Time Travel

by

Alasdair Richmond

• LAST REVIEWED: 26 October 2015
• LAST MODIFIED: 26 October 2015
• DOI: 10.1093/obo/9780195396577-0295

Introduction

Time travel is a philosophical growth industry, with many issues in metaphysics and elsewhere recently transformed by consideration of time travel possibilities. The debate has gradually shifted from focusing on time travel’s logical possibility (which possibility is now generally although not universally granted) to sundry topics including persistence, causation, personal identity, freedom, composition, and natural laws, to name but a few. Besides metaphysical discussions, some time travel works draw on the philosophies of science, spacetime, and computation. Some interesting forerunners notwithstanding, serious physical interest in time travel begins with Gödel’s 1949a demonstration that general relativity permits space-times that are riddled with closed timelike curves (“CTCs” henceforth). A key philosophical text on time travel is Lewis 1976 and its argument for the logical possibility of certain backward time travel journeys and even for the possibility of casual loops. Lewis concludes that time travel could occur in a possible world, albeit perhaps a strange world that would feature (or seem to feature) strange restrictions on actions. In Lewis’s analysis, a traveler can arrive in the past of the same history they come from provided that the traveler’s actions on arrival are consistent with the history that they come from. So other worlds or multiple temporal dimensions are not necessary to make time travel consistent. Granted, the physics, persistence conditions, agency, and epistemology of agents in such worlds might look weird indeed. Since Lewis, philosophical time travel questions include the following: given that a traveler into the past cannot create any paradoxical outcomes on arrival, what then would stay their hand? Are the constraints on a traveler’s actions admissible within our ordinary understanding of physical law or human agency? Is time travel compatible with dynamic time or even with the existence of time itself? Can backward time travel be physically possible within a single history? If a time traveler meets another stage of him- or herself, is the traveler in two places at once, and what theory of persistence can cope with this puzzling multiplication? Can time-travel spacetimes resolve otherwise intractable computational problems?

General Overviews

Despite several hundred philosophical and scientific articles, book chapters, and Internet resources devoted to philosophical problems posed by time travel, there is currently no full-length monograph or anthology on the subject. The best introduction to the topic in general so far is chapter 8 of Dainton (second edition 2010), Dainton 2010 being the best general philosophical resource available on time and space. The key work is Lewis 1976, a defense of the logical possibility of backward time travel, from which a large number of subsequent treatments take their cue. A useful overview, albeit largely from a physical science perspective, is Nahin 1999. Also largely physical in emphasis but comprehensive and thorough is Earman 1995. Richmond 2003 surveys philosophical work on time travel to date. Arntzenius 2006 details the problems of free action and nomological constraint posed by backward time travel. Arntzenius and Maudlin 2005 is helpful on (especially) problems of physical law. Carroll 2008 is perhaps the best single online resource available on any aspect of time travel. Le Poidevin 2003 is a highly commendable introduction to the philosophy of time in general but especially good on problems of time travel. Bourne 2006 offers some useful arguments and clarifications centered on Gödel’s arguments about time travel and the relations between time travel and the status of times themselves. Earman and Wüthrich 2006 offers scientifically well informed but approachable and philosophically cogent discussions of what physics might, and might not, allow by way of time travel.

• Arntzenius, Frank. “Time Travel: Double Your Fun.” Philosophy Compass 6 (2006): 599–616.

  DOI: 10.1111/j.1747-9991.2006.00045.x

  Entertaining survey of the philosophical terrain around time travel that concentrates particularly on the constraints on action likely to be suffered by travelers in the past. An excellent introduction to the nomological contrivance problem and more. Available online for purchase or by subscription.

• Arntzenius, Frank, and Tim Maudlin. “Time Travel and Modern Physics.” In Stanford Encyclopedia of Philosophy. Edited by Edward N. Zalta. 2005.

  Notably acute survey of physical possibilities for time travel, including detailed arguments that backward time travel threatens to create correlations that conflict with standard quantum predictions.

• Bourne, C. A Future for Presentism. Oxford: Oxford University Press, 2006.

  DOI: 10.1093/acprof:oso/9780199212804.001.0001

  Although primarily devoted to defending presentism, chapter 8 offers one of the best treatments of Gödel’s ideality argument around and pp. 132–134 offer some interesting sidelights on the possible compatability of time travel and presentism.

• Carroll, John W. A Time Travel Website. 2008–.

  Extremely thorough, engagingly-written, well-designed, and continually evolving online resource that offers helpful discussions, well-chosen readings, and helpful animations to boot.

• Dainton, Barry. Time and Space. 2d ed. Durham, NC: Acumen, 2010.

  Revised and expanded edition of Dainton’s classic 2001 introduction to the philosophy of space and time. Can be highly recommended but notable here for its extensive, essential treatments of time travel, relativity, and Gödel’s “ideality” argument.

• Earman, John. “Recent Work on Time Travel.” In Time’s Arrows Today. Edited by Steven F. Savitt, 268–310. Cambridge, UK: Cambridge University Press, 1995.

  DOI: 10.1017/CBO9780511622861

  Thorough discussion of the then-current state of play in the philosophical and physical literature on time travel. This is still a valuable resource.

• Earman, John, and Christian Wüthrich. “Time Machines.” In Stanford Encyclopedia of Philosophy. Edited by Edward N. Zalta. 2006.

  Comprehensive discussion of physical resources for time travel, among other intriguing suggestions, develops the view that physically realistic time machines might be uncontrollable even if they become a possiblility.

• Le Poidevin, Robin. Travels in Four Dimensions: The Enigmas of Space and Time. Oxford: Oxford University Press, 2003.

  Engaging and clearly written introduction to the philosophy of space and time. Often offers problems and discussions that lend themselves to time travel interpretation. An excellent introductory and pedagogical resource.

• Lewis, David. “The Paradoxes of Time Travel.” American Philosophical Quarterly 13 (1976): 145–152.

  The philosophical time travel work. Includes Lewis’s discrepancy definition of time travel: the most useful by far. Invokes the notion of compossibility to disambiguate “Grandfather paradox” arguments and argues that backward time travel and causal loops can occur in (nonbranching) possible worlds. Usefully distinguishes between replacement change and counterfactual change. (This is often cited and sometimes rebutted but never refuted.)

• Nahin, Paul. Time Machines: Time Travel in Physics, Metaphysics and Science Fiction. 1st ed. New York: American Institute of Physics, 1999.

  DOI: 10.1007/978-1-4757-3088-3

  Engaging and comprehensive attempt at surveying all the scientific, philosophical, and fictional literature on time travel. Perhaps slightly more at ease with physics and fiction than with philosophy, but this is a detailed and thorough treatment.

• Richmond, Alasdair. “Recent Work: Time Travel.” Philosophical Books 44 (2003): 297–309.

  DOI: 10.1111/1468-0149.00308

  Survey of the time travel debate from Lewis 1976 onward, sketching links with debates in persistence, philosophy of spacetime and temporal topology. Available online by subscription.