2,055 research outputs found
CRISPR-mediated phage resistance and the ghost of coevolution past
The past is never dead. It's not even pastWilliam Faulkner (1951
Classification of image distortions in terms of Petrov types
An observer surrounded by sufficiently small spherical light sources at a
fixed distance will see a pattern of elliptical images distributed over the
sky, owing to the distortion effect (shearing effect) of the spacetime geometry
upon light bundles. In lowest non-trivial order with respect to the distance,
this pattern is completely determined by the conformal curvature tensor (Weyl
tensor) at the observation event. In this paper we derive formulas that allow
to calculate these distortion patterns in terms of the Newman-Penrose
formalism. Then we represent the distortion patterns graphically for all Petrov
types, and we discuss their dependence on the velocity of the observer.Comment: 22 pages, 8 eps-figures; revised version, parts of Introduction and
Conclusions rewritte
Operator-Schmidt decompositions and the Fourier transform, with applications to the operator-Schmidt numbers of unitaries
The operator-Schmidt decomposition is useful in quantum information theory
for quantifying the nonlocality of bipartite unitary operations. We construct a
family of unitary operators on C^n tensor C^n whose operator-Schmidt
decompositions are computed using the discrete Fourier transform. As a
corollary, we produce unitaries on C^3 tensor C^3 with operator-Schmidt number
S for every S in {1,...,9}. This corollary was unexpected, since it
contradicted reasonable conjectures of Nielsen et al [Phys. Rev. A 67 (2003)
052301] based on intuition from a striking result in the two-qubit case. By the
results of Dur, Vidal, and Cirac [Phys. Rev. Lett. 89 (2002) 057901
quant-ph/0112124], who also considered the two-qubit case, our result implies
that there are nine equivalence classes of unitaries on C^3 tensor C^3 which
are probabilistically interconvertible by (stochastic) local operations and
classical communication. As another corollary, a prescription is produced for
constructing maximally-entangled operators from biunimodular functions.
Reversing tact, we state a generalized operator-Schmidt decomposition of the
quantum Fourier transform considered as an operator C^M_1 tensor C^M_2 -->
C^N_1 tensor C^N_2, with M_1 x M_2 = N_1 x N_2. This decomposition shows (by
Nielsen's bound) that the communication cost of the QFT remains maximal when a
net transfer of qudits is permitted. In an appendix, a canonical procedure is
given for removing basis-dependence for results and proofs depending on the
"magic basis" introduced in [S. Hill and W. Wootters, "Entanglement of a pair
of quantum bits," Phys Rev. Lett 78 (1997) 5022-5025, quant-ph/9703041 (and
quant-ph/9709029)].Comment: More formal version of my talk at the Simons Conference on Quantum
and Reversible Computation at Stony Brook May 31, 2003. The talk slides and
audio are available at
http://www.physics.sunysb.edu/itp/conf/simons-qcomputation.html. Fixed typos
and minor cosmetic
The Size Distribution of Trans-Neptunian Bodies
[Condensed] We search 0.02 deg^2 for trans-Neptunian objects (TNOs) with
m<=29.2 (diameter ~15 km) using the ACS on HST. Three new objects are
discovered, roughly 25 times fewer than expected from extrapolation of the
differential sky density Sigma(m) of brighter objects. The ACS and other recent
TNO surveys show departures from a power law size distribution. Division of the
TNO sample into ``classical Kuiper belt'' (CKB) and ``Excited'' samples reveals
that Sigma(m) differs for the two populations at 96% confidence. A double power
law adequately fits all data. Implications include: The total mass of the CKB
is ~0.010 M_Earth, only a few times Pluto's mass, and is predominately in the
form of ~100 km bodies. The mass of Excited objects is perhaps a few times
larger. The Excited class has a shallower bright-end size distribution; the
largest objects, including Pluto, comprise tens of percent of the total mass
whereas the largest CKBOs are only ~2% of its mass. The predicted mass of the
largest Excited body is close to the Pluto mass; the largest CKBO is ~60 times
less massive. The deficit of small TNOs occurs for sizes subject to disruption
by present-day collisions, suggesting extensive depletion by collisions. Both
accretion and erosion appearing to have proceeded to more advanced stages in
the Excited class than the CKB. The absence of distant TNOs implies that any
distant (60 AU) population must have less than the CKB mass in the form of
objects 40 km or larger. The CKB population is sparser than theoretical
estimates of the required precursor population for short period comets, but the
Excited population could be a viable precursor population.Comment: Revised version accepted to the Astronomical Journal. Numerical
results are very slightly revised. Implications for the origins of
short-period comets are substantially revised, and tedious material on
statistical tests has been collected into a new Appendi
Response of selected plant and insect species to simulated solid rocket exhaust mixtures and to exhaust components from solid rocket fuels
The effects of solid rocket fuel (SRF) exhaust on selected plant and and insect species in the Merritt Island, Florida area was investigated in order to determine if the exhaust clouds generated by shuttle launches would adversely affect the native, plants of the Merritt Island Wildlife Refuge, the citrus production, or the beekeeping industry of the island. Conditions were simulated in greenhouse exposure chambers and field chambers constructed to model the ideal continuous stirred tank reactor. A plant exposure system was developed for dispensing and monitoring the two major chemicals in SRF exhaust, HCl and Al203, and for dispensing and monitoring SRF exhaust (controlled fuel burns). Plants native to Merritt Island, Florida were grown and used as test species. Dose-response relationships were determined for short term exposure of selected plant species to HCl, Al203, and mixtures of the two to SRF exhaust
Causes of prehospital misinterpretations of ST elevation myocardial infarction
Objectives: To determine the causes of software misinterpretation of ST elevation myocardial infarction (STEMI) compared to clinically identified STEMI to identify opportunities to improve prehospital STEMI identification. Methods: We compared ECGs acquired from July 2011 through June 2012 using the LIFEPAK 15 on adult patients transported by the Los Angeles Fire Department. Cases included patients ≥18 years who received a prehospital ECG. Software interpretation of the ECG (STEMI or not) was compared with data in the regional EMS registry to classify the interpretation as true positive (TP), true negative (TN), false positive (FP), or false negative (FN). For cases where classification was not possible using registry data, 3 blinded cardiologists interpreted the ECG. Each discordance was subsequently reviewed to determine the likely cause of misclassification. The cardiologists independently reviewed a sample of these discordant ECGs and the causes of misclassification were updated in an iterative fashion. Results: Of 44,611 cases, 50% were male (median age 65; inter-quartile range 52–80). Cases were classified as 482 (1.1%) TP, 711 (1.6%) FP, 43371 (97.2%) TN, and 47 (0.11%) FN. Of the 711 classified as FP, 126 (18%) were considered appropriate for, though did not undergo, emergent coronary angiography, because the ECG showed definite (52 cases) or borderline (65 cases) ischemic ST elevation, a STEMI equivalent (5 cases) or ST-elevation due to vasospasm (4 cases). The sensitivity was 92.8% [95% CI 90.6, 94.7%] and the specificity 98.7% [95% CI 98.6, 98.8%]. The leading causes of FP were ECG artifact (20%), early repolarization (16%), probable pericarditis/myocarditis (13%), indeterminate (12%), left ventricular hypertrophy (8%), and right bundle branch block (5%). There were 18 additional reasons for FP interpretation (<4% each). The leading causes of FN were borderline ST-segment elevations less than the algorithm threshold (40%) and tall T waves reducing the ST/T ratio below threshold (15%). There were 11 additional reasons for FN interpretation occurring ≤3 times each. Conclusion: The leading causes of FP automated interpretation of STEMI were ECG artifact and non-ischemic causes of ST-segment elevation. FN were rare and were related to ST-segment elevation or ST/T ratio that did not meet the software algorithm threshold
Dissecting the Workload of a Major Adult Video Portal
Adult content constitutes a major source of Internet traffic. As with many other platforms, these sites are incentivized to engage users and maintain them on the site. This engagement (e.g., through recommendations) shapes the journeys taken through such sites. Using data from a large content delivery network, we explore session journeys within an adult website. We take two perspectives. We first inspect the corpus available on these platforms. Following this, we investigate the session access patterns. We make a number of observations that could be exploited for optimizing delivery, e.g., that users often skip within video streams
Measurements and ab initio Molecular Dynamics Simulations of the High Temperature Ferroelectric Transition in Hexagonal RMnO3
Measurements of the structure of hexagonal RMnO3 (R=rare earths (Ho) and Y)
for temperatures significantly above the ferroelectric transition temperature
(TFE) were conducted to determine the nature of the transition. The local and
long range structural measurements were complemented by ab initio molecular
dynamics simulations. With respect to the Mn sites in YMnO3 and HoMnO3, we find
no large atomic (bond distances or thermal factors), electronic structure
changes or rehybridization on crossing TFE from local structural methods. The
local symmetry about the Mn sites is preserved. With respect to the local
structure about the Ho sites, a reduction of the average Ho-O bond with
increased temperature is found. Ab initio molecular dynamics calculations on
HoMnO3 reveal the detailed motions of all ions. Above ~900 K there are large
displacements of the Ho, O3 and O4 ions along the z-axis which reduce the
buckling of the MnO3/O4 planes. The changes result in O3/O4 ions moving to
towards central points between pairs of Ho ions on the z-axis. These structural
changes make the coordination of Ho sites more symmetric thus extinguishing the
electric polarization. At significantly higher temperatures, rotation of the
MnO5 polyhedra occurs without a significant change in electric polarization.
The born effective charge tensor is found to be highly anisotropic at the O
sites but does not change appreciably at high temperatures
High-Fidelity Modeling for Health Monitoring in Honeycomb Sandwich Structures
High-Fidelity Model of the sandwich composite structure with real geometry is reported. The model includes two composite facesheets, honeycomb core, piezoelectric actuator/sensors, adhesive layers, and the impactor. The novel feature of the model is that it includes modeling of the impact and wave propagation in the structure before and after the impact. Results of modeling of the wave propagation, impact, and damage detection in sandwich honeycomb plates using piezoelectric actuator/sensor scheme are reported. The results of the simulations are compared with the experimental results. It is shown that the model is suitable for analysis of the physics of failure due to the impact and for testing structural health monitoring schemes based on guided wave propagation
A First Look at CQVID-19 Messages on WhatsApp in Pakistan
The worldwide spread of COVID-19 has prompted extensive online discussions,
creating an `infodemic' on social media platforms such as WhatsApp and Twitter.
However, the information shared on these platforms is prone to be unreliable
and/or misleading. In this paper, we present the first analysis of COVID-19
discourse on public WhatsApp groups from Pakistan. Building on a large scale
annotation of thousands of messages containing text and images, we identify the
main categories of discussion. We focus on COVID-19 messages and understand the
different types of images/text messages being propagated. By exploring user
behavior related to COVID messages, we inspect how misinformation is spread.
Finally, by quantifying the flow of information across WhatsApp and Twitter, we
show how information spreads across platforms and how WhatsApp acts as a source
for much of the information shared on Twitter
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