275 research outputs found
ATNoSFERES revisited
ATNoSFERES is a Pittsburgh style Learning Classifier System (LCS) in which
the rules are represented as edges of an Augmented Transition Network.
Genotypes are strings of tokens of a stack-based language, whose execution
builds the labeled graph. The original ATNoSFERES, using a bitstring to
represent the language tokens, has been favorably compared in previous work to
several Michigan style LCSs architectures in the context of Non Markov
problems. Several modifications of ATNoSFERES are proposed here: the most
important one conceptually being a representational change: each token is now
represented by an integer, hence the genotype is a string of integers; several
other modifications of the underlying grammar language are also proposed. The
resulting ATNoSFERES-II is validated on several standard animat Non Markov
problems, on which it outperforms all previously published results in the LCS
literature. The reasons for these improvement are carefully analyzed, and some
assumptions are proposed on the underlying mechanisms in order to explain these
good results
Relativistic magnetohydrodynamics in one dimension
We derive a number of solution for one-dimensional dynamics of relativistic
magnetized plasma that can be used as benchmark estimates in relativistic
hydrodynamic and magnetohydrodynamic numerical codes.
First, we analyze the properties of simple waves of fast modes propagating
orthogonally to the magnetic field in relativistically hot plasma. The magnetic
and kinetic pressures obey different equations of state, so that the system
behaves as a mixture of gases with different polytropic indices. We find the
self-similar solutions for the expansion of hot strongly magnetized plasma into
vacuum.
Second, we derive linear hodograph and Darboux equations for the relativistic
Khalatnikov potential, which describe arbitrary one-dimensional isentropic
relativistic motion of cold magnetized plasma and find their general and
particular solutions. The obtained hodograph and Darboux equations are very
powerful: system of highly non-linear, relativistic, time dependent equations
describing arbitrary (not necessarily self-similar) dynamics of highly
magnetized plasma reduces to a single linear differential equation.Comment: accepted by Phys. Rev.
Federalism, Democracy, and the 2020 Election
In the aftermath of the 2020 election, the United States has experienced an anti-democratic crisis, with a chief executive attempting to delegitimize the general election and declare victory in an election that all impartial observers stated he lost. In comparative terms, the U.S. election system has been much maligned – it is highly localized and partisan, and lacks the independent, apex institutions such as electoral tribunals that are characteristic of many modern democracies. This brief essay builds off our recent joint work on federalism to argue that state and local governments, which administer elections and have refuted claims of widespread voter fraud, are serving as important bulwarks against this threat. By separating and dispersing the functions of governance—the day to day work of governing—U.S. federalism provides protection against authoritarianism. The decentralization of authority over elections offers one particularly dramatic example of this dynamic in action. Indeed, the U.S. model of dispersing core functions, although messy and costly in other ways, may have important advantages in some contexts over the alternative model of centralized, apex institutions, especially by reducing vulnerability to capture
Liquid-gas and other unusual thermal phase transitions in some large-N magnets
Much insight into the low temperature properties of quantum magnets has been
gained by generalizing them to symmetry groups of order N, and then studying
the large N limit. In this paper we consider an unusual aspect of their finite
temperature behavior--their exhibiting a phase transition between a perfectly
paramagetic state and a paramagnetic state with a finite correlation length at
N = \infty. We analyze this phenomenon in some detail in the large ``spin''
(classical) limit of the SU(N) ferromagnet which is also a lattice
discretization of the CP^{N-1} model. We show that at N = \infty the order of
the transition is governed by lattice connectivity. At finite values of N, the
transition goes away in one or less dimension but survives on many lattices in
two dimensions and higher, for sufficiently large N. The latter conclusion
contradicts a recent conjecture of Sokal and Starinets, yet is consistent with
the known finite temperature behavior of the SU(2) case. We also report closely
related first order paramagnet-ferromagnet transitions at large N and shed
light on a violation of Elitzur's theorem at infinite N via the large q limit
of the q-state Potts model, reformulated as an Ising gauge theory.Comment: 27 pages, 7 figures. Added clarifications requested by a refere
A Rigorous Derivation of Electromagnetic Self-force
During the past century, there has been considerable discussion and analysis
of the motion of a point charge, taking into account "self-force" effects due
to the particle's own electromagnetic field. We analyze the issue of "particle
motion" in classical electromagnetism in a rigorous and systematic way by
considering a one-parameter family of solutions to the coupled Maxwell and
matter equations corresponding to having a body whose charge-current density
and stress-energy tensor scale to zero size
in an asymptotically self-similar manner about a worldline as . In this limit, the charge, , and total mass, , of the body go to
zero, and goes to a well defined limit. The Maxwell field
is assumed to be the retarded solution associated with
plus a homogeneous solution (the "external field") that varies
smoothly with . We prove that the worldline must be a
solution to the Lorentz force equations of motion in the external field
. We then obtain self-force, dipole forces, and spin force
as first order perturbative corrections to the center of mass motion of the
body. We believe that this is the first rigorous derivation of the complete
first order correction to Lorentz force motion. We also address the issue of
obtaining a self-consistent perturbative equation of motion associated with our
perturbative result, and argue that the self-force equations of motion that
have previously been written down in conjunction with the "reduction of order"
procedure should provide accurate equations of motion for a sufficiently small
charged body with negligible dipole moments and spin. There is no corresponding
justification for the non-reduced-order equations.Comment: 52 pages, minor correction
On the Speed of Gravity and Relativistic v/c Corrections to the Shapiro Time Delay
Recent papers by Samuel declared that the linearized post-Newtonian v/c
effects are too small to have been measured in the recent experiment involving
Jupiter and quasar J0842+1845 that was used to measure the ultimate speed of
gravity defined as a fundamental constant entering in front of each time
derivative of the metric tensor in the Einstein gravity field equations. We
describe our Lorentz-invariant formulation of the Jovian deflection experiment
and confirm that v/c effects are do observed, as contrasted to the erroneous
claim by Samuel, and that they vanish if and only if the speed of gravity is
infinite.Comment: 7 pages. Final version published in Physics Letters
Theory of photoinduced charge transfer in weakly coupled donor-acceptor conjugated polymers: application to an MEH-PPV:CN-PPV pair
In a pair of coupled donor-acceptor conjugated polymer chains, it is possible
for an exciton photoexcited on either polymer to decay into a hole in the donor
polymer's valence band and an electron in the conduction band of the acceptor
polymer. We calculate the corresponding exciton decay rate and its dependence
on inter-polymer distance. For a pair of derivatives of poly(phenylene
vinylene), PPV, specifically poly[2-methoxy, 5-(2-ethyl-hexyloxy)-1, 4
PPV], MEH-PPV, and poly(2,5-hexyloxy -phenylene cyanovinylene), CN-PPV, at a
separation of 6 \AA the characteristic decay time is 2.2 ps, whereas at 4 \AA
it is fs.Comment: 9 pages, RevTeX, 4 PS files, to be published in a special issue of
Chem. Phy
Geometric phases for wave packets in a uniform magnetic field
A wave packet of a charged particle always make cyclic circular motion in a
uniform magnetic field, just like a classical particle. The nonadiabatic
geometric phase for an arbitrary wave packet can be expressed in terms of the
mean value of a number operator. For a large class of wave packets, the
geometric phase is proportional to the magnetic flux encircled by the orbit of
the wave packet. For more general wave packets, however, the geometric phase
contains an extra term.Comment: REVTeX4, 7 pages, no figur
Elasticity of Stiff Biopolymers
We present a statistical mechanical study of stiff polymers, motivated by
experiments on actin filaments and the considerable current interest in polymer
networks. We obtain simple, approximate analytical forms for the
force-extension relations and compare these with numerical treatments. We note
the important role of boundary conditions in determining force-extension
relations. The theoretical predictions presented here can be tested against
single molecule experiments on neurofilaments and cytoskeletal filaments like
actin and microtubules. Our work is motivated by the buckling of the
cytoskeleton of a cell under compression, a phenomenon of interest to biology.Comment: Submitted for publication, five pages, three figure
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