# Weekly Papers on Quantum Foundations (51)

Nuclear clocks for testing fundamental physics. (arXiv:2012.09304v1 [quant-ph])

The low-energy, long-lived isomer in $^{229}$Th, first studied in the 1970s as an exotic feature in nuclear physics, continues to inspire a multidisciplinary community of physicists. Using the nuclear resonance frequency, determined by the strong and electromagnetic interactions inside the nucleus, it is possible to build a highly precise nuclear clock that will be fundamentally different from all other atomic clocks based on resonant frequencies of the electron shell. The nuclear clock will open opportunities for highly sensitive tests of fundamental principles of physics, particularly in searches for violations of Einstein’s equivalence principle and for new particles and interactions beyond the standard model. It has been proposed to use the nuclear clock to search for variations of the electromagnetic and strong coupling constants and for dark matter searches.

The $^{229}$Th nuclear optical clock still represents a major challenge in view of the tremendous gap of nearly 17 orders of magnitude between the present uncertainty in the nuclear transition frequency and the natural linewidth. Significant experimental progress has been achieved in recent years, which will be briefly reviewed. Moreover, a research strategy will be outlined to consolidate our present knowledge about essential $^{229\rm{m}}$Th properties, to determine the nuclear transition frequency with laser spectroscopic precision, realize different types of nuclear clocks and apply them in precision frequency comparisons with optical atomic clocks to test fundamental physics. Two avenues will be discussed: laser-cooled trapped $^{229}$Th ions that allow experiments with complete control on the nucleus-electron interaction and minimal systematic frequency shifts, and Th-doped solids enabling experiments at high particle number and in different electronic environments.

A New Concept for the Momentum of a Quantum Mechanical Particle in a Box. (arXiv:2012.09596v1 [quant-ph])

For a particle in a box, the operator $- i \partial_x$ is not Hermitean. We provide an alternative construction of a momentum operator $p = p_R + i p_I$, which has a Hermitean component $p_R$ that can be extended to a self-adjoint operator, as well as an anti-Hermitean component $i p_I$. This leads to a description of momentum measurements performed on a particle that is strictly limited to the interior of a box.

Quantum Measurement of Space-Time Events. (arXiv:2011.11541v2 [quant-ph] UPDATED)

The phase space of a relativistic system can be identified with the future tube of complexified Minkowski space. As well as a complex structure and a symplectic structure, the future tube, seen as an eight-dimensional real manifold, is endowed with a natural positive-definite Riemannian metric that accommodates the underlying geometry of the indefinite Minkowski space metric, together with its symmetry group. These structures are enough to allow one to construct a quantum theory of phase-space events. In particular, a theory of quantum measurement can be formulated in a relativistic setting, based on the use of positive operator valued measures, for the detection of phase-space events, hence allowing one to assign probabilities to the outcomes of joint space-time and four-momentum measurements in a manifestly covariant framework. This leads to a localization theorem for phase-space events in relativistic quantum theory, determined by the associated Compton wavelength.

Quantum-to-Classical transition via Quantum Cellular Automata. (arXiv:2012.04237v2 [quant-ph] UPDATED)

A quantum cellular automaton (QCA) is an abstract model consisting of an array of finite-dimensional quantum systems that evolves in discrete time by local unitary operations. Here we propose a simple coarse-graining map, where the spatial structure of the QCA is merged into effective ones. Starting with a QCA that simulates the Dirac equation we apply this coarse-graining map recursively until we get its effective dynamics in the semi-classical limit, which can be described by a classical cellular automaton. We show that the emergent-effective result of the former microscopic discrete model converges to the diffusion equation and to a classical transport equation under a specific initial condition. Therefore, QCA is a good model to validate the quantum-to-classical transition.

Contextual inferences, (non)locality, and the (in)completeness of quantum mechanics. (arXiv:2012.09736v1 [quant-ph])

Authors: Philippe Grangier

In this article we argue that what is usually called “quantum non locality”, leading to the violation of Bell’s inequality and more generally of classical local realism, can be explained by considering contextual inferences, that are both possible and necessary in quantum physics. They do not show up in classical physics, where contexts may be discarded because physical properties are non-contextual. This analysis allows us to explain why “quantum non locality” does not contradict relativistic causality. Our argument is general from a probabilistic point of view, and it strongly suggests that the usual quantum formalism must be completed, by specifying the measurement context.

Probing Particle Physics with Gravitational Waves. (arXiv:2012.09167v1 [hep-ph])

Authors: Horng Sheng Chia

The direct detection of gravitational waves offers an exciting new window onto our Universe. At the same time, multiple observational evidence and theoretical considerations motivate the presence of physics beyond the Standard Model. In this thesis, we explore new ways of probing particle physics in the era of gravitational-wave astronomy. We focus on the signatures of ultralight bosons on the gravitational waves emitted by binary systems, demonstrating how binary black holes are novel detectors of this class of dark matter. We also discuss probes of other types of new physics through their finite-size imprints on gravitational waveforms, and examine the extent to which current template-bank searches could be used to detect these signals. In the first two chapters of this thesis, we review several aspects of gravitational-wave physics and particle physics at the weak coupling frontier; we hope the reader would find these reviews helpful in delving further into the literature and in their research.

Matter-antimatter asymmetry and non-inertial effects. (arXiv:2012.09245v1 [gr-qc])

We investigate non-inertial effects on $CP$-violating processes using a model, based on the framework of quantum field theory in curved spacetimes, devised to account for the decay of accelerated particles. We show that the $CP$ violation parameter for the decay of accelerated kaons into two pions decreases very slightly as very high accelerations are achieved, implying decreased asymmetry between matter and antimatter in this regime. We discuss the relationship between these results and cosmological processes surrounding matter-antimatter asymmetry and argue that, due to the connection between non-inertial and thermal phenomena established by the Unruh effect, this kind of computation may prove useful in furthering the understanding of thermodynamical effects in curved spacetimes.

Wave mechanics for gravity with point-particles. (arXiv:2012.09781v1 [gr-qc])

Authors: Christian MaesKasper MeertsWard Struyve

We consider non-relativistic point-particles coupled to Einstein gravity and their canonical quantization. From the resulting Wheeler-DeWitt wave equation we determine a quantum version of geometrodynamics, where the coupled evolution of particle positions and 3-metric is guided by the wave function. We find that this quantum dynamics implies a quantum extension of the Einstein equations. The conserved energy-momentum tensor now contains a quantum contribution. This conceptually-simple set up is promising both for deriving semiclassical and weak field approximations to the quantum Einstein equations and is thus important for the development of quantum corrections to computational general relativity.

Quantum Fields, Geometric Fluctuations, and the Structure of Spacetime. (arXiv:1809.08265v4 [gr-qc] UPDATED)

Quantum fluctuations of the vacuum stress-energy tensor are highly non-Gaussian, and can have unexpectedly large effects on spacetime geometry. In this paper, we study a two-dimensional dilaton gravity model coupled to a conformal field, in which the distribution of vacuum fluctuations is well understood. In this model, the fluctuations of the matter field are responsible for the fluctuations of the geometry itself. By analyzing the geodesic deviation in this model, we show that a pencil of massive particles propagating on this fuzzy spacetime eventually converges and collapses. This is consistent with our earlier analysis of null geodesics in [Phys. Rev. Lett.\ 107, 021303 (2011)].

A new relativistic theory for Modified Newtonian Dynamics. (arXiv:2007.00082v2 [astro-ph.CO] UPDATED)

Authors: Constantinos SkordisTom Zlosnik

We propose a relativistic gravitational theory leading to Modified Newtonian Dynamics, a paradigm that explains the observed universal acceleration and associated phenomenology in galaxies. We discuss phenomenological requirements leading to its construction and demonstrate its agreement with the observed Cosmic Microwave Background and matter power spectra on linear cosmological scales. We show that its action expanded to 2nd order is free of ghost instabilities and discuss its possible embedding in a more fundamental theory.

Cartography of the space of theories: An interpretational chart for fields that are both (dark) matter and spacetime

Publication date: Available online 13 December 2020

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

Author(s): Niels C.M. Martens, Dennis Lehmkuhl

Finely tuned models sacrifice explanatory depth

Azhar, Feraz and Loeb, Abraham (2020) Finely tuned models sacrifice explanatory depth. [Preprint]

What Are Symmetries?

Baker, David John (2020) What Are Symmetries? [Preprint]

Introductory Textbooks in the Philosophy of Science: the Transformation of the Study Curriculum From 1937 to 2017

Demin, Maxim (2020) Introductory Textbooks in the Philosophy of Science: the Transformation of the Study Curriculum From 1937 to 2017. [Preprint]

Many-Body Level Statistics of Single-Particle Quantum Chaos

Author(s): Yunxiang Liao, Amit Vikram, and Victor Galitski

We consider a noninteracting many-fermion system populating levels of a unitary random matrix ensemble (equivalent to the q=2 complex Sachdev-Ye-Kitaev model)—a generic model of single-particle quantum chaos. We study the corresponding many-particle level statistics by calculating the spectral form …

[Phys. Rev. Lett. 125, 250601] Published Fri Dec 18, 2020

The rotten core of presentism

Abstract

Recently, some have attempted to reformulate debates in first-order metaphysics, particularly in the metaphysics of time and modality, for reasons due to Williamson (Modal logic as metaphysics, Oxford University Press, 2013). In this paper, we focus on the ways in which the likes of Cameron, Correia and Rosenkranz, Deasy, Ingram, Tallant, Viebahn, inter alia, have initiated and responded to attempts to capture the core of presentism using a formal, logical machinery. We argue that such attempts are doomed to fail because there is no theoretical core to presentism. There is no single view or family of views that is presentism.

Grounding Ontic Structuralism

Bianchi, Silvia and Giannotti, Joaquim (2020) Grounding Ontic Structuralism. [Preprint]