Authors: Danilo Artigas Guimarey, Jakub Mielczarek, Carlo Rovelli

The kinematical phase space of classical gravitational field is flat (affine) and unbounded. Because of this, field variables may tend to infinity leading to appearance of singularities, which plague Einstein’s theory of gravity. The purpose of this article is to study the idea of generalizing the theory of gravity by compactification of the phase space. We investigate the procedure of compactification of the phase space on a minisuperspace gravitational model with two dimensional phase space. In the affine limit, the model reduces to the flat de Sitter cosmology. The phase space is generalized to the spherical case, and the case of loop quantum cosmology is recovered in the cylindrical phase space limit. Analysis of the dynamics reveals that the compactness of the phase space leads to both UV and IR effects. In particular, the phase of re-collapse appears, preventing the universe from expanding to infinite volume. Furthermore, the quantum version of the model is investigated and the quantum constraint is solved. As an example, we analyze the case with the spin quantum number $s=2$, for which we determine transition amplitude between initial and final state of the classical trajectory. The probability of the transition is peaked at $\Lambda=0$.

Authors: Richard Howl, Roger Penrose, Ivette Fuentes

Despite almost a century’s worth of study, it is still unclear how general relativity (GR) and quantum theory (QT) should be unified into a consistent theory. The conventional approach is to retain the foundational principles of QT, such as the superposition principle, and modify GR. This is referred to as `quantizing gravity’, resulting in a theory of `quantum gravity’. The opposite approach is `gravitizing QT’ where we attempt to keep the principles of GR, such as the equivalence principle, and consider how this leads to modifications of QT. What we are most lacking in understanding which route to take, if either, is experimental guidance. Here we consider using a Bose-Einstein condensate (BEC) to search for clues. In particular, we study how a single BEC in a superposition of two locations could test a gravitizing QT proposal where wavefunction collapse emerges from a unified theory as an objective process, resolving the measurement problem of QT. Such a modification to QT due to general relativistic principles is testable near the Planck mass scale, which is much closer to experiments than the Planck length scale where quantum, general relativistic effects are traditionally anticipated in quantum gravity theories. Furthermore, experimental tests of this proposal should be simpler to perform than recently suggested experiments that would test the quantizing gravity approach in the Newtonian gravity limit by searching for entanglement between two massive systems that are both in a superposition of two locations.

Authors: Edoardo G. Carnio, Heinz-Peter Breuer, Andreas Buchleitner

Stunning progresses in the experimental resolution and control of natural or man-made complex systems at the level of their quantum mechanical constituents raises the question, across diverse subdisciplines of physics, chemistry and biology, whether that fundamental quantum nature may condition the dynamical and functional system properties on mesoscopic if not macroscopic scales. But which are the distinctive signatures of quantum properties in complex systems, notably when modulated by environmental stochasticity and dynamical instabilities? It appears that, to settle this question across the above communities, a shared understanding is needed of the central feature of quantum mechanics: wave-particle duality. In this Perspective, we elaborate how randomness induced by this very quantum property can be discerned from the stochasticity ubiquitous in complex systems already on the classical level. We argue that in the study of increasingly complex systems such distinction requires the analysis of single incidents of quantum dynamical processes.

Authors: Michael Silberstein, W.M. Stuckey

We discuss the implications for the determinateness and intersubjective consistency of conscious experience in two gedanken experiments from quantum mechanics (QM). In particular, we discuss Wigner’s friend and the delayed choice quantum eraser experiment with a twist. These are both cases (experiments) where quantum phenomena, or at least allegedly possible quantum phenomena/experiments, and the content/efficacy of conscious experience seem to bear on one another. We discuss why these two cases raise concerns for the determinateness and intersubjective consistency of conscious experience. We outline a 4D-global constraint-based approach to explanation in general and for QM in particular that resolves any such concerns without having to invoke metaphysical quietism (as with pragmatic accounts of QM), objective collapse mechanisms or subjective collapse. In short, we provide an account of QM free from any concerns associated with either the standard formalism or relative-state formalism, an account that yields a single 4D block universe with determinate and intersubjectively consistent conscious experience for all conscious agents. Essentially, the mystery in both experiments is caused by a dynamical/causal view of QM, e.g., time-evolved states in Hilbert space, and as we show this mystery can be avoided by a spatiotemporal, constraint-based view of QM, e.g., path integral calculation of probability amplitudes using future boundary conditions. What will become clear is that rather than furiously seeking some way to make dubious deep connections between quantum physics and conscious experience, the kinds of 4D adynamical global constraints that are fundamental to both classical and quantum physics and the relationship between them, also constrain conscious experience. That is, physics properly understood, already is psychology.

Authors: Andrea Oldofredi

In the history of quantum physics several no-go theorems have been proved, and many of them have played a central role in the development of the theory, such as Bell’s or the Kochen-Specker theorem. A recent paper by F. Laudisa has raised reasonable doubts concerning the strategy followed in proving some of these results, since they rely on the standard framework of quantum mechanics, a theory that presents several ontological problems. The aim of this paper is twofold: on the one hand, I intend to reinforce Laudisa’s methodological point by critically discussing Malament’s theorem in the context of the philosophical foundation of Quantum Field Theory; secondly, I rehabilitate Gisin’s theorem showing that Laudisa’s concerns do not apply to it.

Authors: Andrea Oldofredi

Fuchs and Peres (2000) claimed that standard Quantum Mechanics needs no interpretation. In this essay, I show the flaws of the arguments presented in support to this thesis. Specifically, it will be claimed that the authors conflate QM with Quantum Bayesianism (QBism) – the most prominent subjective formulation of quantum theory; thus, they endorse a specific interpretation of the quantum formalism. Secondly, I will explain the main reasons for which QBism should not be considered a physical theory, being it concerned exclusively with agents’ beliefs and silent about the physics of the quantum regime. Consequently, the solutions to the quantum puzzles provided by this approach cannot be satisfactory from a physical perspective. In the third place, I evaluate Fuchs and Peres arguments contra the non-standard interpretations of QM, showing again the fragility of their claims. Finally, it will be stressed the importance of the interpretational work in the context of quantum theory.

Dewar, Neil (2019) General-Relativistic Covariance. [Preprint]

Johns, Oliver (2019) Validity of the Einstein Hole Argument. [Preprint]

Swanson, Noel (2019) On the Ostrogradski Instability; or, Why Physics Really Uses Second Derivatives. [Preprint]

Norton, Joshua (2019) The Hole Argument Against Everything. [Preprint]

Schindler, Samuel (2019) Naturalnesss in physics: just a matter of aesthetics? Book review. [Preprint]

Author(s): P.-P. Crépin, C. Christen, R. Guérout, V. V. Nesvizhevsky, A.Yu. Voronin, and S. Reynaud

We propose to use quantum interferences to improve the accuracy of the measurement of the free-fall acceleration g¯ of antihydrogen in the gravitational behavior of antihydrogen at rest (GBAR) experiment. This method uses most antiatoms prepared in the experiment and it is simple in its principle, a…

[Phys. Rev. A 99, 042119] Published Mon Apr 22, 2019

Violating Bell’s inequality with remotely connected superconducting qubits

Violating Bell’s inequality with remotely connected superconducting qubits, Published online: 22 April 2019; doi:10.1038/s41567-019-0507-7

A deterministic violation of the Bell inequality is reported between two superconducting circuits, providing a necessary test for establishing strong enough quantum entanglement to achieve secure quantum communications.

Experimental characterization of two-particle entanglement through position and momentum correlations

Experimental characterization of two-particle entanglement through position and momentum correlations, Published online: 22 April 2019; doi:10.1038/s41567-019-0508-6

Single-particle resolved measurements in an ultracold-atom experiment reveal an intuitive picture of quantum correlations, providing strong constraints on the full density matrix of the two-particle system in the presence of interactions.

Frigg, Roman and Nguyen, James (2019) Mirrors Without Warnings. [Preprint]

Filomeno, Aldo (2019) Stable regularities without governing laws? Studies in History and Philosophy of Modern Physics. ISSN 1355-2198