The Time Traveler's Little Guide

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Table of contents

Introduction to time travel

What is time travel?

A very crude definition of time travel would be: the possibility of navigating in time as one does in space. A definition a little more precise (but somewhat obscure) would be to say that it is the ability for the traveler to explore, in short periods of his own (traveler's) proper time, various spans of background (observer's) time which are not in the normal continuity of his observation of time.

Preliminary note (about this page)

As everyone knows, time travel (except, to some extent, toward the future) is impossible. Whether it is impossible in theory or merely in practice remains to be seen, but the question is, in my opinion, uninteresting (and basically unscientific). This page is not concerned about the possibility of time travel, nor at all about its modus operandi. It is more about what time travel could be like if it were possible, or in other words as a series of Gedankenexperimenten. Basically, this is a treatise in science-fiction (and an attempt to lay rigorous foundations for time travel stories), but this is not a reason to be serious about it.

Please, loonies, be gone.

A word of advice to science-fiction authors: when writing a story about time travel, it is very important to decide in advance precisely what kind of time travel is involved, and what it is capable of doing, and this must be done in a very coherent and systematic way. Do not merely wave your hands about this: you must systematically explore all the possibilities and all the consequences involved, and you must make sure that the rules are clear and precise. Otherwise, the careful reader will spot inconsistencies. It is true, however, that nearly all existing science-fiction stories dealing with time travel (especially of the polychronic kind) are grossly incoherent.

Now, please fasten your seat belts, we're going for a ride.

Proper time and background time

Before anything, one must be careful to distinguish the traveler's proper time from the background time reference (for more—scientific—information about time and how it is measured, see my page about time). Here is an attempt at explaining these. The traveler's proper time is the time that he feels passing, and it is also (barring psychological or bizarre physiological effects) the time that his wristwatch will measure: unless my wristwatch is radio-controlled, if I jump one hour in the past (resp. one hour in the future), my watch will appear to me to be ticking regularly, whereas to other observers it will seem that it has become one hour late (resp. early), and if I jump ten thousand years in the future, my watch will not measure these ten thousand years (for if it did feel it then so would I, and I would be far dead by the time I got there: this is no time travel but the mere ordinary passing of time). A background time reference, on the other hand, is time as it is measured by observing non-travelers and non-moving clocks: so the act of traveling in time is the act of traveling in this background time as though it were space. Of course, those who know Einstein's theory of relativity need not be told what proper time is and how it differs from a time coordinate in some arbitrary-but-reasonable coordinate system; but I am not assuming that the reader knows about relativity.

Physical versus “mental” time travel

The kind of time travel I am mostly interested in describing, here, is physical time travel: in other words, the traveler's body, along with whatever objects he takes with him, moves in time (and possibly in space also) to the desired position. Obviously, in such a physical time travel, if one is permitted to travel toward the past, one can encounter—and interact with—oneself, so the traveler can duplicate himself by moving toward the past (or disappear entirely from a certain time segment by moving toward the future). Unless the contrary is explicitly mentioned, time travel, in this page, refers to physical time travel.

There is another kind of time travel that must be mentioned, if only to disregard it, namely mental time travel. Briefly speaking, it means that not the traveler's body (and wristwatch) but merely his mind (his consciousness, with all his memories included) moves in time, staying in the same body (which means, in particular, that you can't travel any further in the future than your death or any further in the past than your birth). Forward (toward the future) mental time travel basically means having your consciousness knocked out for a certain period of time, or perhaps suffering from amnesia for that whole period. Backward (toward the past) mental time travel basically means having some foreknowledge of what will happen (or, in the polychronic version, what could happen) in the future, perhaps with a clear memory of having lived it all before and having been sent “back”. There are numerous variations one can imagine on the “mental time travel” theme.

In any case, one must be careful to keep the two quite separate: when writing a story on time travel, one should clearly decide what kind of traveling is involved.

Whither time travel?

One must also distinguish time travel toward the future and time travel toward the past. Technically speaking, in relativity, one should distinguish three regions of space-time relative to a given point: the future region, the past region, and the elsewhere region (namely that which corresponds to a spatial separation greater than the distance that light can travel in the time separation): travel to the elsewhere region means exactly the same thing as faster-than-light travel.

Now traveling toward the future presents no difficulty; as a matter of fact, that is what we do all the time, at the normal rate of one second every second, but even if that rate were immensely increased (which relativity predicts occurs every time space travel at near-light speeds takes place), or concentrated into a “jump”, there would be no particular conceptual difficulty associated with time travel toward the future. To travel to the future merely means freezing the traveler's (proper) time for a certain duration of (background) time. A good simulation of this is obtained by putting the traveler in hibernation (assuming it can technically be done) for the desired period of time.

Traveling toward the past (back in time, that is), on the other hand, is a whole different matter, and presents all sorts of difficulties. The basic question is this: assuming you can go back in the past, is it possible to modify what took place there? Note that there is no real difference betweeen traveling toward the past and observing the future: the ability to observe the future is equivalent to having some kind of “information” travel back in time toward you, so it is a kind of time travel (although “you” are the observer rather than the traveler); and the basic question becomes: assuming you can see the future, is it possible to change the future (do you see what will happen or what would happen)? According as the answer to this basic question is no or yes, one obtains the monochronic or polychronic model of time travel.

Monochronic versus polychronic time travel

In short, monochronic time travel means you can't modify the past (or, depending on your point of view, change the way things will occur), whereas polychronic time travel means you can.

In other words, monochronic time travel means there is a single time (hence the word, monochronic), or a single realm of reality if you wish, which must be envisioned en bloc, and which, to quote Douglas Adams, fits together like a jigsaw puzzle: things which have taken place have really taken place, once and for all times, and things which (have been seen) will take place will really take place and cannot be avoided. The monochronic version is much simpler to explain, because there are no parallel words or such nonsense, but it is possibly harder to work in because, as soon as backward time travel is possible, causality breaks down, and “grandmother”-kind paradoxes appear (what if you travel back in time and kill your own grandmother before she gave birth to your father?—well, the short answer is you can't, otherwise you would never have been born, but so what prevents you from doing it?).

Polychronic time travel, on the other hand, means that you can modify the past. Not only you can, but in fact you must, because the mere fact of entering the past, even if you don't apparently (actively) “change” anything, will indeed inevitably change the entire future course of history, or at least so it is presumed (the so-called butterfly effect). From the time traveler's point of view, he can go back to the past, change a little something there, and then return to the future, and find it entirely changed (and all the more changed that the past was remote). From the observers' point of view, however, they see the traveler disappear (travel toward the past) and never return, since he returns in a different reality, or a parallel world if you like, in which things are quite different (or say things this way: if they did see him return, they would ask him, well, didn't you change anything? see, things haven't changed here at all!); so polychronic time travel necessarily entails the existence of “parallel worlds” (alternate realities, or whatever you wish to call them). Side note: some people will contend that observers who remain in place while the time traveler goes back to alter the past will see reality around them change, perhaps after a repercussion delay, or, if you wish, that they are “attracted” to the new world which the time traveler has entered, which becomes the new reality; I don't think this makes any sense (I mean: I don't think this idea can be expanded into a full coherent description; for example, what happens if two people go back in time, at different moments, and change the past in incompatible ways?). Also note the polychronic vision of time travel is exempt from the “grandmother”-kind paradox: if you go back in time and kill your grandmother, you are merely preventing the existence of a parallel version of “you”, but that is not your (own) self.

Can the monochronic and polychronic visions of time travel be mixed? The answer is both yes and no. It is yes in the sense that there is no reason why both cannot coexist: maybe mad scientist A invents a monochronic time machine, and mad scientist B invents a polychronic one, and both can operate simultaneously, and interact with each other; this may seem mind-boggling, but really there is nothing more complicated about mixing monochronic and polychronic time machines this way than about describing the “plain” polychronic one: all the complexity lies in the polychronic time travel. The answer is perhaps in the sense that one might have intermediate situations between the monochronic and polychronic extremes; however, they are not what you probably think they are, and they are probably not very interesting (for example, as a mathematician, I think it might make sense to define an Abelian-chronic version of time travel, which is exactly the polychronic version when only one time loop is involved, but which becomes different when two are introduced, because the loops would somehow “commute” with each other). But basically the answer is no for what most science-fiction authors have naïvely tried to do. For example, it doesn't make sense to say that the past can be modified but only with a certain (magical, blessed, specially crafted or otherwise special) object: for as soon as that object is sent back in time, everything is changed from that point on, and it makes no sense to say that such or such a change is brought by the object in question—this is a basic misunderstanding of the butterfly effect. I also do not think it makes much sense to have a sort of metastable reality in which the past can be changed, but with great difficulty (because the meaning of difficutly is highly dubious).