The Most Inscrutable Puzzle of Time Travel?

Geoffrey Gorham teaches Philosophy at Macalester College, Minnesota, and is Resident Fellow at the University of Minnesota Center for Philosophy of Science. His primary interest is the history of early science and metaphysics, especially space and time. He is author of Philosophy of Science: A Beginner’s Guide, co-editor of The Language of Nature: Reassessing the Mathematization of Natural Philosophy in the Seventeenth Century, and author of numerous articles. 

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I. Humdrum Backwards Time Travel: Grandparent Paradox.

Partly owing to the influence of philosophers like David Lewis (1976), Steven Savitt (2005), and Nicholas Smith (2024); fiction writers like H.G. Wells (1895), Ray Bradbury (1952), and Audrey Niffenegger (2003); and numerous films, like La Jetée (1962), Back to the Future (1985), and Primer (2004); the “grandparent paradox” has come to occupy a central place in discussions of the suspected logical inconsistencies of time travel. The inconsistency alleged by this paradox is that, were I able to backwards time travel, I would be able to assassinate Grandparent (or some other biological progenitor), before I was conceived. But in that case, if modern biology is roughly correct about human reproduction, I would prevent my later existence; I would be a successful parricide who never existed.

In his well-known article, I think David Lewis successfully deflates this paradox.¹ First, Lewis points out that this hypothetical scenario—in which when I assassinate Grandparent, in one universe Grandparent survives and in another they die young and I never exist—is not time travel, but space-time travel or multiverse jumping. Whether Grandparent or I have “counterparts” in other universes is beside the point. This is the very same fallacy Bradbury commits, or toys with, in “Sound of Thunder” when he imagines a “butterfly effect” whereby I must return to a very different world because I stepped off the path required by the time-travel agency and squished a butterfly. Second, there is no need to suppose that I must be able to change the past when I time travel, but rather simply participate in it. Perhaps any attempt I make to kill them will necessarily fail; I can’t kill Grandparent earlier if they in fact lived and I can’t kill Grandchild later if they in fact will live.

II. Interesting Backwards Time Travel: Causal Loops.

Serious difficulties remain, however, one of which is broached more recently by Robin Le Poidevin (2003, 180–181). Suppose I backwards time travel in order to meet my 12-year old self, and my partner (in the present) suggests I bring back a diary that a stranger gave to me when I was 12. Lewis sanguinely dismisses such “closed causal loops” as “strange!”—like a rabbit’s foot I receive from my future self, which would then never have been a part of a live rabbit (149). But surely there is a paradox lurking here: how old is the rabbit’s foot? And what is in the diary? Either the young me receives the diary from the old me with the history of my long life intact, or blank. In the former case, would I (absurdly) just trace the diary passages over the next 50 years? And if I did, how old or deep are those traces since I would bring back a diary already traced? If the diary entries disappear when I hand it to my younger self, why has my hair grown, my memories accumulated, my lunch rotted, etc. when I take a trip back?

I am not sure the answer to this. But I am pretty sure “strange!” is not a satisfactory response to causal loops. Perhaps the diary loop can be somehow reduced to the Grandparent paradox. Maybe, for example, there is something about time travel, or the physics or biology it requires, that causes the diary entries, or the rabbit’s foot, to be “restored” to pristine status. If this is the case, we might say that we necessarily, on pain of contradiction, must return to the past just as it was, though then it would remain unclear why the old me doesn’t similarly “revert” biologically to my 12-year old version. 

In a recent, important book on the physics and metaphysics of time travel, Nikk Effingham (2020) addresses what he calls “the Bootstrapping Paradox,” following Robert H. Heinlein’s story, “By His Bootstraps” (1941). In Heinlein’s narrative, the young protagonist “Bob Wilson” becomes involved in several causal loops; most significantly, he gradually becomes the much older “Diktor” who travels back in time when he turns out to be “Bob Wilson.” Effingham suggests that neither Wilson-cum-Diktor, nor inanimate objects, such as Wilson-cum-Diktor’s watch, though caught in causal loops, need arise from “nowhere” or ex nihilo.  Effingham proposes that older Diktor’s watch is like the objects possessed by some other looped time-travelers, such as in Jeannot Szwarcz’s film Somewhere in Time (1980). Effingham writes:

It’s natural to think that bootstrapped objects require their underlying matter to likewise be bootstrapped—thus increasing the overall mass energy of the universe for the period of time that the causal loop involving the object takes place—that needn’t be the case [Hanley 2004: 134–5]. Imagine that the parts of Szwarcz’s watch are each individually crafted in a factory just like any regular watch part. At time t those parts replace the worn-out parts of an old watch. The watch is then sent back in time. Gradually its parts become worn and, at time t, are replaced with (of course!) the past versions of the parts. The spent watch parts are then discarded just like regular watch parts and thrown in a bin. The watch parts, and their underlying matter, are not bootstrapped; the watch itself, however, is. (2020, p. 60)

Effingham allows that the “material” of Szwarcz’s watch may be scavenged, from old watches, but (presumably in order to obey the conservation of matter/energy), only the reconstituted watch itself is bootstrapped. But, first, it is not clear the conservation of matter/energy would be violated by a strict or closed causal loop of Szwarcz’s watch or the motherless body of Wilson-cum-Diktor. The matter of these bodies is not ex nihilo; it is simply transported from another time: the future. Second, even if the total conservation of mass or energy is violated by causal loops, it would not follow that causal loops are logically impossible. Indeed, it would be far more “strange” if a putative loopy time-traveler had to somehow “replace” their possessions and even their own body with scavenged matter. Last, it is not counterintuitive that a person (or a watch) could be sustained during time-travel. Some people (diplomats and flight attendants) travel incessantly but don’t fear for their personal identity. It doesn’t seem, a priori, stranger to cross a temporal border than a spatial one. If we adopt a straightforward explanationist epistemology (following Tallant 2020), then the most obvious reason Diktor can “start again” as Wilson is simply because he had the ingenuity to time travel and start again in a younger body. 

Effingham himself admits that “both bootstrapped objects and bootstrapped matter are possible” (p. 60. cf. 63–65), and yet he is especially concerned to refute what he considers a necessary premise of bootstrapping which he labels INTERACTION: “time travel necessarily involves interacting with one’s own past and bringing about a bootstrapped causal chain.” This concern is unwarranted: I could send back my precious rabbit’s foot or diary in a self-regulating time machine, which is discovered by my 12-year old self, and this would generate a bootstrapped causal loop without me ever time-traveling or interacting with my own past. In any case, Lewis is right that causally interacting with (affecting) my own past is no more problematic than causally impacting my own future, which only a dogmatic fatalist would abjure. (Lewis, 151–2)

In a later chapter, Effingham takes up the Grandparent Paradox specifically. He rightly dismisses statements of the paradox which imply that the parricidal time traveler lacks free will. If there is a logical contradiction in time travel it would be manifested just as much by non-agential backwards time travelers, such as billiard balls which, by whatever means, prevent their future creation (92). Another “less interesting” version of the paradox dismissed by Effingham is explanatory: why is it that the time-traveler cannot kill Grandparent? The explanation could be that his time machine misfired, or went astray, or his gun jammed when he found Grandparent, or he killed the wrong person (93). These frustrations could befall the best (or worst) of us, regardless of time travel. It might just be that I miserably, and improbably, fail in my murderous designs on Grandparent: “there might just be some inexplicable things” (93).

So, two of the most famous and worrisome metaphysical questions about time travel: killing Grandparent/changing the past, and closed causal loops/bootstrapping, do not come close to demonstrating that time travel is logically impossible. Another hypothesis, branching universes, is entirely off the mark. But there is another conceptual problem, that I believe to be more bedeviling, and difficult, than the paradoxes that are usually brought against time travel, which we will next consider.

III. Even Stranger: Transparent Time Travel.

I would now like to raise a puzzle about backwards time travel that has not been as widely discussed as the two discussed above (Grandparent and causal loops). Suppose I build a time machine because I want to see what college classes were like for my partner, when they were at this college around age 20 in 1995. Suppose I build a time machine in 2025 that is just big enough for me and set it in the corner of a classroom where I know they attended lectures (I teach at the same college as their alma mater). I also make the walls of the machine out of plexiglass because I am dubious of stories and movies that depict the “personal time” of the traveler as mired by flashing lights and whirlwinds, with no sensible connection to the real world or “external time.” I want my backwards time journey to be transparent and open to public scrutiny. 

If we consider this journey in “objective” or “external” time, from earlier to later according to the usual movements of clocks, and the thermodynamic history of the universe, my future partner and their classmates would first see me one day in 1995, in the corner, in a plexiglass box. Suppose, shortly after I arrive back then, I decide to exit my time machine for the next two weeks. What is strange is that at that time I would necessarily split or duplicate, since for the next two weeks I am both still backwards time traveling to the date I arrive, and also hanging out on the college campus in 1995. And there is no reason students in the class would not perceive this duplication or splitting of me. I might join their class discussions, sitting next to my future partner, while also sitting in the plexiglass time machine. Also subsequent generations of classes, up to 2025, would see me sitting there, slowly un-growing my hair and beard (since my “personal” or “biological” time runs backwards from 2025 to 1995). Conversely, in 2025, the current students would one day see a middle-aged professor enter the room, in order to commandeer the backwards time machine. Then he would fuse or merge with the time-traveler who, according to campus legend, has been sitting there for decades.

IV. Personal Identity.

The 1995 past time-traveler, who “splits” from (and looks upon) their double a day later, who is still lazily time-traveling, would face a real identity crisis. Call the person still traveling Split-a, and the person who emerges Split-b. Split-a is not yet in a crisis, though they might anticipate that they will be, but Split-b presently gazes on someone who looks exactly like themselves, and whom they know to be their time-traveling double. The unsettling existential question Split-b would face is whether that person behind the plexiglass is me, since they have mostly the same history and memories as me, and will “soon” be asking themselves this very question. Notice that this personal identity crisis is different from those discussed by Lewis (147–8), which involve meeting a childhood temporal part of myself. The other I encounter, when I exit the machine, is nearly exactly the same age as me, with the same body, and mostly the same memories, and is only a week away from being strictly identical with me. By analogy, I can imagine writing a letter to my older self, and putting it into some sort of time capsule, and I encounter my younger self indirectly in old pictures and memories. What is hard to conceive is directly perceiving myself, almost face to face. It is nearly the same, at the 2025 beginning of the journey. Merge-b knows they are about to fuse with a body just like theirs, Merge-a, and with a psychological history exactly like theirs, at the appointed time. They might pause, and wonder, is that really me who has been time-travelling (for all to see) since 1995? There does not seem to be a definitive answer.

V. Conclusion

My main point is that there is an aspect of the notion of time travel that has been neglected in popular culture and by philosophers, namely that time-travel is in principle transparent to outside observers and to the travelers themselves. This transparency, which is usually sidelined, or simply obscured, in fictional depictions of time travel, brings to light the sheer strangeness of time travel, including doubts about personal identity, and self-encountering, and whether the “classic” paradoxes of time travel, such as “multiverses,” “grandfather,” and “causal loops,” really identify what mystifies us most about going to the past. 

notes

1.  Lewis, 1975, 150-152. Grey (1999) defends Lewis’s deflationism about affecting the past but also objects to the empirical implications of closed causal loops. Dowe in turn (2000) defends time travel against the acute objections of Grey. In an article published nearly contemporaneously with Lewis, Horwich (1975) effectively dispenses with a number of alleged objections to time travel, which mostly rely on physical and metaphysical principles. As far as I can tell, none of these authors address exactly the puzzle I describe in Section III below, and following.

References

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