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)

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)

Within the context of general relativity, we consider one definition of a “time machine” proposed by Earman, Smeenk, and Wüthrich. They conjecture that, under their definition, the class of time machine spacetimes is not empty. Here, we prove this conjecture. †To contact the author, please write to: Department of Philosophy, University of Washington, Box 353350, Seattle, WA 98195‐3350; e‐mail: [email protected].

This article shows a clear sense in which general relativity allows for a type of ‘machine’ that can bring about a spacetime structure suitable for the implementation of ‘supertasks’. 1Introduction2Preliminaries3Malament–Hogarth Spacetimes4Machines5Malament–Hogarth Machines6Conclusion.

I investigate the principle *anything goes* within the context of general relativity. After a few preliminaries, I show a sense in which the universe is unknowable from within this context; I suggest that we 'keep our options open' with respect to competing models of it. Given the state of affairs, proceeding counter-inductively seems to be especially appropriate; I use this method to blur some of the usual lines between 'reasonable' and 'unreasonable' models of the universe. Along the way, one is (...) led to a useful collection of variant theories of general relativity -- each theory incompatible with the standard formulation. One may contrast one variant theory with another in order to understand foundational questions within 'general relativity' in a more nuanced way. I close by sketching some of the work ahead if we are to embrace such a pluralistic methodology. (shrink)

In a recent publication in the European Journal for Philosophy of Science, Romero and Pérez claim to reveal new trouble for the already difficult life of presentism in relativistic spacetimes. Their argument purports to demonstrate the impossibility of postulating a viable present in the presence of black holes, in particular the Schwarzschild geometries. I argue that their argument is flawed, and that the Schwarzschild geometries they consider offer no novel threats to presentism. However, if we consider more general black holes, (...) different and more potent threats may be lurking in the dark. I discuss these threats and sketch how a presentist may respond, thus painting a more balanced picture of the way the existence of black holes might impact presentist metaphysics. At present there is no decisive verdict from black holes, but it turns out a close scrutiny of the developments in the relevant physics will be needed to see whether their import will remain so inconclusive. (shrink)

We present a counterexample to Krasnikov's much discussed time machine no-go result. In addition, we prove a positive statement: a time machine existence theorem under a modest "no holes" assumption.

A database of 343 free-response ESP trials conducted at centers in the U.K. was constructed in order to test the hypothesis that the relatively fast varying components of geomagnetic activity, geomagnetic pulsations, might be driving the reported associations between ESP, geomagnetic activity and local sidereal time. Local geomagnetic field measurements taken at 1- second intervals during 99 trials, and at 5-second intervals during 244 trials, were converted by fast Fourier transform into power within five frequency bands. Two patterns were observed: (...) ESP was found to succeed only during periods of enhanced pulsation activity within the 0.2–0.5 Hz band, but ESP effect was absent during the most disturbed periods of activity in the 0.025–0.1 Hz band. The pattern of ESP effect by local sidereal time was similar to that found by Spottiswoode (1997b), and this shape was found to be attributable to the pattern of ESP results by pulsation activity in the 0.2–0.5 Hz band. The observed patterns were demonstrated to have excellent explanatory power in terms of accounting for findings previously reported in the literature. Keywords: ESP—geomagnetic activity—geomagnetic pulsations—local sidereal time. (shrink)