3,287 research outputs found
The Kardar-Parisi-Zhang equation and universality class
Brownian motion is a continuum scaling limit for a wide class of random
processes, and there has been great success in developing a theory for its
properties (such as distribution functions or regularity) and expanding the
breadth of its universality class. Over the past twenty five years a new
universality class has emerged to describe a host of important physical and
probabilistic models (including one dimensional interface growth processes,
interacting particle systems and polymers in random environments) which display
characteristic, though unusual, scalings and new statistics. This class is
called the Kardar-Parisi-Zhang (KPZ) universality class and underlying it is,
again, a continuum object -- a non-linear stochastic partial differential
equation -- known as the KPZ equation. The purpose of this survey is to explain
the context for, as well as the content of a number of mathematical
breakthroughs which have culminated in the derivation of the exact formula for
the distribution function of the KPZ equation started with {\it narrow wedge}
initial data. In particular we emphasize three topics: (1) The approximation of
the KPZ equation through the weakly asymmetric simple exclusion process; (2)
The derivation of the exact one-point distribution of the solution to the KPZ
equation with narrow wedge initial data; (3) Connections with directed polymers
in random media. As the purpose of this article is to survey and review, we
make precise statements but provide only heuristic arguments with indications
of the technical complexities necessary to make such arguments mathematically
rigorous.Comment: 57 pages, survey article, 7 figures, addition physics ref. added and
typo's correcte
Soliton approach to the noisy Burgers equation: Steepest descent method
The noisy Burgers equation in one spatial dimension is analyzed by means of
the Martin-Siggia-Rose technique in functional form. In a canonical formulation
the morphology and scaling behavior are accessed by mean of a principle of
least action in the asymptotic non-perturbative weak noise limit. The ensuing
coupled saddle point field equations for the local slope and noise fields,
replacing the noisy Burgers equation, are solved yielding nonlinear localized
soliton solutions and extended linear diffusive mode solutions, describing the
morphology of a growing interface. The canonical formalism and the principle of
least action also associate momentum, energy, and action with a
soliton-diffusive mode configuration and thus provides a selection criterion
for the noise-induced fluctuations. In a ``quantum mechanical'' representation
of the path integral the noise fluctuations, corresponding to different paths
in the path integral, are interpreted as ``quantum fluctuations'' and the
growth morphology represented by a Landau-type quasi-particle gas of ``quantum
solitons'' with gapless dispersion and ``quantum diffusive modes'' with a gap
in the spectrum. Finally, the scaling properties are dicussed from a heuristic
point of view in terms of a``quantum spectral representation'' for the slope
correlations. The dynamic eponent z=3/2 is given by the gapless soliton
dispersion law, whereas the roughness exponent zeta =1/2 follows from a
regularity property of the form factor in the spectral representation. A
heuristic expression for the scaling function is given by spectral
representation and has a form similar to the probability distribution for Levy
flights with index .Comment: 30 pages, Revtex file, 14 figures, to be submitted to Phys. Rev.
Heuristic derivation of continuum kinetic equations from microscopic dynamics
We present an approximate and heuristic scheme for the derivation of
continuum kinetic equations from microscopic dynamics for stochastic,
interacting systems. The method consists of a mean-field type, decoupled
approximation of the master equation followed by the `naive' continuum limit.
The Ising model and driven diffusive systems are used as illustrations. The
equations derived are in agreement with other approaches, and consequences of
the microscopic dependences of coarse-grained parameters compare favorably with
exact or high-temperature expansions. The method is valuable when more
systematic and rigorous approaches fail, and when microscopic inputs in the
continuum theory are desirable.Comment: 7 pages, RevTeX, two-column, 4 PS figures include
On the universality of the incompressible Euler equation on compact manifolds
The incompressible Euler equations on a compact Riemannian manifold
take the form \begin{align*} \partial_t u + \nabla_u u &= - \mathrm{grad}_g p
\mathrm{div}_g u &= 0. \end{align*} We show that any quadratic ODE , where is a
symmetric bilinear map, can be linearly embedded into the incompressible Euler
equations for some manifold if and only if obeys the cancellation
condition for some positive definite inner
product on . This allows one to construct
explicit solutions to the Euler equations with various dynamical features, such
as quasiperiodic solutions, or solutions that transition from one steady state
to another, and provides evidence for the "Turing universality" of such Euler
flows.Comment: 14 pages, no figures, to appear, Discrete and Continuous Dynamical
Systems. This is the final versio
Fisher Hartwig determinants, conformal field theory and universality in generalised XX models
We discuss certain quadratic models of spinless fermions on a 1D lattice, and
their corresponding spin chains. These were studied by Keating and Mezzadri in
the context of their relation to the Haar measures of the classical compact
groups. We show how these models correspond to translation invariant models on
an infinite or semi-infinite chain, which in the simplest case reduce to the
familiar XX model. We give physical context to mathematical results for the
entanglement entropy, and calculate the spin-spin correlation functions using
the Fisher-Hartwig conjecture. These calculations rigorously demonstrate
universality in classes of these models. We show that these are in agreement
with field theoretic and renormalization group arguments that we provide
Infrared Behaviour of Systems With Goldstone Bosons
We develop various complementary concepts and techniques for handling quantum
fluctuations of Goldstone bosons.We emphasise that one of the consequences of
the masslessness of Goldstone bosons is that the longitudinal fluctuations also
have a diverging susceptibility characterised by an anomalous dimension
in space-time dimensions .In these fluctuations diverge
logarithmically in the infrared region.We show the generality of this
phenomenon by providing three arguments based on i). Renormalization group
flows, ii). Ward identities, and iii). Schwinger-Dyson equations.We obtain an
explicit form for the generating functional of one-particle irreducible
vertices of the O(N) (non)--linear --models in the leading 1/N
approximation.We show that this incorporates all infrared behaviour correctly
both in linear and non-linear -- models. Our techniques provide an
alternative to chiral perturbation theory.Some consequences are discussed
briefly.Comment: 28 pages,2 Figs, a new section on some universal features of
multipion processes has been adde
Geometric Path Integrals. A Language for Multiscale Biology and Systems Robustness
In this paper we suggest that, under suitable conditions, supervised learning
can provide the basis to formulate at the microscopic level quantitative
questions on the phenotype structure of multicellular organisms. The problem of
explaining the robustness of the phenotype structure is rephrased as a real
geometrical problem on a fixed domain. We further suggest a generalization of
path integrals that reduces the problem of deciding whether a given molecular
network can generate specific phenotypes to a numerical property of a
robustness function with complex output, for which we give heuristic
justification. Finally, we use our formalism to interpret a pointedly
quantitative developmental biology problem on the allowed number of pairs of
legs in centipedes
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