7,605 research outputs found
Essence and Cause: Making Something Be What It Is
Aristotle frequently describes essence as a “cause” or “explanation”, thus ascribing to essence some sort of causal or explanatory role. This explanatory role is often explicated by scholars in terms of essence “making the thing be what it is” or “making it the very thing that it is”. I argue that this is problematic, at least on the assumption that “making” expresses an explanatory relation, since it violates certain formal features of explanation. I then consider whether Aristotle is vulnerable to this problem by examining the explanatory role of essence in Posterior Analytics and Metaphysics Z 17
Statistical study of the conductance and shot noise in open quantum-chaotic cavities: Contribution from whispering gallery modes
In the past, a maximum-entropy model was introduced and applied to the study
of statistical scattering by chaotic cavities, when short paths may play an
important role in the scattering process. In particular, the validity of the
model was investigated in relation with the statistical properties of the
conductance in open chaotic cavities. In this article we investigate further
the validity of the maximum-entropy model, by comparing the theoretical
predictions with the results of computer simulations, in which the Schroedinger
equation is solved numerically inside the cavity for one and two open channels
in the leads; we analyze, in addition to the conductance, the zero-frequency
limit of the shot-noise power spectrum. We also obtain theoretical results for
the ensemble average of this last quantity, for the orthogonal and unitary
cases of the circular ensemble and an arbitrary number of channels. Generally
speaking, the agreement between theory and numerics is good. In some of the
cavities that we study, short paths consist of whispering gallery modes, which
were excluded in previous studies. These cavities turn out to be all the more
interesting, as it is in relation with them that we found certain systematic
discrepancies in the comparison with theory. We give evidence that it is the
lack of stationarity inside the energy interval that is analyzed, and hence the
lack of ergodicity that gives rise to the discrepancies. Indeed, the agreement
between theory and numerical simulations is improved when the energy interval
is reduced to a point and the statistics is then collected over an ensemble. It
thus appears that the maximum-entropy model is valid beyond the domain where it
was originally derived. An understanding of this situation is still lacking at
the present moment.Comment: Revised version, minor modifications, 28 pages, 7 figure
Diagnostic reasoning techniques for selective monitoring
An architecture for using diagnostic reasoning techniques in selective monitoring is presented. Given the sensor readings and a model of the physical system, a number of assertions are generated and expressed as Boolean equations. The resulting system of Boolean equations is solved symbolically. Using a priori probabilities of component failure and Bayes' rule, revised probabilities of failure can be computed. These will indicate what components have failed or are the most likely to have failed. This approach is suitable for systems that are well understood and for which the correctness of the assertions can be guaranteed. Also, the system must be such that changes are slow enough to allow the computation
Copper and Barium Abundances in the Ursa Major Moving Group
We present Cu and Ba abundances for 7 G-K dwarf stars, members of the
solar-metallicity, 0.3 Gyr old Ursa Major Moving Group. All analyzed member
stars show [Ba/Fe] excesses of +0.3-plus, associated with [Cu/Fe] deficiencies
of up to -0.23 dex. The present results suggest that there is an
anti-correlation between the abundances of Cu and the heavy elements produced
by the main component of the neutron capture s-process. Other possible
anomalies are Na and C deficiencies with respect to normal solar-metallicity
stars. The new data do not confirm the recent claim that the group member
HR6094 is a Ba dwarf star.Comment: 8 pages, 6 figures, accepted to MNRA
Task planning and control synthesis for robotic manipulation in space applications
Space-based robotic systems for diagnosis, repair and assembly of systems will require new techniques of planning and manipulation to accomplish these complex tasks. Results of work in assembly task representation, discrete task planning, and control synthesis which provide a design environment for flexible assembly systems in manufacturing applications, and which extend to planning of manipulatiuon operations in unstructured environments are summarized. Assembly planning is carried out using the AND/OR graph representation which encompasses all possible partial orders of operations and may be used to plan assembly sequences. Discrete task planning uses the configuration map which facilitates search over a space of discrete operations parameters in sequential operations in order to achieve required goals in the space of bounded configuration sets
Conductance and persistent current in quasi-one-dimensional systems with grain boundaries: Effects of the strongly reflecting and columnar grains
We study mesoscopic transport in the Q1D wires and rings made of a 2D
conductor of width W and length L >> W. Our aim is to compare an impurity-free
conductor with grain boundaries with a grain-free conductor with impurity
disorder. A single grain boundary is modeled as a set of the
2D--function-like barriers positioned equidistantly on a straight line
and disorder is emulated by a large number of such straight lines, intersecting
the conductor with random orientation in random positions. The impurity
disorder is modeled by the 2D -barriers with the randomly chosen
positions and signs. The electron transmission through the wires is calculated
by the scattering-matrix method, and the Landauer conductance is obtained. We
calculate the persistent current in the rings threaded by magnetic flux: We
incorporate into the scattering-matrix method the flux-dependent cyclic
boundary conditions and we introduce a trick allowing to study the persistent
currents in rings of almost realistic size. We mainly focus on the numerical
results for L much larger than the electron mean-free path, when the transport
is diffusive. If the grain boundaries are weakly reflecting, the systems with
grain boundaries show the same (mean) conductance and the same (typical)
persistent current as the systems with impurities, and the results also agree
with the single-particle theories treating disorder as a white-noise-like
potential. If the grain boundaries are strongly reflecting, the typical
persistent currents can be about three times larger than the results of the
white-noise-based theory, thus resembling the experimental results of Jariwala
et al. (PRL 2001). We extend our study to the 3D conductors with columnar
grains. We find that the persistent current exceeds the white-noise-based
result by another one order of magnitude, similarly as in the experiment of
Chandrasekhar et al. (PRL 1991)
Intensity correlations in electronic wave propagation in a disordered medium: the influence of spin-orbit scattering
We obtain explicit expressions for the correlation functions of transmission
and reflection coefficients of coherent electronic waves propagating through a
disordered quasi-one-dimensional medium with purely elastic diffusive
scattering in the presence of spin-orbit interactions. We find in the metallic
regime both large local intensity fluctuations and long-range correlations
which ultimately lead to universal conductance fluctuations. We show that the
main effect of spin-orbit scattering is to suppress both local and long-range
intensity fluctuations by a universal symmetry factor 4. We use a scattering
approach based on random transfer matrices.Comment: 15 pages, written in plain TeX, Preprint OUTP-93-42S (University of
Oxford), to appear in Phys. Rev.
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