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28 pages, 617 KiB

Open AccessArticle

Wormhole Restrictions from Quantum Energy Inequalities

by Eleni-Alexandra Kontou

Universe 2024, 10(7), 291; https://doi.org/10.3390/universe10070291 - 6 Jul 2024

Viewed by 687

Wormhole solutions, bridges that connect different parts of spacetime, were proposed early in the history of General Relativity. Soon after, it was shown that all wormholes violate classical energy conditions, which are non-negativity constraints on contractions of the stress–energy tensor. Since these conditions [...] Read more.

Wormhole solutions, bridges that connect different parts of spacetime, were proposed early in the history of General Relativity. Soon after, it was shown that all wormholes violate classical energy conditions, which are non-negativity constraints on contractions of the stress–energy tensor. Since these conditions are violated by quantum fields, it was believed that wormholes can be constructed in the context of semiclassical gravity. But negative energies in quantum field theory are not without restriction: quantum energy inequalities (QEIs) control renormalized negative energies averaged over a geodesic. Thus, QEIs provide restrictions on the construction of wormholes. This work is a review of the relevant literature, thus focusing on results where QEIs restrict traversable wormholes. Both ‘short’ and ‘long’ (without causality violations) wormhole solutions in the context of semiclassical gravity are examined. A new result is presented on constraints on the Maldacena, Milekhin, and Popov ‘long’ wormhole from the recently derived doubled smeared null energy condition. Full article

(This article belongs to the Special Issue The Physics of Time Travel)

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21 pages, 329 KiB

Open AccessArticle

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

Cited by 1 | Viewed by 879

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)

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14 pages, 538 KiB

Open AccessArticle

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 1671

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

Open AccessArticle

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

Cited by 1 | Viewed by 2295

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

Open AccessEditor’s ChoiceArticle

Closed Timelike Curves Induced by a Buchdahl-Inspired Vacuum Spacetime in

R^{2}

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 5 | Viewed by 1873

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

\tilde{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

\tilde{k}

of higher-derivative characteristic, and reduces to the Schwarzschild metric, for

\tilde{k} = 0

. For the case

\tilde{k} \in (- 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

Open AccessArticle

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 1483

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

Open AccessArticle

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 1439

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

Open AccessCommunication

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 2 | Viewed by 1096

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

Open AccessArticle

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 1223

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

Open AccessCommunication

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 3 | Viewed by 1630

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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