Time Travel
Visiting the Past
In the Fall
of 1993, in the Critical Thinking course (PHIL 001), we were using
the textbook Logical Reasoning (Wadsworth), by
Bradley Dowden.
We had been working through the book, chapter by
chapter. On Oct. 31, I read p. 202, which contains the
following argument:
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Nobody has ever built a time machine that could take a person
back to an earlier time. Nobody should be seriously trying to
build one, either, because a good argument exists for why the
machine can never be built. The argument goes like this.
Suppose you did have a time machine right now, and you could step
into it and travel back to some earlier time. Your actions in
that time might then prevent your grandparents from ever having
met one another. This would make you not born, and thus not step
into the time machine. So, the claim that there could be a time
machine is self-contradictory. |
That evening, I sent an email letter to Bradley Dowden
(with whom I had been corresponding occasionally during the
previous few weeks). Below, with his permission, is a transcript
of our exchange.
Swartz to Dowden
October 31, 1993
Dear Brad,
I have tonight read your chapter nine.
I disagree with your argument at the top of p. 202,
the one which alleges to show that (time) travel into the past is
logically impossible. I would like to suggest that your argument
commits a modal fallacy.
Since it is possible that someone should have prevented your
grandparents from having met one another, and since it is
impossible for you to travel into the past and to have prevented
your grandparents from having met one another, you conclude that
it is thus impossible to travel into the past. Let "P" stand for
"preventing your grandparents from meeting" and "T" stand for
"travel into the past" (patched up as needed to be proper
statements). Then your argument is
The argument is invalid. From the conjoint impossibility of P
and T, and the possibility of P, the impossibility of T does
not follow. (Just to drive the point home, now let "T" stand
for "the coffee table is four-sided" and let "P" stand for "the
coffee table is six-sided".)
What is logically impossible is that BOTH one travels into
the past AND changes the past (from what it was). But so
long as one does not change the past, there is no logical
contradiction in positing travel into the past.
Below I reproduce section 8.11 (pp. 224-227) from my book
Beyond Experience: Metaphysical Theories and Philosophical
Constraints (Toronto: Toronto University Press), 1991. You may
want, also, to look at Possible Worlds: An Introduction to
Logic and Its Philosophy, by Raymond Bradley and myself
(Indianapolis: Hackett), 1979, p. 25, exercise 4, and p. 333,
part C. David Lewis has written a fine paper on time travel. I
have the exact reference on campus. I have another book here at
home that lists a paper by Lewis. If, when I get to campus
tomorrow, I find that there is more than one Lewis paper on time
travel and that the one I list here is not the one I wanted, I
will send along the corrected reference. In the meantime, I
think, the paper is "The Paradoxes of Time Travel", in
American Philosophical Quarterly, vol. 13, 1976, pp. 145-152.
Regards,
–– Norman
[From Beyond Experience by Norman Swartz]
Section 8.11 Time travel
One kind of time travel is so common, so familiar, that it is
rarely ever recognized for what it is. All of us – except those
at death's door – have an ability to travel forward in time. All
we have to do is wait. Waiting is the simplest and most direct
form of time travel. Most parents know this intuitively,
although perhaps without ever having realized that they do. When
youngsters, filled with the anticipation of a birthday party, say
impatiently, "I wish it was tomorrow",37 their parents will
often counsel them by saying, "Just wait; it will be."
But waiting has two drawbacks. First, it is strictly
forward-directed: one can travel into the future by waiting, but
not into the past. Moreover, there does not seem to be any
analogous 'operation' which will take us backward in time. There
is no such thing as 'reverse-waiting' or 'unwaiting'. The second
drawback to waiting as a mode of time travel is that it proceeds
in lockstep with the ticking of the clock. To get from noon
today to noon tomorrow takes twenty-four hours of waiting. What
persons who are seeking 'better' methods of moving about in time
clearly want is a way of getting from noon today to noon tomorrow
without having to spend twenty-four hours in the process. A
minute or two of traveling time is far more attractive to them.
Taylor has shown us one way of traveling forward and
backward in time. Objects which are curved in space can, as we
have seen, perform the temporal equivalent of objects moving back
and forth in space (see pp. 200-3). But that is of scant use to
the person wanting unlimited capacity for time travel. By
bending my body into a "V"-shape – head and toes forward, hips to
the rear (i.e. similar to that of the object pictured in Figure
8.4, p. 202) – and by moving forward at 1 m/sec, then along a
certain path my toes will travel backward in time from my hips by
a fraction of a second. But I cannot use Taylor's method to
transport my present body from now to yesterday, still less to
the year 1750.
The concept of moving forward or backward in time by great
leaps is intelligible. Suppose it takes me eight hours to digest
a meal and suppose that in one day my hair grows 0.06 cm. Now
suppose that I am placed into a 'time machine'. I sit in the
machine for the time it takes me to digest the meal I just ate.
In this same time my hair grows 0.02 cm. In short, my body has
aged eight hours. But suppose when I step out of the machine it
is one year later (or earlier) than when I stepped into the
machine. This would be a case of the sort of time travel which
is depicted in countless science-fiction writings. We will call
this 'accelerated' time travel.
Is accelerated time travel possible? Forward-directed
accelerated time travel is certainly logically possible. It may
even be physically possible. Indeed the technology may be
imminent. If cryogenic freezing (low-temperature 'suspended
animation') can be realized for human beings, it would certainly
qualify as forward time travel. We already possess the
technology to forward accelerate in time certain creatures (e.g.
the fish Dallia pectoralis [145], 19), which can be frozen
alive and subsequently thawed and revived with little or no
permanent damage.38
But the real problem has always been with the notion of
backward-directed time travel. Is accelerated backward time
travel physically possible? There is a certain amount of
empirical evidence that it is not. The best of this evidence is
simply the fact that, so far as we can tell, no one has traveled
to the here and now from any time or place in the future. Of
course such evidence is not conclusive: it may be that future
generations will have destroyed themselves in a war or
environmental disaster; or it may be that they will have enacted
legislation with sufficiently severe sanctions and policing to
prevent time travel to our century; etc. Nonetheless, the very
fact that there are no visitors here and now from the future
strongly suggests that at no time in the future will a means be
found to permit traveling backward in time. And the fact that it
will never be done in turn suggests that it is physically
impossible.
But even if backward time travel were to be physically
impossible, might it still be logically possible? Even if this
world is of such a sort that traveling backward in time cannot be
realized, might there be other possible worlds where traveling
backward in time does occur?
Many persons have thought that traveling backward in time is
logically impossible. Their arguments typically are of this
sort: "If you could travel backward in time, then you could
encounter yourself when you were a youngster. Even if you are
not normally homicidally inclined, it is at least theoretically
possible that you kill that youngster. But if you did, then you
would not have grown up to have reached the age when you traveled
back in time. Thus there would be a contradiction: you both
would and would not have traveled backward in time. Since the
story involves a contradiction, it is logically impossible to
travel backward in time." Such arguments have been around for
years. They are especially tricky because they involve what are
called modal concepts, in particular the notions of
possibility and impossibility. Does the very concept of
travel into the past entail contradictions? Does the possibility
of murdering yourself as a child show that backward-directed time
travel is an impossibility?
The answer is: there is no possibility, if you travel into
the past, of murdering yourself as a child. The very fact that
you are here now logically guarantees that no one – neither
you nor anyone else – murdered you as a child, for there is no
possibility of changing the past.
This notion that one cannot change the past needs careful
attention. There is nothing special about the past in this
particular regard. For you can no more change the past than you
can change the present or change the future. And yet this is not
fatalism. I am not arguing that our deliberations and actions
are futile.
I cannot change the future – by anything I have done, am
doing, or will do – from what it is going to be. But I can
change the future from what it might have been. I may carefully
consider the appearance of my garden, and after a bit of thought,
mulling over a few alternatives, I decide to cut down the apple
tree. By so doing, I change the future from what it might have
been. But I do not change it from what it will be. Indeed, by
my doing what I do, I – in small measure – contribute to making
the future the very way it will be.
Similarly, I cannot change the present from the way it is.
I can only change the present from the way it might have been,
from the way it would have been were I not doing what I am doing
right now. And finally, I cannot change the past from the way it
was. In the past, I changed it from what it might have been,
from what it would have been had I not done what I did.
We can change the world from what it might have been; but in
doing that we contribute to making the world the way it was,
is, and will be. We cannot – on pain of logical
contradiction – change the world from the way it was, is, or will
be.
The application of these logical principles for time travel
becomes clear. If one travels into the past, then one does not
change the past; one does in the past only what in fact happened.
If you are alive today, having grown up in the preceding years,
then you were not murdered. If, then, you or anyone else travels
into the past, then that time traveler simply does not murder
you. What does that time traveler do in the past? From our
perspective, looking backward in time, that traveler does
whatever in fact happened, and that – since you are alive today –
does not include murdering you.
Time travel into the past involves no intrinsic
contradiction. The appearance of contradiction arises only if
one illicitly hypothesizes that the time traveler can change the
past from what it was. But that sort of contradiction has
nothing whatever to do with time travel per se. One would
encounter the same sort of contradiction if one were to
hypothesize that someone now were to change the present from the
way it is or someone in the future were to change the future from
the way it will be. All these latter notions are logically
impossible. But none of them is intrinsic to the concept of time
travel.
One should take care in describing time travelers not to
give them logically impossible capabilities, e.g. the capacity to
change the past from the way it was, the present from the way it
is, or the future from the way it will be. But once one has done
that, then there is no need to think the concept of time travel
to be logically impossible. It just turns out to be a contingent
fact about this actual world that accelerated backward travel in
time does not occur.
Notes
37. The subjunctive mood seems to have disappeared among
today's youth.
38. For a bibliography on 'freeze tolerance' see [198], 79-84.
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Dowden to Swartz
Nov. 1, 1993
Norman,
You make some interesting points about time travel, but let
me offer a counter.
1. The formal modal argument is invalid, but my argument doesn't
properly translate into that modal argument.
2. I do not agree that my reasoning on p. 202 is as follows:
"Since it is possible that someone should have prevented your
grandparents from having met one another, and since it is
impossible for you to travel into the past and to have prevented
your grandparents from having met one another, you conclude that
it is thus impossible to travel into the past." That isn't why I
conclude that time travel into the past is impossible. Why I do
conclude this will be clearer in a moment.
3. I do agree that I am using the same old argument that you
discuss in your book in the following passage:"If you could
travel backward in time, then you could encounter yourself when
you were a youngster. Even if you are not normally homicidally
inclined, it is at least theoretically possible that you kill
that youngster. But if you did, then you would not have grown up
to have reached the age when you traveled back in time. Thus
there would be a contradiction: you both would and would not have
traveled backward in time. Since the story involves a
contradiction, it is logically impossible to travel backward in
time."
4. I agree that your analysis of time travel into the past does
show that "If one travels into the past, then one does not change
the past."
5. However, what we MEAN, or at least what I and most other
people I talk with mean, by travel into the past, is travel in
which the past does change. Statements that are true are made
false. To travel into the past as a disembodied watcher of past
events is not to travel at all.
6. You believe time travel is possible. OK. Let's suppose you
have your time machine and you go back. For example, suppose you
step into a time machine tomorrow and go say hello to your great
grandparents. This supposition leads immediately to a
contradiction because it is true that you never did say hello to
your great grandparents. To say hello is to change the past, but
as you will certainly agree, to make the true be false is absurd,
and to change the past is logically impossible. Whatever is true
is true, period. Therefore, the only way to travel into the past
is to do so in a way that changes nothing. But to travel this
way is thereby not to travel into the past at all because travel
into the past requires changing the past.
7. So I stand by my argument on p. 202.
–– Brad
Swartz to Dowden
November 2, 1993
Dear Brad,
BD> 5. However, what we MEAN, or at least what I and
most other
BD> people I talk with mean, by travel into the past, is
travel
BD> in which the past does change. Statements that are
true are
BD> made false. To travel into the past as a disembodied
BD> watcher of past events is not to travel at all.
This is too easy, and – no offense intended – a bit of a cheat.
Compare with this:
It is logically impossible to build a car that will take one from
Sacramento to Cleveland. What we MEAN, or at least what I and
most other people I talk with mean, by travel to distant places,
is travel in which the distant place is changed. Statements that
are true are made false. To travel to a distant place as a
disembodied watcher of distant events is not to travel at all.
Of course a time machine which allowed one to change the
past
is logically impossible, but making that the
(unrealizable)
goal is to trivialize the problem, to pose it in a
question-begging manner, to stack the deck. Even if one does
stack the deck in this way, there still remains the question: Is
it logically possible to build a time machine which allows travel
into the past where one does not change the past? And the
answer to this latter, nontrivial, version of the question is:
yes. Robert Heinlein, for example, has described the manner
brilliantly in his long book Time Enough for Love and in
his
short story "All You Zombies". I defy anyone to find a logical
contradiction in those tales.
When I argue for the logical possibility of travel into
the
past, I do not mean as a disembodied watcher of past
events.
I mean as a 'real' participant in the activity of the past.
Suppose a visitor were to arrive here and now from the year 2045.
He shakes my hand, and then sits and chats with me about what is
in store during the next 52 years. I take notes and record them
in my diary. A year (1994) from now, I even publish some of
these notes. The visitor from the future (year 2045) has not
changed the past (i.e. the past relative to the year 2045): he
has contributed to making the past just the way it was. By
traveling back to the year 1993, he caused certain events to
occur in 1993 and in 1994. Nothing was changed from the way it
was; but the past was changed from the way it would have been if
he had not traveled back from 2045 to 1993. Nothing true is made
false; there is no logical contradiction.
BD> 6. You believe time travel is possible. OK. Let's
suppose
BD> you have your time machine and you go back. For
example,
BD> suppose you step into a time machine tomorrow and go
say
BD> hello to your great grandparents. This supposition
leads
BD> immediately to a contradiction because it is true
that you
BD> never did say hello to your great grandparents. To
say
BD> hello is to change the past, but as you will certainly
BD> agree, to make the true be false is absurd, and to change
BD> the past is logically impossible. Whatever is true is true,
BD> period. Therefore, the only way to travel into the past is
BD> to do so in a way that changes nothing. But to travel this
BD> way is thereby not to travel into the past at all because
BD> travel into the past requires changing the past.
To be sure, there are ways of telling time travel stories (your
way immediately above) that are self-contradictory. But
to
prove impossibility, it is not sufficient to tell one
story
in which there is a self-contradiction, one must show something
far stronger, viz. that every story in which there is time
travel harbors a contradiction. You have not done this.
Heinlein and many others have offered stories in which there are
no contradictions. And all it takes to demonstrate possibility
is one story free of contradiction. (Many philosophers
misunderstand the methodology of possible-worlds tales. Quinton,
for example, got it wrong in his "Spaces and Times",
Philosophy, vol. 37 no. 140 (Apr. 1962), pp. 130-47. I
explain his error, and the correct methodology, on pp. 218-219 in
my Beyond Experience (referred to earlier).)
Remember the logic of possibility ( ) is just
the logic of
existence [ ( x) ] extended to the set of all
possible worlds.
Failure to find time travel in one possible world does not
show its impossibility, any more than failing to find my
wristwatch in the bedroom would show that it does not exist. To
be sure there are (some) scenarios describing time travel which
are self-contradictory. (You just gave one such, in saying that
you both said hello to your grandparents and did not.) But one
proves the logical impossibility of time travel only if one shows
that every story of time travel is logically
self-contradictory. There is no self-contradiction in the little
story I just told about the visitor from 2045; there are no
self-contradictions in the stories I just mentioned by Robert
Heinlein. In these stories, the time travelers 'really' walk
about, observe, and are actors on the scene, in the past;
however, they change nothing. They no more bring about
contradictions than you and I do by doing whatever it is we do
today.
BD> Therefore, the only way to travel into the past is
BD> to do so in a way that changes nothing.
Yes. I agree completely. But I would also add "Ditto for travel
in space". Travel in space is similarly constrained by the law
of non-contradiction.
BD> But to travel this way is thereby not to travel into
the past
BD> at all because travel into the past requires changing
the
BD> past.
This is too strong, and it is question-begging. The corresponding argument
about space would 'show' (illicitly) that travel in space is
impossible.
–– Norman
Dowden to Swartz
Nov. 2, 1993
Norman,
I found your argument about the visitor from 2045 very
convincing. I guess I've never thought very seriously about the
issue of time travel. Yes, I now believe it is logically
possible to build a time machine which allows travel into the
past where one does not change the past. This would be 'real'
time travel, not just a disembodied existence passively viewing
the scene. The person could contribute to making the past just
the way it was.
Previously when I've thought about time travel into the past
I've thought of that kind of travel that meets the demand "Oh, I
wish I could go back and make the Toronto Blue Jays lose the
World Series, or go back and make Adolf Hitler slip on a banana
peel and die at the age of seven." That sort of time travel, in
which the past does get changed, is logically impossible, and
that is the only kind of time travel I was considering when I
wrote page 202 of my textbook. So I'll change my ways in the
future. I'll change the first sentence at the top of page 202 to
say "Nobody has ever built a time machine that could take a
person back to an earlier time to change what has happened." No,
I believe the whole paragraph should be rewritten.
Thanks for the discussion about this topic of time travel.
–– Brad
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