4,102 research outputs found
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.
The screen representation of vector coupling coefficients or Wigner 3j symbols: exact computation and illustration of the asymptotic behavior
The Wigner symbols of the quantum angular momentum theory are related to
the vector coupling or Clebsch-Gordan coefficients and to the Hahn and dual
Hahn polynomials of the discrete orthogonal hyperspherical family, of use in
discretization approximations. We point out the important role of the Regge
symmetries for defining the screen where images of the coefficients are
projected, and for discussing their asymptotic properties and semiclassical
behavior. Recursion relationships are formulated as eigenvalue equations, and
exploited both for computational purposes and for physical interpretations.Comment: 14 pages, 8 figures, presented at ICCSA 2014, 14th International
Conference on Computational Science and Application
Tribocorrosion of hard-on-hard total hip replacements with metal and ceramic counterfaces under standard and adverse loading conditions
28 mm Metal-on-Metal (MoM) and Metal-on-Ceramic (MoC) Total Hip Replacements were articulated to 1 million cycles under both Standard Gait and Microseparation conditions. The hip simulator was fully instrumented with a three-electrode electrochemical cell to facilitate monitoring of corrosive degradation. The estimated volume loss from corrosion at the bearing surface was seen to increase by nearly an order of magnitude for both devices, representing as much as 17% of total degradation. Anodic current transients also displayed near order of magnitude increases in the peak current for both bearing couples. An adverse loading scenario could cause as much as an order of magnitude increase in the metallic ions released into the joint capsule as well as an increased volume of wear debris
The Effect of Microseparation on Corrosion Rates of Metal-on-Metal Total Hip Replacements
The poor performance of Metal-on-Metal (MoM) bearings has to date been blamed on “adverse loading” conditions. Studies have focused on the effect of cup inclination and microseparation on gravimetric wear rates and highlighted the importance of surgical technique when implanting such devices. Up to four fold increase in the wear rates of MoM bearings subjected to microseparation has been reported during the bedding-in period. The contribution of corrosive processes to overall material degradation during adverse loading has not previously been investigated. In the present study 28 mm HC CoCrMo alloy Total Hip Replacements were tested to 1 Mcycles under standard gait and severe microseparation conditions in an electrochemically instrumented hip simulator. An order of magnitude increase in material lost as a result of oxidation was noted (0.234 to 2.044 mm3/Mcycle) during microseparation. Corrosive degradation may therefore be a much more significant contribution to poor bearing performance under adverse loading than previously considered
Quantization of pure gravitational plane waves
Pure gravitational plane waves are considered as a special case of spacetimes
with two commuting spacelike Killing vector fields. Starting with a
midisuperspace that describes this kind of spacetimes, we introduce
gauge-fixing and symmetry conditions that remove all non-physical degrees of
freedom and ensure that the classical solutions are plane waves. In this way,
we arrive at a reduced model with no constraints and whose only degrees of
freedom are given by two fields. In a suitable coordinate system, the reduced
Hamiltonian that generates the time evolution of this model turns out to
vanish, so that all relevant information is contained in the symplectic
structure. We calculate this symplectic structure and particularize our
discussion to the case of linearly polarized plane waves. The reduced phase
space can then be described by an infinite set of annihilation and creation
like variables. We finally quantize the linearly polarized model by introducing
a Fock representation for these variables.Comment: 11 pages, Revtex, no figure
Pore-Scale Displacement Efficiency during Different Salinity Water Flooding in Hydrophilic and Hydrophobic Microstructures
Previous macroscopic core flooding tests have shown that injecting low-salinity water improves oil recovery in sandstone and carbonate reservoirs through wettability alteration. However, consistent mechanistic clarification of the underlying physicochemical mechanisms involved in oil wettability at the pore-scale level is not fully understood. In this work, a microfluidic approach is used to provide in situ visualization of oil–brine flow to give an indication of the micromechanisms affecting oil sweep efficiency. The potential of enhancing oil recovery by low-salinity flooding at the microscale is also investigated, which would help in predicting a reservoir’s performance before committing to production processes at a large field scale. Two types of crude oils with various acid numbers were used, and hydrophilic and hydrophobic physical microstructures were used to mimic sandstones and carbonates. The results revealed a reduction by 7–10% in the residual oil for the water-wet microstructure when the seawater was diluted twice from its original concentration, apparently due to a decrease in the attractive forces. There is no change in the recovery factor for the oil-wet micromodel for the two kinds of crude oils examined. Tertiary low-salinity flooding did not show any effect on the initial wetting state of the hydrophobic surface, rendering it with a strongly oil-wet condition. It is also observed that flow dynamics of the two microstructures examined are different, as the snap-off–coalesce phenomenon dominants the flow in the water-wet system, while oil moved by a piston-like displacement with a stable or irregular front in the hydrophobic system. In contrast to some of the published macroscopic results, our pore-scale displacement shows that low-salinity flooding seems to be an unsuitable choice for enhanced oil recovery for strongly oil-wet reservoirs
Macroscopic behavior of bidisperse suspensions of noncolloidal particles in yield stress fluids
We study both experimentally and theoretically the rheological behavior of
isotropic bidisperse suspensions of noncolloidal particles in yield stress
fluids. We focus on materials in which noncolloidal particles interact with the
suspending fluid only through hydrodynamical interactions. We observe that both
the elastic modulus and yield stress of bidisperse suspensions are lower than
those of monodisperse suspensions of same solid volume fraction. Moreover, we
show that the dimensionless yield stress of such suspensions is linked to their
dimensionless elastic modulus and to their solid volume fraction through the
simple equation of Chateau et al.[J. rheol. 52, 489-506 (2008)]. We also show
that the effect of the particle size heterogeneity can be described by means of
a packing model developed to estimate random loose packing of assemblies of dry
particles. All these observations finally allow us to propose simple closed
form estimates for both the elastic modulus and the yield stress of bidisperse
suspensions: while the elastic modulus is a function of the reduced volume
fraction only, where is the estimated random loose
packing, the yield stress is a function of both the volume fraction and
the reduced volume fraction
Energy and directional signatures for plane quantized gravity waves
Solutions are constructed to the quantum constraints for planar gravity
(fields dependent on z and t only) in the Ashtekar complex connection
formalism. A number of operators are constructed and applied to the solutions.
These include the familiar ADM energy and area operators, as well as new
operators sensitive to directionality (z+ct vs. z-ct dependence). The
directionality operators are quantum analogs of the classical constraints
proposed for unidirectional plane waves by Bondi, Pirani, and Robinson (BPR).
It is argued that the quantum BPR constraints will predict unidirectionality
reliably only for solutions which are semiclassical in a certain sense. The ADM
energy and area operators are likely to have imaginary eigenvalues, unless one
either shifts to a real connection, or allows the connection to occur other
than in a holonomy. In classical theory, the area can evolve to zero. A quantum
mechanical mechanism is proposed which would prevent this collapse.Comment: 54 pages; LaTe
Projective Invariance and One-Loop Effective Action in Affine-Metric Gravity Interacting with Scalar Field
We investigate the influence of the projective invariance on the
renormalization properties of the theory. One-loop counterterms are calculated
in the most general case of interaction of gravity with scalar field.Comment: 10 pages, LATE
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