# Weekly Papers on Quantum Foundations (19)

This is a list of this week’s papers on quantum foundations published in various journals or uploaded to preprint servers such as arxiv.org and PhilSci Archive.

Physical states in Quantum Einstein-Cartan Gravity. (arXiv:1605.01544v1 [gr-qc])

on 2016-5-06 12:52am GMT

Authors: Francesco Cianfrani

The definition of physical states is the main technical issue of canonical approaches towards Quantum Gravity. In this work, we outline how those states can be found in Einstein-Cartan theory via a continuum limit and they are given by finite dimensional representations of the Lorentz group.

Quantum Gravity and a Time Operator in Relativistic Quantum Mechanics. (arXiv:1605.01659v1 [gr-qc])

on 2016-5-06 12:52am GMT

Authors: M. Bauer

The problem of time in the quantization of gravity arises from the fact that time in Schroedinger’s equation is a parameter. This sets time apart from the spatial coordinates, represented by operators in quantum mechanics (QM). Thus “time” in QM and “time” in General Relativity (GR) are seen as mutually incompatible notions. The introduction of a dy- namical time operator in relativistic quantum mechanics (RQM), that in the Heisenberg representation is also a function of the parameter t (iden- tifed as the laboratory time), prompts to examine whether it can help to solve the disfunction referred to above. In particular, its application to the conditional interpretation of the canonical quantization approach toquantum gravity is developed. 1

$W_\infty$ Algebras, Hawking Radiation and Information Retention by Stringy Black Holes. (arXiv:1605.01653v1 [hep-th])

on 2016-5-06 12:52am GMT

We have argued previously, based on the analysis of two-dimensional stringy black holes, that information in stringy versions of four-dimensional Schwarzschild black holes (whose singular regions are represented by appropriate Wess-Zumino-Witten models) is retained by quantum $W$-symmetries when the horizon area is not preserved due to Hawking radiation. It is key that the exactly-marginal conformal world-sheet operator representing a massless stringy particle interacting with the black hole requires a contribution from $W_\infty$ generators in its vertex function. The latter correspond to delocalised, non-propagating, string excitations that guarantee the transfer of information between the string black hole and external particles. When infalling matter crosses the horizon, these topological states are excited via a process: (Stringy black hole) + infalling matter $\rightarrow$ (Stringy black hole)$^\star$, where the black hole is viewed as a stringy state with a specific configuration of $W_\infty$ charges that are conserved. Hawking radiation is then the reverse process, with conservation of the $W_\infty$ charges retaining information. The Hawking radiation spectrum near the horizon of a Schwarzschild or Kerr black hole is specified by matrix elements of higher-order currents that form a phase-space $W_{1+\infty}$ algebra. We show that an appropriate gauging of this algebra preserves the horizon two-dimensional area classically, as expected because the latter is a conserved Noether charge.

Local Causality in a Friedmann-Robertson-Walker Spacetime

PhilSci-Archive: No conditions. Results ordered -Date Deposited.

on 2016-5-05 6:29am GMT

Christian, Joy (2016) Local Causality in a Friedmann-Robertson-Walker Spacetime. [Preprint]

Black Hole Unitarity and Antipodal Entanglement

Latest Results for Foundations of Physics

on 2016-5-05 12:00am GMT

Abstract

Hawking particles emitted by a black hole are usually found to have thermal spectra, if not exactly, then by a very good approximation. Here, we argue differently. It was discovered that spherical partial waves of in-going and out-going matter can be described by unitary evolution operators independently, which allows for studies of space-time properties that were not possible before. Unitarity dictates space-time, as seen by a distant observer, to be topologically non-trivial. Consequently, Hawking particles are only locally thermal, but globally not: we explain why Hawking particles emerging from one hemisphere of a black hole must be 100 % entangled with the Hawking particles emerging from the other hemisphere. This produces exclusively pure quantum states evolving in a unitary manner, and removes the interior region for the outside observer, while it still completely agrees locally with the laws of general relativity. Unitarity is a starting point; no other assumptions are made. Region I and the diametrically opposite regionII of the Penrose diagram represent antipodal points in a PT or CPT relation, as was suggested before. On the horizon itself, antipodal points are identified. A candidate instanton is proposed to describe the formation and evaporation of virtual black holes of the type described here.

Quantum Mechanics and Narratability

Latest Results for Foundations of Physics

on 2016-5-05 12:00am GMT

Abstract

As has been noted by several authors, in a relativistic context, there is an interesting difference between classical and quantum state evolution. For a classical system, a state history of a quantum system given along one foliation uniquely determines, without any consideration of the system’s dynamics, a state history along any other foliation. This is not true for quantum state evolution; there are cases in which a state history along one foliation is compatible with multiple distinct state histories along some other, a phenomenon that David Albert has dubbed “non-narratability.” In this article, we address the question of whether non-narratability is restricted to the sorts of special states that so far have been used to illustrate it. The results of the investigation suggest that there has been a misplaced emphasis on underdetermination of state histories; though this is generic for the special cases that have up until now been considered, involving bipartite systems in pure entangled states, it fails generically in cases in which more component systems are taken into account, and for bipartite systems that have some entanglement with their environment. For such cases, if we impose relativistic causality constraints on the evolution, then, except for very special states, a state history along one foliation uniquely determines a state history along any other. But this in itself is a marked difference between classical and quantum state evolution, because, in a classical setting, no considerations of dynamics at all are needed to go from a state history along one foliation to a state history along another.

Classical Zero-Point Radiation and Relativity: The Problem of Atomic Collapse Revisited

Latest Results for Foundations of Physics

on 2016-5-05 12:00am GMT

Abstract

The physicists of the early twentieth century were unaware of two aspects which are vital to understanding some aspects of modern physics within classical theory. The two aspects are: (1) the presence of classical electromagnetic zero-point radiation, and (2) the importance of special relativity. In classes in modern physics today, the problem of atomic collapse is still mentioned in the historical context of the early twentieth century. However, the classical problem of atomic collapse is currently being treated in the presence of classical zero-point radiation where the problem has been transformed. The presence of classical zero-point radiation indeed keeps the electron from falling into the Coulomb potential center. However, the old collapse problem has been replaced by a new problem where the zero-point radiation may give too much energy to the electron so as to cause “self-ionization.” Special relativity may play a role in understanding this modern variation on the atomic collapse problem, just as relativity has proved crucial for a classical understanding of blackbody radiation.

Less Decoherence and More Coherence in Quantum Gravity, Inflationary Cosmology and Elsewhere

Latest Results for Foundations of Physics

on 2016-5-05 12:00am GMT

Abstract

In Crull (Found Phys 45:1019–1045, 2015) it is argued that, in order to confront outstanding problems in cosmology and quantum gravity, interpretational aspects of quantum theory can by bypassed because decoherence is able to resolve them. As a result, Crull (Found Phys 45:1019–1045, 2015) concludes that our focus on conceptual and interpretational issues, while dealing with such matters in Okon and Sudarsky (Found Phys 44:114–143, 2014), is avoidable and even pernicious. Here we will defend our position by showing in detail why decoherence does not help in the resolution of foundational questions in quantum mechanics, such as the measurement problem or the emergence of classicality.

The Quantum Computer Puzzle (Expanded Version). (arXiv:1605.00992v1 [quant-ph])

on 2016-5-04 3:32am GMT

Authors: Gil Kalai

Quantum computers are hypothetical devices, based on quantum physics, that would enable us to perform certain computations hundreds of orders of magnitude faster than digital computers. This feature is coined as “quantum supremacy” and one aspect or another of such quantum computational supremacy might be brought about in experiments in the near future: by implementing quantum error-correction, systems of non-interacting bosons, exotic new phases of matter called anyons, quantum annealing, or in various other ways.

A main concern regarding the feasibility of quantum computers is that quantum systems are inherently noisy: we cannot accurately control them, and we cannot accurately describe them. We will describe an optimistic hypothesis of quantum noise that would allow quantum computing and a pessimistic hypothesis that wouldn’t. The quantum computer puzzle is deciding between these two hypotheses.

Here is a brief summary of the author’s pessimistic point of view as explained in the paper: understanding quantum computers in the presence of noise requires consideration of behavior at different scales. In the small scale, standard models of noise from the mid-90s are suitable, and quantum evolutions and states described by them manifest a very low-level computational power. This small-scale behavior has far-reaching consequences for the behavior of noisy quantum systems at larger scales. On the one hand, it does not allow reaching the starting points for quantum fault tolerance and quantum supremacy, making them both impossible at all scales. On the other hand, it leads to novel implicit ways for modeling noise at larger scales and to various predictions on the behavior of noisy quantum systems.

The Arrow of Time and Correlations in Quantum Physics. (arXiv:1605.00926v1 [quant-ph])

on 2016-5-04 3:32am GMT

Authors: Vlatko Vedral

We discuss the arrow of time in terms of the increase of correlations between the system and its environment. Here we show that the existence of the arrow of time, based on deleting correlations, requires a strict absence of initial correlations between the system and the environment. We discuss our work in relation to other approaches addressing the same problem and emphasise similarities and differences.

Relativistic locality and the action reaction principle predict de Broglie fields. (arXiv:1605.00844v1 [quant-ph])

on 2016-5-04 3:32am GMT

Authors: Carlos Lopez

The action reaction principle is violated in the standard formulation of Quantum Mechanics, so that its phase space is incomplete. Moreover, projection of state of a quantum system under indirect measurement, when there are alternative virtual paths and one of them is discarded by negative detection implies, according to the action reaction principle, a reaction on the detector, although its macroscopic state does not change. If all interactions are local, mediated by fields with relativistically causal evolution, some system, different from the particle which follows another path, must locally interact with the detector. Relativistic locality and the action reaction principle predict the existence of de Broglie fields. A formulation of Quantum Mechanics in extended Hilbert spaces is presented, in which kinematic and dynamical representations of physical magnitudes are distinguished.

Do we finally understand Quantum Mechanics?. (arXiv:1605.00672v1 [quant-ph])

on 2016-5-04 3:32am GMT

Authors: Alberto C. de la Torre

The ontology emerging from quantum field theory and the results following from Bell’s theorems allowed the development of an intuitive picture of the microscopic world described by quantum mechanics, that is, we can say that we understand this theory. However there remain several aspects of it that are still mysterious and require more work on the foundations of quantum mechanics.

Physics: Material to meaning

Nature – Issue – nature.com science feeds

on 2016-5-04 12:00am GMT

Physics: Material to meaning

Nature 533, 7601 (2016). doi:10.1038/533034a

Author: Robert P. Crease

Robert P. Crease assesses Sean Carroll’s attempt to construct morality out of quantum field theory.

The future’s not ours to see. (arXiv:1605.00566v1 [physics.hist-ph])

on 2016-5-03 9:15am GMT

Authors: Anthony Sudbery

An account of determinism and indeterminism in physics, addressed to non-physicist readers, leading up to proposals for how to understand statements about the future and single-event probability, motivated by quantum mechanics.

Spacetime-noncommutativity regime of Loop Quantum Gravity. (arXiv:1605.00497v1 [gr-qc])

on 2016-5-03 9:13am GMT

A recent study by Bojowald and Paily provided a path toward the identification of an effective quantum-spacetime picture of Loop Quantum Gravity, applicable in the “Minkowski regime”, the regime where the large-scale (coarse-grained) spacetime metric is flat. A pivotal role in the analysis is played by Loop-Quantum-Gravity-based modifications to the hypersurface deformation algebra, which leave a trace in the Minkowski regime. We here show that the symmetry-algebra results reported by Bojowald and Paily are consistent with a description of spacetime in the Minkowski regime given in terms of the $\kappa$-Minkowski noncommutative spacetime, whose relevance for the study of the quantum-gravity problem had already been proposed for independent reasons.

Experiment Investigating the Connection between Weak Values and Contextuality

PRL: General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

on 2016-5-02 2:00pm GMT

Author(s): F. Piacentini, A. Avella, M. P. Levi, R. Lussana, F. Villa, A. Tosi, F. Zappa, M. Gramegna, G. Brida, I. P. Degiovanni, and M. Genovese

Weak value measurements have recently given rise to a great amount of interest in both the possibility of measurement amplification and the chance for further quantum mechanics foundations investigation. In particular, a question emerged about weak values being proof of the incompatibility between q…

[Phys. Rev. Lett. 116, 180401] Published Mon May 02, 2016

Quantum Thermal Machines Fuelled by Vacuum Forces. (arXiv:1604.08732v1 [quant-ph])

on 2016-5-02 2:31am GMT

We propose a quantum thermal machine composed of two nanomechanical resonators (NMR) (two membranes suspended over a trench in a substrate), placed a few $\mu$m from each other. The quantum thermodynamical cycle is powered by the Casimir interaction between the resonators and the working fluid is the polariton resulting from the mixture of the flexural (out-of-plane) vibrations. With the help of piezoelectric cells, we select and sweep the polariton frequency cyclically. We calculate the performance of the proposed quantum thermal machines and show that high efficiencies are achieved thanks to (i) the strong coupling between the resonators and (ii) the large difference between the membrane stiffnesses. Our findings can be of particular importance for applications in nanomechanical technologies where a sensitive control of temperature is needed.