Moving Backward in Time Fred I. Dretske The Philosophical Review, Vol. 71, No. 1. (Jan., 1962), pp. 94-98. Stable URL: http://links.jstor.org/sici?sici=0031-8108%28196201%2971%3A1%3C94%3AMBIT%3E2.0.CO%3B2-4 The Philosophical Review is currently published by Cornell University.
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http://www.jstor.org Fri May 18 08:38:21 2007
MOVING BACKWARD IN TIME
recent article Bernard Mayo takes considerable pains to establish the complementarity between objects and events.l Once this complementarity is exhibited, the stage is set, according to Mr. Mayo, for undermining the "tenacious doctrine" that we can only move forward in time. I n one respect he is certainly right; this is a tenacious doctrine. The following remarks will, in fact, reveal my own addiction to it. In another respect he is wrong; nothing he has to say about the complementarity between objects and events affects the doctrine that we can only move forward in time. I would like to indicate why this is so and, in addition, sketch my reasons for believing that this alleged "doctrine" is, on the contrary, an inextricable part of our conceptual scheme. The author begins by defining the temporal complement of an object (p. 342). He does this by replacing spatial for temporal terms (and vice versa) in the spatiotemporal specification of an object. Whatever satisfies these new formulas, then, is the temporal complement of an object. A few of the more important (for our purposes) characteristics which these entities possess are the following : (i) they extend over considerable space (corresponding to an object's duration through time); (ii) they may, at different places, occupy the same time (objects may, at different times, occupy the same place) ; and (iii) they may, at different places, occupy different times (objects may, at different times, occupy different places). The second feature is called "occurrence" (or "extension"), and it corresponds to a stationary object; the third is called "propagation," the complement of which is a moving object. Now what, in familiar parlance, could reasonably be said to satisfy the formulas, three of which have just been given, for the temporal complement of an object? Mayo suggests that events, as ordinarily understood, come very close. In support of this he cites the Queen's Christmas Day broadcast as a typical event which possesses the requisite properties. We find, for instance, that this broadcast extends over a considerable space; "it could in principle, according to electromagnetic field theory, extend to the whole of space" (p. 343). Moreover, it propagates (the broadcast would, presumably, be at the sun
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"Objects, Events, and Complementarity,"Philosophical Review, LXX (1g61), 340-36 I .
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-a different place-at a different time), and so forth. Somewhat surprisingly we find that the ordinary notion of the Queen's Christmas Day broadcast is being identified with the electromagnetic vibrations emanating from the broadcasting station (cf. p. 348). I say this is surprising because I doubt whether anyone, if asked where the broadcast took place, would be tempted to reply that it occurred in all those places where it could, theoretically, have been received. Mayo is, of course, compelled to talk in this fashion since he is looking for something that behaves in the way the complement of an object behaves. His selection of a "broadcast" in contrast, say, to a coronation as a typical event was, I am sure, motivated by the ease with which it allowed him to pass to something that does resemble the complement of an object. If he had used a less ambiguous example (a birth, death, arrival, departure, collision, accident, battle, or the like), it would have been more readily apparent that he was not talking about the event, as ordinarily understood, but about the light which is reflected from this event. Events, understood in any ordinary sense, are usually confined to the place of the object or person to which they happen. Accidents happen on street corners, births occur in hospitals and in houses, and coronations take place (the locution "take place" is revealing) in buildings. There is certainly an arbitrariness associated with the spatial location of any event (for example, did the coronation take place in London, in England, or in the Northern Hemisphere?), but once we are given the degree of determinateness (building, city, county, country) with which the specification is to be made, the arbitrariness vanishes. There is only one city in which the coronation took place, and it is simply false, in any ordinary sense of these words, to say that it also took place in every city to which the reflected light could, theoretically, extend. I am not denying that we could talk in the way Mayo suggests, but I am denying that we do talk that way. I think it extremely misleading, therefore, to assert, as Mayo does, that "an event, in the ordinary way of speaking, usually spreads outward. . . " (p. 345). Nevertheless, Mayo is talking about something which is, if we follow the physicists (and who else can we follow on such matters?), often spoken of in language appropriate to events. An electromagnetic vibration is something which seems to happen, and it propagates through space. Electromagiletic signals can be regarded as "disturbances" which spread outward at a certain rate, and a disturbance, once again, is an occurrence or happening. Several questions arise
FRED I. DRETSKE
in this connection, however. What is it that vibrates or is disturbed? Is it the electric (or magnetic) field vector or intensity? If so, are we describing the history of this vector when we describe the propagation, refraction, scattering, and so on of such signals? If this is so, then we can no longer say that the events (that is, the vibrations or disturbances) spread outward or propagate; for they happen to the electric field vector and are, therefore, confined to the place at which this vector is when they happen to it. We are, admittedly, indulging in rather dubious picture thinking when we speak of the disturbances as happening to an electric field vector, but if there is anything to which they happen, it is questionable whether we can find a more plausible candidate (the electromagnetic ether ?) . I propose, however, to grant the legitimacy of talking about these "events" (vibrations or disturbances) in the way Mayo suggests. That is, they spread outward, occupy appreciable volumes of space, and so on. They may, after all, be unique in representing the objectless event or process to which C. D. Broad and others devoted some attention. I want to examine what is said about them and, specifically, how they are alleged to support the idea that we can, in some sense, move backward in time. We are told (p. 359) that since a revisit to a stationary object constitutes a backward movement in space, a revisit to an occurring event ought certainly to count as a movement backwards in time. With the above notion of an event (a propagating disturbance), the possibility of revisiting an event is quickly established. If we could move faster than the wave front of a propagating signal (for example, a pistol report), we could overtake the receding event and revisit it. The fact that we cannot do this with most signals is simply an empirical fact; it is not self-contradictory to suppose that we could. Therefore, a movement backward in time is possible but cannot, according to existing empirical information, be accomplished. Mayo reaches this conclusion only by ignoring everything he has already said about the complementarity between objects and events. Using his own characterization of what it means to move backward in time, we should notice that the revisited event must be an occurring event. An "occurring event" is, as we have already seen, the complement of a stationary object. This is as it should be since Mayo is taking the revisit to a stationary object as his pattern for what it means to move backward in time. An occurring event, however, is an event which does not propagate; it is an event which is not at different places at different times. That is, an occurring event is one that, once it stops happening in the observer's immediate neighborhood, stops
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happening everywhere. If this were not so, it would be propagatingthat is, it would, at different places, occupy different times. Yet, every example given to illustrate the revisit to an event utilizes a propagating event (overtaking a pistol report, moving after receding water ripples.) In order to demonstrate that a movement backward in time is possible, it would have to be shown that we could revisit an event which, at different places, occupies the same time (an occurring event). But this is patently impossible since, by definition, an occurring event will, once it has ceased to occur in the observer's neighborhood, have ceased to occur wherever we might wish to visit. Therefore, there is no question of moving fast enough to overtake an occurring event. I conclude, therefore, that Mayo has not, even granting his own formulation of the question, succeeded in undermining the "tenacious doctrine" that we can only move forward in time. It should be noted that the argument above is independent of any considerations arising- out of relativity theory. I have said that once an occurring event stops happening, it stops happening everywhere, and this might be thought to imply a n absolute simultaneity. If we want to introduce limiting velocities, however, the entire argument can be reformulated in somewhat more elaborate terms. An occurring event, once it stops happening in the observer's immediate neighborhood, must (by definition) stop happening in other places at a rate which precludes our ever visiting these places before it stops; an occurring event must "collapse," so to speak, rapidly enough (and this would depend on the limiting velocity) to disallow our ever "catching" it again in some other place. To suppose otherwise is to suppose that an occurring event is a propagating event, and this is a contradiction in terms. Regardless of what our capacity is for overtaking a propagating event, we cannot, by definition, overtake an occurring event. Therefore, a movement backward in time is impossible, logically impossible, in so far as it depends on our ability, limited or otherwise, for moving swiftly through space. The possibility of revisiting an occurring event in some other sense is, however, still an open question, and I would like to address a few brief remarks to this issue. If a revisit to an occurring event is to count as a movement backward in time at all, the event which is revisisted must have ceased to occur prior to the revisit. If this condition were not satisfied, quite familiar and everyday phenomena would have to be counted as a movement backward in time. For instance, a soldier being sent back into battle would be moving backward in time since he would be revisiting the same event (the battle). It is
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clear that if we are going to mean anything significant at all, we cannot mean a revisit, before its cessation, to a n event which, like battles, coronations, weddings, and so on, persists over considerable intervals. This being so, we must somehow revisit an event after it has ceased (and since it is an occurring event, it will have ceased everywhere). Since revisiting an event means to visit it again later, a revisited event is one which precedes itself (assuming that the visited event is simultaneous with the visit). Hence, some of the familiar properties of the temporal "successor" relation (irreflexivity, asymmetry) would no longer obtain. But we have a choice in this matter; we could abandon these general properties of the temporal ordering relation or we could treat the second visit as a visit to a distinct, but similar, event (thus maintaining the "successor" relation in its familiar form). The latter alternative is, it seems to me, the natural and almost inevitable choice; in fact, it accounts for our tendency to think of the properties of the temporal "successor" relation as something more than empirical truths. We do this because we can always preserve them by an appropriate, and conceptually far less disruptive, distinction between events. What this points up is that we use time (along with the objects to which the events happen) in our individuation and reidentification of events. We "cannot" revisit the same event because the notion of a "revisit" and the notion of "the same event" are, within our conceptual system, mutually incompatible. The notion of a revisit carries with it the implication of temporal succession, and temporal succession is one of our criteria for marking off, when necessary, the emergence of new events. FREDI. DRETSKE
University of Wisconsin
