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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.
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.
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.
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.
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.
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).