Time Travel - Visiting the Past

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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:

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.

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

You are cordially invited to visit Norman Swartz's Homepage and his Philosophical Notes.