machine and then start using it, I might rightly ask myself “how much past is there to travel to?” Or maybe, “are we then yet?” To put the question another way, how big is time? 

To answer that question, I put forward the possibility that space, though it will exist forever, is at all times finite. There is only so much space at any given time, and if one were to travel forever in a single direction one would eventually arrive back where one started. 

I have a couple of reasons for thinking this, and not least among them is the fact that the greatest philosopher of the twentieth century, Albert Einstein, thought something similar. But one can also arrive at this conclusion logically. Consider the contrary position, that space is infinite. A corollary to that assumption would have to be that there are points in space which are infinitely distant from each other. Somewhere in the universe is at least one point-A and at least one point-B between which the distance is equal to infinity; if that is not the case, then in what way is space infinite? How could it be said that there is an infinity of space if every point in space is a finite distance from every other? 

Because we are so used to thinking of space as enormously huge and unfathomable (which, do not mistake me, it very much is) it might be tempting to think of it as infinite. There have been humans who thought the sea was infinite, too, that its bounty would be endless and that it would never be full no matter how much waste and trash we put into it. Neither of these assumptions would prove true, but their existence shows how people who encounter an overwhelmingly large object can take it to be infinite, how “this must go on forever” is a not-uncommon response to the experience of something vast. But there is an important difference between a space being enormous and a space being infinite, for by definition infinity is unreachable and unreducible. If one has a bucket of infinity apples and one removes an apple from the bucket, there are still infinity apples in it, and if one continues removing apples from the bucket over and over again for a billion years, the number of apples in the bucket will remain infinity while the number of apples one has removed will be merely very large. To be infinite is to be irreducible; to be finite is to be reducible. So the question then becomes, can there be two points in space which are irreducibly distant from each other?

Putting the question in this way allows us to remove issues of practicality from our thought experiment. Of course, one can never reach, for example, the Andromeda Galaxy using a spaceship, and the expansion of the universe (of which I will say more in a moment) means that in some billions of years the distance between our galaxy and all of the others will have become so great that no light will ever reach us from anywhere else. And yet, if on a clear day I look up at an empty speck of the sky, pick a location in space some quite unfathomable distance away, and jump, the distance between me and that point in space goes down. From this fact I derive a hypothesis, that no point in space is metaphysically unreachable, that every distance between every point in space is finite, which is to say reducible. Limits like the speed of light do not affect this outcome, because finiteness can be demonstrated by merely reducing the distance, not completely closing it. 

Part of the difficulty in thinking this way is that if we consider the universe as bounded in some way we are tempted to ask, “what bounds it?” We have this idea of a finite universe as some kind of ball of spacetime held in a space of something which it displaces in the act of expanding and in relation to which one can have a “bird’s eye view” of all existence. While this possibility shouldn’t be discounted (how could we possibly know one way or another if it’s true?) it isn’t necessary for a finite model of spacetime since the question “finite in relation to what?” is only askable in relation to spacetime itself. That is to say, the question imposes a model of understanding developed within spacetime in order to imagine an outside to spacetime, but since nobody has ever been outside of spacetime we have no idea what that would look like and no reason to assume that our intuitions would be applicable. In short, unless someone actually gets out it’s all a big question mark. What we know, however, is that the universe has expanded, and this expansion can be understood as the increasing of the distance between points of space, like how two dots on a balloon grow farther apart as it’s inflated. That balloons displace the air around them and occupy more space in the process of expanding is only a limitation of the metaphor – it is the growing distance between the dots which is significant. 

The fact that the universe began as a single point and then expanded outwards in a “big bang” also gives another reason for hypothesizing that spacetime is finite. Space and time began at a particular point and then expanded outwards. We might say that at one point the volume of spacetime was equal to 1, then it became 2, then 3, then 4 and so on, expanding and expanding at an accelerated rate, driven on by whatever force or process scientists have named “dark energy.” One can therefore see the progression of the expansion of spacetime as a kind of counting, where as time passes (grows?) there comes to be more universe, which becomes larger and larger progressively. But you cannot count to infinity. Infinity is not a number on the number line but rather an idealization derived from the number line’s inexhaustibility, the fact that there are always more numbers no matter how long you count. Thus, even if the universe will exist forever, there can be no point in the future where it has existed forever. Since it grows over time and since it cannot grow to infinite size, there will never be infinite space, even if the amount of space is huge. 

This fact brings us back to time. As Einstein shows us, space and time are really the same thing – spacetime – which we falsely perceive as separate. Really, we do not detect time directly at all, but rather notice change and attribute it to time. Time is, really, another dimension; one can map all of existence on a four-dimensional plane showing the start of the big bang and all the physical space the universe has ever occupied/created all along the temporal dimension (much like one can map how spatial points relate to each other on a Cartesian plane). From the fact that time is a dimension philosophers and scientists have developed a model known as a “block universe,” or essentially a universe where the past and future always already exist, and where the passage of time is essentially a figment of our limited perception. In that model there really is nothing new under the sun: all of being was instantiated at once (or may have in fact always existed) and if one somehow had a “God’s-eye-view” of existence then one would see the playing-out of history as a single continuous object. (The Tralfamadorians of Kurt Vonnegut’s Slaughterhouse Five see time in this way.) This model of time has complex and fascinating fictional consequences which science fiction authors should spend more time exploring, but it is not the model that I am proposing. 

How much time is there? The block universe model says “all of it.” All the time that exists and will ever exist was created effectively at once, and the block universe projects on to eternity towards an infinity that can never be reached. Yet space and time are the same thing, and it is at least thinkable that there is a limited amount of space, so perhaps there has also been a limited amount of time. Put another way, the question is what is the total size of the time dimension? What is the current most distant point on that axis from the origin, what we might call the leading edge of time? 

The size of time isn’t easy to measure, but it can be deduced by asking, at what rate does the time dimension grow? And on asking that question, it becomes apparent that it is the same as asking, how long has time existed? Time has existed since the big bang, and it has grown in the act of passing. Time’s size is equal to its age, and it grows at a rate of one second per second. 

So, to sum up before moving on: we have a hypothetical model of the universe in which both space and time are finite, and where, because of the unreachability of infinity, no spatiotemporal dimension can at any point have a non-finite value. From this it follows that time has a leading edge, that there is a point in time which is the latest of all, the “present” of the universe, after which time does not yet exist. It is, in short, a non-eternalist block universe, one where the past and present always exist but the future does not. I cannot think of a way for one to prove that one’s own present is the true present without the intervention of time travel, but if it is the case that the leading edge is the present we occupy then that would answer the question, “if time travel is possible then where are the travellers from the future?” The answer would be that the future doesn’t exist yet, so there is nobody to come for a visit. 

Next, a bit of terminology that will enable a discussion of how time travel can work in this model and why it would not give rise to a grandfather paradox. We will label the instant in which time began as T0 and label the leading edge of time as Tn (n for “now”). To spare us the trouble of specifying the exact point in time of any past moment, we will refer to points between T0 and Tn with a two-digit number indicating which percentage of the totality of time antedates that point. So T50 is the point at which half of all the time which has passed is in the past, while T01 is the first instant after the big bang and T99 is the last instant before the leading edge. Hypothetical times which exist after Tn are indicated with negative numbers, which reflects the fact that those points in time do not yet exist. 

To return, then, to the question of time travel, one question which stories that incorporate the grandfather paradox often play with is how long it takes for a change in the past to affect the present. In Back to the Future, Marty McFly accidentally prevents his parents from meeting and falling in love, which would result in him ceasing to exist and thus never going back in time, which would create a paradox. In a strict grandfather paradox interpretation, Marty should have been done for as soon as he disrupted that meeting – yet he persists, a man outside of time, for long enough that he can get his parents back together and save his future. But in the model I present here, Marty had nothing to worry about: the change in the past would never have reached his future at all. 

To explain why, let us imagine the pace of time paused for a moment: the universal timeline stopped at precisely the extent it happens to occupy at the moment you read these words. (And it is worth contemplating the fact that our image of “stopped time” is actually stopped motion, a world that stays perfectly the same – a testament to the fact that we do not actually see time, but rather see change and attribute it to time.) This stoppage gives us something stable to think with (yet another limitation of the human mind in the face of reality, that it needs its thinking to be in terms of something stable). Let us imagine this frozen time dimension as a line stretching from T0 to Tn. Now, Marty gets into the time machine (a modified DMC DeLorean) at Tn (for him, 1985) and goes back to 1955, which for ease of reference I’m going to call T50 even though 1955 is far more recent than that. In this version, Marty goes back in time and prevents his parents from meeting, and then unlike in the movie fails to get them back together. He then gets back in the DeLorean and goes home, only to find that nothing is different. What happened? 

The answer is that when he changed T50, Marty changed the chain of causality which would result from the events of T50 to create a new future within the same timeline – not an alternate universe but an orphaned present, one which can no longer be constructed from the events anterior to it. One way to imagine this is to think about what happens when you aim a flashlight into space and turn it on. Though the light spreads out and the signal gets weaker and weaker, the photons emitting from that flashlight will continue travelling until they hit something. Indeed, they will continue moving even after the flashlight is turned off. The beam of light would seem to disappear, but that is only because new photons are no longer replacing the ones that have gone out of sight; what appears to us as a steady and present light beam is really a constant replacement of old light with new light, which ends when the light turns off. However, when the light turns off, the beam of light that had already gone out no longer links back continuously with its beginning. The beam is orphaned, lacking a clear origin, but nevertheless continues on its way until something stops it. But there is nothing which can stop time, as far as we know. 

So, if Marty came from time’s leading edge, changed T50, and then returned, he would find that his present, like the beam of light, has continued on despite being orphaned. Yet because the change exists on the same time dimension for all points in history, should Marty wait a little while (e.g. until T-10) then go back again a little less far, to say T60, he will not find the T60 that existed following T50 as it was before he time travelled, but rather the T60 which is “downstream” from his changes – an alternate 1965 which follows his changes to 1955. Why does Marty need to wait in order to see this new T60? Because following his changes, the time following the new T50 still hasn’t happened. Time grows and proceeds at a rate of one second per second, after all, and the past can be thought of as a “place” that exists within spacetime. 

To get even more radical, one can imagine that any act of time travel changes the past at least somewhat, even if just though the air you displace or the heat and carbon dioxide which leaves your body. Indeed, the body heat would amount to new energy being added to the universe in addition to the matter and energy circulating since the big bang, in seeming violation of thermodynamics. This fact implies that if time travel were possible, we could possibly use it to create endless quantities of matter or energy simply by taking it from the past, secure in the knowledge that (if this model were to prove true) we will not affect the present. One could continually harvest and re-harvest the same crops or build and re-build the same commodities, or extract energy directly from the earliest moments of creation (perhaps by using the big bang to boil a lot of water and turn a turbine, since we seem to always end up doing it that way), thereby circumventing the limits of thermodynamics and possibly serving as a basis for humanity to reverse entropy within its local spatiotemporal environment, allowing life to persist indefinitely in defiance of the fading stars. 

What is also implied by the impossibility of not affecting the timeline is that any act of time travel always orphans its own future. More to the point, entire chains of causality can be cut off from both past and future, creating a kind of bubble of events where certain acts and their consequences are only effective within its boundaries. For the sake of nomenclature, and in relation to our paused timeline, we can name such a time bubble according to its temporal range. Thus, a visit to T50 followed by a visit to T60 creates a time bubble T50-60. Of course, because time doesn’t actually stop this bubble would immediately become T51-61, T52-62, and so on, but the point is that the events within that range would be causally cut off from both past and future, a branch in the chain of causality all on its own. 

To make this description more concrete, let us return to Marty. Marty goes back to 1955 and stops his parents from getting together. We consider this a change because, in relation to Tn and assuming an absence of time travel, the course of events leads deterministically to Marty’s parents meeting. Without time travel, the past is always inevitable: what has happened is always what will always have happened, ineluctably, eternally, no matter how much we want it to have been otherwise. So, all of the events from T0 on collectively add up to Marty’s parents falling in love in 1955 and eventually having children. Then, by time travelling, Marty changes the course of events and produces a past result which was not inevitable, which from the perspective of determinism is the same as saying that an event is impossible: a temporal miracle. By causing such a miracle, Marty creates a new chain of events which will, after the passage of time, produce its own future: a 1985 which is different from Marty’s 1985, and which he could visit if he were to travel back again from 2015. That future could then continue indefinitely, always 30 years behind the leading edge, but if Marty were to visit the new 1985 then he would change it again, destroying that trajectory by disrupting it with a new temporal miracle (however small) and so producing yet another future. 

Thus, the course of events he inaugurated in 1955 with his first miracle is disrupted in 1985 by the second, and yet that bubble of 30 years would continue to exist for as long as time does, the consequences of the first miracle always continuing to play out. The downstream effects of his first trip to the past will continue to ensue, developing its own future – not as an alternate timeline but as a space within the one that all time shares. Of course, Marty could visit this space as many times as he wants (T50-60 becoming T50-55 and T56-60) and would never really be able to destroy it. But each visit, each miracle, sub-divides it further, leading perhaps to temporal bubbles of only hours or seconds in scope, a past, present, and future bound to the space of a moment: “Infinity in the palm of your hand / And eternity in an hour.” 

What results is a model of time travel without grandfather paradoxes and without alternate timelines, where every action continues to have consequences forever, and where every change to the timeline, every temporal miracle, results in a history which really happens and which really has meaning for the people living within it. Multiple acts of time travel simply result in time bubbles, orphaned and childless timelines which lack any causal continuity with T0 or Tn, but which nevertheless continue to exist in the future of the former and the past of the latter, and which any time traveller may visit. 

This result also brings us to a question: are we at the leading edge of time? As far as I can fathom (admittedly, not especially far) the only way we could know for sure is to either achieve time travel ourselves or be visited by a time traveller from the future (a traveller from our past would require that a time machine had already been invented). A time traveller from the future means that we are not at Tn, that there is in fact a future ahead of us in the time dimension, which could mean that this collection of hypotheses is false, or that it is true and that we are in someone’s past. Alternatively, if we develop a time machine which is able to go to the past and return, we may find that it is impossible to go forward beyond a certain point, that there is no future to go to because it does not exist. What this would look like I have no idea. Perhaps we would loop around to the beginning of time? Perhaps the machine would simply not work? Perhaps the time travellers would succeed in leaving spacetime and finally see what is beyond it? In any case, as I suggested earlier the lack of time travellers from the future (insofar as we know) could be explained by the fact that we are at the leading edge and so there is no future for them to travel from

An alternate explanation for the lack of time travellers, one that holds even if we are not at the leading edge of time, is that time travel’s main use lies in historical research and resource gathering, not in changing past events. If, for instance, I make some sort of terrible mistake, then a decade later go back in time and stop myself, that will be all well and good for the version of me from T90 who is now spared this grievous error, but the me from Tn who made the time machine and went back to change the past is still out of luck. The premise from films like The Terminator, where robots from the future go back in time to prevent their defeat by humanity, would be impossible in a system like this. Even if the robots succeeded in the past, they would still have lost in the present, which is the point of time they actually care about. The lack of time travellers could well be the result of a simple lack of motivation. 

Let us wonder, then, if humans in the future (assuming there is a future) invented a time machine, what would they use it for? Drawing energy from the big bang might be one: permanently changing the course of the universe, but not for them or their present, resulting in a timeline which is radically cut off from its past but which is thus endowed with inexhaustible resources. Another reason might be historical research. In that case the risk of temporal miracles disrupting the chain of events one wishes to study would require countermeasures which, whatever form they might take, necessarily result in the travellers affecting the world around them as little as possible. If a historian from the year 3,000 wants to watch an important event happening today, they would logically employ whatever means available to evade detection and avoid corrupting their data. Conversely, one might visit the past in order to change it, so to test out counterfactuals and support historical arguments. What would happen if, for example, Archduke Ferdinand had never been assassinated? Would the First World War still happen eventually, or not? If you had a time machine, and if you knew that you couldn’t change your own present, then you could go back, save the Archduke, and then see what happens. History, in short, would become an experimental science. 

What I think is exciting about this model for fiction writing is the way it provides a coherent backdrop for a variety of new stories. It is not just a matter of establishing that the past can be changed, or of cogitating through the relevant questions to the point that the willing suspension of disbelief is achieved, but in serving as a reasoned basis for inspiration, much like science and philosophy have always done for authors of speculative fiction. Time travel stories both develop and develop from the interaction between our intuitive understanding of time and our philosophical and scientific understanding of time, forms of knowledge which grow farther and farther apart the more we learn about the universe. Many of the ideas I expressed in this essay seemed counter-intuitive to me the first time I had them, and now laid out on paper they seem far stranger still. That, if nothing else, suggests that they would make a good story, which is reason enough to write them down. 


About the Writer:

Jeremy Colangelo is an author and scholar from London, Ontario. He is the author of the short story collection Beneath the Statue as well as stories and poems in numerous periodicals. He has also published two academic books on disability and literature and several academic articles.

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