# Weekly Papers on Quantum Foundations (16)

This is a list of this week’s papers on quantum foundations published in the various journals or uploaded to the preprint servers such as arxiv.org and PhilSci Archive.

Quantum Time. (arXiv:1504.04215v1 [quant-ph])

on 2015-4-17 1:44am GMT

We give a consistent quantum description of time, based on Page and Wootters’ conditional probabilities mechanism, that overcomes the criticisms that were raised against similar previous proposals. In particular we show how the model allows to reproduce the correct statistics of sequential measurements performed on a system at different times.

Mean and Variance in Quantum Theory

Latest Results for Foundations of Physics

on 2015-4-16 12:00am GMT

Abstract

Calculation of the mean of an observable in quantum mechanics is typically assumed to require that the state vector be in the domain of the corresponding self-adjoint operator or for a mixed state that the operator times the density matrix be in the trace class. We remind the reader that these assumptions are unnecessary. We state what is actually needed to calculate the mean of an observable as well as its variance.

The Consistent Histories Formalism and the Measurement Problem. (arXiv:1504.03231v1 [quant-ph])

on 2015-4-14 1:17am GMT

Authors: E. OkonD. Sudarsky

In response to a recent rebuttal of [1] presented in [2], we defend the claim that the Consistent Histories formulation of quantum mechanics does not solve the measurement problem. In order to do so, we argue that satisfactory solutions to the problem must not only not contain anthropomorphic terms (such as measurement or observer) at the fundamental level, but also that applications of the formalism to concrete situations (e.g., measurements) should not require any input not contained in the description of the situation at hand at the fundamental level. Our assertion is that the Consistent Histories formalism does not meet the second criterion. We also argue that the so-called second measurement problem, i.e., the inability to explain how an experimental result is related to a property possessed by the measured system before the measurement took place, is only a pseudo-problem. As a result, we reject the claim, defended in [2], that the capacity of the Consistent Histories formalism to solve it should count as an advantage over other interpretations.

Foundations of statistical mechanics from symmetries of entanglement. (arXiv:1504.02797v1 [quant-ph])

on 2015-4-14 1:17am GMT

Authors: Sebastian DeffnerWojciech H. Zurek

Envariance – entanglement assisted invariance – is a recently discovered symmetry of composite quantum systems. In this work, it is shown that thermodynamic equilibrium states are fully characterized by their envariance. In particular, the microcanonical equilibrium of a system $\mathcal{S}$ with Hamiltonian $H_\mathcal{S}$ can be described as an “even”, i.e., envariant under any unitary transformation, fully energetically degenerate quantum state. The representation of the canonical equilibrium then follows from simply counting degenerate energy states. Our conceptually novel approach is free of mathematically ambiguous notions such as probability, ensemble, randomness, etc. .

Can classical physics agree with quantum physics on quantum phenomena?. (arXiv:1504.03207v1 [physics.class-ph])

on 2015-4-14 1:17am GMT

Authors: Michele Marrocco

Classical physics fails where quantum physics prevails. This common understanding applies to quantum phenomena that are acknowledged to be beyond the reach of classical physics. Here, we make an attempt at weakening this solid belief that classical physics is unfit to explain the quantum world. The trial run is the quantization of the free radiation field that will be addressed by following a strategy that is free from operators or quantum-mechanical concepts

Similarity of the Magnetic Force between Dirac Particles to the Quantum Force of Bohmian Mechanics. (arXiv:1409.8271v4 [physics.class-ph] CROSS LISTED)

on 2015-4-14 1:17am GMT

Authors: David C. Lush

In the de Broglie – Bohm formulation of quantum mechanics, the electron is stationary in the ground state of hydrogenic atoms, because the quantum force exactly cancels the Coulomb attraction of the electron to the nucleus. In this paper it is shown that classical electrodynamics similarly predicts the Coulomb force can be effectively canceled by part of the magnetic force that occurs between two particles each consisting of a point charge moving in a circulatory motion at the speed of light. Supposition of such motion is the basis of the {\em Zitterbewegung} interpretation of quantum theory. The magnetic force between two such particles for separation large compared to their circulatory motions contains a radial inverse square law part with magnitude equal to the Coulomb force, sinusoidally modulated by the phase difference between the circulatory motions. This raises a possibility that the quantum force of Bohmian mechanics may be attributable to the magnetic force of classical electrodynamics.

Probing a Gravitational Cat State. (arXiv:1504.03103v1 [quant-ph])

on 2015-4-14 1:17am GMT

Authors: Charis AnastopoulosBei-Lok Hu

We investigate the nature of a gravitational two-state system (G2S) in the simplest setup in Newtonian gravity. In a quantum description of matter a single motionless massive particle can in principle be in a superposition state of two spatially-separated locations. This superposition state in gravity, or gravitational cat state, would lead to fluctuations in the Newtonian force exerted on a nearby test particle. The central quantity of importance for this inquiry is the energy density correlation. This corresponds to the noise kernel in stochastic gravity theory, evaluated in the weak field nonrelativistic limit. In this limit, quantum fluctuations of the stress energy tensor manifest as the fluctuations of the Newtonian force. We describe the properties of such a G2S system and present two ways of measuring the cat state for the Newtonian force, one by way of a classical probe, the other a quantum harmonic oscillator. Our findings include: (i) mass density fluctuations persist even in single particle systems, and they are of the same order of magnitude as the mean; (ii) a classical probe generically records a non-Markovian fluctuating force; (iii) a quantum probe interacting with the G2S system may undergo Rabi oscillations in a strong coupling regime. This simple prototypical gravitational quantum system could provide a robust testing ground to compare predictions from alternative quantum theories, since the results reported here are based on standard quantum mechanics and classical gravity.

Path integrals in configuration space and the emergence of classical behavior for closed systems. (arXiv:1504.02818v1 [quant-ph])

on 2015-4-14 1:17am GMT

Authors: Henrique Gomes

Traditionally, the field of quantum foundations has been preoccupied with different forms of the question “How can an observer be in a state of quantum superposition?”. In this paper, I approach this question by exploring a timeless interpretation of quantum mechanics \emph{of closed systems}, solely in terms of path integrals in non-relativistic timeless configuration space. What prompts a fresh look at the foundational problems in this context, is the advent of multiple gravitational models in which Lorentz symmetry is only emergent. In this setting, I propose a new understanding of records as certain relations between two configurations, the recorded one and the record-holding one. These relations are formalized through a factorization of the amplitude kernel, which forbids unwanted `recoherence’ of branches. On this basis, I show that in simple cases the Born rule emerges from counting the relative density of observers with the same records. Furthermore, unlike what occurs in consistent histories, in this context there is indeed a preferred notion of coarse-grainings: those centered around piece-wise classical paths in configuration space. Thus, this new understanding claims to resolve aspects of the measurement problem which are still deemed controversial in the standard approaches.

Macroscopic noncontextuality as a principle for almost-quantum correlations

PRA: Fundamental concepts

on 2015-4-13 2:00pm GMT

Author(s): Joe Henson and Ana Belén Sainz

Quantum mechanics allows only certain sets of experimental results (or “probabilistic models”) for Bell-type quantum nonlocality experiments. A derivation of this set from simple physical or information theoretic principles would represent an important step forward in our understanding of quantum me…

[Phys. Rev. A 91, 042114] Published Mon Apr 13, 2015