# Weekly Papers on Quantum Foundations (8)

Wigner’s Friend Depends on Self-Contradictory Quantum Amplification

Knight, Andrew (2022) Wigner’s Friend Depends on Self-Contradictory Quantum Amplification. [Preprint]

Remarks on the use of objective probabilities in Bell-CHSH inequalities. (arXiv:2202.08353v1 [quant-ph])

The violation of Bell inequalities is often interpreted as showing that, if hidden variables exist, they must be contextual and non local. But they can also be explained questioning the probability space employed, or the validity of the Kolmogorov axioms. In this article we explore the additional constrains which can be deduced from two widely used objetive probability theories: frequentism and propensities.

One of the strongest objections in the deduction of one version of Bell inequalities goes about the probability space, which assumes the existence of values for the output of the experiment in each run, while only two of the four values can be measured each time, making them counterfactual. It is shown that frequentism rejects the possibility of using counterfactual situations, while long-run propensities allow their use. In this case the introduction of locality and contextuality does not help to explain the violation, and an alternative explanation could point to a failure of the probability.

Single case propensities were designed to associate probabilities to single events, but they need to be conditional to the whole universe, and do not have a clear link with the observed relative frequencies. It heavily limits their use.

Tutorial: Macroscopic QED and vacuum forces. (arXiv:2202.08762v1 [quant-ph])

This tutorial introduces the theory of macroscopic QED, where a Hamiltonian is found that represents the electromagnetic field interacting with a dispersive, dissipative material. Using a one dimensional theory as motivation, we build up the more cumbersome three dimensional theory. Then considering the extension of this theory to moving materials, where the material response changes due to both the Doppler effect and the mixing of electric and magnetic responses, it is shown that one gets the theory of quantum electromagnetic forces for free. We finish by applying macroscopic QED to reproduce Pendry’s expression for the quantum friction force between sliding plates.

A quantum prediction as a collection of epistemically restricted classical predictions. (arXiv:2107.02728v5 [quant-ph] UPDATED)

Spekkens has introduced an epistemically restricted classical theory of discrete systems, based on discrete phase space. The theory manifests a number of quantum-like properties but cannot fully imitate quantum theory because it is noncontextual. In this paper we show how, for a certain class of quantum systems, the quantum description of an experiment can be decomposed into classical descriptions that are epistemically restricted, though in a different sense than in Spekkens’ work. For each aspect of the experiment — the preparation, the transformations, and the measurement — the epistemic restriction limits the form of the probability distribution an imagined classical observer may use. There are also global constraints that the whole collection of classical descriptions must satisfy. Each classical description generates its own prediction regarding the outcome of the experiment. One recovers the quantum prediction via a simple but highly nonclassical rule: the “nonrandom part” of the predicted quantum probabilities is obtained by summing the nonrandom parts of the classically predicted probabilities. By “nonrandom part” we mean the deviation from complete randomness, that is, from what one would expect upon measuring the fully mixed state.

On the conjectured gravity-related collapse rate $E_\Delta/\hbar$ of massive quantum superpositions. (arXiv:2111.04604v2 [quant-ph] UPDATED)

Roger Penrose and the author share the proposal that the spatial superposition $|x_1\rangle+|x_2\rangle$ of a massive object collapses into its localized components $|x_1\rangle$ or $|x_2\rangle$ with the characteristic time $\hbar/E_\Delta$ where $E_\Delta$ is the gravitational self-energy excess of the superposition versus the localized states. Underlying arguments of such radical departure from standard quantum mechanics and different derivations of the rate equation are briefly recapitulated and discussed.

Peaceful coexistence of thermal equilibrium and the emergence of time. (arXiv:2112.04057v3 [quant-ph] UPDATED)

We consider a quantum Universe composed by a small system S and a large environment. It has been demonstrated that, for the vast majority of randomly chosen wave-functions of the Universe satisfying a total energy constraint, the reduced density matrix of the system S is given by the canonical statistical distribution. Here, through the Page and Wootters mechanism, we show that time and non-equilibrium dynamics can emerge as a consequence of the entanglement between the system and the environment present in the (randomly chosen) global wave-function of the Universe. The clock is provided by the environment, which ticks the temporal evolution of S. The paradox of the peaceful coexistence of statistical equilibrium and non-equilibrium dynamics is solved by identifying the trace over the environment degrees of freedom with the temporal trace over the entire history of the system S.

Tadpole Cosmology: Self Tuning Without Degeneracy. (arXiv:2202.08672v1 [astro-ph.CO])

Degeneracy is a method to accommodate exact, low energy vacuum states in scalar-tensor gravitational models despite the presence of an arbitrarily large vacuum energy. However, this approach requires very particular combinations of scalar field and metric couplings in the Lagrangian. In this work we study departures from the restrictive degeneracy condition — starting from a fiducial model containing an exact Minkowski space solution, we break the degeneracy condition in numerous simple ways to test if the resulting models maintain certain key features — specifically the dynamical cancellation of a large vacuum energy by the scalar field and the existence of a low energy vacuum state. We highlight the role the tadpole plays in eliminating the fixed points of the dynamical system, generically rendering both the scalar field and metric time dependent. Our results indicate that when violating the degeneracy condition but preserving shift symmetry, the metric maintains an asymptotic Minkowski state, irrespective of the presence of the cosmological constant. In contrast, when shift symmetry is also broken the asymptotic behaviour can radically alter. Regardless, the non-degenerate models in this work share an attractive quality; harboring low energy, late-time asymptotic states that are independent of the vacuum energy. The tadpole allows for a broader class of non-degenerate, self-tuning models than was previously realized.

The Microphysics of Early Dark Energy. (arXiv:2202.08291v1 [astro-ph.CO])

Authors: Vivian I. SablaRobert R. Caldwell

Early Dark Energy (EDE) relies on scalar field dynamics to resolve the Hubble tension, by boosting the pre-recombination length scales and thereby raising the CMB-inferred value of the Hubble constant into agreement with late universe probes. However, the collateral effect of scalar field microphysics on the linear perturbation spectra appears to preclude a fully satisfactory solution. $H_0$ is not raised without the inclusion of a late universe prior, and the “$S_8$-tension”, a discrepancy between early- and late-universe measurements of the structure growth parameter, is exacerbated. What if EDE is not a scalar field? Here, we investigate whether different microphysics, encoded in the constitutive relationships between pressure and energy density fluctuations, can relieve these tensions. We show that EDE with an anisotropic sound speed can soften both the $H_0$ and $S_8$ tensions while still providing a quality fit to CMB data. Future observations from the CMB-S4 experiment may be able to distinguish the underlying microphysics at the $4\sigma$ level, and thereby test whether a scalar field or some richer physics is at work.

On Acceleration in Three Dimensions. (arXiv:2202.08823v1 [hep-th])

We go “back to basics”, studying accelerating systems in $2+1$ AdS gravity \textit{ab initio}. We find three classes of geometry, which we interpret by studying holographically their physical parameters. From these, we construct stationary, accelerating point particles; one-parameter extensions of the BTZ family resembling an accelerating black hole; and find new solutions including a novel accelerating “BTZ geometry” not continuously connected to the BTZ black hole as well as some black funnel solutions.

Present status and future challenges of non-interferometric tests of collapse models

Nature Physics, Published online: 17 February 2022; doi:10.1038/s41567-021-01489-5

Collapse models predict that the superposition principle of quantum mechanics breaks down at macroscopic scales. This Review discusses constraints on these models from non-interferometric experiments.

Newsworthy neutrinos

Nature Physics, Published online: 14 February 2022; doi:10.1038/s41567-022-01531-0

The analysis of the KATRIN Collaboration’s latest measurement campaign constrains the mass of the elusive neutrino with unprecedented sensitivity.

Relativistic Implications for Physical Copies of Conscious States

Knight, Andrew (2020) Relativistic Implications for Physical Copies of Conscious States. [Preprint]

Incorporating free energy models into mechanisms: the case of predictive processing under the free energy principle

Piekarski, Michał (2022) Incorporating free energy models into mechanisms: the case of predictive processing under the free energy principle. [Preprint]

The potential of a thick present through undefined causality and non-locality

Capurso, Alessandro (2022) The potential of a thick present through undefined causality and non-locality. [Preprint]