1,545 research outputs found
Would Two Dimensions be World Enough for Spacetime?
We consider various curious features of general relativity, and relativistic
field theory, in two spacetime dimensions. In particular, we discuss: the
vanishing of the Einstein tensor; the failure of an initial-value formulation
for vacuum spacetimes; the status of singularity theorems; the non-existence of
a Newtonian limit; the status of the cosmological constant; and the character
of matter fields, including perfect fluids and electromagnetic fields. We
conclude with a discussion of what constrains our understanding of physics in
different dimensions.Comment: 31 pages, 1 figur
The Motion of a Body in Newtonian Theories
A theorem due to Bob Geroch and Pong Soo Jang ["Motion of a Body in General
Relativity." Journal of Mathematical Physics 16(1), (1975)] provides the sense
in which the geodesic principle has the status of a theorem in General
Relativity (GR). Here we show that a similar theorem holds in the context of
geometrized Newtonian gravitation (often called Newton-Cartan theory). It
follows that in Newtonian gravitation, as in GR, inertial motion can be derived
from other central principles of the theory.Comment: 12 pages, 1 figure. This is the version that appeared in JMP; it is
only slightly changed from the previous version, to reflect small issue
caught in proo
Global Burden of Sickle Cell Anaemia in Children under Five, 2010-2050: Modelling Based on Demographics, Excess Mortality, and Interventions
The global burden of sickle cell anaemia (SCA) is set to rise as a consequence of improved survival in high-prevalence low- and middle-income countries and population migration to higher-income countries. The host of quantitative evidence documenting these changes has not been assembled at the global level. The purpose of this study is to estimate trends in the future number of newborns with SCA and the number of lives that could be saved in under-five children with SCA by the implementation of different levels of health interventions.First, we calculated projected numbers of newborns with SCA for each 5-y interval between 2010 and 2050 by combining estimates of national SCA frequencies with projected demographic data. We then accounted for under-five mortality (U5m) projections and tested different levels of excess mortality for children with SCA, reflecting the benefits of implementing specific health interventions for under-five patients in 2015, to assess the number of lives that could be saved with appropriate health care services. The estimated number of newborns with SCA globally will increase from 305,800 (confidence interval [CI]: 238,400-398,800) in 2010 to 404,200 (CI: 242,500-657,600) in 2050. It is likely that Nigeria (2010: 91,000 newborns with SCA [CI: 77,900-106,100]; 2050: 140,800 [CI: 95,500-200,600]) and the Democratic Republic of the Congo (2010: 39,700 [CI: 32,600-48,800]; 2050: 44,700 [CI: 27,100-70,500]) will remain the countries most in need of policies for the prevention and management of SCA. We predict a decrease in the annual number of newborns with SCA in India (2010: 44,400 [CI: 33,700-59,100]; 2050: 33,900 [CI: 15,900-64,700]). The implementation of basic health interventions (e.g., prenatal diagnosis, penicillin prophylaxis, and vaccination) for SCA in 2015, leading to significant reductions in excess mortality among under-five children with SCA, could, by 2050, prolong the lives of 5,302,900 [CI: 3,174,800-6,699,100] newborns with SCA. Similarly, large-scale universal screening could save the lives of up to 9,806,000 (CI: 6,745,800-14,232,700) newborns with SCA globally, 85% (CI: 81%-88%) of whom will be born in sub-Saharan Africa. The study findings are limited by the uncertainty in the estimates and the assumptions around mortality reductions associated with interventions.Our quantitative approach confirms that the global burden of SCA is increasing, and highlights the need to develop specific national policies for appropriate public health planning, particularly in low- and middle-income countries. Further empirical collaborative epidemiological studies are vital to assess current and future health care needs, especially in Nigeria, the Democratic Republic of the Congo, and India
A Categorical Equivalence between Generalized Holonomy Maps on a Connected Manifold and Principal Connections on Bundles over that Manifold
A classic result in the foundations of Yang-Mills theory, due to J. W.
Barrett ["Holonomy and Path Structures in General Relativity and Yang-Mills
Theory." Int. J. Th. Phys. 30(9), (1991)], establishes that given a
"generalized" holonomy map from the space of piece-wise smooth, closed curves
based at some point of a manifold to a Lie group, there exists a principal
bundle with that group as structure group and a principal connection on that
bundle such that the holonomy map corresponds to the holonomies of that
connection. Barrett also provided one sense in which this "recovery theorem"
yields a unique bundle, up to isomorphism. Here we show that something stronger
is true: with an appropriate definition of isomorphism between generalized
holonomy maps, there is an equivalence of categories between the category whose
objects are generalized holonomy maps on a smooth, connected manifold and whose
arrows are holonomy isomorphisms, and the category whose objects are principal
connections on principal bundles over a smooth, connected manifold. This result
clarifies, and somewhat improves upon, the sense of "unique recovery" in
Barrett's theorems; it also makes precise a sense in which there is no loss of
structure involved in moving from a principal bundle formulation of Yang-Mills
theory to a holonomy, or "loop", formulation.Comment: 20 page
The Interaction of Α-Thalassaemia and Haemoglobin G Philadelphia
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73251/1/j.1365-2141.1976.tb00918.x.pd
Dose-response relationship of ICS/fast-onset LABA as reliever therapy in asthma
The dose-response relationship of inhaled corticosteroid (ICS)/fast-onset long acting beta agonist (LABA) reliever therapy has not been formally addressed. The objective of this retrospective analysis is to ascertain from the available evidence whether ICS/fast-onset LABA administered as reliever therapy has a different dose-response relationship than maintenance fixed dose ICS/fast-onset LABA therapy in reducing risk of severe exacerbations
A Relativistic-Plasma Compton Maser
A relativistic pair-plasma which contains a high excitation of electrostatic
turbulence could produce intense radiation at brightness temperature in excess
of 10^22 K by stimulated scattering. Important relativistic effects would
include the broadband frequency response of the plasma and Compton-boosting of
the scattered radiation. In radio-frequency relativistic plasma, the optical
depth can be as small as hundreds of meters. When the plasma wave excitation
and particle distributions are one-dimensional, the frequency-dependent angular
distribution of the emission exhibits characteristics of pulsar emission.Comment: 13 pages, 4 figures; to appear in ApJ 559 (Sept 01
- …