Authors: Morgan H. Lynch, Eliahu Cohen, Yaron Hadad, Ido Kaminer

We examine the radiation emitted by high energy positrons channeled into silicon crystal samples. The positrons are modeled as semiclassical vector currents coupled to an Unruh-DeWitt detector to incorporate any local change in the energy of the positron. In the subsequent accelerated QED analysis, we discover a Larmor formula and power spectrum that are both thermalized by the acceleration. As such, these systems will explicitly exhibit thermalization of the detector energy gap at the celebrated Fulling-Davies-Unruh temperature. Our derived power spectrum, with a nonzero energy gap, is then shown to have an excellent statistical agreement with high energy channeling experiments and therefore provides evidence for the very first observation of the Unruh effect in a non-analogue system.

Authors: Elizabeth Gould, Niayesh Afshordi

It has been suggested that the cosmic history might repeat in cycles, with an infinite series of similar aeons in the past and the future. Here, we instead propose that the cosmic history repeats itself exactly, constructing a universe on a periodic temporal history, which we call Periodic Time Cosmology. In particular, the primordial power spectrum, convolved with the transfer function throughout the cosmic history, would form the next aeon’s primordial power spectrum. By matching the big bang to the infinite future using a conformal rescaling (a la Penrose), we uniquely determine the primordial power spectrum, in terms of the transfer function up to two free parameters. While nearly scale invariant with a red tilt on large scales, using Planck and Baryonic Acoustic Oscillation observations, we find the minimal model is disfavoured compared to a power-law power spectrum at $5.1\sigma$. However, extensions of $\Lambda$CDM cosmic history change the large scale transfer function and can provide better relative fits to the data. For example, the best fit seven parameter model for our Periodic Time Cosmology, with $w=-1.024$ for dark energy equation of state, is only disfavoured relative to a power-law power spectrum (with the same number of parameters) at $1.8\sigma$ level. Therefore, consistency between cosmic history and initial conditions provides a viable description of cosmological observations in the context of Periodic Time Cosmology.

Authors: M.W. Kalinowski

In the paper we consider an interesting possibility of a time as a stochastic process in quantum mechanics.In order to do it we reconsider time as a mechanical quantity in classical mechanics and afterwards we quantize it. We consider continuous and discrete time.

Authors: Detlef Dürr, Ward Struyve

Typicality has always been in the minds of the founding fathers of probability theory when probabilistic reasoning is applied to the real world. However, the role of typicality is not always appreciated. An example is the paper “Foundations of statistical mechanics and the status of Born’s rule in de Broglie-Bohm pilot-wave theory” by Antony Valentini, where he presents typicality and relaxation to equilibrium as distinct approaches to the proof of Born’s rule, while typicality is in fact an overriding necessity. Moreover the “typicality approach” to Born’s rule of “the Bohmian mechanics school” is claimed to be inherently circular. We wish to explain once more in very simple terms why the accusation is off target and why “relaxation to equilibrium” is neither necessary nor sufficient to justify Born’s rule.

Authors: Stephen Boughn

After the development of a self-consistent quantum formalism nearly a century ago, there ensued a quest to understand the often counterintuitive predictions of the theory. These endeavors invariably begin with the assumption of the “truth” of the mathematical formalism of quantum mechanics and then proceed to investigate the theory’s implications for the physical world. One of the outcomes has been endless discussions of the quantum measurement problem, wave/particle duality, the non-locality of entangled quantum states, Schroedinger’s cat, and other philosophical conundrums. In this essay, I take the point of view that quantum mechanics is a mathematical model, a human invention, and rather than pondering what the theory implies about our world, I consider the transposed question: what is it about our world that leads us to a quantum mechanical model of it? One consequence is the realization that discrete quanta, the quantum of action in particular, leads to the wave nature and statistical behavior of matter rather than the other way around.

Ben-Yami, Hanoch (2018) Absolute Distant Simultaneity in Special Relativity. In: UNSPECIFIED.

Abstract

The paper investigates the status of gravitational energy in Newtonian Gravity (NG), developing upon recent work by Dewar and Weatherall. The latter suggest that gravitational energy is a gauge quantity. This is potentially misleading: its gauge status crucially depends on the spacetime setting one adopts. In line with Møller-Nielsen’s plea for a motivational approach to symmetries, we supplement Dewar and Weatherall’s work by discussing gravitational energy–stress in Newtonian spacetime, Galilean spacetime, Maxwell-Huygens spacetime, and Newton–Cartan Theory (NCT). Although we ultimately concur with Dewar and Weatherall that the notion of gravitational energy is problematic in NCT, our analysis goes beyond their work. The absence of an explicit definition of gravitational energy–stress in NCT somewhat detracts from the force of Dewar and Weatherall’s argument. We fill this gap by examining the supposed gauge status of prima facie plausible candidates—NCT analogues of gravitational energy–stress pseudotensors, the Komar mass, and the Bel-Robinson tensor. Our paper further strengthens Dewar and Weatherall’s results. In addition, it sheds more light upon the subtle link between sufficiently rich inertial structure and the definability of gravitational energy in NG.

Hansson Wahlberg, Tobias (2019) The Creation of Institutional Reality, Special Theory of Relativity, and Mere Cambridge Change. [Preprint]

Abstract

I explore two views about the relationship between spatial experience and spatial reality: spatial functionalism and spatial presentationalism. Roughly, spatial functionalism claims that the instantiated spatial properties are those playing a certain causal role in producing spatial experience while spatial presentationalism claims that the instantiated spatial properties include those presented in spatial experience. I argue that each view, in its own way, leads to an ontologically inflationary form of primitivism: whereas spatial functionalism leads to primitivism about phenomenal representation, spatial presentationalism leads to primitivism about spatial properties. I conclude by discussing how to adjudicate between spatial functionalism and spatial presentationalism.

Maudlin, Tim and Okon, Elias and Sudarsky, Daniel (2019) On the Status of Conservation Laws in Physics: Implications for Semiclassical Gravity. [Preprint]

Nature Physics, Published online: 14 October 2019; doi:10.1038/s41567-019-0672-8

Vacuum fluctuations in the vicinity of nanophotonic structures can lead to the conversion of a free electron into a polariton and a high-energy photon, whose frequency can be controlled by the electromagnetic properties of the nanostructure.