_ Paradoxes of Time Travel _ By WassermanRyanOxford University Press, 2018. x + 250 pp.

This article deals with the question of what time reversal means. It begins with a presentation of the standard account of time reversal, with plenty of examples, followed by a popular non-standard account. I argue that, in spite of recent commentary to the contrary, the standard approach to the meaning of time reversal is the only one that is philosophically and physically viable. The article concludes with a few open research problems about time reversal.

Gödel's remarks concerning the ideality of time are examined. In the literature, some of these remarks have been somewhat neglected while others have been heavily criticized. In this note, we propose a clear and defensible sense in which Gödel's work bears on the question of whether there is an objective lapse of time in our world.

It is often thought that there is little that seems more obvious from experience than that time objectively passes, and that time is, in this respect, quite unlike space. Yet nothing in the physical picture of the world seems to correspond to the idea of such an objective passage of time. In this paper, I discuss some attempts to explain this apparent conflict between appearance and reality. I argue that existing attempts to explain the conflict as the result of a (...) perceptual illusion fail, and that it is, in fact, the nature of memory, rather than perception, that explains why we are inclined to think of time as passing. I also offer a diagnosis as to why philosophers have sometimes been tempted to think that an objective passage of time seems to figure directly in perceptual experience, even though it does not. (shrink)

Tensism holds that the present moment has a special status that sets it apart from the past and the future, independently of perceivers. One of the main objections to this view has been Einstein’s argument from special relativity, which aims at showing that absolute simultaneity is a myth. We argue that the moving observer in a causal variant of Einstein’s original thought experiment is subject to a temporal illusion. Owing to the analogy of the cases, this casts doubt on the (...) conjectures that there is no privileged frame of reference and that special relativity poses a problem for tensism. (shrink)

Thinking about time travel is an entertaining way to explore how to understand time and its location in the broad conceptual landscape that includes causation, fate, action, possibility, experience, and reality. It is uncontroversial that time travel towards the future exists, and time travel to the past is generally recognized as permitted by Einstein’s general theory of relativity, though no one knows yet whether nature truly allows it. Coherent time travel stories have added flair to traditional debates over the metaphysical (...) status of the past, the reality of temporal passage, and the existence of free will. Moreover, plausible models of time travel and time machines can be used to investigate the subtle relation between space-time structure and causality. -/- It surveys some philosophical issues concerning time travel and should serves as a quick introduction. It includes a new and improved way to define a time machine. (shrink)

But this picture of a ‘block universe’, composed of a timeless web of ‘world-lines’ in a four-dimensional space, however strongly suggested by the theory of relativity, is a piece of gratuitous metaphysics. Since the concept of change, of something happening, is an inseparable component of the common-sense concept of time and a necessary component of the scientist's view of reality, it is quite out of the question that theoretical physics should require us to hold the Eleatic view that nothing happens (...) in ‘the objective world’. Here, as so often in the philosophy of science, a useful limitation in the form of representation is mistaken for a deficiency of the universe. (shrink)

The aim of this essay is to introduce philosophers of science to some recent philosophical discussions of the nature and origin of the direction of time. The essay is organized around books by Hans Reichenbach, Paul Horwich, and Huw Price. I outline their major arguments and treat certain critical points in detail. I speculate at the end about the ways in which the subject may continue to develop and in which it may connect with other areas of philosophy.

This paper is an enquiry into the logical, metaphysical, and physical possibility of time travel understood in the sense of the existence of closed worldlines that can be traced out by physical objects. We argue that none of the purported paradoxes rule out time travel either on grounds of logic or metaphysics. More relevantly, modern spacetime theories such as general relativity seem to permit models that feature closed worldlines. We discuss, in the context of Gödel's infamous argument for the ideality (...) of time based on his eponymous spacetime, what this apparent physical possibility of time travel means. Furthermore, we review the recent literature on so-called time machines, i.e., of devices that produce closed worldlines where none would have existed otherwise. Finally, we investigate what the implications of the quantum behaviour of matter for the possibility of time travel might be and explicate in what sense time travel might be possible according to leading contenders for full quantum theories of gravity such as string theory and loop quantum gravity. (shrink)

The Many-Worlds Interpretation (MWI) is an approach to quantum mechanics according to which, in addition to the world we are aware of directly, there are many other similar worlds which exist in parallel at the same space and time. The existence of the other worlds makes it possible to remove randomness and action at a distance from quantum theory and thus from all physics.

There is a philosophical tradition of arguing against presentism, the thesis that only presently existing things exist, on the basis of its incompatibility with fundamental physics. I grant that presentism is incompatible with special and general relativity, but argue that presentism is not incompatible with quantum gravity, because there are some theories of quantum gravity that utilize a fixed foliation of spacetime. I reply to various objections to this defense of presentism, and point out a flaw in Gödel's modal argument (...) for the ideality of time. This paper provides an interesting case study of the interplay between physics and philosophy. (shrink)

Since the onset of logical positivism, the general wisdom of the philosophy of science has it that the kantian philosophy of (space and) time has been superseded by the theory of relativity, in the same sense in which the latter has replaced Newton’s theory of absolute space and time. On the wake of Cassirer and Gödel, in this paper I raise doubts on this commonplace by suggesting some conditions that are necessary to defend the ideality of time in the sense (...) of Kant. In the last part of the paper I bring to bear some contemporary physical theories on such conditions. (shrink)

Some have thought that the process of the expansion of the universe can be used to define an absolute ‘cosmic time’ which then serves as the absolute time required by tensed theories of time. Indeed, this is the very reason why many tense theorists are happy to concede that special relativity is incompatible with the tense thesis, because they think that general relativity, which trumps special relativity, and on which modern cosmology rests, supplies the means of defining temporal becoming using (...) cosmic time. I argue that cosmic time is not up to the task, and that these tense theorists should rethink their strategy in dealing with the theories of relativity. Introduction The Mellor-Rees argument against tense theories Can expansion combat such wrinkles? Event and creation horizons Bursting the balloon. (shrink)

In the literature on the compatibility between the time of our experience and the time of physics, the special theory of relativity has enjoyed central stage. By bringing into the discussion the general theory of relativity, I suggest a new analysis of the misunderstood notion of becoming, developed from hints in Gödel's published and unpublished arguments for the ideality of time. I claim that recent endorsements of such arguments, based on Gödel's own ‘rotating’ solution to Einstein's field equation, fail: once (...) understood in the right way, becoming can be shown to be both mind-independent and compatible with spacetime physics. Being a neededtertium quidbetween views of time traditionally regarded as in conflict, such a new approach to becoming should also help to dissolve a crucial aspect of the century-old debate between the so-called A and B theories of time. (shrink)

I wish to discuss a supposed implication of one sort of time travel. The sort of time travel is time travel into one’s past along a closed timelike curve. The implication is that in spacetimes with CTCs there can be no temporal passage or “flow” of time. I will argue that the implication does not hold.

Mark Hinchliff concludes a recent paper, "The Puzzle of Change," with a section entitled "Is the Presentist Refuted by the Special Theory of Relativity?" His answer is "no." I respond by arguing that presentists face great difficulties in merely stating their position in Minkowski spacetime. I round up some likely candidates for the job and exhibit their deficiencies.

A comparison of a recent paper by Giora Hon in this journal with a book I wrote several years ago, on Gödel's philosophy of time, reveals that the substance, and indeed many of the words themselves, appearing in Hon's essay are in fact original to my book—the ideas of which he sadly failed to understand.

_ Belief: A Pragmatic Picture _ By ZimmermanAaron Z.Oxford University Press, 2018. viii + 180 pp.

Two symmetry arguments are discussed, each purporting to show that there is no more room for a preferred division of spacetime into instants of time in general relativistic cosmology than in Minkowski spacetime. The first argument is due to Gödel, and concerns the symmetries of his famous rotating cosmologies. The second turns upon the symmetries of a certain space of relativistic possibilities. Both arguments are found wanting.

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. 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 would give rise to a new type of puzzle. (...) The new puzzles confront perdurantists and can be solved just by shifting to endurantism. (shrink)

A variety of ideas arising in decoherence theory, and in the ongoing debate over Everett's relative-state theory, can be linked to issues in relativity theory and the philosophy of time, specifically the relational theory of tense and of identity over time. These have been systematically presented in companion papers (Saunders 1995; 1996a); in what follows we shall consider the same circle of ideas, but specifically in relation to the interpretation of probability, and its identification with relations in the Hilbert Space (...) norm. The familiar objection that Everett's approach yields probabilities different from quantum mechanics is easily dealt with. The more fundamental question is how to interpret these probabilities consistent with the relational theory of change, and the relational theory of identity over time. I shall show that the relational theory needs nothing more than the physical, minimal criterion of identity as defined by Everett's theory, and that this can be transparently interpreted in terms of the ordinary notion of the chance occurrence of an event, as witnessed in the present. It is in this sense that the theory has empirical content. (shrink)

Abstract In an engaging and ingenious paper, Irvine (1993) purports to show how the resolution of Braess? paradox can be applied to Newcomb's problem. To accomplish this end, Irvine forges three links. First, he couples Braess? paradox to the Cohen?Kelly queuing paradox. Second, he couples the Cohen?Kelly queuing paradox to the Prisoner's Dilemma (PD). Third, in accord with received literature, he couples the PD to Newcomb's problem itself. Claiming that the linked models are ?structurally identical?, he argues that Braess solves (...) Newcomb's problem. This paper shows that Irvine's linkage depends on structural similarities?rather than identities?between and among the models. The elucidation of functional disanalogies illuminates structural dissimilarities which sever that linkage. I claim that the Cohen?Kelly queuing paradox cloaks a fine structure that decouples it from both Braess? paradox and the PD (Marinoff, 1996a). I further assert that the putative reduction of the PD to a Newcomb problem (e.g. Brams, 1975; Lewis, 1979) is seriously flawed. It follows that Braess? paradox does not solve Newcomb's problem via the foregoing and herein sundered chain. I conclude by substantiating a stronger claim, namely that Braess'paradox cannot solve Newcomb's problem at all. (shrink)

I formulate a theory of persistence in the endurantist family and pose a problem for the conjunction of this theory with orthodox versions of special or general relativity. The problem centers around the question: Where are things?

In this paper I argue that the clash of the Sellars’ two images is particularly acute in the case of time. In Time and the World Order Sellars seems embarked on a quest to locate manifest time in Minkowski spacetime. I suggest that he should have argued for the replacement of manifest time with the local, path-dependent time of the “scientific image”, just as he suggests that manifest objects must be replaced by their scientific counterparts.