2,173 research outputs found
DNA viewed as an out-of-equilibrium structure
The complexity of the primary structure of human DNA is explored using
methods from nonequilibrium statistical mechanics, dynamical systems theory and
information theory. The use of chi-square tests shows that DNA cannot be
described as a low order Markov chain of order up to . Although detailed
balance seems to hold at the level of purine-pyrimidine notation it fails when
all four basepairs are considered, suggesting spatial asymmetry and
irreversibility. Furthermore, the block entropy does not increase linearly with
the block size, reflecting the long range nature of the correlations in the
human genomic sequences. To probe locally the spatial structure of the chain we
study the exit distances from a specific symbol, the distribution of recurrence
distances and the Hurst exponent, all of which show power law tails and long
range characteristics. These results suggest that human DNA can be viewed as a
non-equilibrium structure maintained in its state through interactions with a
constantly changing environment. Based solely on the exit distance distribution
accounting for the nonequilibrium statistics and using the Monte Carlo
rejection sampling method we construct a model DNA sequence. This method allows
to keep all long range and short range statistical characteristics of the
original sequence. The model sequence presents the same characteristic
exponents as the natural DNA but fails to capture point-to-point details
Unitary Evolution on a Discrete Phase Space
We construct unitary evolution operators on a phase space with power of two
discretization. These operators realize the metaplectic representation of the
modular group SL(2,Z_{2^n}). It acts in a natural way on the coordinates of the
non-commutative 2-torus, T_{2^n}^2$ and thus is relevant for non-commutative
field theories as well as theories of quantum space-time. The class of
operators may also be useful for the efficient realization of new quantum
algorithms.Comment: 5 pages, contribution to Lattice 2005 (theoretical developments
Nonlinear dynamic systems in the geosciences
Geophysical phenomena are often characterized by complex, random-looking deviations of the relevant variables from their average values. Typical examples of such aperiodicity are the intermittent succession of Quaternary glaciations as revealed by the oxygen isotope record of deep-sea cores of the last 106 years or the pronounced spatial disorder characterizing geologic materials. A major task of the geoscientist is to reconstitute from this type of record the principal mechanisms responsible for the observed behavior. Traditional approaches attribute the complexity encountered in the record of a natural variable to external uncontrollable factors and to poorly known parameters whose presence tends to blur fundamental underlying regularities. Here, we consider that complexity might be an intrinsic property generated by the nonlinear character of the system's dynamics. We review bifurcations, chaos, and fractals, three important mechanisms leading to complex behavior in nonlinear dynamic systems, and stress the role of the theory of nonlinear dynamic systems as a major tool of interdisciplinary research in the geosciences. The general ideas are illustrated on the dynamics of Quaternary glaciations and the dynamics of tracer transport in a sediment
Nonlinear dynamic systems in the geosciences
Geophysical phenomena are often characterized by complex, random-looking deviations of the relevant variables from their average values. Typical examples of such aperiodicity are the intermittent succession of Quaternary glaciations as revealed by the oxygen isotope record of deep-sea cores of the last 106 years or the pronounced spatial disorder characterizing geologic materials. A major task of the geoscientist is to reconstitute from this type of record the principal mechanisms responsible for the observed behavior. Traditional approaches attribute the complexity encountered in the record of a natural variable to external uncontrollable factors and to poorly known parameters whose presence tends to blur fundamental underlying regularities. Here, we consider that complexity might be an intrinsic property generated by the nonlinear character of the system's dynamics. We review bifurcations, chaos, and fractals, three important mechanisms leading to complex behavior in nonlinear dynamic systems, and stress the role of the theory of nonlinear dynamic systems as a major tool of interdisciplinary research in the geosciences. The general ideas are illustrated on the dynamics of Quaternary glaciations and the dynamics of tracer transport in a sediment
Thermostating by Deterministic Scattering: Heat and Shear Flow
We apply a recently proposed novel thermostating mechanism to an interacting
many-particle system where the bulk particles are moving according to
Hamiltonian dynamics. At the boundaries the system is thermalized by
deterministic and time-reversible scattering. We show how this scattering
mechanism can be related to stochastic boundary conditions. We subsequently
simulate nonequilibrium steady states associated to thermal conduction and
shear flow for a hard disk fluid. The bulk behavior of the model is studied by
comparing the transport coefficients obtained from computer simulations to
theoretical results. Furthermore, thermodynamic entropy production and
exponential phase-space contraction rates in the stationary nonequilibrium
states are calculated showing that in general these quantities do not agree.Comment: 16 pages (revtex) with 9 figures (postscript
Thermostating by deterministic scattering: the periodic Lorentz gas
We present a novel mechanism for thermalizing a system of particles in
equilibrium and nonequilibrium situations, based on specifically modeling
energy transfer at the boundaries via a microscopic collision process. We apply
our method to the periodic Lorentz gas, where a point particle moves
diffusively through an ensemble of hard disks arranged on a triangular lattice.
First, collision rules are defined for this system in thermal equilibrium. They
determine the velocity of the moving particle such that the system is
deterministic, time reversible, and microcanonical. These collision rules can
systematically be adapted to the case where one associates arbitrarily many
degrees of freedom to the disk, which here acts as a boundary. Subsequently,
the system is investigated in nonequilibrium situations by applying an external
field. We show that in the limit where the disk is endowed by infinitely many
degrees of freedom it acts as a thermal reservoir yielding a well-defined
nonequilibrium steady state. The characteristic properties of this state, as
obtained from computer simulations, are finally compared to the ones of the
so-called Gaussian thermostated driven Lorentz gas.Comment: 13 pages (revtex) with 10 figures (encapsulated postscript
Thermostatting by deterministic scattering
We present a mechanism for thermalizing a moving particle by microscopic
deterministic scattering. As an example, we consider the periodic Lorentz gas.
We modify the collision rules by including energy transfer between particle and
scatterer such that the scatterer mimics a thermal reservoir with arbitrarily
many degrees of freedom. The complete system is deterministic, time-reversible,
and provides a microcanonical density in equilibrium. In the limit of the disk
representing infinitely many degrees of freedom and by applying an electric
field the system goes into a nonequilibrium steady state.Comment: 4 pages (revtex) with 4 figures (postscript
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