25,262 research outputs found
Water impact loads
Computer program to generate time history of load factor and pressure on conical body of revolution during impact with water is discussed. Program calculates depth of penetration, velocity, force, load factor, maximum pressure at water line, and average pressure. Program is written in FORTRAN 4 Level H for IBM 360/85/195 Release 20.1 computer
Resolving the virial discrepancy in clusters of galaxies with modified Newtonian dynamics
A sample of 197 X-ray emitting clusters of galaxies is considered in the
context of Milgrom's modified Newtonian dynamics (MOND). It is shown that the
gas mass, extrapolated via an assumed model to a fixed radius of 3 Mpc,
is correlated with the gas temperature as predicted by MOND (). The observed temperatures are generally consistent with the inferred
mass of hot gas; no substantial quantity of additional unseen matter is
required in the context of MOND. However, modified dynamics cannot resolve the
strong lensing discrepancy in those clusters where this phenomenon occurs. The
prediction is that additional baryonic matter may be detected in the central
regions of rich clusters.Comment: Submitted to A&A, 4 pages, 3 figures, A&A macro
Modifying gravity with the Aether: an alternative to Dark Matter
There is evidence that Newton and Einstein's theories of gravity cannot
explain the dynamics of a universe made up solely of baryons and radiation. To
be able to understand the properties of galaxies, clusters of galaxies and the
universe on the whole it has become commonplace to invoke the presence of dark
matter. An alternative approach is to modify the gravitational field equations
to accommodate observations. We propose a new class of gravitational theories
in which we add a new degree of freedom, the Aether, in the form of a vector
field that is coupled covariantly, but non-minimally, with the space-time
metric. We explore the Newtonian and non-Newtonian limits, discuss the
conditions for these theories to be consistent and explore their effect on
cosmology.Comment: Updated version: Notation improved - TG
Super- and subradiant emission of two-level systems in the near-Dicke limit
We analyze the stability of super- and subradiant states in a system of
identical two-level atoms in the near-Dicke limit, i.e., when the atoms are
very close to each other compared to the wavelength of resonant light. The
dynamics of the system are studied using a renormalized master equation, both
with multipolar and minimal-coupling interaction schemes. We show that both
models lead to the same result and, in contrast to unrenormalized models,
predict that the relative orientation of the (co-aligned) dipoles is
unimportant in the Dicke limit. Our master equation is of relevance to any
system of dipole-coupled two-level atoms, and gives bounds on the strength of
the dipole-dipole interaction for closely spaced atoms. Exact calculations for
small atom systems in the near-Dicke limit show the increased emission times
resulting from the evolution generated by the strong dipole-dipole interaction.
However, for large numbers of atoms in the near-Dicke limit, it is shown that
as the number of atoms increases, the effect of the dipole-dipole interaction
on collective emission is reduced.Comment: 14 pages, 6 figures, published versio
Giant nonlinearity and entanglement of single photons in photonic bandgap structures
Giantly enhanced cross-phase modulation with suppressed spectral broadening
is predicted between optically-induced dark-state polaritons whose propagation
is strongly affected by photonic bandgaps of spatially periodic media with
multilevel dopants. This mechanism is shown to be capable of fully entangling
two single-photon pulses with high fidelity.Comment: 7 pages, 1 figur
Implications of the first detection of coherent elastic neutrino-nucleus scattering (CEvNS) with Liquid Argon
The CENNS-10 experiment of the COHERENT collaboration has recently reported
the first detection of coherent-elastic neutrino-nucleus scattering (CEvNS) in
liquid Argon with more than significance. In this work, we exploit
the new data in order to probe various interesting parameters which are of key
importance to CEvNS within and beyond the Standard Model. A dedicated
statistical analysis of these data shows that the current constraints are
significantly improved in most cases. We derive a first measurement of the
neutron rms charge radius of Argon, and also an improved determination of the
weak mixing angle in the low energy regime. We also update the constraints on
neutrino non-standard interactions, electromagnetic properties and light
mediators with respect to those derived from the first COHERENT-CsI data.Comment: discussion expanded including light mediators and nuclear
uncertainties, figures added, references added. V3: Fig. 7 corrected,
conclusions unchange
Using a simple expert system to assist a powered wheelchair user
A simple expert system is described that helps wheelchair users to drive their wheelchairs. The expert system takes data in from sensors and a joystick, identifies obstacles and then recommends a safe route. Wheelchair users were timed while driving around a variety of routes and using a joystick controlling their wheelchair via the simple expert system. Ultrasonic sensors are used to detect the obstacles. The simple expert system performed better than other recently published systems. In more difficult situations, wheelchair drivers did better when there was help from a sensor system. Wheelchair users completed routes with the sensors and expert system and results are compared with the same users driving without any assistance. The new systems show a significant improvement
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