Authors: Raphael Bousso, Fernando Quevedo, Steven Weinberg

Joseph Polchinski (1954-2018), one of the the leading theoretical physicists of the past 50 years, was an exceptionally broad and deep thinker. He made fundamental contributions to quantum field theory, advancing the role of the renormalization group, and to cosmology, addressing the cosmological constant problem. Polchinski’s work on D-branes revolutionized string theory and led to the discovery of a nonperturbative quantum theory of gravity. His recent, incisive reformulation of the black hole information paradox presents us with a profound challenge. Joe was deeply devoted to his family, a beloved colleague and advisor, an excellent writer, and an accomplished athlete.

We present a coherent and consistent framework for explicit time-dependence in non-Hermitian quantum mechanics. The area of non-Hermitian quantum mechanics has been growing rapidly over the past twenty years. This has been driven by the fact that $\mathcal{PT}$-symmetric non-Hermitian systems exhibit real energy eigenvalues and unitary time evolution. Historically, the introduction of time into the world of non-Hermitian quantum mechanics has been a conceptually difficult problem to address, as it requires the Hamiltonian to become unobservable. We solve this issue with the introduction of a new observable energy operator and explain why its instigation is a necessary and natural progression in this setting. For the first time, the introduction of time has allowed us to make sense of the parameter regime in which the $\mathcal{PT}$-symmetry is spontaneously broken. Ordinarily, in the time-independent setting, the energy eigenvalues become complex and the wave functions are asymptotically unbounded. We demonstrate that in the time-dependent setting this broken symmetry can be mended and analysis on the spontaneously broken $\mathcal{PT}$ regime is indeed possible. We provide many examples of this mending on a wide range of different systems, beginning with a $2\times2$ matrix model and extending to higher dimensional matrix models and coupled harmonic oscillator systems with infinite Hilbert space. Furthermore, we use the framework to perform analysis on time-dependent quasi-exactly solvable models. We present the “eternal life” of entropy in this thesis. Ordinarily, for entangled quantum systems coupled to the environments, the entropy decays rapidly to zero. However, in the spontaneously broken regime, we find the entropy decays asymptotically to a non-zero value. We create an elegant framework for Darboux and Darboux/Crum transformations for time-dependent non-Hermitian Hamiltonians.

Extended abstract of “Algebraic approach to position-dependent mass systems in both classical and quantum pictures”, a series of three lectures delivered by the author in the VIII School on Geometry and Physics, 24 June-8 June 2019, organized by the Department of Mathematical Physics of the University of Bialystok, in Bialowieza, Poland (this http URL)

It is shown that when properly analyzed using principles consistent with the use of a Hilbert space to describe microscopic properties, quantum mechanics is a local theory: one system cannot influence another system with which it does not interact. Claims to the contrary based on quantum violations of Bell inequalities are shown to be incorrect. A specific example traces a violation of the CHSH Bell inequality in the case of a spin-3/2 particle to the noncommutation of certain quantum operators in a situation where (non)locality is not an issue. A consistent histories analysis of what quantum measurements measure, in terms of quantum properties, is used to identify the basic problem with derivations of Bell inequalities: the use of classical concepts (hidden variables) rather than a probabilistic structure appropriate to the quantum domain. A difficulty with the original Einstein-Podolsky-Rosen (EPR) argument for the incompleteness of quantum mechanics is the use of a counterfactual argument which is not valid if one assumes that Hilbert-space quantum mechanics is complete; locality is not an issue. The quantum correlations that violate Bell inequalities can be understood using local quantum common causes. Wavefunction collapse and Schr\”odinger steering are calculational procedures, not physical processes. A general Principle of Einstein Locality rules out nonlocal influences between noninteracting quantum systems. Some suggestions are made for changes in terminology that could clarify discussions of quantum foundations and be less confusing to students.

The no-signalling principle preventing superluminal communication is a limiting paradigm for physical theories. Within the information-theoretic framework it is commonly understood in terms of admissible correlations in composite systems. Here we unveil its complementary incarnation — the ‘dynamical no-signalling principle’ —, which forbids superluminal signalling via measurements on simple physical objects (e.g. particles) evolving in time. We show that it imposes strong constraints on admissible models of dynamics. The posited principle is universal — it can be applied to any theory (classical, quantum or post-quantum) with well-defined rules of calculating detection statistics in spacetime. As an immediate application we show how one could exploit the Schr\”odinger equation to establish a fully operational superluminal protocol in the Minkowski spacetime. This example illustrates how the principle can be used to identify the limits of applicability of a given model of quantum or post-quantum dynamics.

Publication date: Available online 5 February 2020

Source: Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics

Author(s): Nicholas Danne

Authors: Krishna P B, Titus K Mathew

The spatial expansion of the universe can be described as the emergence of space with the progress of cosmic time, through a simple equation, $\Delta V = \Delta t\left(N_{surf}- N_{bulk}\right)$. This law of emergence suggested by Padmanabhan in the context of general relativity for a flat universe has been generalized by Sheykhi to Gauss Bonnet and Lovelock gravity for a universe with any spacial curvature. We investigate whether this generalized holographic equipartition effectively implies the maximization of horizon entropy. First, we obtain the constraints imposed by the maximization of horizon entropy in Einstein, Gauss Bonnet and Lovelock gravities for a universe with any spacial curvature. We then analyze the consistency of the law of emergence in \cite{Sheykhi}, with these constraints obtained. Interestingly, both the law of emergence and the horizon entropy maximization demands an asymptotically de Sitter universe with $\omega \geq -1$. More specifically, when the degrees of freedom in the bulk $( N_{bulk})$ becomes equal to the degrees of freedom on the boundary surface $(N_{surf}),$ the universe attains a state of maximum horizon entropy. Thus, the law of emergence can be viewed as a tendency for maximizing the horizon entropy, even in a non flat universe. Our results points at the deep connection between the law of emergence and horizon thermodynamics, beyond Einstein gravity irrespective of the spacial curvature.

Authors: Alok Laddha, Siddharth G. Prabhu, Suvrat Raju, Pushkal Shrivastava

We argue that, in a theory of quantum gravity in a four dimensional asymptotically flat spacetime, all information about massless excitations can be obtained from an infinitesimal neighbourhood of the past boundary of future null infinity and does not require observations over all of future null infinity. Moreover, all information about the state that can be obtained through observations near a cut of future null infinity can also be obtained from observations near any earlier cut although the converse is not true. We provide independent arguments for these two assertions. Similar statements hold for past null infinity. These statements have immediate implications for the information paradox since they suggest that the fine-grained von Neumann entropy of the state defined on a segment $(-\infty,u)$ of future null infinity is independent of u. This is very different from the oft-discussed Page curve that this entropy is sometimes expected to obey. We contrast our results with recent discussions of the Page curve in the context of black hole evaporation, and also discuss the relation of our results to other proposals for holography in flat space.

Authors: Michał Eckstein, Paweł Horodecki, Ryszard Horodecki, Tomasz Miller

The no-signalling principle preventing superluminal communication is a limiting paradigm for physical theories. Within the information-theoretic framework it is commonly understood in terms of admissible correlations in composite systems. Here we unveil its complementary incarnation — the ‘dynamical no-signalling principle’ —, which forbids superluminal signalling via measurements on simple physical objects (e.g. particles) evolving in time. We show that it imposes strong constraints on admissible models of dynamics. The posited principle is universal — it can be applied to any theory (classical, quantum or post-quantum) with well-defined rules of calculating detection statistics in spacetime. As an immediate application we show how one could exploit the Schr\”odinger equation to establish a fully operational superluminal protocol in the Minkowski spacetime. This example illustrates how the principle can be used to identify the limits of applicability of a given model of quantum or post-quantum dynamics.

Jacquart, Melissa (2017) Dark Matter and Dark Energy. [Preprint]

Jacquart, Melissa (2020) Observations, Simulations, and Reasoning in Astrophysics. [Preprint]

Abstract

Microphysical laws are time reversible, but macrophysics, chemistry and biology are not. This paper explores how this asymmetry (a classic example of a broken symmetry) arises due to the cosmological context, where a non-local Direction of Time is imposed by the expansion of the universe. This situation is best represented by an Evolving Block Universe, where local arrows of time (thermodynamic, electrodynamic, gravitational, wave, quantum, biological) emerge in concordance with the Direction of Time because a global Past Condition results in the Second Law of Thermodynamics pointing to the future. At the quantum level, the indefinite future changes to the definite past due to quantum wave function collapse events.

Author(s): Susanne Still

This Letter exposes a tight connection between the thermodynamic efficiency of information processing and predictive inference. A generalized lower bound on dissipation is derived for partially observable information engines which are allowed to use temperature differences. It is shown that the rete…

[Phys. Rev. Lett. 124, 050601] Published Thu Feb 06, 2020

Gomori, Marton (2020) On the Very Idea of Distant Correlations. [Preprint]

Nature Physics, Published online: 06 February 2020; doi:10.1038/s41567-020-0801-4

Majorana transparency

Nature Physics, Published online: 06 February 2020; doi:10.1038/s41567-020-0786-z

The certainty of uncertainty

Nature Physics, Published online: 06 February 2020; doi:10.1038/s41567-020-0805-0

Lindblad’s tenfold way

Nature Physics, Published online: 06 February 2020; doi:10.1038/s41567-019-0780-5

Lack of reproducibility is not necessarily bad news; it may herald new discoveries and signal scientific progress.

Authors: Emanuele Goldoni, Ledo Stefanini

In 1919, Eddington and Dyson led two famous expeditions to measure the bending of light during a total solar eclipse. The results of this effort led to the first experimental confirmation of Einstein’s General Relativity and contributed to create its unique and enduring fame. Since then, similar experiments have been carried out all around the world, confirming the predictions of the General Relativity. Later, developments in radio interferometry provided a more accurate way to measure the gravitation deflection. We believe that – after more than a century – starlight deflection caused by the Sun’s gravity still represents a simple and intuitive way to introduce high-school students to General Relativity and its effects. To this aim, we gathered measurements taken during eight eclipses spanning from 1919 to 2017, and we created a single dataset of homogeneous values. Together with the whole dataset, this article provides a blueprint for a possible group activity for students, useful to introduce the theory in physics classes with a playful approach.

Authors: Jeroen van Dongen

As the History of Science Society, which is based in America, holds its annual meeting in Utrecht, one of the key academic centers on the European continent, one may surmise that the field has returned home. Yet, this hardly reflects how today’s world of scholarship is constituted: in the historiography of science, ‘provincializing Europe’ has become an important theme, while the field itself, as is the case across the world of academia, is centered around a predominantly American literature. At the same time, ever since historians of science have emancipated themselves from the sciences a long time ago, they often have appeared, in the public eye, to question rather than to seek to bolster the authority of the sciences. How has this situation come about, and what does it tell us about the world we live in today? What insight is sought and what public benefit is gained by the historical study of science? As we try to answer these questions, we will follow a number of key mid-twentieth century historians–Eduard Dijksterhuis, Thomas Kuhn and Martin Klein–in their Atlantic crossings. Their answers to debates on the constitution of the early modern scientific revolution or the novelty of the work of Max Planck will illustrate how notions of ‘center’ and ‘periphery’ have shifted–and what that may tell us about being ‘in Europe’ today.

Authors: Jeroen van Dongen

Albert Einstein’s practice in physics and his philosophical positions gradually reoriented themselves from more empiricist towards rationalist viewpoints. This change accompanied his turn towards unified field theory and different presentations of himself, eventually leading to his highly programmatic Autobiographical Notes in 1949. Einstein enlisted his own history and professional stature to mold an ideal of a theoretical physicist who represented particular epistemic virtues and moral qualities. These in turn reflected the theoretical ideas of his strongly mathematical unification program and professed Spinozist beliefs.

Authors: R. E. Kastner

The Frauchiger-Renner Paradox is an extension of paradoxes based on the ‘Problem of Measurement,’ such as Schrodinger’s Cat and Wigner’s Friend. All of these paradoxes stem from assuming that quantum theory has only unitary (linear) physical dynamics, and the attendant ambiguity about what counts as a ‘measurement’–i.e., the inability to account for the observation of determinate measurement outcomes from within the theory itself. This paper discusses a basic inconsistency arising in the FR scenario at a much earlier point than the derived contradiction: namely, the inconsistency inherent in treating an improper mixture (reduced density operator) as a proper, epistemic mixture. This is an illegitimate procedure that is nevertheless endemic if quantum theory is assumed to be always unitary. In contrast, under a non-unitary account of quantum state reduction yielding determinate outcomes, the use of a proper mixture for measurement results becomes legitimate, and this entire class of paradoxes cannot be mounted. The conclusion is that the real lesson of the FR paradox is that it is the unitary-only assumption that needs to be critically reassessed.

Authors: Roderick Sutherland

A version of Bohm’s model incorporating retrocausality is presented, the aim being to explain the nonlocality of Bell’s theorem while maintaining Lorentz invariance in the underlying ontology. The strengths and weaknesses of this alternative model are compared with those of the standard Bohm model.

The Frauchiger-Renner Paradox is an extension of paradoxes based on the ‘Problem of Measurement,’ such as Schrodinger’s Cat and Wigner’s Friend. All of these paradoxes stem from assuming that quantum theory has only unitary (linear) physical dynamics, and the attendant ambiguity about what counts as a ‘measurement’–i.e., the inability to account for the observation of determinate measurement outcomes from within the theory itself. This paper discusses a basic inconsistency arising in the FR scenario at a much earlier point than the derived contradiction: namely, the inconsistency inherent in treating an improper mixture (reduced density operator) as a proper, epistemic mixture. This is an illegitimate procedure that is nevertheless endemic if quantum theory is assumed to be always unitary. In contrast, under a non-unitary account of quantum state reduction yielding determinate outcomes, the use of a proper mixture for measurement results becomes legitimate, and this entire class of paradoxes cannot be mounted. The conclusion is that the real lesson of the FR paradox is that it is the unitary-only assumption that needs to be critically reassessed.

We supplement the Lorentz transform $L(v)$ with a new “Tunneling” transform $T(v)$. Application of this new transform to elementary quantum mechanics offers a novel, intuitive insight into the nature of quantum tunneling; in particular, the so called “Klein Paradox” is discussed.

We review the current understanding of the role of topology in non-Hermitian (NH) systems, and its far-reaching physical consequences observable in a range of dissipative settings. In particular, we elucidate how the paramount and genuinely NH concept of exceptional degeneracies, at which both eigenvalues and eigenvectors coalesce, leads to phenomena drastically distinct from the familiar Hermitian realm. An immediate consequence is the ubiquitous occurrence of nodal NH topological phases with concomitant open Fermi-Seifert surfaces, where conventional band-touching points are replaced by the aforementioned exceptional degeneracies. We furthermore discuss new notions of gapped phases including topological phases in single-band systems, and clarify how a given physical context may affect the symmetry-based topological classification. A unique property of NH systems with relevance beyond the field of topological phases consists in the anomalous relation between bulk- and boundary-physics, stemming from the striking sensitivity of NH matrices to boundary conditions. Unifying several complementary insights recently reported in this context, we put together a clear picture of intriguing phenomena such as the NH bulk-boundary correspondence, and the NH skin effect. Finally, we review applications of NH topology in both classical systems including optical setups with gain and loss, electric circuits, mechanical systems, and genuine quantum systems such as electronic transport settings at material junctions, and dissipative cold-atom setups.

Authors: Yangang Chen, Ravi Kunjwal, Heidar Moradi, Yasaman K. Yazdi, Miguel Zilhão

Entanglement entropy of quantum fields in gravitational settings is a topic of growing importance. This entropy of entanglement is conventionally computed relative to Cauchy hypersurfaces where it is possible via a partial tracing to associate a reduced density matrix to the spacelike region of interest. In recent years Sorkin has proposed an alternative, manifestly covariant, formulation of entropy in terms of the spacetime two-point correlation function. This formulation, developed for a Gaussian scalar field theory, is explicitly spacetime in nature and evades some of the possible non-covariance issues faced by the conventional formulation. In this paper we take the first steps towards extending Sorkin’s entropy to non-Gaussian theories where Wick’s theorem no longer holds and one would expect higher correlators to contribute. We consider quartic perturbations away from the Gaussian case and find that to first order in perturbation theory, the entropy formula derived by Sorkin continues to hold but with the two-point correlators replaced by their perturbation-corrected counterparts. This is a non-trivial and, to our knowledge, novel result. Our work also suggests avenues for extensions to more generic interacting theories.

Authors: Basem Kamal El-Menoufi, Sonali Mohapatra

Combining insights from both the effective field theory of quantum gravity and black hole thermodynamics, we derive two novel consistency relations to be satisfied by any quantum theory of gravity. First, we show that a particular combination of the number of massless (light) fields in the theory must take integer values. Second, we show that, once the massless spectrum is fixed, the Wilson coefficient of the Kretschmann scalar in the low-energy effective theory is fully determined by the logarithm of a single natural number.

Authors: Masahiro Hotta, Achim Kempf, Eduardo Martin-Martinez, Takeshi Tomitsuka, Koji Yamaguchi

We prove a non-perturbative duality concerning the dynamics of harmonic-oscillator-type Unruh DeWitt detectors in curved spacetimes. Concretely, using the Takagi transformation we show that the action of a harmonic oscillator Unruh DeWitt detector with one frequency in a spacetime is equal to that of a detector with a different frequency in a conformally related spacetime. As an example, we show that the dynamics of simple stationary detectors in flat spacetime is dual to that of detectors in a cosmological scenario. The non-perturbative duality enables us to investigate entanglement harvesting in new scenarios in curved spacetime by using results obtained in simpler, conformally related spacetimes.

Authors: Kimet Jusufi, Phongpichit Channuie, Mubasher Jamil

In this paper, we investigate the effect of the Generalized Uncertainty Principle (GUP) in the Casimir wormhole spacetime recently proposed by Garattini [Eur. Phys. J. C (2019) 79: 951]. In particular, we consider three types of the GUP relations, firstly the Kempf, Mangano and Mann (KMM) model, secondly the Detournay, Gabriel and Spindel (DGS) model, and finally the so called type II model for GUP principle. To this end, we consider three specific models of the redshift function along with two different EoS of state given by $\mathcal{P}_r(r)=\omega_r(r) \rho(r)$ along with $\mathcal{P}_t(r)=\omega_t (r)\mathcal{P}_r(r)$ and obtain a class of asymptotically flat wormhole solutions supported by Casimir energy under the effect of GUP. Furthermore we check the null, weak, and strong condition at the wormhole throat with a radius $r_0$, and show that in general the classical energy condition are violated by some arbitrary quantity at the wormhole throat. Importantly, we examine the wormhole geometry with semi-classical corrections via embedding diagrams. We also consider the ADM mass of the wormhole, the volume integral quantifier to calculate the amount of the exotic matter near the wormhole throat, and the deflection angle of light.

Authors: Tomer Shushi

In this paper, we discuss a geometrodynamical approach to particle physics, in which quantum mechanics is no more than an approximated model of nature in the microscopic scale. We derive quantum mechanics from the concept of non-local geometrodynamics. Using the concept of superoscillations, we obtain the metric of the particles, which allows mapping this metric into the quantum wavefunction representation.

Authors: Junpei Harada

A connection-independent formulation of general relativity is presented, in which the dynamics does not depend on the choice of connection. The gravity action in this formulation includes one additional scalar term in addition to the Einstein-Hilbert action. No conditions on the connection are imposed. Nevertheless, this formulation yields the Einstein equations, without adding the Gibbons-Hawking-York term even when a manifold has a boundary. Furthermore, this formulation yields a unified description of general relativity, teleparallel gravity, and symmetric teleparallel gravity.

Niestegge, Gerd (2020) Local tomography and the role of the complex numbers in quantum mechanics. [Preprint]

Niestegge, Gerd (2020) A simple and quantum-mechanically motivated characterization of the formally real Jordan algebras. Royal Society Proceedings A, 476. 0-14.

Weinberger, Naftali (2020) Near-Decomposability and the Time-Scale Relativity of Causal Representations. [Preprint]

Abstract

We propose a simple situation in which the magnetic Aharonov–Bohm potential influences the values of the deficiency indices of the initial Schrödinger operator, so determining whether the particle interacts with the solenoid or not. Even with the particle excluded from the magnetic field, the number of self-adjoint extensions of the initial Hamiltonian depends on the magnetic flux. This is a new point of view of the Aharonov–Bohm effect.

We discuss the proposal by Sebens and Carroll to derive the Born rule in Everettian quantum mechanics from a principle they call ‘ESP-QM’. We argue that the proposal fails: ESP-QM is not, as Sebens and Carroll argue, a ‘less general version’ of an independently plausible principle, ESP, and can only be motivated by the empirical success of quantum mechanics, including use of the Born rule. Therefore, ESP-QM cannot have the status of a meta-theoretical principle of reasoning and provides no viable basis for deriving the Born rule.

Author(s): Lucas Tendick, Hermann Kampermann, and Dagmar Bruß

We discuss the relation between entanglement and nonlocality in the hidden nonlocality scenario. Hidden nonlocality signifies nonlocality that can be activated by applying local filters to a particular state that admits a local hidden-variable model in the Bell scenario. We present a fully biseparab…

[Phys. Rev. Lett. 124, 050401] Published Mon Feb 03, 2020

Guralp, Genco (2020) The Evidence for the Accelerating Universe: Endorsement and Robust Consistency. [Preprint]

Abstract

f(R) gravity is examined in the context of a five-dimensional Kaluza-Klein theory with degenerate metric. In this theory electromagnetism is described by two vector fields, and there is a reflection symmetry between them which unifies them with gravitation. For matter, it is shown how the Lagrangian may be any function and still generate the same equations of motion, provided that some simple conditions are satisfied. The field equations are derived, and it is found that f(R) gravity is not consistent with the reflection symmetry.

Abstract

In previous articles we presented a simple set of axioms named “Contexts, Systems and Modalities” (CSM), where the structure of quantum mechanics appears as a result of the interplay between the quantized number of modalities accessible to a quantum system, and the continuum of contexts that are required to define these modalities. In the present article we discuss further how to obtain (or rather infer) Born’s rule within this framework. Our approach is compared with other former and recent derivations, and its strong links with Gleason’s theorem are particularly emphasized.

I will prove a theorem on the precise connection of the time and phase-space average of the Boltzmann equilibrium showing that the behaviour of a dynamical system with a stationary measure and a dominant equilibrium state is qualitatively ergodic. Explicitly, I will show that given a dynamical system with a stationary measure and a region of overwhelming phase-space measure, almost all trajectories spend almost all of their time in that region. Conversely, given that almost all trajectories spend almost all of their time in a certain region, that region is of overwhelming phase-space measure. In total, the time and phase-space average of the equilibrium state approximately coincide. Consequently, equilibrium can be defined equivalently in terms of the time or the phase-space average. Moreover, since the two averages are almost equal, the behaviour of the system is essentially ergodic. While this does not explain the approach to equilibrium, it provides a means to estimate the fluctuation rates.