4,153 research outputs found
A slowly rotating perfect fluid body in an ambient vacuum
A global model of a slowly rotating perfect fluid ball in general relativity
is presented. To second order in the rotation parameter, the junction surface
is an ellipsoidal cylinder. The interior is given by a limiting case of the
Wahlquist solution, and the vacuum region is not asymptotically flat. The
impossibility of joining an asymptotically flat vacuum region has been shown in
a preceding work.Comment: 7 pages, published versio
The quadrupole moment of slowly rotating fluid balls
In this paper we use the second order formalism of Hartle to study slowly and
rigidly rotating stars with focus on the quadrupole moment of the object. The
second order field equations for the interior fluid are solved numerically for
different classes of possible equations of state and these solutions are then
matched to a vacuum solution that includes the general asymptotically flat
axisymmetric metric to second order, using the Darmois-Israel procedure. For
these solutions we find that the quadrupole moment differs from that of the
Kerr metric, as has also been found for some equations of state in other
studies. Further we consider the post-Minkowskian limit analytically. In the
paper we also illustrate how the relativistic multipole moments can be
calculated from a complex gravitational potential.Comment: 13 pages, 5 figure
Variable - temperature scanning optical and force microscope
The implementation of a scanning microscope capable of working in confocal,
atomic force and apertureless near field configurations is presented. The
microscope is designed to operate in the temperature range 4 - 300 K, using
conventional helium flow cryostats. In AFM mode, the distance between the
sample and an etched tungsten tip is controlled by a self - sensing
piezoelectric tuning fork. The vertical position of both the AFM head and
microscope objective can be accurately controlled using piezoelectric coarse
approach motors. The scanning is performed using a compact XYZ stage, while the
AFM and optical head are kept fixed, allowing scanning probe and optical
measurements to be acquired simultaneously and in concert. The free optical
axis of the microscope enables both reflection and transmission experiments to
be performed.Comment: 24 pages, 9 figures, submitted to the journal "Review of Scientific
Instruments
Cognitive Computation sans Representation
The Computational Theory of Mind (CTM) holds that cognitive processes are essentially computational, and hence computation provides the scientific key to explaining mentality. The Representational Theory of Mind (RTM) holds that representational content is the key feature in distinguishing mental from non-mental systems. I argue that there is a deep incompatibility between these two theoretical frameworks, and that the acceptance of CTM provides strong grounds for rejecting RTM. The focal point of the incompatibility is the fact that representational content is extrinsic to formal procedures as such, and the intended interpretation of syntax makes no difference to the execution of an algorithm. So the unique 'content' postulated by RTM is superfluous to the formal procedures of CTM. And once these procedures are implemented in a physical mechanism, it is exclusively the causal properties of the physical mechanism that are responsible for all aspects of the system's behaviour. So once again, postulated content is rendered superfluous. To the extent that semantic content may appear to play a role in behaviour, it must be syntactically encoded within the system, and just as in a standard computational artefact, so too with the human mind/brain - it's pure syntax all the way down to the level of physical implementation. Hence 'content' is at most a convenient meta-level gloss, projected from the outside by human theorists, which itself can play no role in cognitive processing
Renormalized effective actions for the O(N) model at next-to-leading order of the 1/N expansion
A fully explicit renormalized quantum action functional is constructed for
the O(N)-model in the auxiliary field formulation at next-to-leading order
(NLO) of the 1/N expansion. Counterterms are consistently and explicitly
derived for arbitrary constant vacuum expectation value of the scalar and
auxiliary fields. The renormalized NLO pion propagator is exact at this order
and satisfies Goldstone's theorem. Elimination of the auxiliary field sector at
the level of the functional provides with order N^0 accuracy the renormalized
effective action of the model in terms of the original variables. Alternative
elimination of the pion and sigma propagators provides the renormalized NLO
effective potential for the expectation values of the N-vector and of the
auxiliary field with the same accuracy.Comment: RevTeX4, 19 pages, 3 figures. Version published Phys. Rev.
Degenerate distributions in complex Langevin dynamics: one-dimensional QCD at finite chemical potential
We demonstrate analytically that complex Langevin dynamics can solve the sign
problem in one-dimensional QCD in the thermodynamic limit. In particular, it is
shown that the contributions from the complex and highly oscillating spectral
density of the Dirac operator to the chiral condensate are taken into account
correctly. We find an infinite number of classical fixed points of the Langevin
flow in the thermodynamic limit. The correct solution originates from a
continuum of degenerate distributions in the complexified space.Comment: 20 pages, several eps figures, minor comments added, to appear in
JHE
Petrov types of slowly rotating fluid balls
Circularly rotating axisymmetric perfect fluid space-times are investigated
to second order in the small angular velocity. The conditions of various
special Petrov types are solved in a comoving tetrad formalism. A number of
theorems are stated on the possible Petrov types of various fluid models. It is
shown that Petrov type II solutions must reduce to the de Sitter spacetime in
the static limit. Two space-times with a physically satisfactory
energy-momentum tensor are investigated in detail. For the rotating
incompressible fluid, it is proven that the Petrov type cannot be D. The
equation of the rotation function can be solved for the Tolman type
IV fluid in terms of quadratures. It is also shown that the rotating version of
the Tolman IV space-time cannot be Petrov type D.Comment: 14 pages, version to appear in Gen. Rel. Gra
The QCD phase diagram at nonzero quark density
We determine the phase diagram of QCD on the \mu-T plane for small to
moderate chemical potentials. Two transition lines are defined with two
quantities, the chiral condensate and the strange quark number susceptibility.
The calculations are carried out on N_t =6,8 and 10 lattices generated with a
Symanzik improved gauge and stout-link improved 2+1 flavor staggered fermion
action using physical quark masses. After carrying out the continuum
extrapolation we find that both quantities result in a similar curvature of the
transition line. Furthermore, our results indicate that in leading order the
width of the transition region remains essentially the same as the chemical
potential is increased.Comment: 12 pages, 6 figure
Resonant excitations of the 't Hooft-Polyakov monopole
The spherically symmetric magnetic monopole in an SU(2) gauge theory coupled
to a massless Higgs field is shown to possess an infinite number of resonances
or quasinormal modes. These modes are eigenfunctions of the isospin 1
perturbation equations with complex eigenvalues, ,
satisfying the outgoing radiation condition. For , their
frequencies approach the mass of the vector boson, , while
their lifetimes tend to infinity. The response of the monopole to
an arbitrary initial perturbation is largely determined by these resonant
modes, whose collective effect leads to the formation of a long living
breather-like excitation characterized by pulsations with a frequency
approaching and with an amplitude decaying at late times as .Comment: 4 page
An effective chiral Hadron-Quark Equation of State
We construct an effective model for the QCD equation of state, taking into
account chiral symmetry restoration as well as the deconfinement phase
transition. The correct asymptotic degrees of freedom at the high and low
temperature limits are included (quarks hadrons). The model
shows a rapid crossover for both order parameters, as is expected from lattice
calculations. We then compare the thermodynamic properties of the model at
which turn out to be in qualitative agreement with lattice data,
while apparent quantitative differences can be attributed to hadronic
contributions and excluded volume corrections. Furthermore we discuss the
effects of a repulsive vector type quark interaction at finite baryon number
densities on the resulting phase diagram of the model. Our current model is
able to reproduce a first-order liquid gas phase transition as expected, but
does not show any signs of a first order deconfinement or chiral phase
transition. Both transitions rather appear as a very wide crossover in which
heavily medium modified hadron coexist with free quarks.Comment: 19 pages, 13 figures Version accepted by J. Phys.
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