288 research outputs found
Legendrian links, causality, and the Low conjecture
Let be a globally hyperbolic spacetime with Cauchy surface
diffeomorphic to an open subset of . The Legendrian Low conjecture
formulated by Nat\'ario and Tod says that two events x,y\in\ss are causally
related if and only if the Legendrian link of spheres whose points are light geodesics passing through and is
non-trivial in the contact manifold of all light geodesics in . The Low
conjecture says that for the events are causally related if and
only if is non-trivial as a topological link. We
prove the Low and the Legendrian Low conjectures. We also show that similar
statements hold for any globally hyperbolic such that a cover of
its Cauchy surface is diffeomorphic to an open domain in Comment: Version 3 - minor improvements, references added 11 pages, 1 figur
Linking and causality in globally hyperbolic spacetimes
The linking number is defined if link components are zero homologous.
Our affine linking invariant generalizes to the case of linked
submanifolds with arbitrary homology classes. We apply to the study of
causality in Lorentz manifolds. Let be a spacelike Cauchy surface in a
globally hyperbolic spacetime . The spherical cotangent bundle
is identified with the space of all null geodesics in
Hence the set of null geodesics passing through a point gives an
embedded -sphere in called the sky of Low observed
that if the link is nontrivial, then are causally
related. This motivated the problem (communicated by Penrose) on the Arnold's
1998 problem list to apply link theory to the study of causality. The spheres
are isotopic to fibers of They are nonzero
homologous and is undefined when is closed, while is well defined. Moreover, if is not an
odd-dimensional rational homology sphere. We give a formula for the increment
of \alk under passages through Arnold dangerous tangencies. If is
such that takes values in and is conformal to having all
the timelike sectional curvatures nonnegative, then are causally
related if and only if . We show that in
nonrefocussing are causally unrelated iff can be deformed
to a pair of -fibers of by an isotopy through skies. Low
showed that if (\ss, g) is refocussing, then is compact. We show that the
universal cover of is also compact.Comment: We added: Theorem 11.5 saying that a Cauchy surface in a refocussing
space time has finite pi_1; changed Theorem 7.5 to be in terms of conformal
classes of Lorentz metrics and did a few more changes. 45 pages, 3 figures. A
part of the paper (several results of sections 4,5,6,9,10) is an extension
and development of our work math.GT/0207219 in the context of Lorentzian
geometry. The results of sections 7,8,11,12 and Appendix B are ne
The potential impact of orange-fleshed sweetpotatoes on vitamin A intake in Sub-Saharan Africa.
A recent ex ante impact assessment indicates that orangefleshed sweetpotatoes can make a major contribution to alleviating vitamin A malnutrition in Sub-Saharan Africa. Replacing the white-fleshed varieties now grown by farmers with new high ß-carotene cultivars that meet local preferences would benefit an estimated 50 million children under age 6 who are currently at risk. The majority of children in Burundi, Rwanda and Uganda would benefit, as would about half of the children in Tanzania. Children in Ethiopia, Kenya and South Africa would also be affected, though to a lesser degree. The study did not take into account the benefits of the new cultivars to pregnant and lactating women, a population whose health is also likely to improve from the availability of the new plant types. Vitamin A deficiency is a major public health problem throughout the region and is responsible for tens of thousands of deaths annually among young children
Search for a T_20 Analyzer for Deuterons
This work was supported by the National Science Foundation Grant NSF PHY 81-14339 and by Indiana Universit
The twin paradox and Mach's principle
The problem of absolute motion in the context of the twin paradox is
discussed. It is shown that the various versions of the clock paradox feature
some aspects which Mach might have been appreciated. However, the ultimate
cause of the behavior of the clocks must be attributed to the autonomous status
of spacetime, thereby proving the relational program advocated by Mach as
impracticable.Comment: Latex2e, 11 pages, 6 figures, 33 references, no tables. Accepted for
publication in The European Physical Journal PLUS (EPJ PLUS
Theorising Disability: Beyond Common Sense
This article seeks to introduce the topic of disability to political theory via a discussion of some of the literature produced by disability theorists. The author argues that these more radical approaches conceptualise disability in ways that conflict with ‘common-sense’ notions of disability that tend to underpin political theoretical considerations of the topic. Furthermore, the author suggests that these more radical conceptualisations have profound implications for current debates on social justice, equality and citizenship that highlight the extent to which these notions are also currently underpinned by ‘common-sense’ notions of ‘normality’
Measurements of d+p Elastic Scattering Analyzing Powers at 80 MeV
This research was sponsored by the National Science Foundation Grant NSF PHY 87-1440
Intravaginal Practices, Bacterial Vaginosis, and HIV Infection in Women: Individual Participant Data Meta-analysis
Some Properties of a Transient New Coherent Condition of Matter Formed in High--Energy Hadronic Collisions
We investigate the dynamical possibility for the formation of a transient new
coherent condition of matter in high--energy hadronic collisions. The coherent
bosonic amplitude is characterized by a non--zero momentum and is sustained by
--wave interactions of quasi--pions in a dense fermionic medium. We make
quantitative estimates of several essential properties: the condensate momentum
and the fermionic density, the size of the coherent amplitude and the negative
energy density contributed by the condensate, a characteristic proper time for
the system to exist prior to breakdown into a few pions, and a characteristic
extension of the system over the plane perpendicular to the collision axis.
These quantities then allow us to make definite estimates of new signals: a few
pions with anomalously small transverse momenta MeV/c; and a
possible anomalous bremsstrahlung of very soft photons with characteristic
transverse momenta as low as about 4 MeV/c.Comment: 23 pages, LaTeX. A complete postscript file is available via
anonymous ftp at ttpux2.physik.uni-karlsruhe.de (129.13.102.139) as /ttp94-18
/ttp94-18.ps, Local preprint# TTP94-1
Oxidised cosmic acceleration
We give detailed proofs of several new no-go theorems for constructing flat
four-dimensional accelerating universes from warped dimensional reduction.
These new theorems improve upon previous ones by weakening the energy
conditions, by including time-dependent compactifications, and by treating
accelerated expansion that is not precisely de Sitter. We show that de Sitter
expansion violates the higher-dimensional null energy condition (NEC) if the
compactification manifold M is one-dimensional, if its intrinsic Ricci scalar R
vanishes everywhere, or if R and the warp function satisfy a simple limit
condition. If expansion is not de Sitter, we establish threshold
equation-of-state parameters w below which accelerated expansion must be
transient. Below the threshold w there are bounds on the number of e-foldings
of expansion. If M is one-dimensional or R everywhere vanishing, exceeding the
bound implies the NEC is violated. If R does not vanish everywhere on M,
exceeding the bound implies the strong energy condition (SEC) is violated.
Observationally, the w thresholds indicate that experiments with finite
resolution in w can cleanly discriminate between different models which satisfy
or violate the relevant energy conditions.Comment: v2: corrections, references adde
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