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.

A parametrix for quantum gravity?. (arXiv:1510.08821v1 [gr-qc])

on 2015-10-31 7:28am GMT

Authors: Giampiero Esposito

In the sixties, DeWitt discovered that the advanced and retarded Green functions of the wave operator on metric perturbations in the de Donder gauge make it possible to define classical Poisson brackets on the space of functionals that are invariant under the action of the full diffeomorphism group of spacetime. He therefore tried to exploit this property to define invariant commutators for the quantized gravitational field, but the operator counterpart of such classical Poisson brackets turned out to be a hard task. On the other hand, the mathematical literature studies often an approximate inverse, the parametrix, which is, strictly, a distribution. We here suggest that such a construction might be exploited in canonical quantum gravity. We begin with the simplest case, i.e. fundamental solution and parametrix for the linear, scalar wave operator; the next step are tensor wave equations, again for linear theory, e.g. Maxwell theory in curved spacetime. Last, the nonlinear Einstein equations are studied, relying upon the well-established Choquet-Bruhat construction, according to which the fifth derivatives of solutions of a nonlinear hyperbolic system solve a linear hyperbolic system. The latter is solved by means of Kirchhoff-type formulas, while the former fifth-order equations can be solved by means of well-established parametrix techniques for elliptic operators. But then the metric components that solve the vacuum Einstein equations can be obtained by convolution of such a parametrix with Kirchhoff-type formulas. Some basic functional equations for the parametrix are also obtained, that help in studying classical and quantum version of the Jacobi identity.

Observational Exclusion of a Consistent Quantum Cosmological Scenario. (arXiv:1510.08766v1 [gr-qc])

on 2015-10-31 7:28am GMT

Authors: Boris Bolliet, Aurelien Barrau, Julien Grain, Susanne Schander

It is often argued that inflation erases all the information about what took place before it started. Quantum gravity, relevant in the Planck era, seems therefore mostly impossible to probe with cosmological observations. In general, only very \textit{ad hoc} scenarios or hyper fine-tuned initial conditions can lead to observationally testable theories. Here we consider a well-defined and well motivated candidate quantum cosmology model that predicts inflation. Using the most recent observational constraints on the cosmic microwave background B modes, we show that the model is excluded for all its parameter space, without any tuning. Some important consequences are drawn for the \textit{deformed algebra approach} to loop quantum cosmology. We emphasize that neither loop quantum cosmology in general nor loop quantum gravity are disfavored by this study but their falsifiability is established.

on 2015-10-29 8:35am GMT

Authors: Pierre Martinetti

Noncommutative geometry, in its many incarnations, appears at the crossroad of various researches in theoretical and mathematical physics: from models of quantum space-time (with or without breaking of Lorentz symmetry) to loop gravity and string theory, from early considerations on UV-divergencies in quantum field theory to recent models of gauge theories on noncommutative spacetime, from Connes description of the standard model of elementary particles to recent Pati-Salam like extensions. We list several of these applications, emphasizing also the original point of view brought by noncommutative geometry on the nature of time.

This text serves as an introduction to the volume of proceedings of the parallel session “Noncommutative geometry and quantum gravity”, as a part of the conference “Conceptual and technical challenges in quantum gravity” organized at the University of Rome “La Sapienza” in September 2014.

on 2015-10-28 9:46am GMT

Publication date: Available online 27 October 2015

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

Author(s): Alexis de Saint-Ours

After reviewing the problem of time in Quantum Gravity, I compare from a philosophical perspective, both Carlo Rovelli’s and Julian Barbour’s (before Shape Dynamics) understanding of time in Quantum Gravity and in dynamics in general, trying to show that those two relational understandings of time differ. Rovelli argues that there is change without time and that time can be abstracted from any change whereas Barbour claims that some motions are better than others for constituting duration standards and that time is to be abstracted from all change in the universe. I conclude by a few remarks on Bergson׳s criticism of physics in the light of those debates trying to show that both Rovelli and Barbour give surrationalist (as Bachelard understood it) answers to the critique of spatialized time in Physics.

on 2015-10-28 7:48am GMT

Authors: Jason Williams, Sheng-wey Chiow, Holger Mueller, Nan Yu

We describe the Quantum Test of the Equivalence principle and Space Time (QTEST), a concept for an atom interferometry mission on the International Space Station (ISS). The primary science objective of the mission is a test of Einstein’s equivalence principle with two rubidium isotope gases at a precision of better than 10$^{-15}$. Distinct from the classical tests is the use of quantum wave packets and their expected large spatial separation in the QTEST experiment. This dual species atom interferometer experiment will also be sensitive to time-dependent equivalence principle violations that would be signatures for ultralight dark-matter particles. In addition, QTEST will be able to perform photon recoil measurements to better than 10$^{-11}$ precision. This improves upon terrestrial experiments by a factor of 100, enabling an accurate test of the standard model of particle physics and contributing to mass measurement, in the proposed new international system of units (SI), with significantly improved precision. The predicted high measurement precision of QTEST comes from the microgravity environment on ISS, offering extended free fall times in a well-controlled environment. Suppression of systematic errors by use of symmetric interferometer configurations and rejection of common-mode errors drives the QTEST design. It uses Bragg interferometry with a single laser beam at the “magic” wavelength, where the two isotopes have the same polarizability, for mitigating sensitivities to vibrations and laser noise, imaging detection for correcting cloud initial conditions and maintaining contrast, modulation of the atomic hyperfine states for reduced sensitivity to magnetic field gradients, two source regions for simultaneous time reversal measurements and redundancy, and modulation of the gravity vector using a rotating platform to reduce otherwise difficult systematics to below 10$^{-16}$.

Three Lectures On Topological Phases Of Matter. (arXiv:1510.07698v1 [cond-mat.mes-hall])

on 2015-10-28 7:47am GMT

Authors: Edward Witten

These notes are based on lectures at the PSSCMP/PiTP summer school that was held at Princeton University and the Institute for Advanced Study in July, 2015. They are devoted largely to topological phases of matter that can be understood in terms of free fermions and band theory. They also contain an introduction to the fractional quantum Hall effect from the point of view of effective field theory.

A logical account of quantum superpositions. (arXiv:1510.08023v1 [quant-ph])

on 2015-10-28 7:47am GMT

Authors: Decio Krause, Jonas R. B. Arenhart

In this paper we consider the phenomenon of superpositions in quantum mechanics and suggest a way to deal with the idea in a logical setting from a syntactical point of view, that is, as subsumed in the language of the formalism, and not semantically. We restrict the discussion to the propositional level only. Then, after presenting the motivations and a possible world semantics, the formalism is outlined and we also consider within this schema the claim that superpositions may involve contradictions, as in the case of the Schr\”odinger’s cat, which (it is usually said) is both alive and dead. We argue that this claim is a misreading of the quantum case. Finally, we sketch a new form of quantum logic that involves three kinds of negations and present the relationships among them. The paper is a first approach to the subject, introducing some main guidelines to be developed by a `syntactical’ logical approach to quantum superpositions.

on 2015-10-28 7:47am GMT

Authors: Michael B. Heaney

This paper presents a new Relativistic Symmetrical Interpretation (RSI) of the Dirac equation in (1+1)D which postulates: quantum mechanics is intrinsically time-symmetric, with no arrow of time; the fundamental objects of quantum mechanics are transitions; a transition is fully described by a complex transition amplitude density with specified initial and final boundary conditions; and transition amplitude densities never collapse. This RSI is compared to the Copenhagen Interpretation (CI) for the analysis of Einstein’s bubble experiment with a spin-$\frac{1}{2}$ particle. This RSI can predict the future and retrodict the past, has no zitterbewegung, resolves some inconsistencies of the CI, and eliminates some of the conceptual problems of the CI.

Multiplicity in Everett׳s interpretation of quantum mechanics

on 2015-10-27 9:45am GMT

Publication date: Available online 23 October 2015

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

Author(s): Louis Marchildon

Everett׳s interpretation of quantum mechanics was proposed to avoid problems inherent in the prevailing interpretational frame. It assumes that quantum mechanics can be applied to any system and that the state vector always evolves unitarily. It then claims that whenever an observable is measured, all possible results of the measurement exist. This notion of multiplicity has been understood in different ways by proponents of Everett׳s theory. In fact the spectrum of opinions on various ontological questions raised by Everett׳s approach is rather large, as we attempt to document in this critical review. We conclude that much remains to be done to clarify and specify Everett׳s approach.

Presenting Nonreflexive Quantum Mechanics: Formalism and Metaphysics

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

on 2015-10-26 3:51pm GMT

Krause, Décio and Arenhart, Jonas R. B. (2015) Presenting Nonreflexive Quantum Mechanics: Formalism and Metaphysics. [Preprint]

“Formal” vs. “Empirical” Approaches to Quantum-Classical Reduction

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

on 2015-10-25 3:01am GMT

Rosaler, Joshua (2015) “Formal” vs. “Empirical” Approaches to Quantum-Classical Reduction. [Published Article]