723 research outputs found
Model for energy transfer in the solar wind: Formulation of model
The two-fluid solar-wind model is extended by including the collisionless dissipation of hydromagnetic waves originating at the sun. A series of solar wind models is generated, parameterized by the total energy flux of hydromagnetic waves at the base of the model. The resulting properties of propagation and dissipating of hydromagnetic waves on this model are presented
Wind enhanced planetary escape: Collisional modifications
The problem of thermal escape is considered in which both the effects of thermospheric winds at the exobase and collisions below the exobase are included in a Monte Carlo calculation. The collisions are included by means of a collisional relaxation layer of a background gas which models the transition region between the exosphere and the thermosphere. The wind effects are considered in the limiting cases of vertical and horizontal flows. Two species are considered: terrestrial hydrogen and terrestrial helium. In the cases of terrestrial hydrogen the escape fluxes were found to be strongly filtered or throttled by collisions at high exospheric temperatures. The model is applied to molecular hydrogen diffusing through a methane relaxation layer under conditions possible on Titan. The results are similar to the case of terrestrial hydrogen with wind enhanced escape being strongly suppressed by collisions. It is concluded that wind enhanced escape is not an important process on Titan
Signature of the Simplicial Supermetric
We investigate the signature of the Lund-Regge metric on spaces of simplicial
three-geometries which are important in some formulations of quantum gravity.
Tetrahedra can be joined together to make a three-dimensional piecewise linear
manifold. A metric on this manifold is specified by assigning a flat metric to
the interior of the tetrahedra and values to their squared edge-lengths. The
subset of the space of squared edge-lengths obeying triangle and analogous
inequalities is simplicial configuration space. We derive the Lund-Regge metric
on simplicial configuration space and show how it provides the shortest
distance between simplicial three-geometries among all choices of gauge inside
the simplices for defining this metric (Regge gauge freedom). We show
analytically that there is always at least one physical timelike direction in
simplicial configuration space and provide a lower bound on the number of
spacelike directions. We show that in the neighborhood of points in this space
corresponding to flat metrics there are spacelike directions corresponding to
gauge freedom in assigning the edge-lengths. We evaluate the signature
numerically for the simplicial configuration spaces based on some simple
triangulations of the three-sphere (S^3) and three-torus (T^3). For the surface
of a four-simplex triangulation of S^3 we find one timelike direction and all
the rest spacelike over all of the simplicial configuration space. For the
triangulation of T^3 around flat space we find degeneracies in the simplicial
supermetric as well as a few gauge modes corresponding to a positive
eigenvalue. Moreover, we have determined that some of the negative eigenvalues
are physical, i.e. the corresponding eigenvectors are not generators of
diffeomorphisms. We compare our results with the known properties of continuum
superspace.Comment: 24 pages, RevTeX, 4 eps Figures. Submitted to Classical Quantum
Gravit
Conditional probabilities in Ponzano-Regge minisuperspace
We examine the Hartle-Hawking no-boundary initial state for the Ponzano-Regge
formulation of gravity in three dimensions. We consider the behavior of
conditional probabilities and expectation values for geometrical quantities in
this initial state for a simple minisuperspace model consisting of a
two-parameter set of anisotropic geometries on a 2-sphere boundary. We find
dependence on the cutoff used in the construction of Ponzano-Regge amplitudes
for expectation values of edge lengths. However, these expectation values are
cutoff independent when computed in certain, but not all, conditional
probability distributions. Conditions that yield cutoff independent expectation
values are those that constrain the boundary geometry to a finite range of edge
lengths. We argue that such conditions have a correspondence to fixing a range
of local time, as classically associated with the area of a surface for
spatially closed cosmologies. Thus these results may hint at how classical
spacetime emerges from quantum amplitudes.Comment: 26 pages including 10 figures, some reorganization in the
presentation of results, expanded discussion of results in the context of 2+1
gravity in the Witten variables, 3 new reference
Quasiclassical Coarse Graining and Thermodynamic Entropy
Our everyday descriptions of the universe are highly coarse-grained,
following only a tiny fraction of the variables necessary for a perfectly
fine-grained description. Coarse graining in classical physics is made natural
by our limited powers of observation and computation. But in the modern quantum
mechanics of closed systems, some measure of coarse graining is inescapable
because there are no non-trivial, probabilistic, fine-grained descriptions.
This essay explores the consequences of that fact. Quantum theory allows for
various coarse-grained descriptions some of which are mutually incompatible.
For most purposes, however, we are interested in the small subset of
``quasiclassical descriptions'' defined by ranges of values of averages over
small volumes of densities of conserved quantities such as energy and momentum
and approximately conserved quantities such as baryon number. The
near-conservation of these quasiclassical quantities results in approximate
decoherence, predictability, and local equilibrium, leading to closed sets of
equations of motion. In any description, information is sacrificed through the
coarse graining that yields decoherence and gives rise to probabilities for
histories. In quasiclassical descriptions, further information is sacrificed in
exhibiting the emergent regularities summarized by classical equations of
motion. An appropriate entropy measures the loss of information. For a
``quasiclassical realm'' this is connected with the usual thermodynamic entropy
as obtained from statistical mechanics. It was low for the initial state of our
universe and has been increasing since.Comment: 17 pages, 0 figures, revtex4, Dedicated to Rafael Sorkin on his 60th
birthday, minor correction
Flux estimates of ions from the lunar exosphere
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95219/1/grl29279.pd
The Generalized Hartle-Hawking Initial State: Quantum Field Theory on Einstein Conifolds
Recent arguments have indicated that the sum over histories formulation of
quantum amplitudes for gravity should include sums over conifolds, a set of
histories with more general topology than that of manifolds. This paper
addresses the consequences of conifold histories in gravitational functional
integrals that also include scalar fields. This study will be carried out
explicitly for the generalized Hartle-Hawking initial state, that is the
Hartle-Hawking initial state generalized to a sum over conifolds. In the
perturbative limit of the semiclassical approximation to the generalized
Hartle-Hawking state, one finds that quantum field theory on Einstein conifolds
is recovered. In particular, the quantum field theory of a scalar field on de
Sitter spacetime with spatial topology is derived from the generalized
Hartle-Hawking initial state in this approximation. This derivation is carried
out for a scalar field of arbitrary mass and scalar curvature coupling.
Additionally, the generalized Hartle-Hawking boundary condition produces a
state that is not identical to but corresponds to the Bunch-Davies vacuum on
de Sitter spacetime. This result cannot be obtained from the original
Hartle-Hawking state formulated as a sum over manifolds as there is no Einstein
manifold with round boundary.Comment: Revtex 3, 31 pages, 4 epsf figure
Primordial fluctuations from nonlinear couplings
We study the spectrum of primordial fluctuations in theories where the
inflaton field is coupled to massless fields and/or to itself. Conformally
invariant theories generically predict a scale invariant spectrum. Scales
entering the theory through infrared divergences cause logarithmic corrections
to the spectrum, tiltilng it towards the blue. We discuss in some detail
whether these fluctuations are quantum or classical in nature.Comment: 12 pages, Revtex, we added an appendix clarifying our assumptions
about the initial conditions at the beggining of inflatio
Modeling the decoherence of spacetime
The question of whether unobserved short-wavelength modes of the
gravitational field can induce decoherence in the long-wavelength modes (``the
decoherence of spacetime'') is addressed using a simplified model of
perturbative general relativity, related to the Nordstrom-Einstein-Fokker
theory, where the metric is assumed to be conformally flat. For some
long-wavelength coarse grainings, the Feynman-Vernon influence phase is found
to be effective at suppressing the off-diagonal elements of the decoherence
functional. The requirement that the short-wavelength modes be in a
sufficiently high-temperature state places limits on the applicability of this
perturbative approach.Comment: 38 pages, REVTeX; 7 diagrams and 6 PostScript figures included via
epsfig. Final cosmetic changes made at publicatio
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