18,550 research outputs found
A model of quantum reduction with decoherence
The problem of reduction (wave packet reduction) is reexamined under two
simple conditions: Reduction is a last step completing decoherence. It acts in
commonplace circumstances and should be therefore compatible with the
mathematical frame of quantum field theory and the standard model.
These conditions lead to an essentially unique model for reduction.
Consistency with renormalization and time-reversal violation suggest however a
primary action in the vicinity of Planck's length. The inclusion of quantum
gravity and the uniqueness of space-time point moreover to generalized quantum
theory, first proposed by Gell-Mann and Hartle, as a convenient framework for
developing this model into a more complete theory.Comment: 20 pages. To be published in Physical Review
Effective power-law dependence of Lyapunov exponents on the central mass in galaxies
Using both numerical and analytical approaches, we demonstrate the existence
of an effective power-law relation between the mean Lyapunov
exponent of stellar orbits chaotically scattered by a supermassive black
hole in the center of a galaxy and the mass parameter , i.e. ratio of the
mass of the black hole over the mass of the galaxy. The exponent is found
numerically to obtain values in the range --. We propose a
theoretical interpretation of these exponents, based on estimates of local
`stretching numbers', i.e. local Lyapunov exponents at successive transits of
the orbits through the black hole's sphere of influence. We thus predict
with --. Our basic model refers to elliptical
galaxy models with a central core. However, we find numerically that an
effective power law scaling of with holds also in models with central
cusp, beyond a mass scale up to which chaos is dominated by the influence of
the cusp itself. We finally show numerically that an analogous law exists also
in disc galaxies with rotating bars. In the latter case, chaotic scattering by
the black hole affects mainly populations of thick tube-like orbits surrounding
some low-order branches of the family of periodic orbits, as well as its
bifurcations at low-order resonances, mainly the Inner Lindbland resonance and
the 4/1 resonance. Implications of the correlations between and to
determining the rate of secular evolution of galaxies are discussed.Comment: 27 pages, 19 figure
A Central Limit Theorem for biased random walks on Galton-Watson trees
Let be a rooted Galton-Watson tree with offspring distribution
that has , mean and exponential tails.
Consider the -biased random walk on ;
this is the nearest neighbor random walk which, when at a vertex with
offspring, moves closer to the root with probability ,
and moves to each of the offspring with probability . It is
known that this walk has an a.s. constant speed
(where is the distance of from the root), with for and for . For all , we prove
a quenched CLT for |X_n|-n\v. (For the walk is positive
recurrent, and there is no CLT.) The most interesting case by far is
, where the CLT has the following form: for almost every ,
the ratio converges in law as to a
deterministic multiple of the absolute value of a Brownian motion. Our approach
to this case is based on an explicit description of an invariant measure for
the walk from the point of view of the particle (previously, such a measure was
explicitly known only for ) and the construction of appropriate
harmonic coordinates.Comment: 34 pages, 4 figure
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