92,210 research outputs found
The principle of equivalence and projective structure in space-times
This paper discusses the extent to which one can determine the space-time
metric from a knowledge of a certain subset of the (unparametrised) geodesics
of its Levi-Civita connection, that is, from the experimental evidence of the
equivalence principle. It is shown that, if the space-time concerned is known
to be vacuum, then the Levi-Civita connection is uniquely determined and its
associated metric is uniquely determined up to a choice of units of
measurement, by the specification of these geodesics. It is further
demonstrated that if two space-times share the same unparametrised geodesics
and only one is assumed vacuum then their Levi-Civita connections are again
equal (and so the other metric is also a vacuum metric) and the first result
above is recovered.Comment: 23 pages, submitted to Classical and Quantum Gravit
On the Theory of Killing Orbits in Space-Time
This paper gives a theoretical discussion of the orbits and isotropies which
arise in a space-time which admits a Lie algebra of Killing vector fields. The
submanifold structure of the orbits is explored together with their induced
Killing vector structure. A general decomposition of a space-time in terms of
the nature and dimension of its orbits is given and the concept of stability
and instability for orbits introduced. A general relation is shown linking the
dimensions of the Killing algebra, the orbits and the isotropies. The
well-behaved nature of "stable" orbits and the possible miss-behaviour of the
"unstable" ones is pointed out and, in particular, the fact that independent
Killing vector fields in space-time may not induce independent such vector
fields on unstable orbits. Several examples are presented to exhibit these
features. Finally, an appendix is given which revisits and attempts to clarify
the well-known theorem of Fubini on the dimension of Killing orbits.Comment: Latex, 19 pages, no figur
Profile of pulmonary smooth muscle cells and their response to blockade and Stimulations
INTRODUCTION: The tone of the pulmonary arteries is the summation of the activity of each smooth muscle cell (SMC) within a vessel wall and its interaction with the endothelial cells and extracellular matrix (including collagen). There are reported phenotypic differences between SMC in the inner & outer layers of pulmonary artery walls1. To tissue engineer a blood vessel a basic understanding of differences between inner and outer SMC’s in terms of attachment and contraction of 3D matrix is essential. We hypothesised that they will differ in their ability to contract a 3D collagen gel. Using a Culture Force Monitor (CFM) we sought to: (1) quantify the contractile ability of SMC derived from inner and outer normal and hypoxic arteries, harvested from piglet models, over 24 hours, (2) test the effect of cytoskeletal blockade on force retention in collagen gels and (3) quantify cellular contractile response to agonist or antagonist stimulation
Mission analysis of solar powered aircraft
The effect of a real mission scenario on a solar powered airplane configuration which had been developed in previous work were assessed. The mission used was surveillance of crop conditions over a route from Phoenix to Tucson to Tombstone, Arizona. Appendices are attached which address the applicability of existing platforms and payloads to do this mission
Self-similar static solutions admitting a two-space of constant curvature
A recent result by Haggag and Hajj-Boutros is reviewed within the framework
of self-similar space-times, extending, in some sense, their results and
presenting a family of metrics consisting of all the static spherically
symmetric perfect fluid solutions admitting a homothety.Comment: 6 page
Symmetries of Bianchi I space-times
All diagonal proper Bianchi I space-times are determined which admit certain
important symmetries. It is shown that for Homotheties, Conformal motions and
Kinematic Self-Similarities the resulting space-times are defined explicitly in
terms of a set of parameters whereas Affine Collineations, Ricci Collineations
and Curvature Collineations, if they are admitted, they determine the metric
modulo certain algebraic conditions. In all cases the symmetry vectors are
explicitly computed. The physical and the geometrical consequences of the
results are discussed and a new anisitropic fluid, physically valid solution
which admits a proper conformal Killing vector, is given.Comment: 19 pages, LaTex, Accepted for publication in Journal of Mathematical
Physic
Photoelectric observations of Mars and Jupiter with a scanning polarimeter
Photoelectric observations of Mars and Jupiter with scanning polarimete
Modelling economies in transition: an introduction
This paper considers the implications of structural breaks, such as have occurred in many transition economies, for econometric modelling based on the multivariate cointegration paradigm. It outlines recent developments on the identification of linear cointegrated systems, discusses some practical problems, and presents an extension to non-linear systems. This is followed by a discussion of the impact of structural breaks on the identification and estimation of such systems. Finally, it relates these issues to the other papers in this volume.
Pulmonary artery smooth muscle cells response to vaso-active stimulations in a real-time 3-Dimensional model
INTRODUCTION: The tone of the pulmonary arteries is the summation of the activity of each smooth muscle cell (PASMC) within a vessel wall and its interaction with the endothelial cells and extracellular matrix (including collagen). There are reported phenotypic differences between PASMC in the inner & outer layers of pulmonary artery walls1. The response of a tissue engineered blood vessel to contractile and relaxing stimulants in-vitro is essential to predicting the response of the physiological and pathological vessels in-vivo. Previous work showed that pulmonary artery relaxation to nitric oxide is inhibited after exposure to chronic hypoxia 2. We hypothesised that PASMC will differ in their ability to contract or relax a 3D collagen gel. Using a Culture Force Monitor (CFM) we sought to quantify the cellular response of PASMC derived from inner and outer normal and hypoxic arteries, harvested from piglet models, over 24 hours in response to contractile agonists and relaxing antagonists
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