55 research outputs found
The extension problem for partial Boolean structures in Quantum Mechanics
Alternative partial Boolean structures, implicit in the discussion of
classical representability of sets of quantum mechanical predictions, are
characterized, with definite general conclusions on the equivalence of the
approaches going back to Bell and Kochen-Specker. An algebraic approach is
presented, allowing for a discussion of partial classical extension, amounting
to reduction of the number of contexts, classical representability arising as a
special case. As a result, known techniques are generalized and some of the
associated computational difficulties overcome. The implications on the
discussion of Boole-Bell inequalities are indicated.Comment: A number of misprints have been corrected and some terminology
changed in order to avoid possible ambiguitie
Plane waves in quantum gravity: breakdown of the classical spacetime
Starting with the Hamiltonian formulation for spacetimes with two commuting
spacelike Killing vectors, we construct a midisuperspace model for linearly
polarized plane waves in vacuum gravity. This model has no constraints and its
degrees of freedom can be interpreted as an infinite and continuous set of
annihilation and creation like variables. We also consider a simplified version
of the model, in which the number of modes is restricted to a discrete set. In
both cases, the quantization is achieved by introducing a Fock representation.
We find regularized operators to represent the metric and discuss whether the
coherent states of the quantum theory are peaked around classical spacetimes.
It is shown that, although the expectation value of the metric on Killing
orbits coincides with a classical solution, its relative fluctuations become
significant when one approaches a region where null geodesics are focused. In
that region, the spacetimes described by coherent states fail to admit an
approximate classical description. This result applies as well to the vacuum of
the theory.Comment: 11 pages, no figures, version accepted for publication in Phys. Rev.
From Stochastic Differential Equations to Quantum Field Theory
Covariant stochastic partial (pseudo-)differential equations are studied in
any dimension. In particular a large class of covariant interacting local
quantum fields obeying the Morchio-Strocchi system of axioms for indefinite
quantum field theory is constructed by solving the analysed equations. The
associated random cosurface models are discussed and some elementary properties
of them are outlined.Comment: 11 pages, Latex, to appear in: Reports On Mathematical Physics No.X
Vol.XX (199X
Evaluación de la calidad de los materiales educativos digitales aplicados a la enseñanza del Inglés como lengua extranjera.
El desarrollo tecnológico de las últimas décadas ha permitido la implementación de dispositivos cada vez más avanzados en los espacios educativos formales. Estos dispositivos se han constituido en una herramienta fértil para potenciar situaciones de aprendizaje. En los últimos años, las tecnologías de la información y de la comunicación (TIC) han atravesado un período de cuestionamiento sobre su rol e impacto en los procesos de enseñanza y de aprendizaje, generando - tanto en los docentes como en los alumnos - reacciones diversas. Sin embrago, en la actualidad, ya han adquirido un sentido dentro de las prácticas pedagógicas a través de la implementación de las aulas virtuales. No obstante, creemos que la presencia de las TIC, por sí sola, no es garantía de mejora de la calidad de los procesos de enseñanza y de aprendizaje, ya que no todos los tecnofactos cumplen los requisitos de aprovechamiento pedagógico. Por lo tanto, es necesario reflexionar sobre la calidad de dichos instrumentos. En este sentido, el objetivo central de esta comunicación es presentar los resultados de un proyecto de investigación llevado a cabo en la Facultad de Lenguas durante el período 2014-2015, entre cuyos objetivos podemos mencionar los siguientes: (i) definir las características generales de los materiales educativos multimedia; (ii) elaborar criterios consensuados de evaluación que contribuyan a mejorar la calidad de las aulas virtuales destinadas a la enseñanza del inglés como lengua extranjera; (iii) aplicar dichos criterios para detectar fortalezas y debilidades; (iv) diseñar estrategias pedagógicas que contribuyan a superar las debilidades que pudieran haberse detectado.Fil: Negrelli, Fabián H. Universidad Nacional de Córdoba. Facultad de Lenguas; Argentina.Fil: Morchio, María José. Universidad Nacional de Córdoba. Facultad de Lenguas; Argentina.Fil: Capell, Martín Salvador. Universidad Nacional de Córdoba. Facultad de Lenguas; Argentina.Fil: Ferreras, Cecilia R. Universidad Nacional de Córdoba. Facultad de Lenguas; Argentina.Otras Lengua y Literatur
Localization and symmetries
The violation of the Noether relation between symmetries and charges is
reduced to the time dependence of the charge associated to a conserved current.
For the U(1) gauge symmetry a non-perturbative control of the charge
commutators is obtained by an analysis of the Coulomb charged fields. From
this, in the unbroken case we obtain a correct expression for the electric
charge on the Coulomb states, its superselection and the presence of massless
vector bosons; in the broken case, we obtain a general non-perturbative version
of the Higgs phenomenon, i.e. the absence of massless Goldstone bosons and of
massless vector bosons. The conservation of the (gauge dependent) current
associated to the U(1) axial symmetry in QCD is shown to be compatible with the
time dependence of the corresponding charge commutators and a non-vanishing
eta' mass, as a consequence of the non locality of the (conserved) current.Comment: Invited contribution to ``The Quantum Universe'', dedicated to G.
Ghirardi for his 70th birthda
The Quantum Sine-Gordon model in perturbative AQFT
We study the Sine-Gordon model with Minkowski signature in the framework of perturbative algebraic quantum field theory. We calculate the vertex operator algebra braiding property. We prove that in the finite regime of the model, the expectation value—with respect to the vacuum or a Hadamard state—of the Epstein Glaser S-matrix and the interacting current or the field respectively converge, both given as formal power series
A discrete nonetheless remarkable brick in de Sitter: the "massless minimally coupled field"
Over the last ten years interest in the physics of de Sitter spacetime has
been growing very fast. Besides the supposed existence of a "de sitterian
period" in inflation theories, the observational evidence of an acceleration of
the universe expansion (interpreted as a positive cosmological constant or a
"dark energy" or some form of "quintessence") has triggered a lot of attention
in the physics community. A specific de sitterian field called "massless
minimally coupled field" (mmc) plays a fundamental role in inflation models and
in the construction of the de sitterian gravitational field. A covariant
quantization of the mmc field, `a la Krein-Gupta-Bleuler was proposed in [1].
In this talk, we will review this construction and explain the relevance of
such a field in the construction of a massless spin 2 field in de Sitter
space-time.Comment: Proceedings of the XXVII Colloquium on Group Theoretical Methods in
Physics, Yerevan, August 200
Charges in Gauge Theories
In this article we investigate charged particles in gauge theories. After
reviewing the physical and theoretical problems, a method to construct charged
particles is presented. Explicit solutions are found in the Abelian theory and
a physical interpretation is given. These solutions and our interpretation of
these variables as the true degrees of freedom for charged particles, are then
tested in the perturbative domain and are demonstrated to yield infra-red
finite, on-shell Green's functions at all orders of perturbation theory. The
extension to collinear divergences is studied and it is shown that this method
applies to the case of massless charged particles. The application of these
constructions to the charged sectors of the standard model is reviewed and we
conclude with a discussion of the successes achieved so far in this programme
and a list of open questions.Comment: 47 pages, LaTeX, 14 figures, uses feynmp, necessary Metapost files
included. Review to appear in Pramana, Journal of Physics. Minor LaTeX change
to make page numbers visible on "Letter" paper forma
Gauge-fixing parameter dependence of two-point gauge variant correlation functions
The gauge-fixing parameter dependence of two-point gauge variant
correlation functions is studied for QED and QCD. We show that, in three
Euclidean dimensions, or for four-dimensional thermal gauge theories, the usual
procedure of getting a general covariant gauge-fixing term by averaging over a
class of covariant gauge-fixing conditions leads to a nontrivial gauge-fixing
parameter dependence in gauge variant two-point correlation functions (e.g.
fermion propagators). This nontrivial gauge-fixing parameter dependence
modifies the large distance behavior of the two-point correlation functions by
introducing additional exponentially decaying factors. These factors are the
origin of the gauge dependence encountered in some perturbative evaluations of
the damping rates and the static chromoelectric screening length in a general
covariant gauge. To avoid this modification of the long distance behavior
introduced by performing the average over a class of covariant gauge-fixing
conditions, one can either choose a vanishing gauge-fixing parameter or apply
an unphysical infrared cutoff.Comment: 21 pages, RevTe
Ventilation and outcomes following robotic-assisted abdominal surgery: an international, multicentre observational study
Background: International data on the epidemiology, ventilation practice, and outcomes in patients undergoing abdominal robotic-assisted surgery (RAS) are lacking. The aim of the study was to assess the incidence of postoperative pulmonary complications (PPCs), and to describe ventilator management after abdominal RAS. Methods: This was an international, multicentre, prospective study in 34 centres in nine countries. Patients ≥18 yr of age undergoing abdominal RAS were enrolled between April 2017 and March 2019. The Assess Respiratory Risk in Surgical Patients in Catalonia (ARISCAT) score was used to stratify for higher risk of PPCs (≥26). The primary outcome was the incidence of PPCs. Secondary endpoints included the preoperative risk for PPCs and ventilator management. Results: Of 1167 subjects screened, 905 abdominal RAS patients were included. Overall, 590 (65.2%) patients were at increased risk for PPCs. Meanwhile, 172 (19%) patients sustained PPCs, which occurred more frequently in 132 (22.4%) patients at increased risk, compared with 40 (12.7%) patients at lower risk of PPCs (absolute risk difference: 12.2% [95% confidence intervals (CI), 6.8–17.6%]; P<0.001). Plateau and driving pressures were higher in patients at increased risk, compared with patients at low risk of PPCs, but no ventilatory variables were independently associated with increased occurrence of PPCs. Development of PPCs was associated with a longer hospital stay. Conclusions: One in five patients developed one or more PPCs (chiefly unplanned oxygen requirement), which was associated with a longer hospital stay. No ventilatory variables were independently associated with PPCs. Clinical trial registration: NCT02989415
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