6,176 research outputs found
Large quantum gravity effects: Cylindrical waves in four dimensions
Linearly polarized cylindrical waves in four-dimensional vacuum gravity are
mathematically equivalent to rotationally symmetric gravity coupled to a
Maxwell (or Klein-Gordon) field in three dimensions. The quantization of this
latter system was performed by Ashtekar and Pierri in a recent work. Employing
that quantization, we obtain here a complete quantum theory which describes the
four-dimensional geometry of the Einstein-Rosen waves. In particular, we
construct regularized operators to represent the metric. It is shown that the
results achieved by Ashtekar about the existence of important quantum gravity
effects in the Einstein-Maxwell system at large distances from the symmetry
axis continue to be valid from a four-dimensional point of view. The only
significant difference is that, in order to admit an approximate classical
description in the asymptotic region, states that are coherent in the Maxwell
field need not contain a large number of photons anymore. We also analyze the
metric fluctuations on the symmetry axis and argue that they are generally
relevant for all of the coherent states.Comment: Version accepted for publication in Int. J. Mod. Phys.
Involutions on the Algebra of Physical Observables From Reality Conditions
Some aspects of the algebraic quantization programme proposed by Ashtekar are
revisited in this article. It is proved that, for systems with first-class
constraints, the involution introduced on the algebra of quantum operators via
reality conditions can never be projected unambiguously to the algebra of
physical observables, ie, of quantum observables modulo constraints. It is
nevertheless shown that, under sufficiently general assumptions, one can still
induce an involution on the algebra of physical observables from reality
conditions, though the involution obtained depends on the choice of particular
representatives for the equivalence classes of quantum observables and this
implies an additional ambiguity in the quantization procedure suggested by
Ashtekar.Comment: 19 pages, latex, no figure
A Brief Introduction to Loop Quantum Cosmology
In recent years, Loop Quantum Gravity has emerged as a solid candidate for a
nonperturbative quantum theory of General Relativity. It is a background
independent theory based on a description of the gravitational field in terms
of holonomies and fluxes. In order to discuss its physical implications, a lot
of attention has been paid to the application of the quantization techniques of
Loop Quantum Gravity to symmetry reduced models with cosmological solutions, a
line of research that has been called Loop Quantum Cosmology. We summarize its
fundamentals and the main differences with respect to the more conventional
quantization approaches employed in cosmology until now. In addition, we
comment on the most important results that have been obtained in Loop Quantum
Cosmology by analyzing simple homogeneous and isotropic models. These results
include the resolution of the classical big-bang singularity, which is replaced
by a quantum bounce.Comment: 15 pages, published in AIP Conference Proceedings, Volume 1130,
Geometry and Physics: XVII International Fall Workshop on Geometry and
Physic
Corrections to the fluxes of a Neutrino Factory
In view of their physics goals, future neutrino factories from muon decay aim
at an overall flux precision of or better. We analytically study
the QED radiative corrections to the neutrino differential distributions from
muon decay. Kinematic uncertainties due to the divergence of the muon beam are
considered as well. The resulting corrections to the neutrino flux turn out to
be of order , safely below the required precision.Comment: 22 pages, 8 figures. Some references changed. Final version accepted
for publication in EPJ
The calibration and flight test performance of the space shuttle orbiter air data system
The Space Shuttle air data system (ADS) is used by the guidance, navigation and control system (GN&C) to guide the vehicle to a safe landing. In addition, postflight aerodynamic analysis requires a precise knowledge of flight conditions. Since the orbiter is essentially an unpowered vehicle, the conventional methods of obtaining the ADS calibration were not available; therefore, the calibration was derived using a unique and extensive wind tunnel test program. This test program included subsonic tests with a 0.36-scale orbiter model, transonic and supersonic tests with a smaller 0.2-scale model, and numerous ADS probe-alone tests. The wind tunnel calibration was further refined with subsonic results from the approach and landing test (ALT) program, thus producing the ADS calibration for the orbital flight test (OFT) program. The calibration of the Space Shuttle ADS and its performance during flight are discussed in this paper. A brief description of the system is followed by a discussion of the calibration methodology, and then by a review of the wind tunnel and flight test programs. Finally, the flight results are presented, including an evaluation of the system performance for on-board systems use and a description of the calibration refinements developed to provide the best possible air data for postflight analysis work
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