404 research outputs found
Null Geodesic Congruences, Asymptotically Flat Space-Times and Their Physical Interpretation
Shear-free or asymptotically shear-free null geodesic congruences possess a
large number of fascinating geometric properties and to be closely related, in
the context of general relativity, to a variety of physically significant
affects. It is the purpose of this paper to develop these issues and find
applications in GR. The applications center around the problem of extracting
interior physical properties of an asymptotically flat space-time directly from
the asymptotic gravitational (and Maxwell) field itself in analogy with the
determination of total charge by an integral over the Maxwell field at infinity
or the identification of the interior mass (and its loss) by (Bondi's)
integrals of the Weyl tensor, also at infinity. More specifically we will see
that the asymptotically shear-free congruences lead us to an asymptotic
definition of the center-of-mass and its equations of motion. This includes a
kinematic meaning, in terms of the center of mass motion, for the Bondi
three-momentum. In addition, we obtain insights into intrinsic spin and, in
general, angular momentum, including an angular momentum conservation law with
well-defined flux terms. When a Maxwell field is present the asymptotically
shear-free congruences allow us to determine/define at infinity a
center-of-charge world-line and intrinsic magnetic dipole moment.Comment: 98 pages, 6 appendices. v2: typos corrected; v3: significant changes
made to results section using simpler arguments, added discussion of real
structures, additional references, 2 new appendice
Which SMEs seek external support? Business characteristics, management behaviour and external influences in a contingency approach
To improve SME growth and competitiveness, governments often encourage business owner-managers to make use of external sources of support. Whether they seek this depends on the degree to which they perceive themselves to need assistance. Additionally its use can be constrained by market failures. In this paper, we model whether SME owner-managers seek information and advice from formal sources, including public and private providers. In 2011, the researchers conducted a telephone survey of 1202 SMEs (1-249 employees) in England to assess the use and non-use of external support between 2008 and 2011. Using a contingency approach, we model various influences on the use and non-use of formal support and identify those owner-managers who face more concerns but have less confidence in their capabilities. We find the demand for support, especially from private providers, is fuelled by a firmâs objective to grow and a size threshold, although this is moderated by various concerns which increase the likelihood of using public sources. The willingness to take informal advice can act as a stepping stone to using formal sources. Whilst market failures affected less than a fifth of firms, those with women directors were particularly affected as were newly founded firms
A Snapshot of J. L. Synge
A brief description is given of the life and influence on relativity theory
of Professor J. L. Synge accompanied by some technical examples to illustrate
his style of work
A window to quantum gravity phenomena in the emergence of the seeds of cosmic structure
Inflationary cosmology has, in the last few years,received a strong dose of
support from observations. The fact that the fluctuation spectrum can be
extracted from the inflationary scenario through an analysis that involves
quantum field theory in curved space-time, and that it coincides with the
observational data has lead to a certain complacency in the community, which
prevents the critical analysis of the obscure spots in the derivation. We argue
here briefly, as we have discussed in more detail elsewhere, that there is
something important missing in our understanding of the origin of the seeds of
Cosmic Structure, as is evidenced by the fact that in the standard accounts the
inhomogeneity and anisotropy of our universe seems to emerge from an exactly
homogeneous andisotropic initial state through processes that do not break
those symmetries. This article gives a very brief recount of the problems faced
by the arguments based on established physics. The conclusion is that we need
some new physics to be able to fully address the problem. The article then
exposes one avenue that has been used to address the central issue and
elaborates on the degree to which, the new approach makes different predictions
from the standard analyses. The approach is inspired on Penrose's proposals
that Quantum Gravity might lead to a real, dynamical collapse of the wave
function, a process that we argued has the properties needed to extract us from
the theoretical impasse described above.Comment: 13 pages, 3 figures. To appear in DICE 2008 conference proceeding
A practical approach to achieving agility : a theory of constraints perspective
This article documents an action research (AR) project aimed at identifying the practical steps needed to become an agile manufacturer through a combination of the theory of constraints (TOC) and resource- based view (RBV) approaches in a small to medium enterprise (SME) in the Australian manufacturing sector. To date, lean production has been highlighted as a possible catalyst for creating an agile manufacturer, despite the evidence suggesting that lean manufacturing lacks the responsiveness and adaptability to effectively handle a rapidly changing market place and only works well in a stable environment. A more flexible system of production is required to fully encompass the agile characteristics needed to attain a competitive advantage. This research provides empirical evidence that the TOC perspective can be used as a practical approach for becoming an agile manufacturer. The study provides a workable approach for small firms to achieve ‘Agility’ in practice
Direct entropy determination and application to artificial spin ice
From thermodynamic origins, the concept of entropy has expanded to a range of
statistical measures of uncertainty, which may still be thermodynamically
significant. However, laboratory measurements of entropy continue to rely on
direct measurements of heat. New technologies that can map out myriads of
microscopic degrees of freedom suggest direct determination of configurational
entropy by counting in systems where it is thermodynamically inaccessible, such
as granular and colloidal materials, proteins and lithographically fabricated
nanometre-scale arrays. Here, we demonstrate a conditional-probability
technique to calculate entropy densities of translation-invariant states on
lattices using limited configuration data on small clusters, and apply it to
arrays of interacting nanometre-scale magnetic islands (artificial spin ice).
Models for statistically disordered systems can be assessed by applying the
method to relative entropy densities. For artificial spin ice, this analysis
shows that nearest-neighbour correlations drive longer-range ones.Comment: 10 page
Scattering Amplitudes and BCFW Recursion in Twistor Space
Twistor ideas have led to a number of recent advances in our understanding of
scattering amplitudes. Much of this work has been indirect, determining the
twistor space support of scattering amplitudes by examining the amplitudes in
momentum space. In this paper, we construct the actual twistor scattering
amplitudes themselves. We show that the recursion relations of Britto, Cachazo,
Feng and Witten have a natural twistor formulation that, together with the
three-point seed amplitudes, allows us to recursively construct general tree
amplitudes in twistor space. We obtain explicit formulae for -particle MHV
and NMHV super-amplitudes, their CPT conjugates (whose representations are
distinct in our chiral framework), and the eight particle N^2MHV
super-amplitude. We also give simple closed form formulae for the N=8
supergravity recursion and the MHV and conjugate MHV amplitudes. This gives a
formulation of scattering amplitudes in maximally supersymmetric theories in
which superconformal symmetry and its breaking is manifest. For N^kMHV, the
amplitudes are given by 2n-4 integrals in the form of Hilbert transforms of a
product of purely geometric, superconformally invariant twistor delta
functions, dressed by certain sign operators. These sign operators subtly
violate conformal invariance, even for tree-level amplitudes in N=4 super
Yang-Mills, and we trace their origin to a topological property of split
signature space-time. We develop the twistor transform to relate our work to
the ambidextrous twistor diagram approach of Hodges and of Arkani-Hamed,
Cachazo, Cheung and Kaplan.Comment: v2: minor corrections + extra refs. v3: further minor corrections,
extra discussion of signature issues + more ref
Value Creation from Big Data: Looking Inside the Black Box
The advent of big data is fundamentally changing the business landscape. We open the âblack boxâ of the firm to explore how firms transform big data in order to create value and why firms differ in their abilities to create value from big data. Grounded in detailed evidence from China, the worldâs largest digital market, where many firms actively engage in value creation activities from big data, we identify several novel features. We find that it is not the data itself, or individual data scientists, that generate value creation opportunities. Rather, value creation occurs through the process of data management, where managers are able to democratize, contextualize, experiment and execute data insights in a timely manner. We add richness to current theory by developing a conceptual framework of value creation from big data. We also identify avenues for future research and implications for practicing managers
Characteristic Evolution and Matching
I review the development of numerical evolution codes for general relativity
based upon the characteristic initial value problem. Progress in characteristic
evolution is traced from the early stage of 1D feasibility studies to 2D
axisymmetric codes that accurately simulate the oscillations and gravitational
collapse of relativistic stars and to current 3D codes that provide pieces of a
binary black hole spacetime. Cauchy codes have now been successful at
simulating all aspects of the binary black hole problem inside an artificially
constructed outer boundary. A prime application of characteristic evolution is
to extend such simulations to null infinity where the waveform from the binary
inspiral and merger can be unambiguously computed. This has now been
accomplished by Cauchy-characteristic extraction, where data for the
characteristic evolution is supplied by Cauchy data on an extraction worldtube
inside the artificial outer boundary. The ultimate application of
characteristic evolution is to eliminate the role of this outer boundary by
constructing a global solution via Cauchy-characteristic matching. Progress in
this direction is discussed.Comment: New version to appear in Living Reviews 2012. arXiv admin note:
updated version of arXiv:gr-qc/050809
The EROS2 search for microlensing events towards the spiral arms: the complete seven season results
The EROS-2 project has been designed to search for microlensing events
towards any dense stellar field. The densest parts of the Galactic spiral arms
have been monitored to maximize the microlensing signal expected from the stars
of the Galactic disk and bulge. 12.9 million stars have been monitored during 7
seasons towards 4 directions in the Galactic plane, away from the Galactic
center. A total of 27 microlensing event candidates have been found. Estimates
of the optical depths from the 22 best events are provided. A first order
interpretation shows that simple Galactic models with a standard disk and an
elongated bulge are in agreement with our observations. We find that the
average microlensing optical depth towards the complete EROS-cataloged stars of
the spiral arms is , a number that is
stable when the selection criteria are moderately varied. As the EROS catalog
is almost complete up to , the optical depth estimated for the
sub-sample of bright target stars with () is easier to interpret. The set of microlensing events
that we have observed is consistent with a simple Galactic model. A more
precise interpretation would require either a better knowledge of the distance
distribution of the target stars, or a simulation based on a Galactic model.
For this purpose, we define and discuss the concept of optical depth for a
given catalog or for a limiting magnitude.Comment: 22 pages submitted to Astronomy & Astrophysic
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