journal club on aspects of information, quantum theory, and gravity
We have had 53 seminars so far! Here is the complete list.
25 Mar 2026
Andrew J. Groszek and Charles W. Woffinden
What if every known communication channel were blocked: no radio, no light, no sound? Is physics out of options? In this talk, we’ll argue that it isn’t. We explore an unconventional idea: using gravity itself as a wireless communication channel. By simply moving a mass back and forth, a sender can modulate the local static gravitational field, which a distant receiver can detect using an off-the-shelf gravimeter. Unlike electromagnetic signals, gravity cannot be shielded, screened, or turned off—and that makes it a uniquely “unblockable” carrier of information.
We introduce the basic physics behind gravitational communication, show how it can be analysed as an antenna problem with well-defined data rates, directionality, and noise, and then present an experimental demonstration. Using nothing more exotic than an antiquated elevator and a 1980s gravimeter, we have successfully transmitted a 49-bit gravitational message through a brick wall. While this is not the future of high-bandwidth Wi-Fi, it opens up a surprisingly rich intersection of gravitation, information theory, and experimental ingenuity—and raises the delightful possibility that you really could communicate using gravity.
12 Mar 2026
Motivated by the increasing connections between information theory and high-energy physics, particularly in the context of the AdS/CFT correspondence, we explore the information geometry associated to a variety of simple systems. By studying their Fisher metrics, we derive some general lessons that may have important implications for the application of information geometry in holography. We begin by demonstrating that the symmetries of the physical theory under study play a strong role in the resulting geometry, and that the appearance of an AdS metric is a relatively general feature. We then investigate what information the Fisher metric retains about the physics of the underlying theory by studying the geometry for both the classical 2d Ising model and the corresponding 1d free fermion theory, and find that the curvature diverges precisely at the phase transition on both sides. We discuss the differences that result from placing a metric on the space of theories vs. states, using the example of coherent free fermion states. We compare the latter to the metric on the space of coherent free boson states and show that in both cases the metric is determined by the symmetries of the corresponding density matrix. We also clarify some misconceptions in the literature pertaining to different notions of flatness associated to metric and non-metric connections, with implications for how one interprets the curvature of the geometry. Our results indicate that in general, caution is needed when connecting the AdS geometry arising from certain models with the AdS/CFT correspondence, and seek to provide a useful collection of guidelines for future progress in this exciting area.
26 Feb 2026
The infrared sector of field theories exhibits a universal structure that appears to be inherently classical and is closely connected to the phenomenon of memory in asymptotic radiation. Gravitational and electromagnetic memory encode permanent imprints within the radiation carrying gauge fields and are intimately related to asymptotic symmetries and soft theorems. Despite their ubiquity and theoretical robustness, however, these effects have not yet been experimentally verified. In the context of gravitational radiation, frequency-band-limited detectors are not directly sensitive to the defining net memory offset, but only to the associated time-dependent transition. On the other hand, the amplitude of the memory offset in electromagnetic radiation appears too small to be detectable in realistic experiments.
In this talk, I will present recent progress on both these fronts. First, I will discuss a proper theoretical modelling of gravitational memory that enables a confident detection claim with next-generation gravitational-wave detectors. Second, I will present a new and realistically realizable Poincaré symmetry-breaking mechanism that can amplify the amplitude of electromagnetic memory by several orders of magnitude.
27 Nov 2025
Primordial black holes (PBHs) offer a distinctive link between gravitational physics and particle phenomenology. Formed in the early Universe through mechanisms such as large primordial inhomogeneities, phase transitions, topological defects, or bubble collisions, they can span masses from sub-planetary scales to many solar masses—far beyond the range produced by stellar collapse. Their Hawking evaporation provides a concrete manifestation of gravitational particle production, generating a thermal spectrum of all kinematically accessible species and probing high-energy physics beyond laboratory reach. PBHs with initial masses near $10^{15}$ g are of particular interest, as they would be completing their evaporation today in a brief, explosive phase marked by rapidly rising temperatures and intense emission of high-energy particles.
Neutrinos furnish two complementary avenues to investigate such objects: they serve as direct messengers of Hawking radiation, still unverified experimentally, and they allow PBH evaporation to be considered as a potential explanation for rare ultra-high-energy neutrino events. In this context, the hypothesis that the ~220 PeV KM3NeT event arose from a nearby PBH burst has been quantitatively assessed. Incorporating multi-messenger constraints, including expected gamma-ray and neutrino coincident signals, indicates that such an interpretation is strongly disfavored within minimal four-dimensional evaporation scenarios. These studies nevertheless establish how evaporating PBHs provide a quantitative and testable interface between gravitational particle production and high-energy neutrino phenomenology.
25 Sep 2025
We analytically investigate the effects of gravitational waves on the Casimir force between two uncharged metallic plates. The gravitational contribution to the electromagnetic vacuum energy is computed using covariant path integral quantization in a gravitational wave background. Our findings reveal that gravitons are absorbed by the cavity, inducing a repulsive correction to the Casimir force and a decrease in the von Neumann quantum entropy.
We demonstrate that the gravitational wave interaction induces a permanent modification of the vacuum polarization through the absorption of gravitons by confined photons. We establish a novel link between the Weinberg soft graviton theorem and the constant shift in the Casimir energy, further confirming the gravitational memory effect in Casimir cavities. Our results suggest that Casimir cavities serve as sensitive probes of quantum and classical gravity.
11 Sep 2025
We study quasinormal modes of massive scalar perturbations in Kerr black holes using the isomonodromic method. For arbitrary scalar masses $M\mu$ and black hole spins $a/M,$ we numerically determine the quasinormal frequencies for various orbital $\ell$, azimuthal $m$, and overtone $n$ numbers. In particular, we derive an analytic expression for frequencies of the zero-damping modes near the extremal limit $a/M \to 1$. For $\ell=m=1$, we reveal that the fundamental mode becomes a damped mode (rather than a zero-damping mode) if the scalar field is sufficiently heavy. By exploring the parameter space, we find numerical evidence for level-crossing between the longest-living mode and the first overtone at an exceptional point $(M\mu)_c \simeq 0.3704981$ and $(a/M)_c \simeq 0.9994660$.
05 Aug 2025
Throughout the last decade there has been a rising interest in the infrared structure of gravity and gauge theory. This was partially motivated by the unforeseen correspondences between many different results from the 1960s and 1970s—namely asymptotic symmetries, soft theorems, and memory effects—but has found many applications in theoretical high-energy physics, mathematical physics, and general relativity and gravitation. In this seminar (which is a preview for my qualifying exam), I will give an overview of all of these topics, with a particular interest toward the work I have been developing with Landulfo.
15 Jul 2025
We review Kerr’s paper which raises questions regarding the relation between trapped surfaces, null geodesics of finite affine length, and spacetime singularities. His main argument lies on counter-examples to the theorems proven by Hawking and Penrose, in which he analyzes principal null geodesic congruences on Kerr spacetime. However, it is evident that his argument is based on the behavior of a bad coordinate system, and thus, does not represent a fundamental property of spacetime. Nevertheless, Kerr raises an important point on how to actually interpret the singularity theorems, in particular, the claim that all black holes contain singularities. Following this discussion, we also pose a question on the theoretical prediction of black holes and their experimental confirmation.
03 Jun 2025
Levy Bruno do Nascimento Batista
Recently, Wald and collaborators have proposed a novel source of decoherence of charges and masses in a spatial superposition state when placed in a spacetime with Killing horizons. They argued that the effect is primarily caused by the emission of soft particles through these horizons, which will eventually happen even if the process is assumed to be adiabatic. We investigate this claim in a rather different setting, with a gapless detector interacting with a massive scalar field. We introduce how such interaction is described and how to quantify the resulting decoherence. Then, we investigate our proposed model in two seminal cases: when the components of the spatial superposition are inertial and when they are uniformly accelerated in Minkowski space. Working in an analogous regime, we compare our results to those obtained by Wald, highlighting their similarities. Finally, we discuss what we sought to improve with our model and how it might clarify some questions left open in the original work.
20 May 2025
Maitá C. Micol
In this seminar, we will continue the discussion on the covariant phase space (CPS) method, with concrete examples in scalar field theory, electromagnetism and general relativity. The goal of this talk is to develop the intuition behind this formalism and to give a nice application with the derivation of the first law of black hole thermodynamics.
06 May 2025
One of the most significant debates of our time is whether our macroscopic world (i) naturally emerges from quantum mechanics or (ii) requires new physics. We argue for the latter and propose a simple gravitational self-decoherence mechanism. For this purpose, we postulate the existence of a Heisenberg cut such that particles with masses m much smaller and larger than a critical mass $M_C$ (of the order of the Planck mass $M_P$) would be necessarily treated according to quantum and classical rules, respectively. Our effective model is designed to capture the new physics that free quantum particles would experience as their masses approach $M_C$. The purity loss for free quantum particles is evaluated and shown to be highly inefficient for quantum particles with $m « M_C$ but very effective for those with $m \sim M_C$. The physical picture behind it is that coherence would (easily) leak from heavy enough particles to (non-observable) spacetime quantum degrees of freedom. Finally, we contextualize our proposal with state-of-the-art experiments and show how it can be tested in a future Stern-Gerlach-like experiment.
22 Apr 2025
When all thermonuclear sources of energy are exhausted a sufficiently heavy star will collapse. Unless fission due to rotation, the radiation of mass, or the blowing off of mass by radiation, reduce the star’s mass to the order of that of the sun, this contraction will continue indefinitely. In the present paper we study the solutions of the gravitational field equations which describe this process. In I, general and qualitative arguments are given on the behavior of the metrical tensor as the contraction progresses: the radius of the star approaches asymptotically its gravitational radius; light from the surface of the star is progressively reddened, and can escape over a progressively narrower range of angles. In II, an analytic solution of the field equations confirming these general arguments is obtained for the case that the pressure within the star can be neglected. The total time of collapse for an observer comoving with the stellar matter is finite, and for this idealized case and typical stellar masses, of the order of a day; an external observer sees the star asymptotically shrinking to its gravitational radius.
08 Apr 2025
We present an explicit solution to the Einstein field equations sourced by a stress tensor with identically zero energy density. We refer to the solution as the “platypus metric” due to its intriguing properties. It exemplifies a key difference between Newton and Einstein gravity: while Newtonian gravity is sourced exclusively by mass, pressure alone can source relativistic gravitational fields. We also discuss various qualitative aspects of the platypus solution. For example, it has a stable photon ring, all non-radial null geodesics are bounded, it is locally conformally flat, spatially flat with vanishing extrinsic curvature, among other properties. While the platypus spacetime does not appear to represent any physical scenario in the universe, it is an interesting example of the strange behaviors one can obtain within general relativity.
25 Mar 2025
The swirling universe solution is an exact solution of Einstein’s field equation, it is stationary and axially symmetric space-time that is fully characterized by one parameter, the swirling parameter, which is to be understood as the background rotation. The “swirling” name actually came from a preliminary study on the effects of the background on the motion of test particles. This space-time possesses some interesting characteristics, like the space-time frame-dragging that changes its sign with respect to the equatorial plane and the geometry of the ergoregions, which, differently from the well-known Kerr space-time, are two disconnected, non-compact patches, above and below the equatorial plane, that extend all the way to infinity. In this talk, I will demonstrate that the equations of motion for a test particle in the swirling universe can be decoupled using the Hamilton-Jacobi formalism, where a fourth constant of motion is obtained, a method akin to the Carter constant for the Kerr geometry; thus, the geodesic equations can then be analytically integrated using elementary and elliptic functions. A typical orbit is then bounded in the radial direction and escapes to infinity in the z-direction. However, once a Schwarzschild black hole is immersed into a swirling universe, the geodesic equations can no longer be decoupled, and hence, a numerical approach is required to study the motion of test particles, suggesting the emergence of chaotic orbits. Furthermore, this solution has been generalized to include external electromagnetic fields as well, in a solution that is coined as the “electromagnetic-swirling universe”. The motion of charged particles in this novel solution can also be analytically integrated using a similar fashion.
25 Feb 2025
Maitá C. Micol
In this talk I will give a brief introduction and motivation for the covariant phase space (CPS) method. We will be interested in the application of this framework to the study of symmetries and conserved (corner!) charges in gauge theories, with a special focus on the diffeomorphism symmetry of General Relativity. We will start by reviewing the necessary mathematical background from symplectic geometry and discussing the relation between phase spaces in particle mechanics and the CPS method. We conclude by revisiting Wald’s seminal paper on the Bekenstein-Hawking entropy as a Noether charge, giving a new perspective on the first law of BH mechanics.
11 Feb 2025
Caio César Rodrigues Evangelista
General Relativity(GR) is the theory of gravity that better describes how objects fall under the presence of strong gravitational fields. Describing body’s trajectories in the presence of compact object, predicting gravity waves, and in particular, the existence of black holes. The observations made by the Event Horizon Telescope(EHT), about the black holes in the centre of the galaxy M87, and in centre of the Milk Way, Sgr A*, gave rise to a new era in testing black hole theory and GR itself by means of luminosity coming from electromagnetic radiation of the accretion disk. Theoretical analysis and General Relativistic Magnetohydrodynamics(GRMHD) universally agree on two facts: The existence of a photon ring, and a luminosity gradient descent caused by the light rays emiited by the accretion disk that intersect the event horizon(EH), and thus, don’t ever get to the detector. Hence, forming a shadow. This seminar aims to review computational probes around the fact that because of the very few available test within GR, specially for black holes, it becomes extremely necessary the use of ’Shadowgraphy’, e.g, have some better observational viability in astrophysical scenarios, through numerical simulations for light ray geodesics, i.e, the ray tracing protocol, as well as simulating the properties of the accretion disk by the intensity profile distribution functions, so that one can extract optical, geometrical and emission information of the accretion disk.
28 Jan 2025
The memory effect is a simple, yet profound, prediction of general relativity and other field theories with massless propagating modes. Over the last decade, it has been noticed that it bears close connections to topics in the infrared structure of general relativity and gauge theory, and it is expected to be measured in future gravitational wave detectors. In this seminar, I provide a pedagogical introduction to linear memory in many theories of interest, ranging from the scalar wave equation to modified gravity.
16 Sep 2024
The major problem in quantum gravity tends to be seen as the difficulty in obtaining an ultraviolet completion of general relativity. Pure general relativity is a non-renormalizable theory, and therefore loses its predictability on scales close to the Planck scale. Interestingly, however, adding quadratic terms in the curvatures to the action of gravity makes the theory perturbatively renormalizable, a fact known at least since the 1970s (and suspected since the 1960s). In this series of lectures, I intend to revisit general concepts of renormalization in curved spacetimes (in the case of both QFTCS and general relativity) and argue that quadratic terms are to be expected in a quantum theory of gravity. Having done so, I will discuss the advantages, disadvantages, and my criticisms of the quadratic gravity program.
05 Sep 2024
The major problem in quantum gravity tends to be seen as the difficulty in obtaining an ultraviolet completion of general relativity. Pure general relativity is a non-renormalizable theory, and therefore loses its predictability on scales close to the Planck scale. Interestingly, however, adding quadratic terms in the curvatures to the action of gravity makes the theory perturbatively renormalizable, a fact known at least since the 1970s (and suspected since the 1960s). In this series of lectures, I intend to revisit general concepts of renormalization in curved spacetimes (in the case of both QFTCS and general relativity) and argue that quadratic terms are to be expected in a quantum theory of gravity. Having done so, I will discuss the advantages, disadvantages, and my criticisms of the quadratic gravity program.
22 Aug 2024
The major problem in quantum gravity tends to be seen as the difficulty in obtaining an ultraviolet completion of general relativity. Pure general relativity is a non-renormalizable theory, and therefore loses its predictability on scales close to the Planck scale. Interestingly, however, adding quadratic terms in the curvatures to the action of gravity makes the theory perturbatively renormalizable, a fact known at least since the 1970s (and suspected since the 1960s). In this series of lectures, I intend to revisit general concepts of renormalization in curved spacetimes (in the case of both QFTCS and general relativity) and argue that quadratic terms are to be expected in a quantum theory of gravity. Having done so, I will discuss the advantages, disadvantages, and my criticisms of the quadratic gravity program.
08 Aug 2024
The major problem in quantum gravity tends to be seen as the difficulty in obtaining an ultraviolet completion of general relativity. Pure general relativity is a non-renormalizable theory, and therefore loses its predictability on scales close to the Planck scale. Interestingly, however, adding quadratic terms in the curvatures to the action of gravity makes the theory perturbatively renormalizable, a fact known at least since the 1970s (and suspected since the 1960s). In this series of lectures, I intend to revisit general concepts of renormalization in curved spacetimes (in the case of both QFTCS and general relativity) and argue that quadratic terms are to be expected in a quantum theory of gravity. Having done so, I will discuss the advantages, disadvantages, and my criticisms of the quadratic gravity program.
25 Jul 2024
Stephen Hawking’s derivation of Hawking radiation relied on one particular spacetime model, that of a star collapsing into a black hole which then remains in existence forever. He then argued that Hawking radiation implies this model should be thrown away in favour of a different model, that of an evaporating black hole. This aspect of Hawking’s argument is an example of an idealization that is pervasive in the literature on black hole thermodynamics, but which has not yet been widely discussed by philosophers. The aim of this paper is to clarify the nature of Hawking’s idealization, and to show a sense in which it leads to a paradox. After identifying this idealization paradox in classic derivations of Hawking radiation, I go on to show how various research programmes in black hole thermodynamics can be viewed as possible resolutions to the paradox. I give an initial analysis of the prospects for success of these various resolutions, and show how they shed light on both the philosophical foundations of both Hawking radiation on the nature of idealizations in physics.
06 Jun 2024
The memory effect is a prediction in classical general relativity that consists in the fact that, upon the passage of a gravitational wave, a pair of nearby inertial detectors will be permanently displaced. In this seminar, I will review the basic ideas behind the linear memory effect and discuss how it is connected to other infrared aspects of general relativity, such as Weinberg’s soft graviton theorem and the Bondi–Metzner–Sachs group.
23 May 2024
A equação de Schrödinger-Newton nasceu na década de 1980 como uma proposta para explicar o comportamento de objetos macroscópicos através do paradigma quântico. Nessa descrição, partículas quânticas não-relativísticas estariam sujeitas a um potencial gravitacional de auto-interação e, com isso, passariam a ser governadas por uma dinâmica não-linear. Apesar de exótica, a equação de Schrödinger-Newton representa uma decorrência direta da equação semi-clássica de Einstein. Neste sentido, ao considerarmos que os graus de liberdade espaço-temporais são fundamentalmente clássicos e se acoplam com matéria-energia pelo valor esperado do tensor de energia-momento, a equação de Schrödinger-Newton aparece como uma descrição natural para partículas não-relativísticas.
11 Apr 2024
Explaining the behavior of macroscopic objects from the point of view of the quantum paradigm has challenged the scientific community for the past century. A mechanism of gravitational self-interaction, governed by the so-called Schrödinger-Newton equation, is among the proposals that aim to shed some light on it. Despite all efforts, this mechanism has been proven difficult to probe. Here, we consider a Stern-Gerlach-like experiment to try it out. The Schrödinger-Newton equation can be analytically solved under certain proper conditions, and a change-of-phase effect induced by the gravitational self-interacting potential can be calculated.
28 Mar 2024
The Bondi–Van der Burg–Metzner–Sachs (BMS) group is the group of symmetries of null infinity in asymptotically flat spacetimes. While one could naively expect for this group to be simply the Poincaré group, so that general relativity reduces to special relativity far away from any matter sources, it turns out that the correct group is an infinite-dimensional extension of the Poincaré group. In this pedagogical review, we discuss the main ideas behind the BMS group and provide an explicit derivation of it. We also discuss its physical consequences and applications in different topics in fundamental physics.
14 Mar 2024
The Bondi–Van der Burg–Metzner–Sachs (BMS) group is the group of symmetries of null infinity in asymptotically flat spacetimes. While one could naively expect for this group to be simply the Poincaré group, so that general relativity reduces to special relativity far away from any matter sources, it turns out that the correct group is an infinite-dimensional extension of the Poincaré group. In this pedagogical review, we discuss the main ideas behind the BMS group and provide an explicit derivation of it. We also discuss its physical consequences and applications in different topics in fundamental physics.
29 Feb 2024
We discuss the loss of information in black holes in the context of a globally hyperbolic spacetime that maintains unchanged the whole semiclassical picture except for the ‘‘last evaporation breath’’, which actually pertains to full quantum gravity. Although observers outside the black hole will not have access to information that enters the horizon, no information is lost in the sense it is carried over from one Cauchy surface to the next one (provided the evolution is unitary). In this token, the loss of information in black holes becomes as benign as in the usual classical stellar collapse into a non-evaporating black hole.
15 Feb 2024
The boundary between Quantum Theory and General Relativity has been an ubiquitous discussion in the physics community since the earliest days of these models. Finding a consistent way to combine them is one of the greatest quests in contemporary physics, and recent quantum-informational approaches are playing an important role to understand how we should describe the symbiosis between these theories [1,2]. In this seminar I intend to give a brief overview of why it’s interesting to employ Quantum Foundations techniques to approach the non-classical nature of gravity, and, in particular, to emphasize the use of Generalized Probabilistic Theories as strong apparatus to extract no-go theorems regarding classicality, entanglement, and (if time permits) reversibility.
01 Feb 2024
The boundary between Quantum Theory and General Relativity has been an ubiquitous discussion in the physics community since the earliest days of these models. Finding a consistent way to combine them is one of the greatest quests in contemporary physics, and recent quantum-informational approaches are playing an important role to understand how we should describe the symbiosis between these theories [1,2]. In this seminar I intend to give a brief overview of why it’s interesting to employ Quantum Foundations techniques to approach the non-classical nature of gravity, and, in particular, to emphasize the use of Generalized Probabilistic Theories as strong apparatus to extract no-go theorems regarding classicality, entanglement, and (if time permits) reversibility.
11 Sep 2023
The main goal of this paper is to provide some insight into how promising the Schrödinger-Newton equation would be to explain the emergence of classicality. Based on the similarity of the Newton and Coulomb potentials, we add an electric self-interacting term to the Schrödinger-Newton equation for the hydrogen atom. Our results rule out the possibility that single electrons self-interact through their electromagnetic field. Next, we use the hydrogen atom to get insight into the intrinsic difficulty of testing the Schrödinger-Newton equation itself and conclude that the Planck scale must be approached before sound constraints are established. Although our results cannot be used to rule out the Schrödinger-Newton equation at all, they might be seen as disfavoring it if we base our reasoning on the resemblance between the gravitational and electromagnetic interactions at low energies.
14 Aug 2023
We show that if a massive (or charged) body is put in a quantum superposition of spatially separated states in the vicinity of a black hole or cosmological horizon, the mere presence of the horizon will eventually destroy the coherence of the superposition. This occurs because, in effect, the long-range fields sourced by the superposition register on the horizon which forces the emission of entangling “soft gravitons/photons” through the horizon. This enables the horizon to harvest “which path” information about the superposition. We provide estimates of the decoherence time for such quantum superpositions in the presence of a black hole and cosmological horizon. Additionally, we show that this decoherence is distinct from—and larger than—the decoherence resulting from the presence of thermal radiation from the horizon. Finally, we further sharpen and generalize this mechanism by recasting the gedankenexperiment in the language of (approximate) quantum error correction. This yields a complementary picture where the decoherence is due to an “eavesdropper” (Eve) inside the black hole attempting to obtain “which path” information by measuring the long-range fields of the superposed body. We compute the quantum fidelity to determine the amount of information such an interior observer can obtain, and use the information-disturbance tradeoff to give a direct relationship between the Eve’s information and the decoherence of the superposition in the exterior. In particular, we show that the decoherence of the superposition corresponds to the “optimal” measurement performable in the black hole interior. We comment on how this phenomenon can be interpreted as a low-energy probe of the so-called central dogmas of black hole and cosmological horizons.
16 Dec 2022
Eduardo Amâncio Barbosa Oliveira
The manner in which states of some quantum systems become effectively classical is of great significance for the foundations of quantum physics, as well as for problems of practical interest such as quantum engineering. In the past two decades it has become increasingly clear that many (perhaps all) of the symptoms of classicality can be induced in quantum systems by their environments. Thus decoherence is caused by the interaction in which the environment in effect monitors certain observables of the system, destroying coherence between the pointer states corresponding to their eigenvalues. This leads to environment-induced superselection or einselection, a quantum process associated with selective loss of information. Einselection enforces classicality by imposing an effective ban on the vast majority of the Hilbert space, eliminating especially the flagrantly nonlocal “Schrödinger-cat states.” The classical structure of phase space emerges from the quantum Hilbert space in the appropriate macroscopic limit. Combination of einselection with dynamics leads to the idealizations of a point and of a classical trajectory. In measurements, einselection replaces quantum entanglement between the apparatus and the measured system with the classical correlation. Only the preferred pointer observable of the apparatus can store information that has predictive power. When the measured quantum system is microscopic and isolated, this restriction on the predictive utility of its correlations with the macroscopic apparatus results in the effective “collapse of the wave packet.” Spreading of the correlations with the effectively classical pointer states throughout the environment allows one to understand “classical reality” as a property based on the relatively objective existence of the einselected states. Effectively classical pointer states can be “found out” without being re-prepared, e.g, by intercepting the information already present in the environment. The redundancy of the records of pointer states in the environment (which can be thought of as their “fitness” in the Darwinian sense) is a measure of their classicality.
30 Nov 2022
Eduardo Amâncio Barbosa Oliveira
The manner in which states of some quantum systems become effectively classical is of great significance for the foundations of quantum physics, as well as for problems of practical interest such as quantum engineering. In the past two decades it has become increasingly clear that many (perhaps all) of the symptoms of classicality can be induced in quantum systems by their environments. Thus decoherence is caused by the interaction in which the environment in effect monitors certain observables of the system, destroying coherence between the pointer states corresponding to their eigenvalues. This leads to environment-induced superselection or einselection, a quantum process associated with selective loss of information. Einselection enforces classicality by imposing an effective ban on the vast majority of the Hilbert space, eliminating especially the flagrantly nonlocal “Schrödinger-cat states.” The classical structure of phase space emerges from the quantum Hilbert space in the appropriate macroscopic limit. Combination of einselection with dynamics leads to the idealizations of a point and of a classical trajectory. In measurements, einselection replaces quantum entanglement between the apparatus and the measured system with the classical correlation. Only the preferred pointer observable of the apparatus can store information that has predictive power. When the measured quantum system is microscopic and isolated, this restriction on the predictive utility of its correlations with the macroscopic apparatus results in the effective “collapse of the wave packet.” Spreading of the correlations with the effectively classical pointer states throughout the environment allows one to understand “classical reality” as a property based on the relatively objective existence of the einselected states. Effectively classical pointer states can be “found out” without being re-prepared, e.g, by intercepting the information already present in the environment. The redundancy of the records of pointer states in the environment (which can be thought of as their “fitness” in the Darwinian sense) is a measure of their classicality.
19 Oct 2022
Newton’s equations of motion tell us that a mass at rest at the apex of a dome with the shape specified here can spontaneously move. It has been suggested that this indeterminism should be discounted since it draws on an incomplete rendering of Newtonian physics, or it is “unphysical,” or it employs illicit idealizations. I analyze and reject each of these reasons.
21 Sep 2022
João Lucas Rodrigues
This paper discusses correlations between the results of measurements performed on physical systems which are widely separated, but have interacted in the past. It is shown that quantum correlations are stronger than classical correlations. This property leads to the following paradox, known as Bell’s theorem: Let us assume that the outcome of an experiment performed on one of the systems is independent of the choice of the experiment performed on the other. Now, let us try to imagine the results of alternative measurements, which could have been performed on the same systems instead of the actual measurements. Then there is no way of contriving these hypothetical results so that they will satisfy all the quantum correlations with the results of the actual measurements. However, the weaker classical correlations can be satisfied.
09 Sep 2022
We provide a setup by which one can recover the geometry of spacetime from local measurements of quantum particle detectors coupled to a quantum field. Concretely, we show how one can recover the field’s correlation function from measurements on the detectors. Then, we are able to recover the invariant spacetime interval from the measurement outcomes, and hence reconstruct a notion of spacetime metric. This suggest that quantum particle detectors are the experimentally accessible devices that could replace the classical ‘rulers’ and ‘clocks’ of general relativity.
24 Jun 2022
A great deal of evidence has been mounting over the years showing a deep connection between acceleration, radiation, and the Unruh effect. Indeed, the fact that the Unruh effect can be codified in the Larmor radiation emitted by the charge was used to propose an experiment to experimentally confirm the existence of the Unruh thermal bath. However, such connection has two puzzling issues: (1) how the quantum Unruh effect can be codified in the classical Larmor radiation and (2) the fundamental role played by zero-Rindler-energy modes of the Unruh thermal bath in such a context. Here we generalize a recent analysis made for the scalar case to the more realistic case of Maxwell electrodynamics and settle these two puzzling issues.
29 Apr 2022
A mecânica quântica é de 1926. Logo depois começou-se a busca pela quantização dos campos fundamentais. Um dos primeiros a falar sobre a necessidade de se quantizar a gravitação foi o russo Matvei Bronstein, em 1933, pouco antes de ser preso (1937) e executado (1938), no grande expurgo de Stalin. Desde então, a linha adotada pelos físicos para quantizar a gravitação foi inspirada em maior ou menor escala pelo sucesso da eletrodinâmica quântica, onde se assume um espaço-tempo de fundo sobre o qual perturbações são quantizadas (sendo que diferentes escolas diferiam sobretudo sobre quais seriam os graus de liberdade que deveriam ser quantizados). Mas depois de 90 anos de um fracasso miserável, as tentativas têm sido cada vez mais desesperadas – desde se culpar a quântica até se assumir que gravitação seria clássica. A minha ideia aqui é discutir mais o que não fazer do que dizer o que fazer. Minha única convicção está enraizada no fato que um gênio das cavernas faria muito menos à ciência dedicando a vida para chegar ao Sol, do que seu amigo mais modesto que procurasse entender o que é o foguinho que ele gera diariamente friccionando gravetos.
01 Apr 2022
It is no secret that quantum mechanics often produces weirdness. While some of its unexpected predictions are seemingly inconsequential, sidestepping confrontation with experiments, others could, in principle, be game-changers. Or couldn’t they? In this talk, we focus on quantum mechanical violations of the classical energy conditions, i.e., reasonable expectations on the classical matter that, for example, prevent one from building a time machine to meet oneself in the past. Without compliance with energy conditions, it may seem intriguing why there aren’t negative masses roaming around us. Based on the Casimir effect, we argue that it may be fundamentally impossible to observe an object with a negative mass, despite the presence of static negative energy densities. We identify the specific elements that outweigh the Casimir energies, making the entire apparatus yield an attractive gravitational force on distant bodies.
18 Mar 2022
When quantum fields are subject to the effects of gravitational fields, interesting effects happen. Among them, one of the most famous is Hawking’s prediction that black holes might evaporate, a process which allows the evolution of the field’s quantum state from a pure state to a mixed state. This has led a number of physicists to dispute the result claiming it goes against the principles of Quantum Mechanics, giving this prediction the name “Black Hole Information Loss Paradox” as a consequence. We shall quickly review the “paradox” and its possible “solutions” by following Unruh & Wald 2017, which concludes there is a single “conservative” approach to avoid the conclusion: the possibility that the lost information comes out near the end of the evaporation process as a final “burst”. This approach has some complications due to the need to release great amounts of information without much energy available. However, recent work with radiation due to moving mirrors suggests it might be possible if the information comes out by means of correlations of the Hawking radiation with vacuum fluctuations at late times. In most of the seminar we shall then see how Wald 2019 concludes after a deeper analysis that this approach is very unlikely to work in the black hole case, hence leading us to either accept the “paradox” as a prediction or to radically modify our knowledge of Physics to avoid it.
04 Mar 2022
When quantum fields are subject to the effects of gravitational fields, interesting effects happen. Among them, one of the most famous is Hawking’s prediction that black holes might evaporate, a process which allows the evolution of the field’s quantum state from a pure state to a mixed state. This has led a number of physicists to dispute the result claiming it goes against the principles of Quantum Mechanics, giving this prediction the name “Black Hole Information Loss Paradox” as a consequence. We shall quickly review the “paradox” and its possible “solutions” by following Unruh & Wald 2017, which concludes there is a single “conservative” approach to avoid the conclusion: the possibility that the lost information comes out near the end of the evaporation process as a final “burst”. This approach has some complications due to the need to release great amounts of information without much energy available. However, recent work with radiation due to moving mirrors suggests it might be possible if the information comes out by means of correlations of the Hawking radiation with vacuum fluctuations at late times. In most of the seminar we shall then see how Wald 2019 concludes after a deeper analysis that this approach is very unlikely to work in the black hole case, hence leading us to either accept the “paradox” as a prediction or to radically modify our knowledge of Physics to avoid it.
18 Feb 2022
When quantum fields are subject to the effects of gravitational fields, interesting effects happen. Among them, one of the most famous is Hawking’s prediction that black holes might evaporate, a process which allows the evolution of the field’s quantum state from a pure state to a mixed state. This has led a number of physicists to dispute the result claiming it goes against the principles of Quantum Mechanics, giving this prediction the name “Black Hole Information Loss Paradox” as a consequence. We shall quickly review the “paradox” and its possible “solutions” by following Unruh & Wald 2017, which concludes there is a single “conservative” approach to avoid the conclusion: the possibility that the lost information comes out near the end of the evaporation process as a final “burst”. This approach has some complications due to the need to release great amounts of information without much energy available. However, recent work with radiation due to moving mirrors suggests it might be possible if the information comes out by means of correlations of the Hawking radiation with vacuum fluctuations at late times. In most of the seminar we shall then see how Wald 2019 concludes after a deeper analysis that this approach is very unlikely to work in the black hole case, hence leading us to either accept the “paradox” as a prediction or to radically modify our knowledge of Physics to avoid it.
06 Dec 2021
Em um conjunto de dois artigos Fewster e Verch propõem um framework para descrever de maneira bastante geral medições locais em TQC utilizando a formulação algébrica de TQC’s (TQCA) onde se foi possível se beneficiar de métodos para a descrição de observáveis e operações locais desenvolvidos nesse formalismo, além disso também permite a descrição de medições de uma sonda em um sistema quântico em meio a um espaço tempo possivelmente curvo, no segundo artigo consequências da adoção desse framework implicam na impossibilidade da emergência de sinais supraluminais em medições locais.
22 Nov 2021
The study of black hole physics revealed a fundamental connection between thermodynamics, quantum mechanics, and gravity. Today, it is known that black holes are thermodynamical objects with well-defined temperature and entropy. Although black hole radiance gives us the mechanism from which we can associate a well-defined temperature to the black hole, the origin of its entropy remains a mystery. Here we investigate how the quantum fluctuations from the fields that render the black hole its temperature contribute to its entropy. By using the exact renormalization group equation for a self-interacting real scalar field in a spacetime possessing a bifurcate Killing horizon, we find the renormalization group flow of the total gravitational entropy. We show that throughout the flow one can split the quantum field contribution to the entropy into a part coming from the entanglement between field degrees of freedom inside and outside the horizon and a part due to the quantum corrections to the Wald entropy coming from the Noether charge. The renormalized black hole entropy is shown to be constant throughout the flow while the balance between the effective black hole entropy at low energies and the infrared entanglement entropy changes. A similar conclusion is valid for the Wald entropy part of the total entropy. Additionally, our calculations show that there is no mismatch between the renormalization of the coupling constants (in particular, the minimal coupling $\xi$ with the scalar curvature) coming from the effective action or the total gravitational entropy, showing that once the theory is renormalized the total gravitational entropy comes out automatically finite.
08 Nov 2021
The study of black hole physics revealed a fundamental connection between thermodynamics, quantum mechanics, and gravity. Today, it is known that black holes are thermodynamical objects with well-defined temperature and entropy. Although black hole radiance gives us the mechanism from which we can associate a well-defined temperature to the black hole, the origin of its entropy remains a mystery. Here we investigate how the quantum fluctuations from the fields that render the black hole its temperature contribute to its entropy. By using the exact renormalization group equation for a self-interacting real scalar field in a spacetime possessing a bifurcate Killing horizon, we find the renormalization group flow of the total gravitational entropy. We show that throughout the flow one can split the quantum field contribution to the entropy into a part coming from the entanglement between field degrees of freedom inside and outside the horizon and a part due to the quantum corrections to the Wald entropy coming from the Noether charge. The renormalized black hole entropy is shown to be constant throughout the flow while the balance between the effective black hole entropy at low energies and the infrared entanglement entropy changes. A similar conclusion is valid for the Wald entropy part of the total entropy. Additionally, our calculations show that there is no mismatch between the renormalization of the coupling constants (in particular, the minimal coupling $\xi$ with the scalar curvature) coming from the effective action or the total gravitational entropy, showing that once the theory is renormalized the total gravitational entropy comes out automatically finite.
18 Oct 2021
Sasa Salmen and João Victor Balieiro da Silva
I describe the work done in collaboration with A. Belenchia, F. Giacomini, E. Castro-Ruiz, C. Bruckner and M. Aspelmeyer that analyzes a gedanken experiment involving a massive body that is put into a quantum superposition. Remarkably, even for a nonrelativistic body, both vacuum fluctuations of the gravitational field and the quantization of gravitational radiation are essential in order to avoid inconsistencies. In addition, it is essential that the quantum body be viewed as entangled with its own Newtonian-like gravitational field in order to understand how the body may become entangled with other massive bodies via gravitational interactions.
27 Sep 2021
I describe the work done in collaboration with A. Belenchia, F. Giacomini, E. Castro-Ruiz, C. Bruckner and M. Aspelmeyer that analyzes a gedanken experiment involving a massive body that is put into a quantum superposition. Remarkably, even for a nonrelativistic body, both vacuum fluctuations of the gravitational field and the quantization of gravitational radiation are essential in order to avoid inconsistencies. In addition, it is essential that the quantum body be viewed as entangled with its own Newtonian-like gravitational field in order to understand how the body may become entangled with other massive bodies via gravitational interactions.
13 Sep 2021
We investigate the transmission of both classical and quantum information between two arbitrary observers in globally hyperbolic spacetimes using a quantum field as a communication channel. The field is supposed to be in some arbitrary quasifree state and no choice of representation of its canonical commutation relations is made. Both sender and receiver possess some localized two-level quantum system with which they can interact with the quantum field to prepare the input and receive the output of the channel, respectively. The interaction between the two-level systems and the quantum field is such that one can trace out the field degrees of freedom exactly and thus obtain the quantum channel in a nonperturbative way. We end the paper determining the unassisted as well as the entanglement-assisted classical and quantum channel capacities.
30 Aug 2021
By means of a series of toy models constructed within Classical Mechanics, we shall review what is the so-called Ostrogradsky ghost in higher-order theories, i.e., theories whose Lagrangian involves higher-order time derivatives of dynamical variables. After some motivation for the consideration of such theories is provided, we shall follow (Ganz & Noui 2021) to figure out what ghosts are, which theories they haunt, and when and why one should be scared of them. In particular, we will walk through the derivation of Ostrogradsky’s Instability Theorem, see how it leads to an unbounded Hamiltonian and how it could be fixed by coupling the problematic dynamical variable to a variable without higher derivatives. This then allows one to use higher-order theories without the appearance of spooky ghosts. Still following (Ganz & Noui 2021), we then consider how the ghosts might or not manifest in quantum and/or covariant theories. At last, if time allows it, we shall briefly take a look at (Hawking & Hertog 2002) to analyze what consequences higher-derivatives might have in a quantum theory of gravity.
16 Aug 2021
We analyze potential violations of causality in Unruh-DeWitt-type detector models in relativistic quantum information. We proceed by first studying the relation between faster-than-light signaling and the causal factorization of the dynamics for multiple detector-field interactions. We show in what way spatially extended nonrelativistic detector models predict superluminal propagation of the field’s initial conditions. We draw parallels between this characteristic of detector models, stemming from their nonrelativistic dynamics, and Sorkin’s “impossible measurements on quantum fields” [2]. Based on these features, we discuss the validity of measurements in quantum field theory when performed with nonrelativistic particle detectors.
02 Aug 2021
The works by Rohrlich and Boulware shed a lot of light onto the problem of electromagnetic radiation produced by point-like charges. We will discuss the papers in historical order, first by defining in a covariant fashion the momentum and energy of the field produced by a moving charge, to then establish a criteria to define radiation. After this we move to analyze the equivalence principle as a whole, providing a rigorous formulation of it and its implications for uniformly accelerated charges. Finally we tie up all of this concepts by studying the reason of why the co-accelerated observers will not report any radiation coming from the charge.
19 Jul 2021
The evaporation of black holes raises a number of conceptual issues, most of them related to the final stages of evaporation, where the interplay between the central singularity and Hawking radiation cannot be ignored. Regular models of black holes replace the central singularity with a nonsingular spacetime region, in which an effective classical geometric description is available. It has been argued that these models provide an effective, but complete,description of the evaporation of black holes at all times up to their eventual disappearance. However, here we point out that known models fail to be self-consistent: the regular core is exponentially unstable against perturbations with a finite timescale, while the evaporation time is infinite,therefore making the instability impossible to prevent. We also discuss how to overcome these difficulties, highlighting that this can be done only at the price of accepting that these models cannot be fully predictive regarding the final stages of evaporation.