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time travel: presentism, going nowhere and personal time

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Hey gang,

Following our discussion on presentism and time travel i decided to take things one step further,

Hope this provides a degree of amusement,

Can a Presentist Believe in Time Travel?


Keller and Nelson argue that presentism can accommodate time-travel, contrary to previous thinking. They attempt to translate Lewis's theory of time travel into presentist terminology. Arguably, if time-travel is compatible with four-dimensionalism, it is also compatible with presentism because they can tell an equally coherent story, using tensed sentences. This essay will examine the conceivability of travelling to a non-existent past, causal relations at temporal distance, personal time and propose a solution to deal with Keller and Nelson's shortfalls by formulating 'relative-presentism'. Granting both the internal coherence of presentism and all that is needed for time-travel to work for eternalism, we will question whether presentism can accommodate time-travel.

We must first ascertain the basic dispute. Presentism asserts that there is only one ontologically real temporal location, the presently existing moment. Secondly, tensed statements are irreducible, for there are no past or future objects to which they may be reduced. In turn, four-dimensionalists retort that objects possess both spatial and temporal parts, thereby providing the necessary foundations for temporal travel.

Presentism appears incompatible with time-travel; one cannot travel to a non-existent past. Backwards causation seems incongruous, if there is no past on which to have an effect. This objection implies that presentists cannot even talk about past events. Even if the presentist can consistently refer to past events, this scarcely entails the coherence of travelling to a non-existent entity. 

The so-called 'Nowhere Argument' fails to disprove presentism's accommodation of time-travel. "Proving too much" , it appears to negate the coherence of presentism. Even ordinary presentist temporal-becoming requires travel to a non-existent temporal region. There is nothing specific to ordinary passage that allows for the travel of non-existent times, which denies the same to past or distant futures. We can clearly overcome the most obvious objection to presentism's compatibility with time-travel.

Accommodation of the four-dimensional interpretation is perhaps merely a matter of translating Lewis's references to personal time into tensed terms. This reformulation can be illustrated by my hypothetical time-travelling to view my own conception. The eternalist, believing the past exists, would say that "it is the case that nineteen years ago I am viewing my conception". Rather than assert the existence of a conception-viewing by me, temporally located some nineteen years ago, the presentist translates this concept, so "it is the case that nineteen years ago that I was viewing my conception". The presentist substitutes two-place tense operators for the four-dimensionalist's use of two-place causal relations 

Using the model that "because 'x' it was the case 'y' units of time ago that 'z', the presentist can thereby translate eternalist terminology. In this example, the presentist substitutes "my entry into the time machine" and "my viewing my conception" for "because I am entering a time-machine, it was the case nineteen years ago that I am watching my conception". So, the four-dimensionalist statement that I disappear in 2005 and reappear in 1986 is transliterated into a presentist account; hence I "will disappear three minutes from now" and "I did reappear in 1986", with the appropriate memories. The presentist's account appears coherent; concepts remain constant despite minor linguistic adjustments. 

However, mere translation seems insufficient to deal with temporal sequence. Although my reappearance, in 1986, precedes my disappearance in external time, the actuality of reappearance is only realised in my personal time. This paradox can only be refuted if we can show the compatibility of two distinct temporal sequences. The presentist must account for the different ordering of the same events.  

However, such compatibility is undermined by presentism's anti-reductionist stance on the direction of time; clearly distinguishing between external and personal time. Lewis argues that travelling into the external past is in one's personal future. Lewis's account requires that personal time is analogous to external time, but the presentist must deny their similarity. Perhaps Keller and Nelson's interpretation of personal time no longer conforms to Lewis's requirements.

The presentist identifies personal time using tensed descriptions of causal and qualitative facts. In contrast, the four-dimensionalist illustrates personal time with reference to causal relations and qualitative differences between existent person-stages. For the presentist, each personal future is constituted by past-tense causal statements, specific to the individual in question. Their notion of personal future is thereby distinct from any idea of my external future, concerning future-tensed statements.

Upon further evaluation, this issue of personal time undermines the presentism's compatibility with time-travel. Personal time, for presentists, does not "play the role that time plays in the life of a common person" . Adopting a presentist stance, I would be unable to consistently declare that "in three minutes time I will watch my own conception". A presentist is limited to uttering that "watching the activity is in the future in my personal time". However, this new translation seems unsatisfactory. By diminishing the similarity between personal and external time, the presentist subverts personal time into an "invented quality" . My personal future plans, to view my conception, consist in me once viewing their activity, courtesy of the causal time-machine. The presentist's account of personal time implies the fallacy that I am about to see the moment of my conception. The time-machine is only capable of making it the case that I once the moment of my conception, rather than that I will see such an event. The requirements of time-travel seem inconsistent with the presentist's principles. 

Even if we admit backwards causation, the presentist may have difficulty accommodating temporally distant causation. Lewis's account of temporal personal identity assumes the possibility of causal relations that lack a chain of causal dependence across time. For instance, my hopping on a time-machine supposedly causes the temporally distant event of me watching my conception. However, presentism, predicated on the objective notion of temporal becoming, seems unable to accommodate causation at a temporal distance. The presentist is instead confined to limiting causal relations to the now, to temporally adjacent events. 

Perhaps this objection misunderstands the nature of causation. The presentist can accommodate causation, at temporal distance, provided it is recognised as a relation rather than an event. For the four-dimensionalist, event X at t2 causes Y at t1. In contrast, the presentist asserts that X 'will occur' at t2 and that Y 'did occur' at t1; Y 'was the effect of X' and 'X will be the cause of Y'. Despite linguistic disparity, perhaps both can accommodate cross-temporal relations.

Arguably the issue of cross-temporal relations poses more problems than mere linguistics. It is incongruous to suggest I can cause some effect ten minutes from now, without causing anything in-between.  Denying this conflict does not deny the conceivability of time-travel, because the eternalist can accommodate the problem with reference to warping space-time. The eternalist considers time to be a fourth-dimension, thereby grounding her solution. However, this option is unavailable to the presentist. In denying time as a fourth-dimension, the presentist denies that there is a dimension to warp, or a past to be contiguous with. 

Presentism denies the warping of space-time; past events cannot be contiguous with the present. To use an analogy, suppose you picked up a piece of paper; wrote 'future' at the top and 'past' at the bottom and then moved your hands closer together, to reveal bending, warping, and space-time curvature. The future would be contiguous with the past. However, this analogy is not available to the presentist because they do not believe in 'paper' (aka time as a fourth-dimension).

Even if we recognise the presentist's denial of time as a fourth-dimension, she still believes in time, as a constantly changing now. The 'constantly changing now' could be understood as personal time. If we recall warping as the bending of the paper, the 'warping' thereby exists in virtue of someone's now being that 'warp-path'.  For the presentist, time exists as the now, so whatever is now, is time. The time-traveler takes the now with them, making the now relative to her own personal time.

Time-travel requires that there is a multitude of 'now's, which seems inconsistent with the presentism. However, we can reformulate presentism, making the now relative to personal time. On this reading, the time-traveler has a now, in 1986, just as her friends in 2005 continue to have a distinct now. Yet this interpretation of presentism presupposes a dimension which allows for the existence of both nows. My 'personal-time-relative-reformulation' contradicts orthodox presentism by entailing that in 2005 there is a past that exists. However, my 'relative presentism' explicitly denies that a past exists relative to the people in 2005, for that past only exists potentially, relative to a potential time-traveler. In addition, my solution also accommodates traveling into the past without invoking the troubling issue of temporally distant causation; for the traveler's personal time ensures causal continuity. Perhaps presentism, reformulated, can thereby accommodate time-travel.

My resolution of the presentist's conflict seems to imply multiple presents and potentially infinite existences; seemingly reducing presentism to nothing. However, presentism can still be understood, in contrast to the eternalist picture, provided it is interpreted in 'relative terms'. Each time-traveler's personal time is distinct from external time, each exhibiting a distinct now. Yet it seems unlikely that any hardcore presentist would really accept such revision for, though necessary to accommodate time-travel, it undermines the presentist's picture.

Presentism cannot accommodate time-travel. Backwards causation requires a particular kind of cross-temporal relations and, in turn, the existent passage of time. My attempt to revise presentism, though compatible with time-travel, renders presentism meaningless, rebuking its foundational premises. Each time-traveler must person occupy their own version of the now but given that this entails infinite existences, which presentism profusely denies, it seems that a thorough-going cannot really believe in time-travel, unless they are just a bit ditzy.


Bibliography

LEWIS, David, "The Paradoxes of Time Travel," in LEWIS, David, "Philosophical Papers: Volume 2". Oxford: Oxford University Press, 1986. 

KELLER, Simon, and NELSON Michael, "Presentists Should Believe in Time-Travel," Australasian Journal of Philosophy 79 (2001), pp. 333-345. 

SIDER, Ted, "Presentism and Ontological Commitment," Journal of Philosophy 96 (1999), pp. 325-347. 



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