4,378 research outputs found
The high energy limit of the trajectory representation of quantum mechanics
The trajectory representation in the high energy limit (Bohr correspondence
principle) manifests a residual indeterminacy. This indeterminacy is compared
to the indeterminacy found in the classical limit (Planck's constant to 0)
[Int. J. Mod. Phys. A 15, 1363 (2000)] for particles in the classically allowed
region, the classically forbiden region, and near the WKB turning point. The
differences between Bohr's and Planck's principles for the trajectory
representation are compared with the differences between these correspondence
principles for the wave representation. The trajectory representation in the
high energy limit is shown to go to neither classical nor statistical
mechanics. The residual indeterminacy is contrasted to Heisenberg uncertainty.
The relationship between indeterminacy and 't Hooft's information loss and
equivalence classes is investigated.Comment: 12 pages of LaTeX. No figures. Incorporated into the "Proceedings of
the Seventh International Wigner Symposium" (ed. M. E. Noz), 24-29 August
2001, U. of Maryland. Proceedings available at
http://www.physics.umd.edu/robo
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Disabled people and the Internet: experiences, barriers and opportunities
The UK government aims to make all its information and transactions available electronically by 2005. General use of the Internet also continues to grow. This report investigates the Internet's barriers and benefits for disabled people, and considers whether it acts as a means to reduce their social exclusion.
The study surveys the views and experiences of disabled people, both Internet users and non-users. Topics covered include: what the Internet is used for; use of and difficulties with assistive devices (special equipment and adaptations needed by some disabled people in order to use computers); how people learn to use the Internet; views of website accessibility; advantages and disadvantages of Internet usage.
It also explores participants' restrictions on using the Internet, and people's reasons for not using it. There has been little previous research in this area, but the authors review the existing literature. The report finds that many practical problems - such as the cost of training, finding appropriate assistive devices, website accessibility - all inhibit Internet opportunities for many disabled people
Expanded concept of the milliped family Spirobolidae (Diplopoda: Spirobolida: Spirobolidea): Proposals of Aztecolini n. tribe and Floridobolinae/ini and Tylobolini n. stats.; re)descriptions of Floridobolus and F. penneri, both Causey, 1957, and F. orini n. sp.; hypotheses on origins and affinities
The endemic Floridian milliped genus, Floridobolus Causey, 1957, more closely related to tylobolinines in the western United States (US), Mexico, and Guatemala than syntopic spirobolines, is incorporated into Spirobolidae (Spirobolida: Spirobolidea). With taxonomic priority by one year, its monotypic family is reduced to Floridobolinae, n. stat., comprising Floridobolini and Tylobolini, n. stats., the counterpart to Spirobolinae, comprising Spirobolini and Aztecolini, n. tribe; relationships are Floridobolini + (Tylobolini + (Aztecolini + Spirobolini)). Like F. penneri Causey, 1957, 208 km (130 mi) to the south in the Lake Wales Ridge, Polk and Highlands counties (cos.), F. orini n. sp., inhabits âBig Scrubâ environments in the Ocala National Forest, Marion Co. Biogeographic reconstructions, compatible with broader hypotheses on the classâ evolutionary history, indicate that, from a presumptive source area in northern Mexico where the subfamilies overlap, spirobolid stock penetrated the âproto-USâ four times, once per tribe, before the Western Interior Seaway developed in the Cretaceous Period, Mesozoic Era. Three expansions headed northeastward into future âAppalachia,â from which taxa spread southward as the Seaway receded. Floridobolini, the fi rst invader, had to be in âproto-Georgiaâ and positioned to penetrate Florida when the sand dunes that comprise the âCentral Highlandsâ emerged from the sea in the Oligocene (Cenozoic), ~25 mya. As sea levels rose and fell, the dunes fragmented into islands and the subcontinuous Floridobolus population was partitioned. The southernmost became F. penneri; F. orini inhabited a northern island; and a graduate student is investigating other insular remnants for additional species. Shortly after Floridobolini began spreading, Hiltonius/ Tylobolini arose and expanded both southward to Guatemala and northwestward to California; Tylobolus Cook, 1904, diverged in the latter area and dispersed northward to Washington and eastward to Utah/Arizona. The third invader, and the second to disperse northeastward, was Aztecolini, which probably eradicated Floridobolini from some of its established range and was partitioned into Mexican (Aztecolus Chamberlin, 1943) and US (Chicobolus Chamberlin, 1947) taxa by the Seaway. The fi nal invader, Spirobolini, dispersed northwestward and northeastward to both the Pacifi c and Atlantic coasts; instead of Trans-Beringia, we prefer penetration of the Asian part of âAsiamerica,â when it temporarily formed during the Cretaceous, to explain the Mongolian fossil genus, Gobiulus Dzik, 1975, herein assigned to Tylobolini, and the occurrence of Spirobolus Brandt, 1833, in China and Taiwan today. In the east, Narceus Rafi nesque, 1820, spread across Appalachia, eradicated most remaining populations of Floridobolus and Chicobolus, and expanded to Maine and QuĂ©bec after retreat of the Wisconsin glaciation. Chicobolus and Narceus also penetrated earliest Florida; the former established itself in the Central Highlands, spread through the widening peninsula as sea levels fell, and remained on insular refugia when waters rose. Apparently fueled by the different Floridian environments, Narceus underwent time-consuming speciation; consequently, Floridobolus and Chicobolus still survive on the peninsula, and an allopatric population of the latter inhabits coastal South Carolina. However, N. gordanus (Chamberlin, 1943) occurs syntopically with both in peninsular Florida and may be actively eradicating them from their last stronghold. Trigoniulus niger, takahasii, and segmentatus, all by Takakuwa, 1940, are removed from Spirobolidae and returned toTrigoniulidae (Trigoniulidea). New records in the Appendix include the fi rst of Aztecolus from Durango and Jalisco, Mexico
Vibration Characteristics Of Free Thin Cylindrical Shells
This paper considers the flexural vibrations of free thin circular cylinders. A frequency equation is derived using free-free characteristic beam functions to represent the variation of mid-surface shell displacement components, u, v and w, with respect to the axial direction. Timoshenko strain-displacement relations for thin cylinders are used to determine elastic vibratory strain energy. Energy methods are applied to obtain the frequency equation and associated amplitude ratios for each of its roots. This energy solution is checked experimentally using a vibration exciter and numerically using the SABOR IV finite element program. With minor modification, the frequency equation conforms to the one obtained in a similar way by Arnold and Warburton for cylinders with clamped ends and simply supported ends. Thus the proposed form of frequency equation, by accommodating a greater variety of boundary conditions, simplifies the task of determining cylinder vibration characteristics. © 1974 by ASME
TCP throughput guarantee in the DiffServ Assured Forwarding service: what about the results?
Since the proposition of Quality of Service architectures by the IETF, the
interaction between TCP and the QoS services has been intensively studied. This
paper proposes to look forward to the results obtained in terms of TCP
throughput guarantee in the DiffServ Assured Forwarding (DiffServ/AF) service
and to present an overview of the different proposals to solve the problem. It
has been demonstrated that the standardized IETF DiffServ conditioners such as
the token bucket color marker and the time sliding window color maker were not
good TCP traffic descriptors. Starting with this point, several propositions
have been made and most of them presents new marking schemes in order to
replace or improve the traditional token bucket color marker. The main problem
is that TCP congestion control is not designed to work with the AF service.
Indeed, both mechanisms are antagonists. TCP has the property to share in a
fair manner the bottleneck bandwidth between flows while DiffServ network
provides a level of service controllable and predictable. In this paper, we
build a classification of all the propositions made during these last years and
compare them. As a result, we will see that these conditioning schemes can be
separated in three sets of action level and that the conditioning at the
network edge level is the most accepted one. We conclude that the problem is
still unsolved and that TCP, conditioned or not conditioned, remains
inappropriate to the DiffServ/AF service
Hidden Symmetry of Higher Dimensional Kerr-NUT-AdS Spacetimes
It is well known that 4-dimensional Kerr-NUT-AdS spacetime possesses the
hidden symmetry associated with the Killing-Yano tensor. This tensor is
"universal" in the sense that there exist coordinates where it does not depend
on any of the free parameters of the metric. Recently the general higher
dimensional Kerr-NUT-AdS solutions of the Einstein equations were obtained. We
demonstrate that all these metrics with arbitrary rotation and NUT parameters
admit a universal Killing-Yano tensor. We give an explicit presentation of the
Killing-Yano and Killing tensors and briefly discuss their properties.Comment: 4 pages, some discussion and references are adde
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