arXiv subjects

Daniel A. T. Vanzella

Publications and source records attributed to Daniel A. T. Vanzella.

At least 19 recordsLinked to original sources

arXiv2026

Larmor radiation as a witness to the Unruh effect

We discuss the emission of radiation from general sources in quantum scalar, electromagnetic and gravitational fields using the Rindler coordinate frame, which is suitable for a uniformly accelerated observer, in the Minkowski vacuum. In particular, we point out that, to recover the usual Larmor radiation in the interaction picture, it is necessary to incorporate the Unruh effect. Thus, the observation of classical Larmor radiation could be seen as vindicating the Unruh effect in the sense that it is not correctly reproduced in this context unless the Unruh effect is taken into account.

gr-qc↗

arXiv2025

The stress-energy tensor of an Unruh-DeWitt detector

We propose a model for a finite-size particle detector, which allows us to derive its stress-energy tensor. This tensor is obtained from a covariant Lagrangian that describes not only the quantum field that models the detector, $ϕ_{\text{d}}$, but also the systems responsible for its localization: a complex scalar field, $ψ_{\text{c}}$, and a perfect fluid. The local interaction between the detector and the complex field ensures the square integrability of the detector modes, while the fluid serves to define the spatial profile of $ψ_{\text{c}}$, localizing it in space. We then demonstrate that, under very general conditions, the resulting energy tensor -- incorporating all components of the system -- is physically reasonable and satisfies the energy conditions.

quant-ph↗

arXiv2025

Globally hyperbolic evaporating black hole and the information loss issue

We discuss the information loss issue for completely evaporating black holes in the context of a globally hyperbolic spacetime that maintains unchanged the entire semiclassical picture except for the "last evaporation breath," which pertains to full quantum gravity. Even though observers outside the black hole cannot access information that enters the horizon, there is no actual loss of information imposed by the spacetime causal structure since it is carried over from one Cauchy surface to the next (provided the local evolution is unitary).

gr-qc↗

arXiv2024

The number of fundamental constants from a spacetime-based perspective

We revisit Duff, Okun, and Veneziano's divergent views on the number of fundamental constants and argue that the issue can be set to rest by having spacetime as the starting point. This procedure disentangles the resolution in what depends on the assumed spacetime (whether relativistic or not) from the theories built over it. By defining that the number of fundamental constants equals the minimal number of independent standards necessary to express all observables, as assumed by Duff, Okun, and Veneziano, it is shown that the same units fixed by the apparatuses used to construct the spacetimes are enough to express all observables of the physical laws defined over them. As a result, the number of fundamental constants equals two in Galilei spacetime and one in relativistic spacetimes.

gr-qc↗

arXiv2023

Gravity theories with local energy-momentum exchange: a closer look at Rastall-like gravity

Einstein's famous equivalence principle is certainly one of the most striking features of the gravitational interaction. In a strict reading, it states that the effects of gravity can be made to disappear $locally$ by a convenient choice of reference frame. As a consequence, no covariantly-defined gravitational force should exist and energy-momentum of all matter and interaction fields combined, with gravity $excluded$, should be locally conserved. Although elegant, this separate conservation law represents a strong constraint on the dynamics of a gravitating system and it is only logical to question its naturality and observational basis. This is the purpose of the present work. For concreteness sake, we analyze, in the context of metric theories of gravity, the simplest phenomenological model which allows for local energy-momentum exchange between the spacetime and matter/interaction fields while preserving the seemingly more natural principle of general covariance. This concrete model turns out to be a generalization of the socalled Rastall's theory, with one important advantage: criticisms made to the latter, which are often used to dismiss it as a viable or interesting model, do not apply to the former in a universe containing "dark" ingredients, such as ours -- a connection which seems to have been overlooked thus far. We conclude by exploring the consequences of our Rastall-like model to standard (astrophysical and cosmological) gravitational scenarios.

gr-qc↗

arXiv2019

Probing the Unruh effect with an accelerated extended system

It has been proved in the context of quantum fields in Minkowski spacetime that the vacuum state is a thermal state according to uniformly accelerated observers -- a seminal result known as the Unruh effect. Recent claims, however, have challenged the validity of this result for extended systems, thus casting doubts on its physical reality. Here, we study the dynamics of an extended system, uniformly accelerated in the vacuum. We show that its reduced density matrix evolves to a Gibbs thermal state with local temperature given by the Unruh temperature. This proves that the vacuum state does induce thermalization of an accelerated extended system -- which is all one can expect of a legitimate thermal reservoir.

gr-qc↗

arXiv2018

Relativistic spring-mass system

The harmonic oscillator plays a central role in physics describing the dynamics of a wide range of systems close to stable equilibrium points. The nonrelativistic one-dimensional spring-mass system is considered a prototype representative of it. It is usually assumed and galvanized in textbooks that the equation of motion of a relativistic harmonic oscillator is given by the same equation as the nonrelativistic one with the mass $M$ at the tip multiplied by the relativistic factor $1/(1 - v^2/c^2)^{1/2}$. Although the solution of such an equation may depict some physical systems, it does not describe, in general, one-dimensional relativistic spring-mass oscillators under the influence of elastic forces. In recognition to the importance of such a system to physics, we fill a gap in the literature and offer a full relativistic treatment for a system composed of a spring attached to an inertial wall, holding a mass $M$ at the end.

physics.class-ph↗

arXiv2018

The Unruh effect for mixing neutrinos

Recently, the inverse $β$-decay rate calculated with respect to uniformly accelerated observers (experiencing the Unruh thermal bath) was revisited. Concerns have been raised regarding the compatibility of inertial and accelerated observers' results when neutrino mixing is taken into account. Here, we show that these concerns are unfounded by discussing the properties of the Unruh thermal bath with mixing neutrinos and explicitly calculating the decay rates according to both sets of observers and confirming that they are in agreement. The Unruh effect is perfectly valid for mixing neutrinos.

gr-qc↗

arXiv2018

Quest for a 'direct' observation of the Unruh effect with classical electrodynamics: In honor of Atsushi Higuchi 60th anniversary

The Unruh effect is essential to keep the consistency of quantum field theory in inertial and uniformly accelerated frames. Thus, the Unruh effect must be considered as well tested as quantum field theory itself. In spite of it, it would be nice to realize an experiment whose output could be directly interpreted in terms of the Unruh effect. This is not easy because the linear acceleration needed to reach a temperature 1 K is of order $10^{20} {\rm m/s}^2$. We discuss here a conceptually simple experiment reachable under present technology which may accomplish this goal. The inspiration for this proposal can be traced back to Atsushi Higuchi's Ph.D. thesis, which makes it particularly suitable to pay tribute to him on occasion of his $60^{\rm th}$ anniversary.

gr-qc↗

arXiv2017

Proposal for observing the Unruh effect with classical electrodynamics

The Unruh effect -- according to which linearly accelerated observers with proper acceleration a= constant in the (no-particle) vacuum state of inertial observers experience a thermal bath of particles with temperature $T_U = a \hbar / (2 πk_B c)$ -- has just completed its 40$^{th}$ anniversary. A 'direct' experimental confirmation of the Unruh effect has been seen with concern because the linear acceleration needed to reach a temperature $1 K$ is of order $10^{20} m/s^2$. Although the Unruh effect can be rigorously considered as well tested as free quantum field theory itself, it would be satisfying to observe some lab phenomenon which could evidence its existence. Here, we propose a simple experiment reachable under present technology whose result may be directly interpreted in terms of the Unruh thermal bath. Then, instead of waiting for experimentalists to perform the experiment, we use standard classical electrodynamics to anticipate its output and show that it reveals the presence of a thermal bath with temperature $T_U$ in the accelerated frame. Unless one is willing to question the validity of classical electrodynamics, this must be seen as a virtual observation of the Unruh effect. Regardless of doubts still raised by some voices, the Unruh effect lives among us.

gr-qc↗

arXiv2016

Rescuing the concept of swimming in curved spacetime

It has been argued that an extended, quasi-rigid body evolving freely in curved spacetime can deviate from its natural trajectory by simply performing cyclic deformations. More interestingly, in the limit of rapid cycles, the amount of deviation, per cycle, would depend on the sequence of deformations but not on how fast they are performed -- like the motion of a swimmer at low Reynolds number. Here, however, we show that the original analysis which supported this idea is inappropriate to investigate the motion of extended bodies in the context of general relativity, rendering its quantitative results invalid and casting doubts on the reality of this swimming effect. We illustrate this by showing that the original analysis leads to a non-zero deviation even in a scenario where no swimming can possibly occur. Notwithstanding, by applying a fully covariant, local formalism, we show that swimming in curved spacetime is indeed possible and that, in general, its magnitude can be of the same order as (fortuitously) anticipated -- although it is highly suppressed in the particular scenario where it was originally investigated.

gr-qc↗

arXiv2016

Instability of nonminimally coupled scalar fields in the spacetime of thin charged shells

We investigate the stability of a free scalar field nonminimally coupled to gravity under linear perturbations in the spacetime of a charged spherical shell. Our analysis is performed in the context of quantum field theory in curved spacetimes. This paper completes previous analyses which considered the exponential enhancement of vacuum fluctuations in the spacetime of massive shells.

gr-qc↗

arXiv2015

From quantum to classical instability in relativistic stars

It has been shown that gravitational fields produced by realistic classical-matter distributions can force quantum vacuum fluctuations of some nonminimally coupled free scalar fields to undergo a phase of exponential growth. The consequences of this unstable phase to the background spacetime have not been addressed so far due to known difficulties concerning backreaction in semiclassical gravity. It seems reasonable to believe, however, that the quantum fluctuations will "classicalize" when they become large enough, after which backreaction can be treated in the general-relativistic context. Here we investigate the emergence of a classical regime out of the quantum field evolution during the unstable phase. By studying the appearance of classical correlations and loss of quantum coherence, we show that by the time backreaction becomes important the system already behaves classically. Consequently, the gravity-induced instability leads naturally to initial conditions for the eventual classical description of the backreaction. Our results give support to previous analyses which treat classically the instability of scalar fields in the spacetime of relativistic stars, regardless whether the instability is triggered by classical or quantum perturbations.

gr-qc↗

arXiv2014

Instability of nonminimally coupled scalar fields in the spacetime of slowly rotating compact objects

Nonminimally coupled free scalar fields may be unstable in the spacetime of compact objects. Such instability can be triggered by classical seeds or, more simply, by quantum fluctuations giving rise to the so-called {\em vacuum awakening effect}. Here, we investigate how the parameter space which characterizes the instability is affected when the object gains some rotation. For this purpose, we focus on the stability analysis of nonminimally coupled scalar fields in the spacetime of slowly spinning matter shells.

gr-qc↗

arXiv2014

Quantum versus classical instability of scalar fields in curved backgrounds

General-relativistic stable spacetimes can be made unstable under the presence of certain nonminimally coupled free scalar fields. In this paper, we analyze the evolution of linear scalar-field perturbations in spherically symmetric spacetimes and compare the classical stability analysis with a recently discussed quantum field one. In particular, it is shown that vacuum fluctuations lead to natural seeds for the unstable phase, whereas in the classical framework the presence of such seeds in the initial conditions must be assumed.

gr-qc↗

arXiv2013

Reply to "Comment on `Quantum versus classical instability of scalar fields in curved backgrounds'"

We show that the reasoning which led the author of arXiv:1310.6252 to reach his conclusions relies on an incorrect criterion for the existence of normalizable bound solutions. We reinforce that the general result derived in the Appendix of our paper (arXiv:1310.2185), namely, that "there are no tachyonic (i.e., unstable) modes for minimally coupled scalar fields in asymptotically flat spherically symmetric static spacetimes containing no horizons" is indeed correct.

gr-qc↗

arXiv2013

Awaking the vacuum with spheroidal shells

It has been shown that well-behaved spacetimes may induce the vacuum fluctuations of some nonminimally coupled free scalar fields to go through a phase of exponential growth. Here, we discuss this mechanism in the context of spheroidal thin shells emphasizing the consequences of deviations from spherical symmetry.

gr-qc↗

arXiv2012

Comment on "Trouble with the Lorentz law of force: Incompatibility with special relativity and momentum conservation"

It has been recently argued that the Lorentz force is incompatible with Special Relativity and should be amended in the presence of magnetization and polarization in order to avoid a paradox involving a magnet in the presence of an electric field. Here we stress the well-known fact among relativists that such an incompatibility is simply impossible and show that the appearance of such a "paradox" is a mere consequence of not fully considering the relativistic consequences of the covariant form of the Lorentz force. It should be mentioned, though, that this criticism does not invalidate the debate on which is the law of force followed by Nature, which is an experimental issue.

physics.gen-ph↗