Time travel has been a staple of science fiction. With the advent of general relativity it has been entertained by serious physicists. But, especially in the philosophy literature, there have been arguments that time travel is inherently paradoxical. The most famous paradox is the grandfather paradox: you travel back in time and kill your grandfather, thereby preventing your own existence. To avoid inconsistency some circumstance will have to occur which makes you fail in this attempt to kill your grandfather. Doesn't (...) this require some implausible constraint on otherwise unrelated circumstances? We examine such worries in the context of modern physics. (shrink)
Suppose that you travel back in time to talk to your younger self in order to tell her that she (you) should have done some things in her (your) life differently. Of course, you will not be able to make this plan work, we know that from the many versions of 'the grandfather paradox' that populate the philosophical literature about time travel. What will be my centre of interest in this paper is the conversation between you and ... you – (...) i.e. the older you that travelled back in time and the younger you, when you first meet. As we shall see, given this situation, endurantists will have to endorse a strange consequence of their view : you will turn out to be a universal while your properties will turn out to be particulars. (shrink)
Mental time travel and theory of mind develop, both phylo- and ontogenetically, at the same stage. We argue that this synchrony is due to the emergence of a shared competence, namely, the ability to become aware of frames of reference.
The self-visitation paradox is one paradox of time travel. As Ted Sider puts it, “Suppose I travel back in time and stand in a room with my sitting 10-year-old self. I seem to be both sitting and standing, but how can that be?” (2001, 101). So as not to beg any questions, let us label what is sitting B and what is standing C. The worry is about how B can be C in light of the looming contradiction that this (...) one person would be sitting and standing. Sider’s own approach is perdurantist, and holds that B is not C. My concern, though, is with solutions offered by, or on behalf of, endurantists–more 1 specifically, with solutions holding that B is C. The endurantist answer I shall criticize is a relativizer position maintaining that the sitting and the standing need to be relativized to the personal time or proper time of the time traveler. This manner of solution has been offered by Paul Horwich (1975, 433-435 ; 1987, 114-115) and also by Simon Keller and Michael Nelson (2001, 344). I will show that such a view has a linguistically suspect element and that there are three further reasons why relativizing only in this way falls short of solving the paradox. This will be enough to squash the relativizer position because it will not be clear how additional relativization could help, and furthermore any additional relativization would only make the linguistic matter worse. I will also present some considerations in favor of a non-contradiction endurantist alternative; this view eliminates the need for any relativization by denying that sitting and standing are contradictory properties. (shrink)
Ted Sider aptly and concisely states the self-visitation paradox thus: 'Suppose I travel back in time and stand in a room with my sitting 10-year-old self. I seem to be both sitting and standing, but how can that be?' (2001, 101). I will explore a relativist resolution of this paradox offered by, or on behalf of, endurantists.1 It maintains that the sitting and the standing are relative to the personal time or proper time of the time traveler and is intended (...) to yield the result that Ted is sitting at a certain initial personal/proper time but is not standing relative to that time. Similarly, it is also supposed to yield that Ted is standing relative to a later personal/proper time, but not sitting relative to that .. (shrink)
Dear ‘Time Machine’ Research Group; if in order to travel to the past one has to have been there already, and if one can only do what has already been done, then why build a time machine in the first place? À quoi bon l'effort?
The physics behind the limerick is that within Einstein’s special theory of relativity there is a subtle connection between faster-than-light and backwards-in-time travel. If you could do one, then in principle you could also do the other. But relativity is carefully contrived to prevent superluminal and back-in-time travel and communication.
The territory of time travel has, from the days of H. G. Wells to the mid-1980's, been the exclusive province of writers of science fiction and fantasy. SF critics have even argued that time travel stories are so scientifically unlikely that they should be considered fantasy, not science fiction.
Science fiction writers, to avoid undue delays in the story's plot line, need a way of beating the speed of light speed limit of the universe. Most readers of this magazine are familiar with the gimmicks that have been used for faster than light travel: warp drives, detours through hyperspace, matter to tachyon conversion, trans spatial jumps, and dives past the singularity of a rotating black hole. But perhaps the faster than light mechanism which has the best credentials in orthodox (...) physics is the wormhole, also known as a Schwartzschild wormhole or an Einstein Rosen bridge. (shrink)
We consider three possible reasons why humans might accord a privileged status to emotional information when mentally traveling backward or forward in time. First, mental simulation of emotional situations helps one to make adaptive decisions. Second, it can serve an emotion regulation function. Third, it helps people to construct and maintain a positive view of the self.
Here I defend the compatibility of presentism and time travel against a few objections. Keller and Nelson argue that, if presentism is at all plausible, presentism and time travel are as compatible as eternalism and time travel. But Miller and Sider are not convinced. I reply that for their concerns to have merit, Miller and Sider must assume presentists are committed to positions they need not be; I explain why presentists are not so committed and, in the process, defend Keller (...) and Nelson’s position that there is no roadblock to presentist time travel that does not also apply to eternalist time travel. (shrink)
In this paper I consider two objections raised by Nick Smith (1997) to an argument against the probability of time travel given by Paul Horwich (1995, 1987). Horwich argues that time travel leads to inexplicable and improbable coincidences. I argue that one of Smith's objections fails, but that another is correct. I also consider an instructive way to defend Horwich's argument against the second of Smith's objections, but show that it too fails. I conclude that unless there is something faulty (...) in the conception of explanation implicit in Horwich's argument, time travel presents us with nothing that is inexplicable. (shrink)
This idea of time travel has long given philosophers difficulties. Most recently, in his paper ‘Troubles with Time Travel’ William Grey presents a number of objections to time travel, some well known in the philosophical literature, others quite novel. In particular Grey's ‘no destinations’ and ‘double occupation’ objections I take to be original, while what I will call the ‘times paradox’ and the ‘possibility restriction argument’ are versions of well known objections. I show how each of these can be answered, (...) thereby defending the plausibility of time travel. (shrink)
This paper examines various philosophical arguments to do with time travel. It argues that time travel has not been shown to be logically impossible. It then considers whether time travel would give rise to improbable strings of coincidences, or closed causal loops. Finally, it considers whether we could ever be justified in believing someone who claimed to be a time traveller, or whether we would always be more justified in believing that the claimant was either deluded or trying to deceive (...) us. For this last issue the Terry Gilliam film ‘Twelve Monkeys’ is used as an example. (shrink)
Recently, Cody Gilmore has deployed an ingenious case involving backwards time travel to highlight an apparent conflict between the theory that objects persist by perduring, and the thesis that wholly coincident objects are impossible. However, careful attention to the concepts of location and parthood that Gilmore’s cases involve shows that the perdurantist faces no genuine objection from these cases, and that the perdurantist has a number of plausible and dialectically appropriate ways to avoid the supposed conflict.
We address the question of whether it is possible to operate a time machine by manipulating matter and energy so as to manufacture closed timelike curves. This question has received a great deal of attention in the physics literature, with attempts to prove no-go theorems based on classical general relativity and various hybrid theories serving as steps along the way towards quantum gravity. Despite the effort put into these no-go theorems, there is no widely accepted definition of a time machine. (...) We explain the conundrum that must be faced in providing a satisfactory definition and propose a resolution. Roughly, we require that all extensions of the time machine region contain closed timelike curves; the actions of the time machine operator are then sufficiently “potent” to guarantee that closed timelike curves appear. We then review no-go theorems based on classical general relativity, semi-classical quantum gravity, quantum field theory on curved spacetime, and Euclidean quantum gravity. Our verdict on the question of our title is that no result of sufficient generality to underwrite a confident “yes” has been proven. Our review of the no-go results does, however, highlight several foundational problems at the intersection of general relativity and quantum physics that lend substance to the search for an answer. (shrink)
The Multiverse Thesis is a proposed solution to the Grandfather Paradox. It is popular and well promulgated, found in fiction, philosophy and (most importantly) physics. I first offer a short explanation on behalf of its advocates as to why it qualifies as a theory of time travel (as opposed to mere 'universe hopping'). Then I argue that the thesis nevertheless has an unwelcome consequence: that extended objects cannot travel in time. Whilst this does not demonstrate that the Multiverse Thesis is (...) false, the consequence should give pause for concern. Even if it does not lead one to reject the thesis, I briefly detail some reasons to think it is interesting nonetheless. (shrink)
This paper argues that, in light of certain scenarios involving time travel, Sider’s definition of ‘instantaneous temporal part’ cannot be accepted in conjunction with a semantic thesis that perdurantists often assume. I examine a rejoinder from Sider, as well as Thomson’s alternative definition of ‘instantaneous temporal part’, and show how neither helps. Given this, we should give up on the perdurantist semantic thesis. I end by recommending that, once we no longer accept such semantics, we should accept a new set (...) of definitions, which are superior in certain respects to Sider’s original set. (shrink)
I have previously argued in a paper with Robson that a particular time travel scenario favours perdurantism over endurantism on the grounds that endurantists must give up on the Weak Supplementation Principle. Smith has responded, arguing that the reasons we provided are insufficient to warrant this conclusion. This paper agrees with that conclusion (for slightly different reasons: that even the perdurantist has to give up on the Weak Supplementation Principle) but argues that the old argument can be supplanted with a (...) new one. (shrink)
In this paper, we argue that time travel is problematic for the endurantist. For it appears to be possible, given time travel, to construct a wall out of a single time travelling brick. This commits the endurantist to one of the following: (a) the wall is composed of the time travelling brick many times over; (b) the wall does not in fact exist at all; (c) the wall is identical to the brick. We argue that each of these options is (...) unsatisfactory. (shrink)
Memory theories of personal identity are subject to the difficulty that distinct simultaneous person stages may both stand in the memory relation to an earlier person stage. Apparently, Such theories entail that these two duplicate person stages are stages of the same person, A claim argued to be "obviously false". In this paper, I argue that the characteristics of these duplication cases usually cited to support this claim do not provide adequate evidence to make it cogent.
I distinguish paradoxes and hypodoxes among the conundrums of time travel. I introduce ‘hypodoxes’ as a term for seemingly consistent conundrums that seem to be related to various paradoxes, as the Truth-teller is related to the Liar. In this article, I briefly compare paradoxes and hypodoxes of time travel with Liar paradoxes and Truth-teller hypodoxes. I also discuss Lewis’ treatment of time travel paradoxes, which I characterise as a Laissez Faire theory of time travel. Time travel paradoxes are impossible according (...) to Laissez Faire theories, while it seems hypodoxes are possible. (shrink)
A course-grained theory of event individuation is defended by arguing that events are spatiotemporal particulars with an ontological affinity to coarse-grained physical objects and by demonstrating that the metalinguistic correlate to one set of adequate identity conditions for events is most plausibly iterpreted as coarsely individuating events. Such coarse-grained events, it is argued, do admit of divisibility proliferation, much like the proliferation of physical objects entailed by Goodman's calculus of individuals. This coase-grained, divisibility proliferation account of events is then used (...) to resolve Davidson's paradox concerning the poisoned space traveller who is killed long befor he dies. (shrink)
The role of time in episodic memory and mental time travel is considered in light of findings on humans' temporal memory and anticipation. Time is not integral or uniform in memory for the past or anticipation of the future. The commonalities of episodic memory and anticipation require further study.
Patients with damage to the ventromedial prefrontal cortex (VMPFC) are often described as having impaired ability for planning and decision making despite retaining intact capacities for explicit reasoning. The somatic marker hypothesis is that the VMPFC associates implicitly represented affective information with explicit representations of actions or outcomes. Consequently, when the VMPFC is damaged explicit reasoning is no longer scaffolded by affective information, leading to characteristic deficits. These deficits are exemplified in performance on the Iowa Gambling Task (IGT) in which (...) subjects with VMPFC perform significantly worse than neurotypicals in a task which requires them learn from rewarding and punishing experience to make decisions. The somatic marker theory adopts a canonical theory of emotion, in which emotions function as part of a valencing system, to explain the role of affective processes. The first part of the paper argues against this canonical account. The second part provides a different account of the role of the role of the VMPFC in decision-making which does not depend on the canonical account of emotion. Together the first and second parts of the paper provide the basis for a different interpretation of results on the Iowa Gambling Task (IGT). In fact the IGT may be probing a deficit in what has been called mental time travel: the ability to access and use information from previous experience and imaginatively rehearse future experiences as part of the process of deliberation. (shrink)
Existing puzzles about coinciding objects can be divided into two types, corresponding to the manner in which they bear upon the endurantism v. perdurantism debate. (Endurantism is the view that material objects lack temporal extent and persist through time by being wholly present at each moment of their careers. Perdurantism is the opposing view that material objects persist by being temporally extended and having different temporal parts located at different times.) Puzzles of the first type, which involve temporary (...) spatial co-location, can be solved simply by abandoning endurantism in favor of perdurantism, whereas those of the second type, which involve career-long spatial co-location, remain equally puzzling on both views. I show that the possibility of backward time travel (either via discontinuous jumps or via closed timelike curves) would give rise to a new type of puzzle. The new puzzles confront perdurantists and can be solved just by shifting to endurantism. (shrink)
Some philosophers argue that any attempt to model changing the past will either be contradictory or really model avoiding the past. Using Nicholas Smith's (1997) argument as a basis, I formulate a generic version of this Avoidance Argument. I argue that the Avoidance Argument fails because (i) it involves an equivocation of what is meant by ‘bifurcation of the time of an event’ and (ii) resolving the equivocation results in the falsity of at least one of the premises. Hence, the (...) Avoidance Argument will not support the claim that changing the past is logically impossible. (shrink)
Robert Casati and Achille C. Varzi, argue that time machines would be useless or have no practical applications on the grounds that travelling to the past would involve doing what has already been done. I argue that the sense in which travelling to the past involves doing what has already been done fails to support the claim that time machines would have no practical applications.