410 research outputs found
Morgan-Morgan-NUT disk space via the Ehlers transformation
Using the Ehlers transformation along with the gravitoelectromagnetic
approach to stationary spacetimes we start from the Morgan-Morgan disk
spacetime (without radial pressure) as the seed metric and find its
corresponding stationary spacetime. As expected from the Ehlers transformation
the stationary spacetime obtained suffers from a NUT-type singularity and the
new parameter introduced in the stationary case could be interpreted as the
gravitomagnetic monopole charge (or the NUT factor). As a consequence of this
singularity there are closed timelike curves (CTCs) in the singular region of
the spacetime. Some of the properties of this spacetime including its particle
velocity distribution, gravitational redshift, stability and energy conditions
are discussed.Comment: 18 pages, 5 figures, RevTex 4, replaced with the published versio
Aichelburg-Sexl boost of an isolated source in general relativity
A study of the Aichelburg--Sexl boost of the Schwarzschild field is described
in which the emphasis is placed on the field (curvature tensor) with the metric
playing a secondary role. This is motivated by a description of the Coulomb
field of a charged particle viewed by an observer whose speed relative to the
charge approaches the speed of light. Our approach is exemplified by carrying
out an Aichelburg-- Sexl type boost on the Weyl vacuum gravitational field due
to an isolated axially symmetric source. Detailed calculations of the boosts
transverse and parallel to the symmetry axis are given and the results, which
differ significantly, are discussed.Comment: 25 pages, LateX2
Teleparallel Energy-Momentum Distribution of Static Axially Symmetric Spacetimes
This paper is devoted to discuss the energy-momentum for static axially
symmetric spacetimes in the framework of teleparallel theory of gravity. For
this purpose, we use the teleparallel versions of Einstein, Landau-Lifshitz,
Bergmann and Mller prescriptions. A comparison of the results shows
that the energy density is different but the momentum turns out to be constant
in each prescription. This is exactly similar to the results available in
literature using the framework of General Relativity. It is mentioned here that
Mller energy-momentum distribution is independent of the coupling
constant . Finally, we calculate energy-momentum distribution for the
Curzon metric, a special case of the above mentioned spacetime.Comment: 14 pages, accepted for publication in Mod. Phys. Lett.
Brownian Carnot engine
The Carnot cycle imposes a fundamental upper limit to the efficiency of a
macroscopic motor operating between two thermal baths. However, this bound
needs to be reinterpreted at microscopic scales, where molecular bio-motors and
some artificial micro-engines operate. As described by stochastic
thermodynamics, energy transfers in microscopic systems are random and thermal
fluctuations induce transient decreases of entropy, allowing for possible
violations of the Carnot limit. Despite its potential relevance for the
development of a thermodynamics of small systems, an experimental study of
microscopic Carnot engines is still lacking. Here we report on an experimental
realization of a Carnot engine with a single optically trapped Brownian
particle as working substance. We present an exhaustive study of the energetics
of the engine and analyze the fluctuations of the finite-time efficiency,
showing that the Carnot bound can be surpassed for a small number of
non-equilibrium cycles. As its macroscopic counterpart, the energetics of our
Carnot device exhibits basic properties that one would expect to observe in any
microscopic energy transducer operating with baths at different temperatures.
Our results characterize the sources of irreversibility in the engine and the
statistical properties of the efficiency -an insight that could inspire novel
strategies in the design of efficient nano-motors.Comment: 7 pages, 7 figure
Relativistic Static Thin Disks with Radial Stress Suport
New solutions for static non-rotating thin disks of finite radius with
nonzero radial stress are studied. A method to introduce either radial pressure
or radial tension is presented. The method is based on the use of conformal
transformations.Comment: 19 pages, LaTeX, 7 figures, submitted to Class. Quan. Gra
Performance of discrete heat engines and heat pumps in finite time
The performance in finite time of a discrete heat engine with internal
friction is analyzed. The working fluid of the engine is composed of an
ensemble of noninteracting two level systems. External work is applied by
changing the external field and thus the internal energy levels. The friction
induces a minimal cycle time. The power output of the engine is optimized with
respect to time allocation between the contact time with the hot and cold baths
as well as the adiabats. The engine's performance is also optimized with
respect to the external fields. By reversing the cycle of operation a heat pump
is constructed. The performance of the engine as a heat pump is also optimized.
By varying the time allocation between the adiabats and the contact time with
the reservoir a universal behavior can be identified. The optimal performance
of the engine when the cold bath is approaching absolute zero is studied. It is
found that the optimal cooling rate converges linearly to zero when the
temperature approaches absolute zero.Comment: 45 pages LaTeX, 25 eps figure
Chaos in Static Axisymmetric Spacetimes I : Vacuum Case
We study the motion of test particle in static axisymmetric vacuum spacetimes
and discuss two criteria for strong chaos to occur: (1) a local instability
measured by the Weyl curvature, and (2) a tangle of a homoclinic orbit, which
is closely related to an unstable periodic orbit in general relativity. We
analyze several static axisymmetric spacetimes and find that the first
criterion is a sufficient condition for chaos, at least qualitatively. Although
some test particles which do not satisfy the first criterion show chaotic
behavior in some spacetimes, these can be accounted for the second criterion.Comment: More comments for the quantitative estimation of chaos are added, and
some inappropriate terms are changed. This will appear on Class. Quant. Gra
Relativistic Static Thin Disks: The Counter-Rotating Model
A detailed study of the Counter-Rotating Model (CRM) for generic finite
static axially symmetric thin disks with nonzero radial pressure is presented.
We find a general constraint over the counter-rotating tangential velocities
needed to cast the surface energy-momentum tensor of the disk as the
superposition of two counter-rotating perfect fluids. We also found expressions
for the energy density and pressure of the counter-rotating fluids. Then we
shown that, in general, there is not possible to take the two counter-rotating
fluids as circulating along geodesics neither take the two counter-rotating
tangential velocities as equal and opposite. An specific example is studied
where we obtain some CRM with well defined counter-rotating tangential
velocities and stable against radial perturbations. The CRM obtained are in
agree with the strong energy condition, but there are regions of the disks with
negative energy density, in violation of the weak energy condition.Comment: 19 pages, 6 figures. Submitted to Physical Review
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