174 research outputs found
What Makes the Difference
Reflections on a summer internship in the Chicago office of U.S. Senator Paul Simon
An optical mechanism for aberration of starlight
We present a physical-optics based theory of the physical mechanism for
aberration of starlight. We apply non-relativistic and relativistic theories
for wavefront image formation and include the effects of optically transmitting
media within the sensor. We show that the sensors imaging properties combined
with finite velocity of light fully accounts for aberration. That is, the
influence of the moving sensor on the incident wavefront from the star fully
explains aberration. Our treatment differs from all previous derivations
because we include wavefront-imaging physics within the sensor model. Our
predictions match Earth-sensor based measurements, but differ at larger sensor
speeds from predictions of the special relativistic-based theory. While
experimental uncertainty resulting from the low Earth-orbital velocity prevents
experimental confirmation of the special relativistic model of aberration, we
find that Earth-based sensors containing refractive optical media could
experimentally differentiate between these competing descriptions and, in
addition, yield an independent test of time dilation. We derive and present the
details of such an experiment
Recommended from our members
Point-of-care assessment of platelet reactivity in the emergency department may facilitate rapid rule-out of acute coronary syndromes: a prospective cohort pilot feasibility study
Objective: Accurate, efficient and cost-effective disposition of patients presenting to emergency departments (EDs) with symptoms suggestive of acute coronary syndromes (ACS) is a growing priority. Platelet activation is an early feature in the pathogenesis of ACS; thus, we sought to obtain an insight into whether point-of-care testing of platelet function: (1) may assist in the rule-out of ACS; (2) may provide additional predictive value in identifying patients with non-cardiac symptoms versus ACS-positive patients and (3) is logistically feasible in the ED. Design: Prospective cohort feasibility study. Setting: Two urban tertiary care sites, one located in the USA and the second in Argentina. Participants: 509 adult patients presenting with symptoms of ACS. Main outcome measures Platelet reactivity was quantified using the Platelet Function Analyzer-100, with closure time (seconds required for blood, aspirated under high shear, to occlude a 150 µm aperture) serving as the primary endpoint. Closure times were categorised as ‘normal’ or ‘prolonged’, defined objectively as the 90th centile of the distribution for all participants enrolled in the study. Diagnosis of ACS was made using the standard criteria. The use of antiplatelet agents was not an exclusion criterion. Results: Closure times for the study population ranged from 47 to 300 s, with a 90th centile value of 138 s. The proportion of patients with closure times ≥138 s was significantly higher in patients with non-cardiac symptoms (41/330; 12.4%) versus the ACS-positive cohort (2/105 (1.9%); p=0.0006). The specificity of ‘prolonged’ closure times (≥138 s) for a diagnosis of non-cardiac symptoms was 98.1%, with a positive predictive value of 95.4%. Multivariate analysis revealed that the closure time provided incremental, independent predictive value in the rule-out of ACS. Conclusions: Point-of-care assessment of platelet reactivity is feasible in the ED and may facilitate the rapid rule-out of ACS in patients with prolonged closure times
Spontaneous decompactification
Positive vacuum energy together with extra dimensions of space imply that our
four-dimensional Universe is unstable, generically to decompactification of the
extra dimensions. Either quantum tunneling or thermal fluctuations carry one
past a barrier into the decompactifying regime. We give an overview of this
process, and examine the subsequent expansion into the higher- dimensional
geometry. This is governed by certain fixed-point solutions of the evolution
equations, which are studied for both positive and negative spatial curvature.
In the case where there is a higher-dimensional cosmological constant, we also
outline a possible mechanism for compactification to a four-dimensional de
Sitter cosmology.Comment: 27 pages, 5 figures, harvmac. v2: refs added, minor notation change
Atom Interferometers
Interference with atomic and molecular matter waves is a rich branch of
atomic physics and quantum optics. It started with atom diffraction from
crystal surfaces and the separated oscillatory fields technique used in atomic
clocks. Atom interferometry is now reaching maturity as a powerful art with
many applications in modern science. In this review we first describe the basic
tools for coherent atom optics including diffraction by nanostructures and
laser light, three-grating interferometers, and double wells on AtomChips. Then
we review scientific advances in a broad range of fields that have resulted
from the application of atom interferometers. These are grouped in three
categories: (1) fundamental quantum science, (2) precision metrology and (3)
atomic and molecular physics. Although some experiments with Bose Einstein
condensates are included, the focus of the review is on linear matter wave
optics, i.e. phenomena where each single atom interferes with itself.Comment: submitted to Reviews of Modern Physic
Variability of Individual Platelet Reactivity Over Time in Patients Treated With Clopidogrel Insights From the ELEVATE–TIMI 56 Trial
AbstractBackgroundThe degree of antiplatelet response to clopidogrel has been associated with clinical outcomes. Studies have investigated whether adjustment of antiplatelet therapies based on a single platelet function test is beneficial.ObjectivesThe aim of the study was to test the stability of platelet reactivity measurements over time among patients treated with standard and double doses of clopidogrel.MethodsThe ELEVATE–TIMI 56 (Escalating Clopidogrel by Involving a Genetic Strategy–Thrombolysis In Myocardial Infarction 56) investigators genotyped 333 patients with coronary artery disease and randomized them to various clopidogrel regimens. Patients with at least 2 platelet function results on the same maintenance dose of clopidogrel (75 mg or 150 mg) were analyzed. Platelet aggregation was measured using P2Y12 reaction units (PRU).ResultsIn total, the mean platelet reactivity and the total number of nonresponders (PRU ≥230) with clopidogrel did not change between 2 periods for the 75-mg (22.4% vs. 21.9%; p = 0.86) and 150-mg doses of clopidogrel (11.5% vs. 11.5%; p = 1.00). In contrast, when evaluating each patient individually, 15.7% of patients taking clopidogrel 75 mg and 11.4% of patients taking 150 mg had a change in their responder status when tested at 2 different time points (p < 0.001). Despite being treated with the same dose of clopidogrel, >40% of patients had a change in PRU >40 on serial sampling, which approximates the average PRU difference caused by increasing the clopidogrel dose from 75 mg to 150 mg.ConclusionsMeasurements of platelet reactivity vary over time in a significant proportion of patients. Thus, treatment adjustment according to platelet function testing at a single time point might not be sufficient for guiding antiplatelet therapy in clinical or research settings. (Escalating Clopidogrel by Involving a Genetic Strategy–Thrombolysis In Myocardial Infarction 56 [ELEVATE–TIMI 56]; NCT01235351
A High Statistics Search for Ultra-High Energy Gamma-Ray Emission from Cygnus X-3 and Hercules X-1
We have carried out a high statistics (2 Billion events) search for
ultra-high energy gamma-ray emission from the X-ray binary sources Cygnus X-3
and Hercules X-1. Using data taken with the CASA-MIA detector over a five year
period (1990-1995), we find no evidence for steady emission from either source
at energies above 115 TeV. The derived upper limits on such emission are more
than two orders of magnitude lower than earlier claimed detections. We also
find no evidence for neutral particle or gamma-ray emission from either source
on time scales of one day and 0.5 hr. For Cygnus X-3, there is no evidence for
emission correlated with the 4.8 hr X-ray periodicity or with the occurrence of
large radio flares. Unless one postulates that these sources were very active
earlier and are now dormant, the limits presented here put into question the
earlier results, and highlight the difficulties that possible future
experiments will have in detecting gamma-ray signals at ultra-high energies.Comment: 26 LaTeX pages, 16 PostScript figures, uses psfig.sty to be published
in Physical Review
Fermi Large Area Telescope Constraints on the Gamma-ray Opacity of the Universe
The Extragalactic Background Light (EBL) includes photons with wavelengths
from ultraviolet to infrared, which are effective at attenuating gamma rays
with energy above ~10 GeV during propagation from sources at cosmological
distances. This results in a redshift- and energy-dependent attenuation of the
gamma-ray flux of extragalactic sources such as blazars and Gamma-Ray Bursts
(GRBs). The Large Area Telescope onboard Fermi detects a sample of gamma-ray
blazars with redshift up to z~3, and GRBs with redshift up to z~4.3. Using
photons above 10 GeV collected by Fermi over more than one year of observations
for these sources, we investigate the effect of gamma-ray flux attenuation by
the EBL. We place upper limits on the gamma-ray opacity of the Universe at
various energies and redshifts, and compare this with predictions from
well-known EBL models. We find that an EBL intensity in the optical-ultraviolet
wavelengths as great as predicted by the "baseline" model of Stecker et al.
(2006) can be ruled out with high confidence.Comment: 42 pages, 12 figures, accepted version (24 Aug.2010) for publication
in ApJ; Contact authors: A. Bouvier, A. Chen, S. Raino, S. Razzaque, A.
Reimer, L.C. Reye
A population of gamma-ray emitting globular clusters seen with the Fermi Large Area Telescope
Globular clusters with their large populations of millisecond pulsars (MSPs)
are believed to be potential emitters of high-energy gamma-ray emission. Our
goal is to constrain the millisecond pulsar populations in globular clusters
from analysis of gamma-ray observations. We use 546 days of continuous
sky-survey observations obtained with the Large Area Telescope aboard the Fermi
Gamma-ray Space Telescope to study the gamma-ray emission towards 13 globular
clusters. Steady point-like high-energy gamma-ray emission has been
significantly detected towards 8 globular clusters. Five of them (47 Tucanae,
Omega Cen, NGC 6388, Terzan 5, and M 28) show hard spectral power indices and clear evidence for an exponential cut-off in the range
1.0-2.6 GeV, which is the characteristic signature of magnetospheric emission
from MSPs. Three of them (M 62, NGC 6440 and NGC 6652) also show hard spectral
indices , however the presence of an exponential cut-off
can not be unambiguously established. Three of them (Omega Cen, NGC 6388, NGC
6652) have no known radio or X-ray MSPs yet still exhibit MSP spectral
properties. From the observed gamma-ray luminosities, we estimate the total
number of MSPs that is expected to be present in these globular clusters. We
show that our estimates of the MSP population correlate with the stellar
encounter rate and we estimate 2600-4700 MSPs in Galactic globular clusters,
commensurate with previous estimates. The observation of high-energy gamma-ray
emission from a globular cluster thus provides a reliable independent method to
assess their millisecond pulsar populations that can be used to make
constraints on the original neutron star X-ray binary population, essential for
understanding the importance of binary systems in slowing the inevitable core
collapse of globular clusters.Comment: Accepted for publication in A&A. Corresponding authors: J.
Kn\"odlseder, N. Webb, B. Pancraz
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