6.7.2008
On a recent train journey I witnessed by chance an interesting
conversation about time travel. One of the participants championed the idea
that time travel might be possible; the other seemed to oppose that view.
Although I'll have to render the discussion from memory, and thus introduce
some inevitable inaccuracies, I hope to be able to report the lines of the
argument clearly and faithfully enough to get an impression across.
The Champion opened the discussion with an
eloquent portrayal of time travel and the unusual experiences one would have
during the trip. (I'm not going to repeat it here - quite a few descriptions
in that manner can be found in literature, and even in journalistic writing
indeed.)
The Opponent then answered:
"I certainly
admire your enthusiastic and suggestive portrayal. Who would not want to try
out moving through time now, visiting one's own past, or even one's future?
And there seems to be a wide-spread desire to do so, to change our temporal
location at will. But, if you don't mind, isn't there some misconception at
work here?
I noticed, both in your description and generally in much of the
science fiction which features time travel, a certain view of the
past: it is depicted in a way that we might call it 'the past world',
or 'the world of the past'.
The past somehow appears then to be a 'world' in the same way as the
present world, and the two seem to be related to each other like different
parts of a country: they both exist somewhere, and someone who happens to
be located in one of them may travel to the other by means of some
transportation device. Obviously, to get from a part of a country to another
part, the means of transportation would be one that allows traveling in
space - a train, for example. But to get from one time to another, one would
have to use a machine of which we don't know anything yet - it hasn't been
invented so far. Therefore, the technical details must remain subject
to poetic license.
Talking, as we do, about fiction, this is fine of course. We should note,
however, that time travel fiction tends to emphasize the similarities
between traveling in space and in time, while all the differences we would
expect are often simply attributed to the mysterious nature of the time
machine (of which, by definition, we don't know anything).
Well, I'm not really convinced by these sketchy analogies. And it doesn't
really get better once we attempt to flesh them out. First of all, I cannot
see how your world of the past is really not just another place, with
characteristics that don't have to do with time, or with the past; it is a
fantasy world, which just conveniently happens to be populated with
characters from your memories, and from what you know of, or imagine about,
history.
Let's say the time traveler enters a glimmering circle and suddenly finds
herself in an unknown environment. How does she know that she is in a
'different time' now? Surely she can't tell from anything she immediately
perceives: the heart doesn't beat faster; thoughts are still following each
other in (more or less) sequential order; if she drops an item to the floor
the events of her letting the item go, the falling down and the impact will
happen in proper sequential order and with the expected durations. In short,
everything that gives us the perception that time passes by, under normal
circumstances, has not changed at all.
And this is plausible, too. If these things would have changed, we would
very probably take this not as an indicator for her being at a different
time, but we would think she is now at a different place - a place where
the physical conditions that form our familiar environment do not apply.
(Some science fiction stories that describe the effects of an altered
gravity, such as on the moon or on a different planet, give us a similar
account of the changed experiences in their characters.)
So what does a science fiction writer have to do to convince us now that
our time traveler really has arrived 'in the past'? As it seems, there are
two typical strategies:
-
The time traveler takes her surroundings to indicate that she must be
in an entirely different age, a historical one: the things he perceives,
the way people behave, their clothes, their language, the streets and
houses, everything just looks precisely how the time traveler (and we,
the audience) would have pictured something from that period.
Obviously, this requires some shared historical knowledge, and certainly
also a few assumptions and cliches about that historical period - the
science fiction writer has to presuppose this in the audience, and the
time-traveling character must be equipped with at least some of this
background knowledge also. In the end, it is exactly this knowledge that
make us and the time traveler believe that he has arrived in a different
time. (The science fiction writer will also have to go some way to
convince us and the time traveler that she isn't just dreaming, or in an
elaborate illusion theater, or the like.)
-
The author may appeal to memories. You have used such a suggestion
yourself, just a few minutes ago: 'One may travel back to the time of
one's on youth and be able to look once more at one's birthplace - a
house that is now destroyed.' The memory of the house is what does the
trick here - if you would merely travel somewhere and look at a certain
house, this wouldn't give you the idea that you have traveled in time;
but if you recognize it as the house that you remember as the one where
you spent your childhood, that idea would suddenly start to make sense
under the circumstances (the house being destroyed meanwhile etc.).
Note that neither of these two variants has actually to do with the
concept of time, or the metaphysical status of the past (the question whether
it is real or not). The only thing that matters here in the description is
the way the world looks, the way people behave, and so on. That is, we are
exclusively talking about how things are at that place, how they would look
to you if you would visit it.
To sum up: the account given of the past in all these time travel stories
presents it as if it were another place, not another time. That place is then
fitted out with features that make it look as if it wasn't just a different
place, but also a different place at another time. The idea of travel
through time is thus produced only by retrospective, guided re-interpretation;
actually, it's not much more than an elaborate theater that is just designed
to give us that illusion.
But in reality the past is not a different location. (The same applies to
the future.)
The champion replied:
As you yourself said, we may well speak of a 'world of the past'. And it's
important: The past is not another place - it's another world.
(Or perhaps even a whole series of different worlds.)
A world comprises many places, which are all there at the same time - in
that world. The present is (as is every single past or future) not a place.
It's a world. The past into which you can travel to visit your birthplace on
your birthday is a different world: complete with all the places,
including of course your birthplace (and including yourself also, just
being born there). This is the world of the past, a very apt phrase
indeed...
The opponent answered:
A very misleading phrase, I should say, if taken as literally as you take it.
I think I'd rather revoke my using it; there is no 'past world', or 'world
of the past', in that sense. There is the present world - and that is
the world. It is really there, at every moment, but at any moment it is the
only world which exists then. Each later moment, of course, sees the same
present world (albeit it has changed). Those later versions of the same world
are, for now, still in the future, exactly as earlier versions are now
already past. The world exists in every moment, and then it is present -
there are no other (past or future) 'worlds' at the same time.
Champion:
Now it almost sounds as if you want to say that the past itself doesn't exist
at all! I think that's absurd. There is so much in the past that influences,
and even determines, the present, as well as the future. What happens in the
world (the 'present world', if you insist on putting it so) is constrained:
not just anything could happen, and that is precisely because there is a past
that has made present things what they are now.
Opponent (slightly amused, unless I misread
him):
Although it is, as you're insisting, fully a separate world - something
very different from just another place. And yet there seems quite an
interchange to be going on between these 'complete' worlds.
True, we sometimes talk about people, things or events that are 'in the
past'. This is a figure of speech that can be misleading. These things have
of course really existed or happened. But that doesn't mean that they have to
exist (or happen) still, somewhere else. They don't exist or happen
anymore now. To say that they have, in the past, doesn't mean that
they are (now) somewhere different ('in the past'); it just means that
they have been, some time before. Nothing in this implies that they cannot
have consequences in the present or the future.
You exist because your grandfather existed. Now, it may be or not be the
case that your grandfather is still alive, but this fact does not make any
difference with respect to the dependency of your existence on his existence.
In order for you to exist, he must have existed. But he doesn't have
to exist any more in the present.
When we refer to events, things, or persons as being 'in the past'.
we speak of the past as if it was something fixed, a sort of super-thing,
against which we can compare our memories, and generally all statements in
the past tense, in order to tell whether they are true or not. But that
picture is again wrong; if a statement in the past tense is true in a
verifiable sense at all, it is certainly not because it matches some
reference object; statements in the past tense are true or false not
because they they correspond to some object, or state of affairs, called
'the past'.
Champion:
Well, I agree that we must be careful not to read to much into the way we
ordinarily talk. Still, a given statement in the past tense must be either
true or false; how could it be so if it is not true or false in the
past, that is, it holds in the past world (or doesn't hold, in case it
is a falsehood). It gets its truth-value from the past world, it is made
true or false by the facts that obtain in that world. Statements in the
past tense are made true or false in a way similar to how a statement is made
true or false by the present world, if it is in the present tense.
Opponent:
I doubt that - statements in the past tense are still made in the present.
That is precisely why they have to be in a different tense after all. And
they are not identical with statements that have been made in the past
(about the same thing). They do have something in common with them indeed,
because they are about the same thing. But it doesn't even follow that
they have to have the same truth value; it certainly doesn't follow that they
are true or false on the same grounds - on the contrary: since they are
uttered at different times, what makes them true or false must have different
grounds (which also establish those truth-value links).
Champion:
Look at it this way: there is a certain parallel between the world of the
past and a possible world. A possible world is just as much a complete
world; it is what makes statements true if they are qualified with "It's
possible that ...". The qualifier plays the same role as the past tense plays
for sentences about something past.
Something you say in past tense is true or false in the past world - which
is like the real world in some aspects, and different in others. Your
sentences are made true or false by either of them; they are both fully
determined worlds. Likewise, something you state to be possible is true in a
certain possible world, which is also a world that is similar to the actual
world in some respects, and different from it in others.
Possible worlds are usually somehow derived from the actual world by
stipulating some selected facts to be different compared to the actual
world, and assuming the rest of them just remains as they actually are. And
it's quite the same with the past: it is taken to differ from the present
precisely by the facts that have changed since, the rest is still the same.
(Only that we obviously cannot stipulate how the past differs from
the present.)
Opponent:
So we can imagine how the world might have been different - well, but I don't
think this means that there must be, as it were, additional worlds,
that is, all the 'possible worlds', all of them existing alongside the actual
one. When we talk about possible worlds, we're just saying that the world
(the single one there is) might have been otherwise in this and that way.
It only means that we can think of things and events in the world as
varying. A 'possible world', then, is not another world in any substantial
sense, but only a set of stipulations telling us what would be different
in a given scenario. (Plus the tacit assumption that the rest is to be taken
the way it actually is.)
And we do the same when talking about the past: we describe what was
different in the past, what has happened then. Especially when we refer
to the more recent past (as opposed to some historical age, about which
we only know from books and costume films), we just take it that the world
hasn't changed so much since then. (Most of the changes that actually have
happened may also be irrelevant for our respective purposes.) So we just
talk about it in a way as if it were still the same, except for the specific
things we want to refer to. This is because only the latter are relevant,
and we express it by putting them in the past tense. There is no need for
assuming an entire, complete world around them just for the purpose of
making a few remarks about them.
Champion:
But there is! There are, as you said, many things that we cannot simply
stipulate about the past. What has happened in the past forces us to
accept specific differences between the past and the present.
Opponent:
Or so it seems. But let us consider this carefully. What accounts for this
determinateness? Wouldn't this be exclusively things in the present, which
we can currently make out? This is why I think that the truth of statements
in the past tense is grounded in what is the case in the present.
For instance, the fact that you were born is something that we both have
good evidence for - after all, you're sitting right here. Maybe you have
also some more direct evidence, but whatever that is, it must be something
that you can look at, or listen to, right now, that is, in the present.
Evidence that only was there sometime, but of which nobody doesn't
know anymore, is no evidence at all. (And if it appears again - well, then
it's back in the present.) As a sidenote: we have to take the term 'evidence'
widely, of course, so that it would include memories, written or spoken
reports, any form of records, and so on.
Still, that all evidence that we may have is always present evidence
doesn't mean that we can change it at will. We cannot decide this or that to
have happened in the past - it either has or it hasn't. But if we know
about that event at all, then we know it from present evidence. If there was
no evidence at all, then what would it mean to 'know' about it anyway?
This lets us attribute a certain reality to the past: what we think
about what happened is true or false, depending on whether things have
happened indeed the way we think they happened. Yet our only way to find
out is to verify our beliefs and theories about past things against what
we know (independently) about it in the present.
This is also a difference between your 'past world' and the possible
worlds that we can use to talk about scenarios, ways the world might have
been. A description of a possible world in itself is neither true nor false
(as long as we keep to the logically possible and do not allow any
contradictions into the description). But how we describe the past is,
provided that there is enough evidence at all to determine the truth or
falsity of the things we say about it.
Champion:
But aren't you contradicting yourself now? You just said that there are
elements in the past that make some statements true or false. We can't just
describe the past as we like, we must describe it as it was. (And this is
where the world of the past is different from any possible world.) But how
does that fit with the claim that the truth of statements about the past is
grounded in the present?
Opponent:
This evidence, unfortunately, is not nearly as complete as it would have to
be, if we wanted to derive a full (whatever 'full' can mean in this
connection) picture of the past from it, even for a single moment's snapshot.
There isn't a single instant about which we know everything (not that we
know everything about the present, for that matter). Even if we count
everything in that we might learn in the future, even if we did, in fact,
count everything in that we possibly could learn (i.e. every bit and
piece of information that hasn't been destroyed yet), we wouldn't be near a
complete picture without any gaps whatsoever. We have to face the fact, that
most information there could have been about the things past is already
irretrievably gone.
Unfortunately, I had to leave the train at this point.
