The Physics of Time Travel

A special issue of Universe (ISSN 2218-1997). This special issue belongs to the section "Gravitation".

Deadline for manuscript submissions: closed (31 January 2024) | Viewed by 13656

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Max Planck Institute for Gravitational Physics (Albert-Einstein-Institute) Potsdam, Brandenburg, Germany
Interests: high-energy physics; general relativity; quantum field theory; special and general relativity; quantum physics; cosmology; gravitational physics; fundamental physics; quantum cosmology; black holes; wormholes; universe; multiverse; thermodynamics; relativistic quantum information; philosophy of science
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School of Mathematics and Statistics, Victoria University of Wellington, Wellington, New Zealand
Interests: general relativity; gravitational physics; black holes; cosmology; universe; multiverse; special relativity; quantum field theory; quantum physics; fundamental physics; wormholes; warp drives

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School of Mathematics and Statistics, Victoria University of Wellington, Wellington, New Zealand
Interests: general relativity; gravitational physics; black holes; thermodynamics; cosmology; universe; special relativity; quantum field theory; warp drives; tractor beams

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Institute of Theoretical Physics, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic
Interests: general relativity; black holes; curved space-time quantum field theory; analogue spacetimes; exotic solutions to general relativity

Special Issue Information

Dear Colleagues,

The concept of time travel has been a staple of science fiction for some 125 years. Likewise, in physics, notions of non-trivial causal structure were already implicit in special relativity and general relativity, developed some 115 and 105 years ago, respectively. They became much more explicit with Gödel's cosmology of 1949 and the much more recent 1988 introduction of traversable wormholes. 

Since then, a significant body of scientific literature has developed concerning our understanding of possible violations of causality in relativity. These ideas have also been easily carried over into other fields of physics, and discussions of possible causality violations can now be found in quantum theory, modified theories of gravity, particle physics, cosmology, and many other arenas. In addition to the thought-provoking concept of time travel itself, understanding its underlying physics will have deep implications for our current theories of nature.

This Special Issue of Universe is dedicated to exploring and carefully analyzing such unusual causal structures, including those in general relativity and in its various modifications, but also in quantum physics (with or without relativistic considerations). Our aim is to summarize extant frameworks, document the state of the art, chart its challenges, and stimulate discussions.

We plan for this Special Issue to cover a wide variety of topics, focusing on fundamental discussions on the viability of various models. Possible themes will include arguments for and against Hawking's chronology protection conjecture, the formulation of causal hierarchies in both standard and modified general relativity (such as rainbow gravity and Hořava-like gravity), quantum field theory in the presence of time machines, quantum information concerns, the collapse of the wave function in quantum physics, and whether Feynman's “sum over histories” approach should or should not include histories containing closed timelike curves.

We aim to collect articles from this wide range of frameworks—everything from classical general relativity to semi-classical gravity, traversable wormholes, and quantum gravity, leaving open the possibility of pure quantum discussions that make no specific commitment to spacetime paradigms. We also aim to build bridges to relativistic quantum information, as well as have philosophers of science add their perspectives and insights. This will provide the community with a complete picture of the foundational, phenomenological, and mathematical implications that the many demands and challenges of time travel place on physics.

Dr. Ana Alonso-Serrano
Prof. Dr. Matt Visser
Dr. Jessica Santiago
Dr. Sebastian Schuster
Guest Editors

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Universe is an international peer-reviewed open access monthly journal published by MDPI.

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Keywords

  • time travel
  • chronology protection
  • closed causal curves
  • causality
  • inconsistent histories
  • quantum signalling
  • quantum ontology
  • wormholes
  • warp drives
  • faster than light

Published Papers (9 papers)

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Research

21 pages, 334 KiB  
Article
Gravitational Wormholes
by Mengqi Lu, Jiayue Yang and Robert B. Mann
Universe 2024, 10(6), 257; https://doi.org/10.3390/universe10060257 - 10 Jun 2024
Viewed by 275
Abstract
Spacetime wormholes are evidently an essential component of the construction of a time machine. Within the context of general relativity, such objects require, for their formation, exotic matter—matter that violates at least one of the standard energy conditions. Here, we explore the possibility [...] Read more.
Spacetime wormholes are evidently an essential component of the construction of a time machine. Within the context of general relativity, such objects require, for their formation, exotic matter—matter that violates at least one of the standard energy conditions. Here, we explore the possibility that higher-curvature gravity theories might permit the construction of a wormhole without any matter at all. In particular, we consider the simplest form of a generalized quasi topological theory in four spacetime dimensions, known as Einsteinian Cubic Gravity. This theory has a number of promising features that make it an interesting phenomenological competitor to general relativity, including having non-hairy generalizations of the Schwarzschild black hole and linearized equations of second order around maximally symmetric backgrounds. By matching series solutions near the horizon and at large distances, we find evidence that strong asymptotically AdS wormhole solutions can be constructed, with strong curvature effects ensuring that the wormhole throat can exist. Full article
(This article belongs to the Special Issue The Physics of Time Travel)
14 pages, 538 KiB  
Article
Falling into the Past: Geodesics in a Time Travel Metric
by Colin MacLaurin, Fabio Costa and Timothy C. Ralph
Universe 2024, 10(2), 95; https://doi.org/10.3390/universe10020095 - 16 Feb 2024
Viewed by 1276
Abstract
We investigate timelike and null geodesics within the rotating “time machine” spacetime proposed by Ralph, T.C.; et al. Phys. Rev. D 2020, 102, 124013. This is a rotating analogue of Alcubierre’s warp drive spacetime. We obtain geodesics that begin and [...] Read more.
We investigate timelike and null geodesics within the rotating “time machine” spacetime proposed by Ralph, T.C.; et al. Phys. Rev. D 2020, 102, 124013. This is a rotating analogue of Alcubierre’s warp drive spacetime. We obtain geodesics that begin and end in the surrounding flat space region, yet achieve time travel relative to static observers there. This is a global property, as the geodesics remain locally future-pointing, as well as timelike or null. Full article
(This article belongs to the Special Issue The Physics of Time Travel)
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20 pages, 2810 KiB  
Article
Emergent Time and Time Travel in Quantum Physics
by Ana Alonso-Serrano, Sebastian Schuster and Matt Visser
Universe 2024, 10(2), 73; https://doi.org/10.3390/universe10020073 - 2 Feb 2024
Viewed by 1539
Abstract
Entertaining the possibility of time travel will invariably challenge dearly-held concepts in fundamental physics. It becomes relatively easy to construct multiple logical contradictions using differing starting points from various well-established fields of physics. Sometimes, the interpretation is that only a full theory of [...] Read more.
Entertaining the possibility of time travel will invariably challenge dearly-held concepts in fundamental physics. It becomes relatively easy to construct multiple logical contradictions using differing starting points from various well-established fields of physics. Sometimes, the interpretation is that only a full theory of quantum gravity will be able to settle these logical contradictions. Even then, it remains unclear if the multitude of problems could be overcome. Yet as definitive as this seems to the notion of time travel in physics, such recourse to quantum gravity comes with its own, long-standing challenge to most of these counter-arguments to time travel: These arguments rely on time, while quantum gravity is (in)famously stuck with the problem of time. One attempt to answer this problem within the canonical framework resulted in the Page–Wootters formalism, and its recent gauge-theoretic reinterpretation as an emergent notion of time. Herein, we will begin a program to study toy models implementing the Hamiltonian constraint in quantum theory, with an aim toward understanding what an emergent notion of time can tell us about the (im)possibility of time travel. Full article
(This article belongs to the Special Issue The Physics of Time Travel)
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15 pages, 804 KiB  
Article
Closed Timelike Curves Induced by a Buchdahl-Inspired Vacuum Spacetime in R2 Gravity
by Hoang Ky Nguyen and Francisco S. N. Lobo
Universe 2023, 9(11), 467; https://doi.org/10.3390/universe9110467 - 30 Oct 2023
Cited by 3 | Viewed by 1531
Abstract
The recently obtained special Buchdahl-inspired metric Phys. Rev. D 107, 104008 (2023) describes asymptotically flat spacetimes in pure Ricci-squared gravity. The metric depends on a new (Buchdahl) parameter k˜ of higher-derivative characteristic, and reduces to the Schwarzschild metric, for [...] Read more.
The recently obtained special Buchdahl-inspired metric Phys. Rev. D 107, 104008 (2023) describes asymptotically flat spacetimes in pure Ricci-squared gravity. The metric depends on a new (Buchdahl) parameter k˜ of higher-derivative characteristic, and reduces to the Schwarzschild metric, for k˜=0. For the case k˜(1,0), it was shown that it describes a traversable Morris–Thorne–Buchdahl (MTB) wormhole Eur. Phys. J. C 83, 626 (2023), where the weak energy condition is formally violated. In this paper, we briefly review the special Buchdahl-inspired metric, with focuses on the construction of the Kruskal–Szekeres (KS) diagram and the situation for a wormhole to emerge. Interestingly, the MTB wormhole structure appears to permit the formation of closed timelike curves (CTCs). More specifically, a CTC straddles the throat, comprising of two segments positioned in opposite quadrants of the KS diagram. The closed timelike loop thus passes through the wormhole throat twice, causing two reversals in the time direction experienced by the (timelike) traveller on the CTC. The key to constructing a CTC lies in identifying any given pair of antipodal points (T,X) and (T,X) on the wormhole throat in the KS diagram as corresponding to the same spacetime event. It is interesting to note that the Campanelli–Lousto metric in Brans–Dicke gravity is known to support two-way traversable wormholes, and the formation of the CTCs presented herein is equally applicable to the Campanelli–Lousto solution. Full article
(This article belongs to the Special Issue The Physics of Time Travel)
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13 pages, 380 KiB  
Article
Superluminal Local Operations in Quantum Field Theory: A Ping-Pong Ball Test
by Albert Much and Rainer Verch
Universe 2023, 9(10), 447; https://doi.org/10.3390/universe9100447 - 11 Oct 2023
Cited by 2 | Viewed by 1207
Abstract
It is known that, in quantum field theory, localized operations, e.g., given by unitary operators in local observable algebras, may lead to non-causal, or superluminal, state changes within their localization region. In this article, it is shown that, both in quantum field theory [...] Read more.
It is known that, in quantum field theory, localized operations, e.g., given by unitary operators in local observable algebras, may lead to non-causal, or superluminal, state changes within their localization region. In this article, it is shown that, both in quantum field theory as well as in classical relativistic field theory, there are localized operations which correspond to “instantaneous” spatial rotations (leaving the localization region invariant) leading to superluminal effects within the localization region. This shows that “impossible measurement scenarios” which have been investigated in the literature, and which rely on the presence of localized operations that feature superluminal effects within their localization region, do not only occur in quantum field theory, but also in classical field theory. Full article
(This article belongs to the Special Issue The Physics of Time Travel)
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16 pages, 416 KiB  
Article
On the Inaccessibility of Time Machines
by Marija Tomašević
Universe 2023, 9(4), 159; https://doi.org/10.3390/universe9040159 - 24 Mar 2023
Cited by 1 | Viewed by 1184
Abstract
We will explain why time machines, although allowed in General Relativity, cannot be accessed by observers once we include quantum effects. Moreover, we will show that traversable wormholes cannot be turned into time machines without invoking the effects of quantum gravity. Full article
(This article belongs to the Special Issue The Physics of Time Travel)
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6 pages, 315 KiB  
Communication
A Causality Preserving Evolution of a Pair of Strings
by Serguei Krasnikov
Universe 2022, 8(12), 640; https://doi.org/10.3390/universe8120640 - 1 Dec 2022
Cited by 1 | Viewed by 889
Abstract
As follows from Gott’s discovery, a pair of straight string-like singularities moving in opposite directions, when they have suitable speed and impact parameter, produce closed timelike curves. I argue in this paper that there always is a not-so-frightening alternative: the Universe may prefer [...] Read more.
As follows from Gott’s discovery, a pair of straight string-like singularities moving in opposite directions, when they have suitable speed and impact parameter, produce closed timelike curves. I argue in this paper that there always is a not-so-frightening alternative: the Universe may prefer to produce a certain (surprisingly simple and absolutely mild) singularity instead. Full article
(This article belongs to the Special Issue The Physics of Time Travel)
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6 pages, 262 KiB  
Article
Analogue Non-Causal Null Curves and Chronology Protection in a dc-SQUID Array
by Carlos Sabín
Universe 2022, 8(9), 452; https://doi.org/10.3390/universe8090452 - 29 Aug 2022
Cited by 2 | Viewed by 1001
Abstract
We propose an analogue quantum simulator of a 1 + 1D spacetime containing non-causal curves, in particular null geodesics going back in time, by means of a dc-SQUID array embedded on an open superconducting transmission line. This is achieved by mimicking the spatial [...] Read more.
We propose an analogue quantum simulator of a 1 + 1D spacetime containing non-causal curves, in particular null geodesics going back in time, by means of a dc-SQUID array embedded on an open superconducting transmission line. This is achieved by mimicking the spatial dependence of the metric with the propagation speed of the electromagnetic field in the simulator, which can be modulated by an external magnetic flux. We show that it is possible to simulate a spacetime region containing non-causal null geodesics, but not a full spacetime containing a chronological horizon separating regions with non-causal null geodesics and regions without them. This is in agreement with a recent suggestion of an analogue-gravity chronology protection mechanism by Barceló et al. Full article
(This article belongs to the Special Issue The Physics of Time Travel)
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8 pages, 633 KiB  
Communication
On the Counter-Rotation of Closed Time-like Curves
by Yuanyuan Duan, Fangxun Liu, Yu Wang and Yen Chin Ong
Universe 2022, 8(1), 28; https://doi.org/10.3390/universe8010028 - 4 Jan 2022
Cited by 1 | Viewed by 1368
Abstract
While it is tempting to think of closed time-like curves (CTCs) around rotating bodies, such as a black hole, as being “caused” by the rotation of the source, Andréka et al. pointed out that the underlying physics are not as straightforward as this, [...] Read more.
While it is tempting to think of closed time-like curves (CTCs) around rotating bodies, such as a black hole, as being “caused” by the rotation of the source, Andréka et al. pointed out that the underlying physics are not as straightforward as this, since such CTCs are “counter-rotating”, i.e., the time orientation (the opening of the local light cones) of the CTCs is opposite to the direction in which the singularity or the ergosphere rotates. It was also suggested that this is a generic phenomenon that calls for a deeper intuitive physical understanding. In this short note, we point out—with Kerr–Taub–NUT as an example—that CTCs are counter-rotating with respect to the local angular velocity of the spacetime, not the global angular momentum, nor the angular velocity of the black hole horizon, which makes the physical interpretation of CTCs being “caused” by a rotating source even more problematic. Full article
(This article belongs to the Special Issue The Physics of Time Travel)
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