Authors: Mordehai Milgrom

MOND is a paradigm that contends to account for the mass discrepancies in the Universe without invoking `dark’ components, such as `dark matter’ and `dark energy’. It does so by supplanting Newtonian dynamics and General Relativity, departing from them at very low accelerations. Having in mind historians and philosophers of science, as well as physicists and astronomers, I describe in this review the main aspects of MOND — its statement, its basic tenets, its main predictions, and the tests of these predictions — contrasting it with the dark-matter paradigm. I then discuss possible wider ramifications of MOND, for example the potential significance of the MOND constant, $a_0$, and its possible implications for the roots of MOND in cosmology. Along the way I point to parallels with several historical instances of nascent paradigms. In particular, with the emergence of the Copernican world picture, that of quantum physics, and that of relativity. I point to analogies between these paradigms as regards their initial advent, their development, their schematic structure, and their ramifications. For example, the interplay between theories and their corollary laws, and the centrality of a new constant with converging values as deduced from seemingly unrelated manifestations of these laws. I parallel the struggle of the new with the old paradigms, and the appearance of hybrid paradigms at such times of struggle. I also try to identify in the history of those established paradigms a stage that can be likened to that of MOND today.

Authors: Antonio Vassallo

The paper discusses from a metaphysical standpoint the nature of the dependence relation underpinning the talk of mutual action between material and spatiotemporal structures in general relativity. It is shown that the standard analyses of dependence in terms of causation or grounding are ill-suited for the general relativistic context. Instead, a non-standard analytical framework in terms of structural equation modeling is exploited, which leads to the conclusion that the kind of dependence encoded in the Einstein field equations is a novel one.

Authors: S.Deser

To celebrate Roman Jackiw’s 80th birthday, herewith some comments on gravity and gauge theory models in D=3, the chief focus of many of our joint efforts.

Authors: Huiping Zhan, Gaoxiang Li, Huatang Tan

In this paper, we propose a feasible scheme for generating the Schr\”{o}dinger cat states of a macroscopic mechanical resonator in pulsed cavity optomechanics. Starting with cooling the mechanical oscillator to its ground state, a red and a blue pulses with different powers are simultaneously employed to drive the cavity to achieve squeezed mechanical states. Subsequently, a second red pulse is utilized to generate the macroscopic mechanical quantum superpositions, conditioned on the detection of cavity output photons. Finally, after being stored in the resonator for a period of time, the mechanical state is mapped, with a third red pulse, to the cavity output field used for state verification. Our approach is generic and can also be used to produce other kinds of non-Gaussian mechanical states, like optical-catalysis nonclassical states.

Authors: Yurii V. Brezhnev

Clarifying the nature of the quantum state $|\Psi\rangle$ is at the root of the problems with insight into (counterintuitive) quantum postulates. We provide a direct-and math-axiom free-empirical derivation of this object as an element of a vector space. Establishing the linearity of this structure-quantum superposition-is based on a set-theoretic creation of ensemble formations and invokes the following three principia: $(\textsf{I})$ quantum statics, $(\textsf{II})$ doctrine of a number in the physical theory, and $(\textsf{III})$ mathematization of matching the two observations with each other; quantum invariance.

All of the constructs rest upon a formalization of the minimal experimental entity: observed micro-event, detector click. This is sufficient for producing the $\mathbb C$-numbers, axioms of linear vector space (superposition principle), statistical mixtures of states, eigenstates and their spectra, and non-commutativity of observables. No use is required of the concept of time. As a result, the foundations of theory are liberated to a significant extent from the issues associated with physical interpretations, philosophical exegeses, and mathematical reconstruction of the entire quantum edifice.

Authors: Yurii V. Brezhnev

We deduce the Born rule. No use is required of quantum postulates. One exploits only rudimentary quantum mathematics–a linear, not Hilbert’, vector space–and empirical notion of the statistical length of a state. Its statistical nature comes from the experimental detector-clicks being formalized into the abstract quantum micro-events. We also comment on that it is not only that the use has not been made of some quantum axioms when deriving the rule but, in a sense, their invoking would be inconsistent.

Authors: Vitor Cardoso

Because of the very definition of black holes — no light escapes them and falling objects get infinitely faint when approaching — it is impossible to ever prove that they exist. However, electromagnetic and gravitational-wave observatories have now `seen’ black holes. Datasets from these observations, released in 2019 and late 2018, give important hints about the environment, origin and growth of black holes.

Authors: Andrea Dapor, Klaus Liegener, Tomasz Pawłowski

We reexamine a set of existing procedures aimed at recovering the effective description of the dynamics of LQG in the context of cosmological solutions. In particular, the studies of those methods, to which the choice of cuboidal graphs and graph-preserving Hamiltonian is central, result in the formulation of a set of no-go statements, severely limiting the possibility of recovering a physically consistent effective dynamics this way.

Authors: Steven Duplij (University of Münster)

Supergravity is a remarkable theory combining supersymmetry and general relativity. While the theory has many developers from across the globe, we wish to address the question who were the real originators of this fantastic idea.

Author(s): Robert Pisarczyk, Zhikuan Zhao, Yingkai Ouyang, Vlatko Vedral, and Joseph F. Fitzsimons

The capacity of a channel is known to be equivalent to the highest rate at which it can generate entanglement. Analogous to entanglement, the notion of a causality measure characterizes the temporal aspect of quantum correlations. Despite holding an equally fundamental role in physics, temporal quan…

[Phys. Rev. Lett. 123, 150502] Published Thu Oct 10, 2019

Abstract

The relationship whereby one physical theory encompasses the domain of empirical validity of another is widely known as “reduction.” Elsewhere I have argued that one influential methodology for showing that one physical theory reduces to another, associated with the so-called “Bronstein cube” of theories, rests on an oversimplified and excessively vague characterization of the mathematical relationship between theories that typically underpins reduction. I offer what I claim is a more precise characterization of this relationship, which here is based on a more basic notion of reduction between distinct models (one from each theory) of a single physical system. Reduction between two such models, I claim, rests on a particular type of approximation relationship between group actions over the models’ state spaces, characterized by a particular function between the model state spaces and a particular subset of the more encompassing model’s state space. Within this approach, I show formally in what sense and under what conditions reduction is transitive, so that reduction of a model 1 to another model 2 and reduction of model 2 to a third model 3 entails direct reduction of model 1 to model 3. Building on this analysis, I consider cases in which reduction of a model 1 to a model 3 may be effected via distinct intermediate models 2a and 2b, and motivate a set of formal consistency requirements between distinct “reduction paths” having the same models as their “end points”. These constraints are explicitly shown to hold in the reduction of a model of non-relativistic classical mechanics (model 1) to a model of relativistic quantum mechanics (model 3), which may be effected by a composite reduction that proceeds either via a model of non-relativistic quantum mechanics (model 2a) or a model of relativistic classical mechanics (model 2b). I offer some brief speculations as to whether and how this sort of consistency requirement might serve to constrain the reductions relating other theories and models, including the relationship that the Standard Model and general relativity must bear to any viable unification of these frameworks.

Author(s): Wei-Wei Pan, Xiao-Ye Xu, Yaron Kedem, Qin-Qin Wang, Zhe Chen, Munsif Jan, Kai Sun, Jin-Shi Xu, Yong-Jian Han, Chuan-Feng Li, and Guang-Can Guo

Researchers have directly measured the components of a nonlocal, entangled wave function, rather than relying on indirect tomographic or reconstructive techniques.

[Phys. Rev. Lett. 123, 150402] Published Wed Oct 09, 2019

Nature Physics, Published online: 07 October 2019; doi:10.1038/s41567-019-0657-7

Non-Hermitian systems with gain and loss give rise to exceptional points with exceptional properties. An experiment with superconducting qubits now offers a first step towards studying these singularities in the quantum domain.