154 research outputs found
Exploiting Evolution for an Adaptive Drift-Robust Classifier in Chemical Sensing
Gas chemical sensors are strongly affected by drift, i.e., changes in sensors' response with time, that may turn statistical models commonly used for classification completely useless after a period of time. This paper presents a new classifier that embeds an adaptive stage able to reduce drift effects. The proposed system exploits a state-of-the-art evolutionary strategy to iteratively tweak the coefficients of a linear transformation able to transparently transform raw measures in order to mitigate the negative effects of the drift. The system operates continuously. The optimal correction strategy is learnt without a-priori models or other hypothesis on the behavior of physical-chemical sensors. Experimental results demonstrate the efficacy of the approach on a real problem
Exterior and interior metrics with quadrupole moment
We present the Ernst potential and the line element of an exact solution of
Einstein's vacuum field equations that contains as arbitrary parameters the
total mass, the angular momentum, and the quadrupole moment of a rotating mass
distribution. We show that in the limiting case of slowly rotating and slightly
deformed configuration, there exists a coordinate transformation that relates
the exact solution with the approximate Hartle solution. It is shown that this
approximate solution can be smoothly matched with an interior perfect fluid
solution with physically reasonable properties. This opens the possibility of
considering the quadrupole moment as an additional physical degree of freedom
that could be used to search for a realistic exact solution, representing both
the interior and exterior gravitational field generated by a self-gravitating
axisymmetric distribution of mass of perfect fluid in stationary rotation.Comment: Latex, 15 pages, 3 figures, final versio
A source of a quasi--spherical space--time: The case for the M--Q solution
We present a physically reasonable source for an static, axially--symmetric
solution to the Einstein equations. Arguments are provided, supporting our
belief that the exterior space--time produced by such source, describing a
quadrupole correction to the Schwarzschild metric, is particularly suitable
(among known solutions of the Weyl family) for discussing the properties of
quasi--spherical gravitational fields.Comment: 34 pages, 9 figures. To appear in GR
Approximate gravitational field of a rotating deformed mass
A new approximate solution of vacuum and stationary Einstein field equations
is obtained. This solution is constructed by means of a power series expansion
of the Ernst potential in terms of two independent and dimensionless parameters
representing the quadrupole and the angular momentum respectively. The main
feature of the solution is a suitable description of small deviations from
spherical symmetry through perturbations of the static configuration and the
massive multipole structure by using those parameters. This quality of the
solution might eventually provide relevant differences with respect to the
description provided by the Kerr solution.Comment: 16 pages. Latex. To appear in General Relativity and Gravitatio
Recycling bins, garbage cans or think tanks? Three myths regarding policy analysis institutes
The phrase 'think tank' has become ubiquitous – overworked and underspecified – in the political lexicon. It is entrenched in scholarly discussions of public policy as well as in the 'policy wonk' of journalists, lobbyists and spin-doctors. This does not mean that there is an agreed definition of think tank or consensual understanding of their roles and functions. Nevertheless, the majority of organizations with this label undertake policy research of some kind. The idea of think tanks as a research communication 'bridge' presupposes that there are discernible boundaries between (social) science and policy. This paper will investigate some of these boundaries. The frontiers are not only organizational and legal; they also exist in how the 'public interest' is conceived by these bodies and their financiers. Moreover, the social interactions and exchanges involved in 'bridging', themselves muddy the conception of 'boundary', allowing for analysis to go beyond the dualism imposed in seeing science on one side of the bridge, and the state on the other, to address the complex relations between experts and public policy
On the black hole limit of rotating discs and rings
Solutions to Einstein's field equations describing rotating fluid bodies in
equilibrium permit parametric (i.e. quasi-stationary) transitions to the
extreme Kerr solution (outside the horizon). This has been shown analytically
for discs of dust and numerically for ring solutions with various equations of
state. From the exterior point of view, this transition can be interpreted as a
(quasi) black hole limit. All gravitational multipole moments assume precisely
the values of an extremal Kerr black hole in the limit. In the present paper,
the way in which the black hole limit is approached is investigated in more
detail by means of a parametric Taylor series expansion of the exact solution
describing a rigidly rotating disc of dust. Combined with numerical
calculations for ring solutions our results indicate an interesting universal
behaviour of the multipole moments near the black hole limit.Comment: 18 pages, 4 figures; Dedicated to Gernot Neugebauer on the occasion
of his 70th birthda
Towards a formalism for mapping the spacetimes of massive compact objects: Bumpy black holes and their orbits
Observations have established that extremely compact, massive objects are
common in the universe. It is generally accepted that these objects are black
holes. As observations improve, it becomes possible to test this hypothesis in
ever greater detail. In particular, it is or will be possible to measure the
properties of orbits deep in the strong field of a black hole candidate (using
x-ray timing or with gravitational-waves) and to test whether they have the
characteristics of black hole orbits in general relativity. Such measurements
can be used to map the spacetime of a massive compact object, testing whether
the object's multipoles satisfy the strict constraints of the black hole
hypothesis. Such a test requires that we compare against objects with the
``wrong'' multipole structure. In this paper, we present tools for constructing
bumpy black holes: objects that are almost black holes, but that have some
multipoles with the wrong value. The spacetimes which we present are good deep
into the strong field of the object -- we do not use a large r expansion,
except to make contact with weak field intuition. Also, our spacetimes reduce
to the black hole spacetimes of general relativity when the ``bumpiness'' is
set to zero. We propose bumpy black holes as the foundation for a null
experiment: if black hole candidates are the black holes of general relativity,
their bumpiness should be zero. By comparing orbits in a bumpy spacetime with
those of an astrophysical source, observations should be able to test this
hypothesis, stringently testing whether they are the black holes of general
relativity. (Abridged)Comment: 16 pages + 2 appendices + 3 figures. Submitted to PR
A Fermi Surface study of BaKBiO
We present all electron computations of the 3D Fermi surfaces (FS's) in
BaKBiO for a number of different compositions based on the
selfconsistent Korringa-Kohn-Rostoker coherent-potential-approximation
(KKR-CPA) approach for incorporating the effects of Ba/K substitution. By
assuming a simple cubic structure throughout the composition range, the
evolution of the nesting and other features of the FS of the underlying
pristine phase is correlated with the onset of various structural transitions
with K doping. A parameterized scheme for obtaining an accurate 3D map of the
FS in BaKBiO for an arbitrary doping level is developed. We
remark on the puzzling differences between the phase diagrams of
BaKBiO and BaPbBiO by comparing aspects
of their electronic structures and those of the end compounds BaBiO,
KBiO and BaPbO. Our theoretically predicted FS's in the cubic phase are
relevant for analyzing high-resolution Compton scattering and
positron-annihilation experiments sensitive to the electron momentum density,
and are thus amenable to substantial experimental verification.Comment: 12 pages, 7 figures, to appear in Phys. Rev.
Spatial infinity in higher dimensional spacetimes
Motivated by recent studies on the uniqueness or non-uniqueness of higher
dimensional black hole spacetime, we investigate the asymptotic structure of
spatial infinity in n-dimensional spacetimes(). It turns out that the
geometry of spatial infinity does not have maximal symmetry due to the
non-trivial Weyl tensor {}^{(n-1)}C_{abcd} in general. We also address static
spacetime and its multipole moments P_{a_1 a_2 ... a_s}. Contrasting with four
dimensions, we stress that the local structure of spacetimes cannot be unique
under fixed a multipole moments in static vacuum spacetimes. For example, we
will consider the generalized Schwarzschild spacetimes which are deformed black
hole spacetimes with the same multipole moments as spherical Schwarzschild
black holes. To specify the local structure of static vacuum solution we need
some additional information, at least, the Weyl tensor {}^{(n-2)}C_{abcd} at
spatial infinity.Comment: 6 pages, accepted for publication in Physical Review D, published
versio
Time-Independent Gravitational Fields
This article reviews, from a global point of view, rigorous results on time
independent spacetimes. Throughout attention is confined to isolated bodies at
rest or in uniform rotation in an otherwise empty universe. The discussion
starts from first principles and is, as much as possible, self-contained.Comment: 47 pages, LaTeX, uses Springer cl2emult styl
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