44 research outputs found
The harmonic background paradigm, or why gravity is attractive
In a work by Visser, Bassett and Liberati (VBL) [Nucl. Phys. B (Proc. Suppl.)
88, 267 (2000)] a relation was suggested between a null energy condition and
the censorship of superluminal behaviour. Their result was soon challenged by
Gao and Wald [Class. Quantum Grav. 17, 4999, (2000)] who argued that this
relation is gauge dependent and therefore lacks physical significance. In this
paper, we clear up this controversy by showing that both papers are correct but
need to be interpreted in distinct paradigms. In this context, we introduce a
new paradigm to interpret gravitational phenomena, which we call the Harmonic
Background Paradigm. This harmonic background paradigm starts from the idea
that there exists a more fundamental background causality provided by a flat
spacetime geometry. One of the consequences of this paradigm is that the VBL
relation provides an explanation of why gravity is attractive in all standard
weak-field situations.Comment: 33 pages, no figure
Electromagnetism as an emergent phenomenon: a step-by-step guide
We give a detailed description of electrodynamics as an emergent theory from
condensed-matter-like structures, not only {\it per se} but also as a warm-up
for the study of the much more complex case of gravity. We will concentrate on
two scenarios that, although qualitatively different, share some important
features, with the idea of extracting the basic generic ingredients that give
rise to emergent electrodynamics and, more generally, to gauge theories. We
start with Maxwell's mechanical model for electrodynamics, where Maxwell's
equations appear as dynamical consistency conditions. We next take a superfluid
He-like system as representative of a broad class of fermionic quantum
systems whose low-energy physics reproduces classical electrodynamics (Dirac
and Maxwell equations as dynamical low-energy laws). An important lesson that
can be derived from both analyses is that the vector potential has a
microscopic physical reality and that it is only in the low-energy regime that
this physical reality is blurred in favour of gauge invariance, which in
addition turns out to be secondary to effective Lorentz invariance.Comment: 41 pages, 4 figures; v2: references added, version accepted for
publicatio
The lifetime problem of evaporating black holes: mutiny or resignation
It is logically possible that regularly evaporating black holes exist in
nature. In fact, the prevalent theoretical view is that these are indeed the
real objects behind the curtain in astrophysical scenarios. There are several
proposals for regularizing the classical singularity of black holes so that
their formation and evaporation do not lead to information-loss problems. One
characteristic is shared by most of these proposals: these regularly
evaporating black holes present long-lived trapping horizons, with absolutely
enormous evaporation lifetimes in whatever measure. Guided by the discomfort
with these enormous and thus inaccessible lifetimes, we elaborate here on an
alternative regularization of the classical singularity, previously proposed by
the authors in an emergent gravity framework, which leads to a completely
different scenario. In our scheme the collapse of a stellar object would result
in a genuine time-symmetric bounce, which in geometrical terms amounts to the
connection of a black-hole geometry with a white-hole geometry in a regular
manner. The two most differential characteristics of this proposal are: i) the
complete bouncing geometry is a solution of standard classical general
relativity everywhere except in a transient region that necessarily extends
beyond the gravitational radius associated with the total mass of the
collapsing object; and ii) the duration of the bounce as seen by external
observers is very brief (fractions of milliseconds for neutron-star-like
collapses). This scenario motivates the search for new forms of stellar
equilibrium different from black holes. In a brief epilogue we compare our
proposal with a similar geometrical setting recently proposed by Haggard and
Rovelli.Comment: 20 pages, 2 figures; v2: published version, references adde
Hawking tunneling and boomerang behaviour of massive particles with E < m
Copyright © 2012 American Institute of PhysicsTowards New Paradigms: Proceeding of the Spanish Relativity Meeting 2011 (ERE2011), 29 August–2 September 2011, Madrid, SpainMassive particles are radiated from black holes through the Hawking mechanism together with the more familiar radiation of massless particles. For E ≥ m, the emission rate is identical to the massless case. But E < m particles can also tunnel across the horizon. A study of the dispersion relation and wave packet simulations show that their classical trajectory is similar to that of a boomerang. The tunneling formalism is used to calculate the probability for detecting such E < m particles, for a Schwarzschild black hole of astrophysical size or in an analogue gravity experiment, as a function of the distance from the horizon and the energy of the particle
La narrativa como herramienta docente dentro de la gamificación de la estadística en el Grado en Turismo
La gamificación consiste en ludificar los contenidos de una determinada materia o asignatura a través de dinámicas, mecánicas y componentes. Con este proceso se puede lograr una mayor motivación e implicación del alumno, así como fomentar el desarrollo de competencias y del aprendizaje autónomo. Así, el presente texto describe el desarrollo de esta herramienta para la asignatura “Estadística Aplicada al Sector Turístico” del Grado en Turismo con el objetivo de lograr presentar un proceso formativo dinámico y atractivo para el alumno. No obstante, se elabora desde un punto transversal que permita su implementación en asignaturas similares implementadas en enseñanzas superiores de grado. En más detalle, se desarrolla una narrativa entorno a la promoción profesional del alumno dentro de una empresa hotelera ficticia donde irá alcanzando mayor rango profesional a medida que avanza el juego y va superando una serie de pruebas/eventos. Esta narrativa fomenta el interés intrínseco en la actividad por parte del alumno, al mismo tiempo que refuerza la utilidad del propio juego de cara a futuras experiencias profesionales
Hawking radiation and the boomerang behaviour of massive modes near a horizon
Copyright © 2011 American Physical SocietyWe discuss the behaviour of massive modes near a horizon based on a study of the dispersion relation and wave packet simulations of the Klein-Gordon equation. We point out an apparent paradox between two (in principle equivalent) pictures of black hole evaporation through Hawking radiation. In the picture in which the evaporation is due to the emission of positive-energy modes, one immediately obtains a threshold for the emission of massive particles. In the picture in which the evaporation is due to the absorption of negative-energy modes, such a threshold apparently does not exist. We resolve this paradox by tracing the evolution of the positive-energy massive modes with an energy below the threshold. These are seen to be emitted and move away from the black hole horizon, but they bounce back at a "red horizon" and are re-absorbed by the black hole, thus compensating exactly for the difference between the two pictures. For astrophysical black holes, the consequences are curious but do not affect the terrestrial constraints on observing Hawking radiation. For analogue gravity systems with massive modes, however, the consequences are crucial and rather surprising
Do transient white holes have a place in Nature?
The white-hole sector of Kruskal's solution is almost never used in physical applications. However, it can provide a radically different take on the gravitational collapse process, avoiding the problems appearing within the standard paradigm. In this contribution we will try to draw attention to some bouncing geometries that make a democratic usage of the black and white sectors of Kruskal's solution. We will argue that this type of behaviour could be perfectly natural in some approaches to the next physical level beyond classical General Relativity. © Published under licence by IOP Publishing Ltd.Peer Reviewe
Do stars die too long?
MG14, University of Rome "La Sapienza" - Rome, July 12-18, 2015; http://www.icra.it/mg/mg14/Current proposals for regularizing the classical singularity of black holes present longlived
trapping horizons, with enormous inaccessible evaporation lifetimes. We propose
an alternative regularization model, inspired in condensed matter gravitational analogues,
in which the collapse of a stellar object would result in a genuine time-symmetric
bounce. In geometrical terms this amounts to the connection of a black-hole geometry
with a white-hole geometry in a regular manner. The complete bouncing geometry is a
solution of standard classical general relativity everywhere except in a transient region
that necessarily extends beyond the gravitational radius. The duration of the bounce
as seen by external observers is very brief. This motivates the search for new forms of
stellar equilibrium.Financial support was provided by the Spanish MICINN through Projects No. FIS2011-30145-C03-01 and FIS2011-30145-C03-02 (with FEDER contribution), and by the Junta de Andalucía through Project No. FQM219. R. C-R. acknowledges
support from CSIC through the JAE-predoc program, cofunded by FSE.Peer Reviewe