arXiv subjects

Rodrigo Andrade e Silva

Publications and source records attributed to Rodrigo Andrade e Silva.

11 recordsLinked to original sources

arXiv2026

Perfect quantum reflection from a cliff: a potential unbounded from below

We explain how a potential that is well-defined everywhere on the positive half-line, but is nowhere positive and diverges to $-\infty$ as $x\rightarrow 0^+$, can nevertheless confine a particle to the half-line and lead to well-defined dynamics. Such perfect reflection from a "cliff" potential is achieved by a purely dynamical mechanism, without the need to impose any boundary conditions at the edge. We discuss in detail the role of self-adjointness in ensuring dynamical closure at the quantum level, and advocate the principle that, when no boundary conditions or other physical data are given, the formal quantized Hamiltonian is most naturally defined on its maximal domain. We then construct an explicit cliff potential with the claimed properties, showing that the Hamiltonian is self-adjoint on this maximal domain and therefore defines a dynamically closed quantum system. Finally, we study its energy eigenstates, spectrum, and the phase shift associated with the reflection.

quant-ph↗

arXiv2026

Guiding center quantization of a quantum Hall analog of Hawking radiation

We revisit the quantum Hall analog of Hawking radiation in which a Fermi sea of electrons occupying half of a plane, subjected to a quadrupolar electric potential, gives rise to analog Hawking radiation of chiral edge modes. We show that the phenomenon is accurately captured by quantization of the classical guiding center theory, where the gyroscopic motion of the electrons is coarse grained and only the drift motion is resolved. The quantum dynamics takes place on a non-commutative plane, which requires an electron localized in the half-plane to be supported everywhere along the edge direction. This kinematical constraint on the quantum state leads directly to the radiation, which propagates in both directions away from the origin along the edge. The radiation is thermal with respect to laboratory time, which is equal (up to a constant factor) to the boost angle in the analog Minkowski spacetime in which the chiral edge modes propagate, so in fact it corresponds to analog Unruh radiation.

gr-qc↗

arXiv2026

Quantizing the guiding center: do quantization and coarse graining commute?

We develop a nonperturbative, group-theoretic quantization of the effective guiding center theory of a charged particle drifting in a magnetic field in two spatial dimensions, and compare the resulting quantum theory with a corresponding coarse graining of the underlying microscopic theory. In the classical effective theory, the small gyro motion is not resolved, while the motion of the center of the gyro orbit remains observable. The reduced phase space is the physical space itself, so quantization leads to noncommuting spatial coordinates, and the effective theory loses access to the metric structure of physical space, retaining only its area structure. By formulating a prescription to match between the microscopic and effective quantum theories, we find that the predictions of the quantized effective theory are generally consistent with those of the microscopic theory. However, for closed isomagnetic contours the effective theory predicts a quantization of ``radius'', a spatial discreteness absent from the microscopic theory. This illustrates that quantization of an effective theory may yield spurious nonperturbative predictions, even if that quantum theory shows no internal signs of breakdown.

quant-ph↗

arXiv2026

An algebra of proper observables at null infinity: Dirac brackets, Memory and Goldstone probes

We develop a rigorous evaluation of Dirac brackets for classical observables on the phase space of radiative gravitational modes at null infinity that naturally incorporates memory effects. Considering the Ashtekar-Streubel phase space, with boundary conditions in time given by vanishing {\it news} and purely electric {\it shear}, and taking into account the infinite dimensionality of the phase space, we identify the algebra of proper observables (understood as functions on phase space that can be associated with smooth symplectic flows). We show that the action of supertranslation charges generate the correct transformations on the shear. We also show that the conventional definition of the ``Goldstone mode'' adopted in the literature cannot be associated with a proper observable, but nevertheless there exists an infinite family of proper observables, which we call {\it Goldstone probes}, that are capable of measuring the Goldstone mode. We notice that there are no Goldstone probes constructed only out of the shear {\it or} the news, providing a possible explanation for why attempts to construct a (separable) Hilbert space with different memory states have failed so far. Finally, we derive formulas for distributional Dirac brackets between local shear and news, and show that they contain non-local corrections.

hep-th↗

arXiv2026

Quantization of causal diamonds in (2+1)-dimensional gravity -- Part I: Classical reduction

We develop the non-perturbative reduced phase space quantization of causal diamonds in (2+1)-dimensional gravity with a nonpositive cosmological constant. In this Part I we focus on the classical reduction process, and the description of the reduced phase space, while in Part II we discuss the quantization of the phase space and quantum aspects of the causal diamonds. The system is defined as the domain of dependence of a spacelike topological disk with fixed boundary metric. By solving the constraints in a constant-mean-curvature time gauge and removing all the spatial gauge redundancy, we find that the phase space is the cotangent bundle of Diff^+(S^1)/PSL(2,R), i.e., the group of orientation-preserving diffeomorphisms of the circle modulo the projective special linear subgroup. Classically, the states correspond to causal diamonds embedded in AdS_3 (or Mink_3 if $Λ= 0$), with fixed corner length, and whose Cauchy surfaces have the topology of a disc.

hep-th↗

arXiv2025

Quantization of causal diamonds in (2+1)-dimensional gravity -- Part II: Group-theoretic quantization

We develop the non-perturbative reduced phase space quantization of causal diamonds in (2+1)-dimensional gravity with a nonpositive cosmological constant. In Part I we described the classical reduction process and the reduced phase space, $\widetilde{\mathcal P} = T^*(\text{Diff}^+\!(S^1)/\text{PSL}(2, \mathbb R))$, while in Part II we discuss the quantization of the phase space and quantum aspects of the causal diamonds. Because the phase space does not have a natural linear structure, a generalization of the standard canonical (coordinate) quantization is required. In particular, as the configuration space is a homogeneous space for the $\text{Diff}^+\!(S^1)$ group, we apply Isham's group-theoretic quantization scheme. We propose a quantization based on (projective) unitary irreducible representations of the $\text{BMS}_3$ group, which is obtained from a natural prescription for extending $\text{Diff}^+\!(S^1)$ into a transitive group of symplectic symmetries of the phase space. We find a class of suitable quantum theories labelled by a choice of a coadjoint orbit of the Virasoro group and an irreducible unitary representation of the corresponding little group. The most natural choice, justified by a Casimir matching principle, corresponds to a Hilbert space realized by wavefunctions on $\text{Diff}^+\!(S^1)/\text{PSL}(2, \mathbb R)$ valued in some unitary irreducible representation of $\text{SL}(2, \mathbb R)$. A surprising result is that the twist of the diamond corner loop is quantized in terms of the ratio of the Planck length to the corner perimeter.

hep-th↗

arXiv2025

General theory of swimming in curved spacetimes

Swimming in curved spacetimes is a phenomenon whereby free bodies in curved spacetimes are able to propel themselves by performing cyclic internal motions. When originally proposed, it was further suggested that, in the limit of fast internal cycles, the net motion would display a simple geometric-phase character, in which the displacement per cycle would not depend on the time progression of the internal motions but only on the sequence of shapes assumed by the body, like a swimmer in a non-turbulent viscous fluid (low Reynolds number). In this paper we develop a general, covariant theory of swimming in curved spacetimes, describing a technique to study the motion of free, small, light, articulated bodies in general relativity by mapping the problem to an analogue in special relativity. We give considerable attention to the limit of fast cycles and investigate the conditions in which the overall motion could display such geometric-phase behavior. The conclusion, however, is that this simple behavior is only realized in very specific circumstances, depending on the structure of the body, characteristics of internal motions, initial conditions, and symmetries of the spacetime; whereas, in general, our formulas predict a more complicated dynamics.

gr-qc↗

arXiv2022

Causal diamonds in 2+1 dimensional quantum gravity

We develop the reduced phase space quantization of causal diamonds in pure 2+1 dimensional gravity with a non-positive cosmological constant. The system is defined as the domain of dependence of a topological disc with fixed boundary metric. By solving the initial value constraints in a constant-mean-curvature time gauge and removing all the spatial gauge redundancy, we find that the phase space is the cotangent bundle of Diff^+(S^1)/PSL(2,R). To quantize this phase space we apply Isham's group-theoretic quantization scheme, with respect to a BMS_3 group, and find that the quantum theory can be realized by wavefunctions on some coadjoint orbit of the Virasoro group, with labels in irreducible unitary representations of the corresponding little group. We find that the twist of the diamond boundary loop is quantized in integer or half-integer multiples of the ratio of the Planck length to the boundary length.

hep-th↗

arXiv2021

Particle on the sphere: group-theoretic quantization in the presence of a magnetic monopole

The problem of quantizing a particle on a 2-sphere has been treated by numerous approaches, including Isham's global method based on unitary representations of a symplectic symmetry group that acts transitively on the phase space. Here we reconsider this simple model using Isham's scheme, enriched by a magnetic flux through the sphere via a modification of the symplectic form. To maintain complete generality we construct the Hilbert space directly from the symmetry algebra, which is manifestly gauge-invariant, using ladder operators. In this way, we recover algebraically the complete classification of quantizations, and the corresponding energy spectra for the particle. The famous Dirac quantization condition for the monopole charge follows from the requirement that the classical and quantum Casimir invariants match. In an appendix we explain the relation between this approach and the more common one that assumes from the outset a Hilbert space of wave functions that are sections of a nontrivial line bundle over the sphere, and show how the Casimir invariants of the algebra determine the bundle topology.

quant-ph↗

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↗

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↗