570 research outputs found
The twin paradox in compact spaces
Twins travelling at constant relative velocity will each see the other's time
dilate leading to the apparent paradox that each twin believes the other ages
more slowly. In a finite space, the twins can both be on inertial, periodic
orbits so that they have the opportunity to compare their ages when their paths
cross. As we show, they will agree on their respective ages and avoid the
paradox. The resolution relies on the selection of a preferred frame singled
out by the topology of the space.Comment: to be published in PRA, 3 page
The Copernican Principle in Compact Spacetimes
Copernicus realised we were not at the centre of the universe. A universe
made finite by topological identifications introduces a new Copernican
consideration: while we may not be at the geometric centre of the universe,
some galaxy could be. A finite universe also picks out a preferred frame: the
frame in which the universe is smallest. Although we are not likely to be at
the centre of the universe, we must live in the preferred frame (if we are at
rest with respect to the cosmological expansion). We show that the preferred
topological frame must also be the comoving frame in a homogeneous and
isotropic cosmological spacetime. Some implications of topologically
identifying time are also discussed.Comment: 5 page
Design and Experimental Validation of a Ducted Counter-rotating Axial-flow Fans System
An experimental study on the design of counter-rotating axial-flow fans was
carried out. The fans were designed using an inverse method. In particular, the
system is designed to have a pure axial discharge flow. The counter-rotating
fans operate in a ducted-flow configuration and the overall performances are
measured in a normalized test bench. The rotation rate of each fan is
independently controlled. The relative axial spacing between fans can vary from
17% to 310%. The results show that the efficiency is strongly increased
compared to a conventional rotor or to a rotor-stator stage. The effects of
varying the rotation rates ratio on the overall performances are studied and
show that the system has a very flexible use, with a large patch of high
efficient operating points in the parameter space. The increase of axial
spacing causes only a small decrease of the efficiencyComment: accepted in Journal of Fluid Engineerin
There was movement that was stationary, for the four-velocity had passed around
Is the Doppler interpretation of galaxy redshifts in a
Friedmann-Lemaitre-Robertson-Walker (FLRW) model valid in the context of the
approach to comoving spatial sections pioneered by de Sitter, Friedmann,
Lemaitre and Robertson, i.e. according to which the 3-manifold of comoving
space is characterised by both its curvature and topology? Holonomy
transformations for flat, spherical and hyperbolic FLRW spatial sections are
proposed. By quotienting a simply-connected FLRW spatial section by an
appropriate group of holonomy transformations, the Doppler interpretation in a
non-expanding Minkowski space-time, obtained via four-velocity parallel
transport along a photon path, is found to imply that an inertial observer is
receding from herself at a speed greater than zero, implying contradictory
world-lines. The contradiction in the multiply-connected case occurs for
arbitrary redshifts in the flat and spherical cases, and for certain large
redshifts in the hyperbolic case. The link between the Doppler interpretation
of redshifts and cosmic topology can be understood physically as the link
between parallel transport along a photon path and the fact that the comoving
spatial geodesic corresponding to a photon's path can be a closed loop in an
FLRW model of any curvature. Closed comoving spatial loops are fundamental to
cosmic topology.Comment: 8 pages, 2 figures; v2: missing dot added to Defn 1, minor
corrections; submitted to MNRA
Homotopy symmetry in the multiply connected twin paradox of special relativity
In a multiply connected space, the two twins of the special relativity twin
paradox move with constant relative speed and meet a second time without
acceleration. The twins' situations appear to be symmetrical despite the need
for one to be younger due to time dilation. Here, the suggestion that the
apparent symmetry is broken by homotopy classes of the twins' worldlines is
reexamined using space-time diagrams. It is found that each twin finds her own
spatial path to have zero winding index and that of the other twin to have
unity winding index, i.e. the twins' worldlines' relative homotopy classes are
symmetrical. Although the twins' apparent symmetry is in fact broken by the
need for the non-favoured twin to non-simultaneously identify spatial domain
boundaries, the non-favoured twin {\em cannot} detect her disfavoured state if
she only measures the homotopy classes of the two twins' projected worldlines,
contrary to what was previously suggested. We also note that for the
non-favoured twin, the fundamental domain can be chosen by identifying time
boundaries (with a spatial offset) instead of space boundaries (with a temporal
offset).Comment: 11 pages, 9 figures, v6: more elementary algebra, extra figure,
accepted in MNRAS, the definitive version will be available at
http://www.blackwell-synergy.co
Artificial intelligence exceeds humans in epidemiological job coding
BACKGROUND: Work circumstances can substantially negatively impact health. To explore this, large occupational cohorts of free-text job descriptions are manually coded and linked to exposure. Although several automatic coding tools have been developed, accurate exposure assessment is only feasible with human intervention. METHODS: We developed OPERAS, a customizable decision support system for epidemiological job coding. Using 812,522 entries, we developed and tested classification models for the Professions et Catégories Socioprofessionnelles (PCS)2003, Nomenclature d'Activités Française (NAF)2008, International Standard Classifications of Occupation (ISCO)-88, and ISCO-68. Each code comes with an estimated correctness measure to identify instances potentially requiring expert review. Here, OPERAS' decision support enables an increase in efficiency and accuracy of the coding process through code suggestions. Using the Formaldehyde, Silica, ALOHA, and DOM job-exposure matrices, we assessed the classification models' exposure assessment accuracy. RESULTS: We show that, using expert-coded job descriptions as gold standard, OPERAS realized a 0.66-0.84, 0.62-0.81, 0.60-0.79, and 0.57-0.78 inter-coder reliability (in Cohen's Kappa) on the first, second, third, and fourth coding levels, respectively. These exceed the respective inter-coder reliability of expert coders ranging 0.59-0.76, 0.56-0.71, 0.46-0.63, 0.40-0.56 on the same levels, enabling a 75.0-98.4% exposure assessment accuracy and an estimated 19.7-55.7% minimum workload reduction. CONCLUSIONS: OPERAS secures a high degree of accuracy in occupational classification and exposure assessment of free-text job descriptions, substantially reducing workload. As such, OPERAS significantly outperforms both expert coders and other current coding tools. This enables large-scale, efficient, and effective exposure assessment securing healthy work conditions
Chemical composition of B-type supergiants in the OB8, OB10, OB48, OB78 associations of M31
Absolute and differential chemical abundances are presented for the largest
group of massive stars in M31 studied to date. These results were derived from
intermediate resolution spectra of seven B-type supergiants, lying within four
OB associations covering a galactocentric distance of 5 - 12 kpc. The results
are mainly based on an LTE analysis, and we additionally present a full
non-LTE, unified model atmosphere analysis of one star (OB78-277) to
demonstrate the reliability of the differential LTE technique. A comparison of
the stellar oxygen abundance with that of previous nebular results shows that
there is an offset of between ~0.15 - 0.4 dex between the two methods which is
critically dependent on the empirical calibration adopted for the R23 parameter
with [O/H]. However within the typical errors of the stellar and nebular
analyses (and given the strength of dependence of the nebular results on the
calibration used) the oxygen abundances determined in each method are fairly
consistent. We determine the radial oxygen abundance gradient from these stars,
and do not detect any systematic gradient across this galactocentric range. We
find that the inner regions of M31 are not, as previously thought, very 'metal
rich'. Our abundances of C, N, O, Mg, Si, Al, S and Fe in the M31 supergiants
are very similar to those of massive stars in the solar neighbourhood.Comment: 15 pages, 9 figures and 9 tables. Submitted to A&A April 200
Calcium intake and colorectal adenoma risk: Dose-response meta-analysis of prospective observational studies.
Evidence from randomized controlled trials suggests that calcium may protect against recurrence of colorectal adenomas, which could lead to the subsequent prevention of cancer. Yet, because the trials used only a large single dose and were of small sizes, knowledge of the dose-response relationship and influence on high-risk adenomas is limited. To address these issues, we conducted linear and non-linear dose-response meta-analyses primarily based on prospective observational studies published up to July 2014 identified from PubMed and Embase. Summary relative risks (RRs) and 95% confidence intervals (CIs) were calculated for total and supplemental calcium intake, respectively, using a random-effects model. For total calcium intake, summary RR for each 300mg/day increase was 0.95(95% CI=0.92-0.98; I(2) =45%; 8 studies with 11,005 cases; range of intake=333-2,229 mg/day). Evidence of non-linearity was indicated: approximately, compared to 550 mg/day of total calcium intake, the summary RR was 0.92(95% CI=0.89-0.94) at 1000 mg/day and 0.87(95% CI=0.84-0.90) at 1450 mg/day (Pnon -linearity <0.01). Associations were stronger for high-risk adenomas (≥1 cm in diameter, (tubulo)villous histology, dysplasia, or multiplicity): approximately, compared to 550 mg/day of total calcium intake, the summary RR was 0.77(95% CI=0.74-0.81) at 1000 mg/day and reduced to 0.69(95% CI=0.66-0.73) at 1450 mg/da (Pnon -linearity <0.01). For supplemental calcium intake, summary RR of total adenoma risk for each 300mg/day increase was 0.96(95% CI=0.93-0.99; I(2) =0%; 3 studies with 4,548 cases; range of supplementation=0-1,366 mg/day). In conclusion, calcium intake may continue to decrease the risk of adenomas, particularly high-risk adenomas, over a wide range of calcium intake
Selling Through Referrals
We endogenize intermediaries' choice to operate as agents or merchants in a market where there are frictions due to asymmetric information about consumption values. A seller has an object for sale and can reach buyers only through intermediaries. Intermediaries can either mediate the transaction by buying and reselling—the merchant mode—or refer buyers to the seller for a fee—the referral mode. When the seller has a strong bargaining position and can condition the asking price to the intermediaries' business model choice, all intermediaries specialize in agency. The seller's and intermediaries' joint profits equal the seller's profits when he has access to all buyers. When the seller does not have such bargaining power, the level of the referral fee and the degree of competition among intermediaries determine the business mode adoption. A hybrid agency–merchant mode may be adopted in equilibrium. Banning the referral mode can decrease welfare because the merchant mode is associated with additional allocative distortions due to asymmetric information
AntigenDB: an immunoinformatics database of pathogen antigens
The continuing threat of infectious disease and future pandemics, coupled to the continuous increase of drug-resistant pathogens, makes the discovery of new and better vaccines imperative. For effective vaccine development, antigen discovery and validation is a prerequisite. The compilation of information concerning pathogens, virulence factors and antigenic epitopes has resulted in many useful databases. However, most such immunological databases focus almost exclusively on antigens where epitopes are known and ignore those for which epitope information was unavailable. We have compiled more than 500 antigens into the AntigenDB database, making use of the literature and other immunological resources. These antigens come from 44 important pathogenic species. In AntigenDB, a database entry contains information regarding the sequence, structure, origin, etc. of an antigen with additional information such as B and T-cell epitopes, MHC binding, function, gene-expression and post translational modifications, where available. AntigenDB also provides links to major internal and external databases. We shall update AntigenDB on a rolling basis, regularly adding antigens from other organisms and extra data analysis tools. AntigenDB is available freely at http://www.imtech.res.in/raghava/antigendb and its mirror site http://www.bic.uams.edu/raghava/antigendb
- …