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«TIME TRAVEL AND THE MOVABLE PRESENT1 Sara Bernstein Science fiction has made time travel seem familiar and natural. Usually, time travel goes ...»

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Sara Bernstein

Science fiction has made time travel seem familiar and natural. Usually, time travel

goes something like the following: a time traveler enters a time machine, flips a switch, and

exits the time machine in, say, 1960. Then the time traveller embarks on adventures that

create vexing logical and metaphysical paradoxes: killing her own grandfather, or throwing

herself a 10th birthday party that didn’t occur in the first place. Philosophical literature on time travel focuses largely on paradoxes generated by time travel to the past (and more rarely to the future). But less attention is paid to what the time travel itself consists in.

This paper develops one such conception by extracting a principle from a recently proposed model of time travel, and building on it. In “Changing the Past” (2010), Peter van Inwagen argues that a time traveler can change the past without paradox in a growing block universe. After erasing the portion of past existence that generates paradox, a new, nonparadox-generating block can be “grown” after the temporal relocation of the time traveler.

This discussion articulates and expands on the underlying mechanism of Van Inwagen’s model: the time traveler’s control over the location of the objective present. Van Inwagen’s discussion is aimed at preventing paradox by changing the past, but I argue that it provides metaphysical tools for a new model of time travel that can be generalized to all theories of time.2 Roadmap: in section 1, I lay out the requisite conceptual tools for discussing time travel. I also give a brief summary of Van Inwagen’s view. In section 2, I distinguish between two different conceptions of time travel: (i) a traditional conception in which the time traveler does not move the location of the objective present, and (ii) time travel in which the time traveler shifts the location of the objective present. I then articulate and explore the latter model, which I call movable objective present, or MOP. After generalizing MOP to presentist, moving spotlight, and growing block theories of time, I present interesting possible errors of MOP travel.

I owe thanks to audiences at the 2014 Pacific APA and the Gargnano Philosophy of Time Conference for feedback on this paper. I am also grateful to Melissa Schumacher and Heather Wallace for helpful comments.

2 Hudson and Wasserman (2010) argue for a similar conclusion, but do not identify or utilize the more general mechanism of the movable objective present. They argue that hypertime is the key feature that allows the generalization of the model; here I argue that MOP is also essential to the model.


Before delving into models of time travel, it will be helpful to discuss relevant views on the metaphysics of time. Eternalism holds that all times are equally real. There is no meaningful metaphysical distinction between the past, present, and future; Abraham Lincoln is just as real as the iPhone 47. Whether or not one is located in the present is merely a matter of perspective, akin to whether or not one is located “here” in the spatial manifold.

Presentism holds that only the present is real. Everything that exists is present; nonpresent things do not exist.

Growing block holds that the past and present are real. The growing blockist shares the eternalist’s belief in the reality of the past, but stipulates that the present is the “growing edge” of being. With every passing moment, the total amount of being expands.

There are additional questions about the existence and metaphysical status of the objective present. Roughly, the existence of an “objective present” implies that a present time slice is metaphysically privileged in some way. Eternalism, presentism, and growing block vary with respect to the existence of a privileged or objective present. Eternalism denies the existence of such a privileged present, though often includes a subjective present that consists in the perspective of being present without metaphysical privilege. Moving spotlight holds that the present is privileged within the eternalist block by a kind of spotlight that “illuminates” temporally successive slices of the universe. Presentism implies the existence of an objective present by holding that what exists just is the objective present.

Growing block holds that the objective present corresponds with the forward-moving bleeding edge of existence.

Whether or not there is an objective present has deep and important implications for models of time travel, and I will not give an argument for its existence here. Nor will I take a stand on which view of time is correct. I will only work with theories of time that accept an objective present: presentism, growing block, and moving spotlight theory. I will also assume that we can make sense of the idea of an objective present, and that it can ground a certain type of time travel.

Onto Van Inwagen’s model. Using the growing block theory of time, Van Inwagen proposes a model of time travel according to which a time traveler relocates herself to an earlier temporal location in the existing block and annihilates the part of the block between her temporal points of departure and arrival. From the point of arrival, a new, paradox-free block is generated.

Let us examine this model in more detail. Consider a normal growing block universe,

represented by the following diagram:

(a) Growing block universe

–  –  –

Let t represent the beginning of time, and 2013 be the present. Imagine that Bianca, living in 2013, regrets not attending Woodstock in 1969. Here we have the timeline as it occurred,

without Bianca having attended Woodstock:

(b) No time travel

–  –  –

Now, according to Van Inwagen’s model, a time traveller can travel to an earlier point in the block and erase the portions of the block in between the temporal points of departure and arrival. Suppose that Bianca travels from 2013 to 1969 (arriving very shortly before Woodstock so that she can attend the event). In doing so, she annihilates the portion of

existence between 1969 and 2013, like so:

(c) Time travel

–  –  –

Post-time travel, Existence is 44 years smaller than it was before. Bianca attends Woodstock, which she did not do the first time around. Whereas “normal” time travel generates a paradox between Bianca’s initial non-attendance and later attendance at Woodstock, Van Inwagen’s scenario avoids such a problem: Bianca erases the portion of the block in which her initial nonattendance at Woodstock (plus all of the relevant after effects) occurs. After

Bianca’s time travel, the portion of the block beginning with her arrival is regenerated like so:

(d) Time travel plus regeneration

–  –  –

The year 1969 onward occurs a second time around. Only Bianca remembers the “first” time 1969 occurred. But others do not, for she has removed their post-1969 time slices from existence in virtue of having removed all of post-first-time 1969 from existence. And given that Bianca changes the events in second-time-around-1969 with her very presence, nothing else is guaranteed to go the way it did the first time. The end of the Vietnam War, the invention of the microchip, sequencing the human genome: they may not happen, given changes to the past and new regeneration of the block.

Van Inwagen’s model makes use of hypertime. Hypertime, roughly, is a temporal manifold within which another temporal manifold exists. To make this idea clearer, van Inwagen considers an Intelligence that exists outside of time but which sees events in reality unfold in the order in which they occur in hypertime. However time is affected by time travel, the Intelligence will see the unfolding of events hyperchronologically in the passage of hypertime. For example, suppose that a time traveller “rewinds” Reality from t5 to t3. And suppose that the time traveller lives out her years happily in t3, “regenerating” existence

from t3 onwards. From within the temporal manifold, events occur in the following order:

t3, t4, t5, t3, t4, t5. From the hypertemporal perspective, events occur in the following order:

ht3, ht4, ht5, ht6, ht7, ht8. Events whose temporal orders are shifted in time nonetheless have a linear progression in hypertime.

Without time travel, time and hypertime agree on the order of events. According to van Inwagian time travel, time and hypertime disagree on the order of events. Consequently, events can also have different temporal and hypertemporal tenses: the van Inwagian time traveler can attend an event that is in the past and in the hyperfuture, such as Woodstock.

Similarly, van Inwagian time travel results in a durational discrepancy between time and hypertime. Suppose that hypertime is finite and that it lasts for 1000 temporal units.

Normally, the quantity of existence of the growing block universe grows steadily in such a way that time and hypertime agree on its duration: when existence is 400 temporal units long, hyperexistence is also 400 temporal units long. But time travel in which a portion of the growing block is erased generates hypertemporal duration that exceeds temporal duration. Consider our time traveller’s erasure of the span of existence between 1969-2013.

The duration of time extends to 1969, but hypertime marches on past hyper 2013—44 more hyperyears than the temporal manifold (assuming that is the only erasure).


2.1 The Traditional Model: Relocation. Suppose that Bianca travels to 1969 from 2013.

According to the traditional model of time travel, she removes herself from the present day and relocates herself to 1969, leaving the extant temporal manifold intact. The traditional model conceptualizes travel through time as similar to everyday travel through space: just as one does not change the fundamental nature of space by changing spatial locations, one does not change the fundamental nature of time by changing temporal locations.3 One space travels by moving through space, but not by changing space itself. On the traditional model of time travel, one travels through time by moving through time either backwards or forwards, but does not change the fundamental temporal manifold by doing so.

For example, when Bianca leaves 2013 for 1969, she leaves one temporal part of the universe behind (2013), according to the traditional model, and arrives in another temporal part of the universe (1969).4 Like travel through space, the mechanism of traditional time travel is simple relocation in which the time traveller removes herself from one part of the temporal manifold and places herself at another. Just as one does not destroy or affect the rest of the spatial manifold by driving to a coffee shop, one does not destroy or affect the temporal manifold by moving from one location to another.

Importantly, Bianca also leaves others “on the ground” while she travels. Barring well-known paradoxes involving parent and grandparent-killing, she does not change her mother’s existence in 2013 merely by travelling through time. Rather, Bianca leaves her mother “behind” in 2013 as she relocates herself to 1969, much as I leave my home behind when I head for work. This feature reflects the traditional conceptualization of time travel as a kind of relocation of the time traveller similar to spatial relocation.

According to Lewis (1976), time travel also involves a disagreement between personal time and external time. Personal time, roughly speaking, is the time on the time 3 One might change the size and shape of spacetime by time travelling in this way, but not the fundamental nature of time itself.

4 Here I set aside well-known issues involving “second time around” time travel, i.e., the idea that the time traveler must have always been in 1969.

traveler’s watch. External time is the time elapsed on the basic timeline. Suppose that travel from 2013 to 1969 takes Bianca exactly one year. One year of personal time has elapsed for Bianca for the 53 external years she has travelled. Accounts differ on whether personal time is “real” time, but the idea is that traditional time travel involves timelines split between the time traveller and the external timeline. Similarly, the van Inwagian model involves a disagreement in the durations of time and hypertime: the time traveler relocates the edge of time, while hypertime lurches forward.

2.2 The New Model: Movable Objective Present (MOP). I propose that the distinctive feature of van Inwagen’s model is the time traveler’s control over the location of the objective present. When the time traveler flips the switch in the time machine, she does not simply relocate herself to a different spot in the temporal manifold, as on the traditional

model. Rather, she changes the location of the objective present for the entire temporal manifold:

she shifts the location of the objective present to the past of the temporal manifold, as hypertime marches on. I call this sort of time travel movable objective present (hereafter: MOP).

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