323 research outputs found

### Order reductions of Lorentz-Dirac-like equations

We discuss the phenomenon of preacceleration in the light of a method of
successive approximations used to construct the physical order reduction of a
large class of singular equations. A simple but illustrative physical example
is analyzed to get more insight into the convergence properties of the method.Comment: 6 pages, LaTeX, IOP macros, no figure

### Tracking solutions in tachyon cosmology

We perform a thorough phase-plane analysis of the flow defined by the
equations of motion of a FRW universe filled with a tachyonic fluid plus a
barotropic one. The tachyon potential is assumed to be of inverse square form,
thus allowing for a two-dimensional autonomous system of equations. The
Friedmann constraint, combined with a convenient choice of coordinates, renders
the physical state compact. We find the fixed-point solutions, and discuss
whether they represent attractors or not. The way the two fluids contribute at
late-times to the fractional energy density depends on how fast the barotropic
fluid redshifts. If it does it fast enough, the tachyonic fluid takes over at
late times, but if the opposite happens, the situation will not be completely
dominated by the barotropic fluid; instead there will be a residual
non-negligible contribution from the tachyon subject to restrictions coming
from nucleosynthesis.Comment: 5 pages, 4 figure

### An interaction Lagrangian for two spin 1/2 elementary Dirac particles

The kinematical formalism for describing spinning particles developped by the
author is based upon the idea that an elementary particle is a physical system
with no excited states. It can be annihilated by the interaction with its
antiparticle but, if not destroyed, its internal structure can never be
modified. All possible states of the particle are just kinematical
modifications of any one of them. The kinematical state space of the
variational formalism of an elementary particle is necessarily a homogeneous
space of the kinematical group of spacetime symmetries. By assuming Poincare
invariance we have already described a model of a classical spinning particle
which satisfies Dirac's equation when quantized. We have recently shown that
the spacetime symmetry group of this Dirac particle is larger than the Poincare
group. It also contains spacetime dilations and local rotations. In this work
we obtain an interaction Lagrangian for two Dirac particles, which is invariant
under this enlarged spacetime group. It describes a short- and long-range
interaction such that when averaged, to supress the spin content of the
particles, describes the instantaneous Coulomb interaction between them. As an
application, we analyse the interaction between two spinning particles, and
show that it is possible the existence of metastable bound states for two
particles of the same charge, when the spins are parallel and provided some
initial conditions are fulfilled. The possibility of formation of bound pairs
is due to the zitterbewegung spin structure of the particles because when the
spin is neglected, the bound states vanish

### General behaviour of Bianchi VI_0 solutions with an exponential-potential scalar field

The solutions to the Einstein-Klein-Gordon equations without a cosmological
constant are investigated for an exponential potential in a Bianchi VI_0
metric. There exists a two-parameter family of solutions which have a power-law
inflationary behaviour when the exponent of the potential, k, satisfies k^2<2.
In addition, there exists a two-parameter family of singular solutions for all
k^2 values. A simple anisotropic exact solution is found to be stable when
2<k^2.Comment: 10 pages, no figures. To be published in General Relativity and
Gravitatio

### On the Solutions of the Lorentz-Dirac Equation

We discuss the unstable character of the solutions of the Lorentz-Dirac
equation and stress the need of methods like order reduction to derive a
physically acceptable equation of motion. The discussion is illustrated with
the paradigmatic example of the non-relativistic harmonic oscillator with
radiation reaction. We also illustrate removal of the noncasual
pre-acceleration with the introduction of a small correction in the
Lorentz-Dirac equation.Comment: 4 eps figs. to be published in GR

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