5,447 research outputs found
How much measurement independence is needed in order to demonstrate nonlocality?
If nonlocality is to be inferred from a violation of Bell's inequality, an
important assumption is that the measurement settings are freely chosen by the
observers, or alternatively, that they are random and uncorrelated with the
hypothetical local variables. We study the case where this assumption is
weakened, so that measurement settings and local variables are at least
partially correlated. As we show, there is a connection between this type of
model and models which reproduce nonlocal correlations by allowing classical
communication between the distant parties, and a connection with models that
exploit the detection loophole. We show that even if Bob's choices are
completely independent, all correlations obtained from projective measurements
on a singlet can be reproduced, with the correlation (measured by mutual
information) between Alice's choice and local variables less than or equal to a
single bit.Comment: 5 pages, 1 figure. v2 Various improvements in presentation. Results
unchange
The transport of cosmic rays in self-excited magnetic turbulence
The process of diffusive shock acceleration relies on the efficacy with which
hydromagnetic waves can scatter charged particles in the precursor of a shock.
The growth of self-generated waves is driven by both resonant and non-resonant
processes. We perform high-resolution magnetohydrodynamic simulations of the
non-resonant cosmic-ray driven instability, in which the unstable waves are
excited beyond the linear regime. In a snapshot of the resultant field,
particle transport simulations are carried out. The use of a static snapshot of
the field is reasonable given that the Larmor period for particles is typically
very short relative to the instability growth time. The diffusion rate is found
to be close to, or below, the Bohm limit for a range of energies. This provides
the first explicit demonstration that self-excited turbulence reduces the
diffusion coefficient and has important implications for cosmic ray transport
and acceleration in supernova remnants.Comment: 8 pages, 8 figures, accepted for publication in MNRA
Suppressing subordinate reproduction provides benefits to dominants in cooperative societies of meerkats.
In many animal societies, a small proportion of dominant females monopolize reproduction by actively suppressing subordinates. Theory assumes that this is because subordinate reproduction depresses the fitness of dominants, yet the effect of subordinate reproduction on dominant behaviour and reproductive success has never been directly assessed. Here, we describe the consequences of experimentally preventing subordinate breeding in 12 groups of wild meerkats (Suricata suricatta) for three breeding attempts, using contraceptive injections. When subordinates are prevented from breeding, dominants are less aggressive towards subordinates and evict them less often, leading to a higher ratio of helpers to dependent pups, and increased provisioning of the dominant's pups by subordinate females. When subordinate breeding is suppressed, dominants also show improved foraging efficiency, gain more weight during pregnancy and produce heavier pups, which grow faster. These results confirm the benefits of suppression to dominants, and help explain the evolution of singular breeding in vertebrate societies
The electronic structure of amorphous silica: A numerical study
We present a computational study of the electronic properties of amorphous
SiO2. The ionic configurations used are the ones generated by an earlier
molecular dynamics simulations in which the system was cooled with different
cooling rates from the liquid state to a glass, thus giving access to
glass-like configurations with different degrees of disorder [Phys. Rev. B 54,
15808 (1996)]. The electronic structure is described by a tight-binding
Hamiltonian. We study the influence of the degree of disorder on the density of
states, the localization properties, the optical absorption, the nature of
defects within the mobility gap, and on the fluctuations of the Madelung
potential, where the disorder manifests itself most prominently. The
experimentally observed mismatch between a photoconductivity threshold of 9 eV
and the onset of the optical absorption around 7 eV is interpreted by the
picture of eigenstates localized by potential energy fluctuations in a mobility
gap of approximately 9 eV and a density of states that exhibits valence and
conduction band tails which are, even in the absence of defects, deeply located
within the former band gap.Comment: 21 pages of Latex, 5 eps figure
Modelling light-cone distribution amplitudes from non-relativistic bound states
We calculate light-cone distribution amplitudes for non-relativistic bound
states, including radiative corrections from relativistic gluon exchange to
first order in the strong coupling constant. We distinguish between bound
states of quarks with equal (or similar) mass, m_1 ~ m_2, and between bound
states where the quark masses are hierarchical, m_1 >> m_2. For both cases we
calculate the distribution amplitudes at the non-relativistic scale and discuss
the renormalization-group evolution for the leading-twist and 2-particle
distributions. Our results apply to hard exclusive reactions with
non-relativistic bound states in the QCD factorization approach like, for
instance, (B_c -> eta_c l nu) or (e^+ e^- -> J/psi eta_c). They also serve as a
toy model for light-cone distribution amplitudes of light mesons or heavy B and
D mesons, for which certain model-independent properties can be derived. In
particular, we calculate the anomalous dimension for the B meson distribution
amplitude phi_B^-(w) in the Wandzura-Wilczek approximation and derive the
according solution of the evolution equation at leading logarithmic accuracy.Comment: 27 pages, 15 figures, discussion around Eq.(83,84) extende
Homogeneous Photometry V: The Globular Cluster NGC 4147
New BVRI broad-band photometry and astrometry are presented for the globular
cluster NGC 4147, based upon measurements derived from 524 ground-based CCD
images mostly either donated by colleagues or retrieved from public archives.
We have also reanalysed five exposures of the cluster obtained with WFPC2 on
the Hubble Space Telescope in the F439W and F555W (B and V) filters. We present
calibrated color-magnitude and color-color diagrams. Analysis of the
color-magnitude diagram reveals morphogical properties generally consistent
with published metal-abundance estimates for the cluster, and an age typical of
other Galactic globular clusters of similar metallicity. We have also
redetermined the periods and mean magnitudes for the RR Lyrae variables,
including a new c-type variable reported here for the first time. Our data do
not show clear evidence for photometric variability in candidate V18, recently
reported by Arellano Ferro et al. (2004). These observations also support the
non-variable status of candidates V5, V9, and V15. The union of our light-curve
data with those of Newburn (1957), Mannino (1957) and Arellano Ferro et al.
(op. cit.) permits the derivation of significantly improved periods. The mean
periods and the Bailey period-amplitude diagrams support the classification of
the cluster as Oosterhoff I despite its predominantly blue horizontal branch.
The number ratio of c- to ab-type RR Lyrae stars, on the other hand, is
unusually high for an Oosterhoff I cluster. The calibrated results have been
made available through the first author's web site.Comment: 30 pages, 30 figures. PASP, in press. Uses 11/2004 version of
emulateapj (included). For the full-resolution preprint, the reader is
encouraged to download the ps file (123 pages, manuscript format) available
at http://cadcwww.hia.nrc.ca/stetson/NGC4147/ms.ps.gz, or the pdf file (30
pages, preprint format) available at
http://www.astro.puc.cl/~mcatelan/N4147/full-res.pd
Selected Papers on Protoplanetary Disks
Three papers present studies of thermal balances, dynamics, and electromagnetic spectra of protoplanetary disks, which comprise gas and dust orbiting young stars. One paper addresses the reprocessing, in a disk, of photons that originate in the disk itself in addition to photons that originate in the stellar object at the center. The shape of the disk is found to strongly affect the redistribution of energy. Another of the three papers reviews an increase in the optical luminosity of the young star FU Orionis. The increase began in the year 1936 and similar increases have since been observed in other stars. The paper summarizes astronomical, meteoric, and theoretical evidence that these increases are caused by increases in mass fluxes through the inner portions of the protoplanetary disks of these stars. The remaining paper presents a mathematical-modeling study of the structures of protostellar accretion disks, with emphasis on limits on disk flaring. Among the conclusions reached in the study are that (1) the radius at which a disk becomes shadowed from its central stellar object depends on radial mass flow and (2) most planet formation has occurred in environments unheated by stellar radiation
Kōrero Mai: Kaiako experiences of synchronous online teaching and learning in New Zealand
Online teaching and learning programmes allow ākonga who live in isolated areas, or who
have differing learning requirements, to study by distance. Maintaining student engagement
in the online environment is an important aspect. This article explores how kaiako (teachers)
can engage their ākonga (students) better in online environments. The article has a particular
emphasis on supporting Māori learners, who represent 25% of the New Zealand school
population. Five kaiako were interviewed about their experience of teaching New Zealand
secondary school students online. The study found that the kaiako had some awareness of
bicultural values and practices, but lacked confidence in embedding it in their online
teaching, which was limited to synchronous timetabled sessions with some communication
by text and email. The time provided for online students was considerably less than for the
secondary students in traditional classrooms
Radial Velocity Studies of Close Binary Stars. IX
Radial-velocity measurements and sine-curve fits to the orbital velocity
variations are presented for the eighth set of ten close binary systems: AB
And, V402 Aur, V445 Cep, V2082 Cyg, BX Dra, V918 Her, V502 Oph, V1363 Ori, KP
Peg, V335 Peg. Half of the systems (V445 Cep, V2082 Cyg, V918 Her, V1363 Ori,
V335 Peg) were discovered photometrically by the Hipparcos mission and all
systems are double-lined (SB2) contact binaries. The broadening function method
permitted improvement of the orbital elements for AB And and V502 Oph. The
other systems have been observed for radial velocity variations for the first
time; in this group are five bright (V<7.5) binaries: V445 Cep, V2082 Cyg, V918
Her, KP Peg and V335 Peg. Several of the studied systems are prime candidates
for combined light and radial-velocity synthesis solutions.Comment: 17+ pages, 2 tables, 4 figure
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