Weekly Papers on Quantum Foundations (40)

Authors: A. S. Sanz

Since its inception Bohmian mechanics has been generally regarded as a hidden-variable theory aimed at providing an objective description of quantum phenomena. To date, this rather narrow conception of Bohm’s proposal has caused it more rejection than acceptance. Now, after 65 years of Bohmian mechanics, should still be such an interpretational aspect the prevailing appraisal? Why not favoring a more pragmatic view, as a legitimate picture of quantum mechanics, on equal footing in all respects with any other more conventional quantum picture? These questions are used here to introduce a discussion on an alternative way to deal with Bohmian mechanics at present, enhancing its aspect as an efficient and useful picture or formulation to tackle, explore, describe and explain quantum phenomena where phase and correlation (entanglement) are key elements. This discussion is presented through two complementary blocks. The first block is aimed at briefly revisiting the historical context that gave rise to the appearance of Bohmian mechanics, and how this approach or analogous ones have been used in different physical contexts. This discussion is used to emphasize a more pragmatic view to the detriment of the more conventional hidden-variable (ontological) approach that has been a leitmotif within the quantum foundations. The second block focuses on some particular formal aspects of Bohmian mechanics supporting the view presented here, with special emphasis on the physical meaning of the local phase field and the associated velocity field encoded within the wave function. As an illustration, a simple model of Young’s two-slit experiment is considered. The simplicity of this model allows to understand in an easy manner how the information conveyed by the Bohmian formulation relates to other more conventional concepts in quantum mechanics. This sort of pedagogical application is also aimed at …

Authors: Raphael Bousso

I share some memories and offer a personal perspective on Jacob Bekenstein’s legacy, focussing on black hole entropy and the Bekenstein bound. I summarize a number of fascinating recent developments that grew out of Bekenstein’s pioneering contributions, from the Ryu-Takayanagi proposal to the Quantum Null Energy Condition.

Authors: Gia Dvali

An explicit microscopic realization of the phenomenon of holography is provided by a class of simple quantum theories of a bosonic field inhabiting a d-dimensional space and experiencing a momentum dependent attractive interaction. An exact mode counting reveals a family of holographic states. In each a set of gapless modes emerges with their number equal to the area of a (d-1)-dimensional sphere. These modes store an exponentially large number of patterns within a microscopic energy gap. The resulting micro-state entropy obeys the area-law reminiscent of a black hole entropy. We study the time-evolution of the stored patterns and observe the following phenomenon: Among the degenerate micro-states the ones with heavier loaded memories survive longer than those that store emptier patterns. Thus, a state gets stabilized by the burden of its own memory. From time to time the information pattern gets off-loaded from one holographic state into another but cannot escape the system. During this process the pattern becomes highly entangled and scrambled. We suggest that this phenomenon is universal in systems with enhanced memory storage capacity, such as black holes or critical neural networks. This universality sheds an interesting light on the puzzle of why, despite the evaporation, is a black hole forced to maintain information internally for a very long time.

Authors: Stephon AlexanderRaúl Carballo-Rubio

A central aspect of the cosmological constant problem is to understand why vacuum energy does not gravitate. In order to account for this observation, while allowing for nontrivial dynamics of the quantum vacuum, we motivate a novel background independent theory of gravity. The theory is an extension of unimodular gravity that is described in geometric terms by means of a conformal (light-cone) structure and differential forms of degree one and two. We show that the subset of the classical field equations describing the dynamics of matter degrees of freedom and the conformal structure of spacetime are equivalent to that of unimodular gravity. The sector with vanishing matter fields and flat conformal structure is governed by the field equations of BF theory and contains topological invariants that are influenced by quantum vacuum fluctuations. Perturbative deviations from this sector lead to classical solutions that necessarily display relatively small values of the cosmological constant with respect to the would-be contribution of quantum vacuum fluctuations. This feature that goes beyond general relativity (and unimodular gravity) offers an interpretation of the smallness of the currently observed cosmological constant.

Authors: Julian SchmidtRoderich Tumulka

While fundamental physically realistic Hamiltonians should be invariant under time reversal, time asymmetric Hamiltonians can occur as mathematical possibilities or effective Hamiltonians. Here, we study conditions under which non-relativistic Hamiltonians involving particle creation and annihilation, as come up in quantum field theory (QFT), are time asymmetric. It turns out that the time reversal operator T can be more complicated than just complex conjugation, which leads to the question which criteria determine the correct action of time reversal. We use Bohmian trajectories for this purpose and show that time reversal symmetry can be broken when charges are permitted to be complex numbers, where `charge’ means the coupling constant in a QFT that governs the strength with which a fermion emits and absorbs bosons. We pay particular attention to the technique for defining Hamiltonians with particle creation based on interior-boundary conditions, and we find them to generically be time asymmetric. Specifically, we show that time asymmetry for complex charges occurs whenever not all charges have equal or opposite phase. We further show that, in this case, the corresponding ground states can have non-zero probability currents, and we determine the effective potential between fermions of complex charge.

Fankhauser, Johannes (2017) Taming the Delayed Choice Quantum Eraser. [Preprint]
Fankhauser, Johannes (2017) Gravitational redshift, inertia, and the role of charge. [Preprint]

Author(s): Jakub Rembieliński and Jacek Ciborowski

We introduce a variant of quantum and classical electrodynamics formulated on the grounds of a hypothesis of existence of a preferred frame of reference—a formalism complementary to that regarding the structure of the space of photonic states, presented by us recently [Phys. Rev. A 97, 062106 (2018)

[Phys. Rev. A 98, 042107] Published Thu Oct 04, 2018

Teitel, Trevor (2018) Background Independence: Lessons for Further Decades of Dispute. [Preprint]
Dawid, Richard (2018) Chronical Incompleteness, Final Theory Claims, and the Lack of Free Parameters in String Theory. [Preprint]
Lia, den Daas (2018) Spontaneous Symmetry Breaking and Quantum Measurement. UNSPECIFIED.

Many-body localization and quantum thermalization

Many-body localization and quantum thermalization, Published online: 03 October 2018; doi:10.1038/s41567-018-0305-7

It is the common wisdom that time evolution of a many-body system leads to thermalization and washes away quantum correlations. But one class of system — referred to as many-body localized — defy this expectation.

Unscrambling the physics of out-of-time-order correlators

Unscrambling the physics of out-of-time-order correlators, Published online: 03 October 2018; doi:10.1038/s41567-018-0295-5

Quantitative tools for measuring the propagation of information through quantum many-body systems, originally developed to study quantum chaos, have recently found many new applications from black holes to disordered spin systems.


In physics, one is often misled in thinking that the mathematical model of a system is part of or is that system itself. Think of expressions commonly used in physics like “point” particle, motion “on the line”, “smooth” observables, wave function, and even “going to infinity”, without forgetting perplexing phrases like “classical world” versus “quantum world”…. On the other hand, when a mathematical model becomes really inoperative in regard with correct predictions, one is forced to replace it with a new one. It is precisely what happened with the emergence of quantum physics. Classical models were (progressively) superseded by quantum ones through quantization prescriptions. These procedures appear often as ad hoc recipes. In the present paper, well defined quantizations, based on integral calculus and Weyl–Heisenberg symmetry, are described in simple terms through one of the most basic examples of mechanics. Starting from (quasi-) probability distribution(s) on the Euclidean plane viewed as the phase space for the motion of a point particle on the line, i.e., its classical model, we will show how to build corresponding quantum model(s) and associated probabilities (e.g. Husimi) or quasi-probabilities (e.g. Wigner) distributions. We highlight the regularizing rôle of such procedures with the familiar example of the motion of a particle with a variable mass and submitted to a step potential.

Does gravity come from quantum information?

Does gravity come from quantum information?, Published online: 03 October 2018; doi:10.1038/s41567-018-0297-3

Recent developments have seen concepts originally developed in quantum information theory, such as entanglement and quantum error correction, come to play a fundamental role in understanding quantum gravity.

New horizons towards thermalization

New horizons towards thermalization, Published online: 03 October 2018; doi:10.1038/s41567-018-0326-2

Ideas from theorists in fields as disparate as quantum gravity, quantum information and many-body localization are finding common ground, as we explore in this month’s Focus issue on quantum thermalization.

Swanson, Noel (2018) Can Quantum Thermodynamics Save Time? In: UNSPECIFIED.

Authors: Dmitry V. ZhdanovDenys I. BondarTamar Seideman

A quantum analog of friction (understood as a completely positive, Markovian, translation-invariant and phenomenological model of dissipation) is known to be in odds with the detailed balance in the thermodynamic limit. We show that this is not the case for quantum systems with internal (e.g. spin) states non-adiabatically coupled to translational dynamics. For such systems, a quantum master equation is derived which phenomenologically accounts for the frictional effect of a uniform zero temperature environment. A simple analytical example is provided. Conjectures regarding the finite temperature case are also formulated. The results are important for efficient simulations of complex molecular dynamics and quantum reservoir engineering applications.

Authors: Tejinder P. Singh

A brief non-technical account of the current status of collapse models.


Dualism holds (roughly) that some mental events are fundamental and non-physical. I develop a prima facie plausible causal argument for dualism. The argument has several significant implications. First, it constitutes a new way of arguing for dualism. Second, it provides dualists with a parity response to causal arguments for physicalism. Third, it transforms the dialectical role of epiphenomenalism. Fourth, it refutes the view that causal considerations prima facie support physicalism but not dualism. After developing the causal argument for dualism and drawing out these implications, I subject the argument to a battery of objections. Some prompt revisions to the argument. Others reveal limitations in scope. It falls out of the discussion that the causal argument for dualism is best used against physicalism as a keystone in a divide and conquer strategy.


The Horizon Quantum Mechanics is an approach that allows one to analyse the gravitational radius of spherically symmetric systems and compute the probability that a given quantum state is a black hole. We first review the (global) formalism and show how it reproduces a gravitationally inspired GUP relation. This results leads to unacceptably large fluctuations in the horizon size of astrophysical black holes if one insists in describing them as (smeared) central singularities. On the other hand, if they are extended systems, like in the corpuscular models, no such issue arises and one can in fact extend the formalism to include asymptotic mass and angular momentum with the harmonic model of rotating corpuscular black holes. The Horizon Quantum Mechanics then shows that, in simple configurations, the appearance of the inner horizon is suppressed and extremal (macroscopic) geometries seem disfavoured.


It is shown that the nonlocal anomalous effective actions corresponding to the quantum breaking of the conformal symmetry can lead to observable modifications of Einstein’s equations. The fact that Einstein’s general relativity is in perfect agreement with all observations including cosmological or recently observed gravitational waves imposes strong restrictions on the field content of possible extensions of Einstein’s theory: all viable theories should have vanishing conformal anomalies. It is shown that a complete cancellation of conformal anomalies in \(D=4\) for both the \(C^2\) invariant and the Euler (Gauss–Bonnet) invariant can only be achieved for N-extended supergravity multiplets with \(N \ge 5\) .

Chen, Eddy Keming (2018) Quantum Mechanics in a Time-Asymmetric Universe: On the Nature of the Initial Quantum State. [Preprint]

Volume 4, Issue 4, pages 235-246

A. I. Arbab [Show Biography]

Arbab Ibrahim studied physics at Khartoum University and high energy physics at the International Cenetr for Theoretical Physics (ICTP), Italy. He has taught physics at Khartoum University and Qassim University, and he is currently a Professor of Physics. He has been a visiting scholar at University of Illinois, Urbana-Champaign, Towson University, and Sultan Qaboos University. His work concentrates on the formulation of quantum mechanics and electromagnetism using Quaternions. He has publications in wide range of theoretical physics. He is an active reviewer for many international journals.

By expressing the Schrödinger wavefunction in the form ψ=Re^iS, where R and S are real functions, we have shown that the expectation value of S is conserved. The amplitude of the wave (R) is found to satisfy the Schrödinger equation while the phase (S) is related to the energy conservation. Besides the quantum potential that depends on R,  we have obtained a phase potential that depends on the phase S derivative. The phase force is a dissipative force. The quantum potential may be attributed to the interaction between the two subfields S and R comprising the quantum particle. This results in splitting (creation/annihilation) of these subfields, each having a mass mc² with an internal frequency of 2mc²/h, satisfying the original wave equation and endowing the particle its quantum nature. The mass of one subfield reflects the interaction with the other subfield. If in Bohmian ansatz R satisfies the Klein-Gordon equation, then S must satisfies the wave equation. Conversely, if R satisfies the wave equation, then S yields the Einstein relativistic energy momentum equation.

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Volume 4, Issue 4, pages 247-267

Sebastian Fortin [Show Biography] and Olimpia Lombardi [Show Biography]

Oimpia Lombardi obtained her degree in Electronic Engineering and in Philosophy at the University of Buenos Aires, and her PhD in Philosophy at the same university. She is Principal Researcher at the National Scientific and Technical Research Council of Argentina. She is member of the Academie Internationale de Philosophie des Sciences and of the Foundational Questions Institute. She is the director of the Group of the Philosohy of Science at the University of Buenos Aires. Areas of interest: foundations of statistical mechanics, the problem of the arrow of time, interpretation of quantum mechanics, the nature of information, philosophy of chemistry.

Sebastian Fortin has a degree and a PhD in Physics at the University of Buenos Aires and a PhD in Epistemology and History of Science at the National University of Tres de Febrero, Argentina. He is Researcher at the National Scientific and Technical Research Council of Argentina and assistant professor at the Physics Department of the Faculty of Exact and Natural Sciences at the University of Buenos Aires. His field of interest is philosophy of physics, particularly foundations of quantum mechanics.

If decoherence is an irreversible process, its physical meaning might be clarified by comparing quantum and classical irreversibility. In this work we carry out this comparison, from which a unified view of the emergence of irreversibility arises, applicable both to the classical and to the quantum case. According to this unified view, in the two cases the irreversible macro-level arises from the reversible micro-level as a coarse description that can be understood in terms of the concept of projection. This position supplies an understanding of the phenomenon of decoherence different from that implicit in most presentations: the reduced state is not the quantum state of the open system, but a coarse state of the closed composite system; as a consequence, decoherence should be understood not as a phenomenon resulting from the interaction between an open system and its environment, but rather as a coarse evolution that emerges from disregarding certain degrees of freedom of the whole closed system.

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Petkov, Vesselin On Relativistic Mass. UNSPECIFIED.

Volume 4, Issue 4, pages 223-234

Mohammed Sanduk [Show Biography]

Mohammed Sanduk is an Iraqi born British physicist. He was educated at University of Baghdad and University of Manchester. Before attending his undergraduate study, he pub-lished a book in particle physics entitled “Mesons”. Sanduk has worked in industry and academia, and his last post in Iraq was head of the Laser and Opto-electronics Engineering department at Nahrain University in Baghdad. Owing to his interest in the philosophy of science, and he was a member of the academic staff of Pontifical Babel College for Philosophy. Sanduk is working with the department of chemical and process engineering at the University of Surrey. Sanduk is interested in transport of charged particles, Magnetohydro-dynamics, and the renewable energy technology. In addition to that, Sanduk is interested in the foundation of Quantum mechanics, and the philosophy of science & technology.

In the last article, an approach was developed to form an analogy of the wave function and derive analogies for both the mathematical forms of the Dirac and Klein-Gordon equations. The analogies obtained were the transformations from the classical real model forms to the forms in complex space. The analogous of the Klein-Gordon equation was derived from the analogous Dirac equation as in the case of quantum mechanics. In the present work, the forms of Dirac and Klein-Gordon equations were derived as a direct transformation from the classical model. It was found that the Dirac equation form may be related to a complex velocity equation. The Dirac’s Hamiltonian and coefficients correspond to each other in these analogies. The Klein-Gordon equation form may be related to the complex acceleration equation. The complex acceleration equation can explain the generation of the flat spacetime. Although this approach is classical, it may show a possibility of unifying relativistic quantum mechanics and special relativity in a single model and throw light on the undetectable æther.

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Dardashti, Radin and Dawid, Richard and Gryb, Sean and Thebault, Karim P Y (2018) On the Empirical Consequences of the AdS/CFT Duality. [Preprint]

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