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Notes to Time Travel and Modern Physics

1. There is a large philosophical literature on the first two paradoxes (and others), see, e.g., the entry on time travel , Wasserman (2018), and Effingham (2020), but very little on the easy knowledge paradox (emphasized by Deutsch 1991, discussed further below). Our approach differs from the literature surveyed in these two books by focusing on the physical—rather than metaphysical—possibility of time travel.

2. Multiple collisions are handled in the obvious way by continuity considerations: just continue straight lines through the collision point and identify which particle is which by their ordering in space.

3. The dynamics here is radically non-time-reversible. Indeed, the dynamics is deterministic in the future direction but not in the past direction.

4. One might hope that fixed point theorems can be used to prove the existence of solutions in this type of cases too. Consider, for instance, a fixed initial state of motion I of the ball. Then consider all the possible velocities and locations and times \(\langle v,x,t\rangle\) at which such a ball could enter mouth 1 of the wormhole. Each such triple \(\langle v,x,t\rangle\) will determine the trajectory of that ball out of mouth 2. One can then look at the continuation of the trajectory from state I and that from state s , and see whether these trajectories collide. Then one can see for each possible triple \(\langle v,x,t\rangle\) whether the ball that starts in state I will be collided into mouth 1, and if it is, with which speed at what location and at which time this will occur. Thus given state I , each triple \(\langle v,x,t\rangle\) maps onto another triple \(\langle v',x',t'\rangle\). One might then suggest appealing to a fixed point theorem to argue that there must be a solution for each initial state I . However, in the first place the set of possible speeds and times are open sets. And in the second place there can be multiple wormhole traversals. Thus the relevant total state-space of wormhole mouth crossings consists of discretely many completely disconnected state-spaces (with increasing numbers of dimensions). So standard fixed point theorems do not apply directly. It should be noted that the results that have been achieved regarding this case do make use of fixed points theorems quite extensively. But their application is limited to certain sub-problems, and do not yield a fully general proof of the lack of constraints for arbitrary I .

5. This argument, especially the second illustration of it, is similar to the one in Horwich (1987: 124–128). However, we do not share Horwich’s view that it only tells against time travel of humans into their local past.

6. Recently physicists have developed a similar computational approach (called the process matrix formalism) to describe interactions among systems with indefinite causal structure. See Adlam (unpublished) for a philosophical discussion and references to the physics literature.

7. In this section we will presume familiarity with quantum mechanics. See, for example, the entry on quantum mechanics , and its extremely useful guide to further reading, for an entry point to this subject.

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The Quantum Physics of Time Travel

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1994, Scientific American

Related Papers

Adrian Nita

Abstract: we examine the arguments for time travel, in order to appreciate the force of these arguments. We arrive at the conclusion that the arguments for are weaker than the arguments against of the conceptual possibility of the time travel. Away to be a strange world, the world in which took place the travel in time is an impossible world. The basic idea on which is based the rejection of the conceptual possibility of time travel is the violation of the law of identity regarding the world: the world from which the time-traveller goes is not the same with the world in which he arrive. In other words, he cannot travel in the local past or in the local future of the one and the same world.

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This is the table of contents and preface for The Paradoxes of Time Travel, which is forthcoming with Oxford University Press.

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This chapter examines 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, arguing that none of the purported paradoxes rule out time travel on the grounds of either logic or metaphysics. More relevantly, modern space–time theories such as general relativity seem to permit models that feature closed worldlines. The chapter discusses what this apparent physical possibility of time travel means, and, furthermore, reviews the recent literature on so-called time machines, of devices that produce closed worldlines where none would have existed otherwise. Finally, it investigates what the implications of the quantum behavior of matter might be for the possibility of time travel, and explicates 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.

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Ordinarily, philosophers arguing for the possibility of time travel restrict themselves to defending time travel against allegations of inconsistency and contradiction. These objections are usually based on particular theories about time and causality. I believe, however, that this way of arguing can be turned on its head. By using the conceivability thesis – that is, the thesis that if something is conceivable, then it is also possible – we can put forward a positive argument for the possibility of time travel, and then consider how this should inform our metaphysical views. I do this by assuming the truth of the conceivability thesis and then presenting a simple piece of time travel fiction. I argue for the conceivability of this story and, in doing so, attempt to show that the time travel journey it describes is logically possible. I then develop this argument by considering other more controversial cases of time travel. I make minor alterations to the original time travel story in order to show that there are a number of different kinds of time travel journeys (including cases of both forwards and backwards time travel) which are conceivable and, therefore, logically possible. Finally, I ask how the conceivability of different types of time travel should affect the metaphysical views we choose to adopt. I argue that since the conceivability of time travel entails the logical possibility of journeys to other times, any plausible theory of time must be able to accommodate such journeys. I also explain how the conceivability of time travel entails the logical possibility of two particularly unusual cases of causation. I argue that the conceivability of instantaneous time travel entails the logical possibility of causation at a distance, and that the conceivability of backwards time travel entails the logical possibility of backwards causation. Any plausible theory of causation must therefore be able to account for the possibility of these two types of causation. I consider one particular theory of causation which does not (namely, that put forward by D.H. Mellor), and then attempt to establish where it goes wrong.

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I argue that time travel to the past is impossible, given a certain metaphysical theory, namely, The Dynamic Theory of Time. I first spell out my particular way of capturing the difference between The Dynamic Theory of Time and its rival, The Static Theory of Time. Next I offer four different arguments for the conclusion that The Dynamic Theory is inconsistent with the possibility of time travel to the past. Then I argue that, even if I am wrong about this, it will still be true that The Dynamic Theory entails that you should not want to travel back to the past. Finally, I conclude by considering a puzzle that arises for those who believe that time travel to the past is metaphysically impossible: What exactly are we thinking about when we seem to be thinking about traveling back in time? For it certainly does not feel like we are thinking about something that is metaphysically impossible.

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The Physics of Stargates: Parallel Universes, Time Travel and the Enigma of Wormhole Physics

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A Groundbreaking Scientific Discovery Just Gave Humanity the Keys to Interstellar Travel

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If a superluminal—meaning faster than the speed of light—warp drive like Alcubierre’s worked, it would revolutionize humanity’s endeavors across the universe , allowing us, perhaps, to reach Alpha Centauri, our closest star system, in days or weeks even though it’s four light years away.

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  1. Time Travel and Modern Physics

    Time Travel and Modern Physics. First published Thu Feb 17, 2000; substantive revision Mon Mar 6, 2023. 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 ...

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    physics literature on time travel and time machines has been inspired by intriguing connections with quantum eld theory and quantum gravity (the topics of x7 and x8). ... So what is time travel? The standard answer among philosophers, given by David Lewis (1976, 68), is that time travel occurs in case the temporal separation between departure ...

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    2. The Page-Wootters Formalism. Let us consider a quantum system ('the universe') that separates into two subsystems, 'clock' system and a 'residual' system. The PW formalism is our first, and most important ingredient for studying time travel without time (or rather: time travel with only an emergent notion of time).

  4. PDF The Physics of Time Travel

    models time (or is complete) but in my opinion, this would be the best model that avoids a lot of paradoxes of time travel assuming time travel is possible. Finally, I explore several paradoxes of time and explain how my model of time could solve them to a certain extent. Keywords: Time Travel, Physics, Biological Clock, Anti-Particle, History ...

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    1. PDF. Time travel has traditionally been the domain of science fiction, not physics. Fortunately, however, at least within Einstein's theories of relativity, discussions of time travel are open to physicists as well. Special relativity unifies the concepts of time and space. General relativity goes beyond unification and allows time and space ...

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    Chris Smeenk Christian Wuthrich. 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.

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  12. Deutsch, David The quantum physics of time travel.pdf (PDFy mirror

    Deutsch, David The quantum physics of time travel.pdf (PDFy mirror) Publication date 2014-01-01 Topics mirror, pdf.yt Collection pdfymirrors; additional_collections Language English. This public document was automatically mirrored from PDFy. Original filename: Deutsch, David The quantum physics of time travel.pdf;

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  14. (PDF) Emergent Time and Time Travel in Quantum Physics

    Download full-text PDF Read full-text. Download full-text PDF. Read full-text. Download citation. Copy link Link copied. ... Yet as definitive as this seems to the notion of time travel in physics ...

  15. PDF TIME TRAVEL AND THEORIES OF TIME

    time travel (a) to be compatible with the laws of physics, (b) to not imply backward causation and (c) to not be open to a rereading on which no time travel takes place. Science is not primarily concerned with logical contradictions, as (a) would secure this; neither with the counter-intuitive aspects of time travel, as relativity is at odd

  16. Notes to Time Travel and Modern Physics

    Notes to Time Travel and Modern Physics. 1. There is a large philosophical literature on the first two paradoxes (and others), see, e.g., the entry on time travel , Wasserman (2018), and Effingham (2020), but very little on the easy knowledge paradox (emphasized by Deutsch 1991, discussed further below).

  17. PDF Emergent Time and Time Travel in Quantum Physics

    understanding what an emergent notion of time can tell us about the (im)possibility of time travel. Keywords: time travel; quantum gravity; minisuperspace; Page-Wootters formalism; emergent time; relational dynamics 1. Introduction As fascinating as the notion of time travel is, its long list of problematic issues, ranging

  18. PDF Can we time travel? A theoretical physicist provides some answers

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  20. (PDF) The Quantum Physics of Time Travel

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    the past 500 years, found a new solution to Einstein's own equations which allowed for time travel! The "river of time" now had whirlpools in which time could wrap itself into a circle. Goedel's solution was quite ingenious: it postulated a universe filled with a rotating fluid. Anyone walking along the direction of rotation would find themselves

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