330 research outputs found
Quantization of generally covariant systems with extrinsic time
A generally covariant system can be deparametrized by means of an
``extrinsic'' time, provided that the metric has a conformal ``temporal''
Killing vector and the potential exhibits a suitable behavior with respect to
it. The quantization of the system is performed by giving the well ordered
constraint operators which satisfy the algebra. The searching of these
operators is enlightned by the methods of the BRST formalism.Comment: 10 pages. Definite published versio
Geometric Bounds in Spherically Symmetric General Relativity
We exploit an arbitrary extrinsic time foliation of spacetime to solve the
constraints in spherically symmetric general relativity. Among such foliations
there is a one parameter family, linear and homogeneous in the extrinsic
curvature, which permit the momentum constraint to be solved exactly. This
family includes, as special cases, the extrinsic time gauges that have been
exploited in the past. These foliations have the property that the extrinsic
curvature is spacelike with respect to the the spherically symmetric superspace
metric. What is remarkable is that the linearity can be relaxed at no essential
extra cost which permits us to isolate a large non - pathological dense subset
of all extrinsic time foliations. We identify properties of solutions which are
independent of the particular foliation within this subset. When the geometry
is regular, we can place spatially invariant numerical bounds on the values of
both the spatial and the temporal gradients of the scalar areal radius, .
These bounds are entirely independent of the particular gauge and of the
magnitude of the sources. When singularities occur, we demonstrate that the
geometry behaves in a universal way in the neighborhood of the singularity.Comment: 16 pages, revtex, submitted to Phys. Rev.
Time reparameterization in Bianchi type I spinor cosmology
The problem of time reparameterization is addressed at both the classical and
quantum levels in a Bianchi-I universe in which the matter source is a massive
Dirac spinor field. We take the scale factors of the metric as the intrinsic
time and their conjugate momenta as the extrinsic time. A scalar character of
the spinor field is identified as a representation of the extrinsic time. The
construction of the field equations and quantization of the model is achieved
by solving the Hamiltonian constraint after time identification has been dealt
with. This procedure leads to a true Hamiltonian whose exact solutions for the
above choices of time are presentedComment: 16 pages, no figures, to appear in Annals of Physic
Selection rules for the Wheeler-DeWitt equation in quantum cosmology
Selection of physically meaningful solutions of the Wheeler-DeWitt equation
for the wavefunction in quantum cosmology, can be attained by a reduction of
the theory to the sector of true physical degrees of freedom and their
canonical quantization. The resulting physical wavefunction unitarily evolving
in the time variable introduced within this reduction can then be raised to the
level of the cosmological wavefunction in superspace of 3-metrics. We apply
this technique in several simple minisuperspace models and discuss both at
classical and quantum level physical reduction in {\em extrinsic} time -- the
time variable determined in terms of extrinsic curvature. Only this extrinsic
time gauge can be consistently used in vicinity of turning points and bounces
where the scale factor reaches extremum. Since the 3-metric scale factor is
canonically dual to extrinsic time variable, the transition from the physical
wavefunction to the wavefunction in superspace represents a kind of the
generalized Fourier transform. This transformation selects square integrable
solutions of the Wheeler-DeWitt equation, which guarantee Hermiticity of
canonical operators of the Dirac quantization scheme. Semiclassically this
means that wavefunctions are represented by oscillating waves in classically
allowed domains of superspace and exponentially fall off in classically
forbidden (underbarrier) regions. This is explicitly demonstrated in flat FRW
model with a scalar field having a constant negative potential and for the case
of phantom scalar field with a positive potential. The FRW model of a scalar
field with a vanishing potential does not lead to selection rules for solutions
of the Wheeler-DeWitt equation, but this does not violate Hermiticity
properties, because all these solutions are anyway of plane wave type and
describe cosmological dynamics without turning points and bounces.Comment: final version, to appear in Physical Review
Two-dimensional dilaton gravity in a unitary gauge
Reduced phase space formulation of CGHS model of 2d dilaton gravity is
studied in en extrinsic time gauge. The corresponding Hamiltonian can be
promoted into a Hermitian operator acting in the physical Hilbert space,
implying a unitary evolution for the system. Consequences for the black hole
physics are discussed. In particular, this manifestly unitary theory rules out
the Hawking scenario for the endpoint of the black hole evaporation process.Comment: 12 pages, LaTex file, Imperial-TP/92-93/8 and QMW/PH/92/1
Ashtekar Formulation of 2+1 Gravity on a Torus
Pure (2+1)-dimensional Einstein gravity is analysed in the Ashtekar
formulation, when the spatial manifold is a torus. We have found a set of
globally defined observables, forming a closed algebra. This allowed us to
solve the quantum constraints, and to show that the reduced phase space of the
Ashtekar formulation is greater then the corresponding space of the Witten
formulation. Furthermore, we have found a globally defined time variable which
satisfies all the requiriments of an extrinsic time variable in quantum
gravity.Comment: 15 page
Entropic sampling dynamics of the globally-coupled kinetic Ising model
The entropic sampling dynamics based on the reversible information transfer
to and from the environment is applied to the globally coupled Ising model in
the presence of an oscillating magnetic field. When the driving frequency is
low enough, coherence between the magnetization and the external magnetic field
is observed; such behavior tends to weaken with the system size. The time-scale
matching between the intrinsic time scale, defined in the absence of the
external magnetic field, and the extrinsic time scale, given by the inverse of
the driving frequency, is used to explain the observed coherence behavior.Comment: 8 page
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