Time travel has captured the imaginations of countless persons throughout history, although many or most may regard it as complete science fiction rather than even having a marginal possibility of realization. This is not a discussion of the physical mechanics of time travel, but of the identity and nature of time from a philosophical standpoint and the causal ramifications thought inherent to any sort travel through time.
First of all, before even broaching the subject of whether or not it is theoretically feasible to travel through time, one must consider if it is even logically possible. There are many ways of looking at time’s identity, but two I consider central to this discussion are time as an unreality and time as a fourth dimension.
J. E. McTaggart discusses time in two ways, called the “A series” and the “B series.”[i] The A series uses the stages ‘past,’ ‘present,’ and ‘future’ to refer to temporal events and moments. What is now present was once future and will be past. In this series events are always changing. They move from one stage to another in a certain necessary order (that is, order determined solely by definition) and cannot be in more than one stage at any single moment. The B series describes temporal order. If K occurred before L and after J, J is earlier than K and K is earlier than L. This order will never change, no matter what moment of reference is used. The moments J, K, and L could be in one’s past, future, or present, but the order is, was, and will remain unaffected.
McTaggart thinks that the A series is essential to time. Time cannot exist without the existence of the A series and we cannot claim that events exist without using the A series to describe them.[ii] The B series is not necessary in the same way. When explaining the dispensable nature of the B series, McTaggart introduces the “C series.”[iii] The C series is like the B series without the temporal aspect, i.e. a pure sequence. If the order of the series is P, Q, R, S it will also be S, R, Q, P, but not Q, R, P, S or any other combination.
Having described these three series, McTaggart reminds his reader that he does not think time is real. This is because he claims the A series does not exist and without it time does not exist. The A series is unreal because it is self-defining and thus given to circular reasoning. One cannot define or even describe the terms ‘past,’ ‘present,’ and ‘future’ without referring to verb tenses that are themselves characterized by those three terms. He then explains that the B series does not exist either, but that the C series does. Therefore, in actuality, events have sequence but not a temporal order or a description by the three terms of the A series.[iv]
In contrast, David Lewis likens time to a railway, and more specifically a “mountain railway” when described in conjunction with time travel.[v] This sort of time is a fourth dimension, and time travel is not a logical impossibility because times other than the present do exist in actuality. This is not to say that travel is possible – yet – but that there are at least destinations. One could move temporally in a linear way because the temporal moments commonly referred to as the past and future do exist, and not just in the memory or imagination. To further describe this vision of time, it is useful to go into its causal properties, which will be discussed later on.
Also, Lewis introduces the distinctions of “personal time” and “external time” which are almost necessary when considering the paradoxes of time travel.[vi] Personal time is the time traveler’s own time, which can be thought of as the elapsed time on her own wristwatch, which may differ from the external time, along which she is traveling. If the traveler takes ten minutes of her personal time to go linearly in external time, say 50 years, for her only ten minutes have passed but the world is in a different time by a measure of 50 years.
Lewis’ vision of time allows for time travel while McTaggart’s does not. This is most simply explained by noticing that for McTaggart there is nowhere to go, because time does not exist. McTaggart’s main argument is that the A series is necessary for time and that the A series does not exist, therefore time does not exist. I do not think that this is necessarily so. The A series may be a useful construction for describing time in everyday conversation but it may not be requisite for time’s existence. The C series may be enough to fulfill that particular requirement. I do not have the ability to prove this conclusively at the moment, but if we are to further discuss time travel it is imperative that we take Lewis’ vision of the identity of time and carry on with its nature.
Of the nature of time I will also mention only two positions in the interest of simplicity and brevity. The Heraclitean position entails the constant fluctuation of time. According to this outlook, trying to move to a destination in the future would be impossible because the future truly does not yet exist. Travel to this sort of past is not possible because the traveler could not “gain causal access to it.”[vii] On the other hand, the Parmenidean view holds that time is eternal;[viii] the past and the future exist in the same way the present exists. It is in this sort of system that time travel is logically feasible, and this is the perspective we shall take.
This brings us to the problems and paradoxes involved in the causal effectualness of time travel. Can one really change the past? One common-sense argument is that no, one cannot, because the past has already occurred. But this is a Heraclitean view. Another argument is that time is accessible because it is eternal. However, this does not intrinsically imply that one can change the past, or the future for that matter. It is commonly held that the future has not yet happened, therefore one can easily ‘change’ or affect it. According to the Parmenidean view, this is not so. The future exists always, in the same way the past exists. The answer to the question above is yes, of course, because if one is going to change the past, one will have already changed it and will always have changed it.
But if the change has already existed, doesn’t that mean that there wasn’t ever anything from which to change it? This question can be addressed by thinking of time in branches.[ix] Consider the famous grandfather paradox: can Wil go back in time and kill his own grandfather before his own father is conceived? When thinking of time in branches, this sort of scenario is certainly possible.
Imagine there exist two timelines, Alpha and Beta. In timeline Alpha, Wil exists; in Beta he does not. Alpha Wil goes back along line Alpha and kills his own grandfather. Beta branches off Alpha the moment before Wil’s grandfather’s death on line Beta. In this way, Wil’s grandfather does not die on line Alpha and so Wil can go back to kill him. However, after this moment Alpha Wil exists on line Beta. This is another pivotal point for us; there are two choices of perspective regarding temporal causal efficacy. Line Alpha may forever be lost to Alpha Wil; even if he goes forward along the line he will not find a time where a Beta Wil exists. The alternative is that he can indeed jump branches of time laterally as well as in the ‘normal’ linear sense.
To illustrate these two types of time travel I will refer to Isaac Asimov’s The End of Eternity. The primary or ‘normal’ type discussed in the novel is like Wil’s movement along line Alpha; that is, linearly along a single timeline without the ability to jump branches. Indeed, Asimov’s principal character Andrew Harlan does not even realize that the other branches are permanent just as the eternal timeline is permanent. His purpose – his vocation – is to change the timeline for the betterment of humanity. In some cases he can affect change with a simple movement of a book on a shelf, but sometimes he must integrate himself into that particular when for a duration of his own personal time. Harlan - and Harlan’s colleagues - believe that once the timeline has been changed, the ‘former’ one cannot be retrieved. It is not until the end of the novel that Harlan discovers a worker who deals not only with the line of time, but with its infinite branches.[x]
Wil’s story assumes that Wil’s presence in an earlier time has only the effect of his grandfather’s demise. Unfortunately causal theory has not been developed to the point where one can be certain that this would be the case. Even the smallest action could have ramifications of colossal proportions a few years down the line. In Asimov’s world, the relationships of cause and effect are much more firmly established than any causal system we presently possess. This issue ties ethics to time travel.
Ethically, one would suppose that a time traveler whose mission was to change a single event would prefer not to affect any other changes on the timeline, to result in as few branches as possible. A succinct statement of this responsible desire is expressed in the Temporal Prime Directive of Starfleet, a military organization from Gene Roddenberry’s Star Trek series: “All Starfleet personnel are strictly forbidden from directly interfering with historical events and required to maintain the timeline and prevent history from being altered.”[xi]
This is a noble goal, but the prevailing state of causal theory is not at a level to make this aspiration attainable beyond a reasonable doubt. Current science has brought time travel out of the realm of pure science fiction and into that of scientific possibility. There is much yet to be discovered about the nature of time. Metaphysics and physics grow ever closer; the divide between science and spirituality is narrowing. What has been a philosophical subject – the identity and nature of time – will soon become a scientific topic. Time travel may become an actuality. Before this happens, I believe causation should be fervently studied to become as highly developed as possible, to the point of an empirical science.
1. J. Ellis McTaggart, “The Unreality of Time.” Mind 17, no. 68 (Oct. 1908): 458-9. http://links.jstor.org/sici?sici=0026-4423%28190810%292%3A17%3A68%3C457%3ATUOT%3E2.0.CO%3B2-Y
3. Ibid., 461-2.
4. Ibid., 467-9.
5. David Lewis, “The Paradoxes of Time Travel.” American Philosophical Quarterly 13, no. 2 (Apr. 1976): 147.
6. Ibid., 146-7.
7. William Grey, “Troubles with Time Travel.” Philosophy 74, no 287 (1999): 57.
8. Ibid., 56.
9. Lewis, “Paradoxes,” 152.
10. Isaac Asimov, The End of Eternity. Garden City,
11. “Temporal Prime Directive,” Memory Alpha, http://memory-alpha.org/en/index.php?title=Temporal_Prime_Directive&oldid=303254.
Bibliography
Asimov, Isaac. The End of Eternity. Garden City,
