1,231 research outputs found
Canonical Transformation Path to Gauge Theories of Gravity
In this paper, the generic part of the gauge theory of gravity is derived,
based merely on the action principle and on the general principle of
relativity. We apply the canonical transformation framework to formulate
geometrodynamics as a gauge theory. The starting point of our paper is
constituted by the general De~Donder-Weyl Hamiltonian of a system of scalar and
vector fields, which is supposed to be form-invariant under (global) Lorentz
transformations. Following the reasoning of gauge theories, the corresponding
locally form-invariant system is worked out by means of canonical
transformations. The canonical transformation approach ensures by construction
that the form of the action functional is maintained. We thus encounter amended
Hamiltonian systems which are form-invariant under arbitrary spacetime
transformations. This amended system complies with the general principle of
relativity and describes both, the dynamics of the given physical system's
fields and their coupling to those quantities which describe the dynamics of
the spacetime geometry. In this way, it is unambiguously determined how spin-0
and spin-1 fields couple to the dynamics of spacetime.
A term that describes the dynamics of the free gauge fields must finally be
added to the amended Hamiltonian, as common to all gauge theories, to allow for
a dynamic spacetime geometry. The choice of this "dynamics Hamiltonian" is
outside of the scope of gauge theory as presented in this paper. It accounts
for the remaining indefiniteness of any gauge theory of gravity and must be
chosen "by hand" on the basis of physical reasoning. The final Hamiltonian of
the gauge theory of gravity is shown to be at least quadratic in the conjugate
momenta of the gauge fields -- this is beyond the Einstein-Hilbert theory of
General Relativity.Comment: 16 page
New Technologies for the Utilization of Biologically Based Raw Materials for Feed and Food Production
Within the activities undertaken by IIASA's Food and Agriculture Program one part has been directed to assessing the role which new technologies for protein production could play in the future in covering the global demand.
This report contains the main papers submitted to the Task Force Meeting on "New Technologies for the Utilization of Biologically Based Raw Materials for Feed and Food Production" held at Tbilisi, Georgia, USSR in August 1981. The meeting was the second in a series of meetings dealing with the problems of new technologies for the utilization of agricultural wastes.
The main topics for discussion at the Tbilisi meeting were defined during the first meeting held at IIASA in September 1980. Furthermore the network of collaborating institutions and teams, established after this first meeting, produced interesting background material in the form of answers to the questionnaires distributed by IIASA. (See Table 6 of this report) .
The meeting was seen as a further step towards the assessment of the new technologies on protein production and the basis for future collaboration was outlined and the proposal for holding the next meeting was submitted
The Nature of the Dense Core Population in the Pipe Nebula: Thermal Cores Under Pressure
In this paper we present the results of a systematic investigation of an
entire population of starless dust cores within a single molecular cloud.
Analysis of extinction data shows the cores to be dense objects characterized
by a narrow range of density. Analysis of C18O and NH3 molecular-line
observations reveals very narrow lines. The non-thermal velocity dispersions
measured in both these tracers are found to be subsonic for the large majority
of the cores and show no correlation with core mass (or size). Thermal pressure
is thus the dominate source of internal gas pressure and support for most of
the core population. The total internal gas pressures of the cores are found to
be roughly independent of core mass over the entire range of the core mass
function (CMF) indicating that the cores are in pressure equilibrium with an
external source of pressure. This external pressure is most likely provided by
the weight of the surrounding Pipe cloud within which the cores are embedded.
Most of the cores appear to be pressure confined, gravitationally unbound
entities whose nature, structure and future evolution are determined by only a
few physical factors which include self-gravity, the fundamental processes of
thermal physics and the simple requirement of pressure equilibrium with the
surrounding environment. The observed core properties likely constitute the
initial conditions for star formation in dense gas. The entire core population
is found to be characterized by a single critical Bonnor-Ebert mass. This mass
coincides with the characteristic mass of the Pipe CMF indicating that most
cores formed in the cloud are near critical stability. This suggests that the
mass function of cores (and the IMF) has its origin in the physical process of
thermal fragmentation in a pressurized medium.Comment: To appear in the Astrophysical Journa
Expectations in Micro Data: Rationality Revisited
An increasing number of longitudinal data sets collect expectations information regarding a variety of future individual level events and decisions, providing researchers with the opportunity to explore expectations over micro variables in detail. We provide a theoretical framework and an econometric methodology to use that type of information to test the Rational Expectations hypothesis in models of individual behavior, and present tests using two different panel data sets.
Periodic boundary conditions for the simulation of 3D domain patterns in tetragonal ferroelectric material
Periodic domain patterns in tetragonal ferroelectrics are explored using a phase field model calibrated for barium titanate. In this context, we discuss the standard periodic boundary condition and introduce the concept of reverse periodic boundary conditions. Both concepts allow the assembly of cubic cells in accordance with mechanical and electrical conditions. However, application of the reverse periodic boundary condition is due to an increased size of the RVE and enforces more complex structures compared to the standard condition. This may be of particular interest for other multiphysics simulations. Additionally, we formulate mechanical side conditions with minimal spherical (hydrostatic) stress, or conditions with controlled average strain. It is found that in sufficiently small periodic cells, only a uniform single domain, or the simplest stripe domains constitute equilibrium states. However, once the periodic cells are of order 20 domain wall widths in size, more complex, 3-dimensional patterns emerge. Some of these patterns are known from prior studies, but we also identify other domain patterns with long, ribbon-like domains threaded through them and some vortex-like structures. Keywords: Periodic boundary conditions; RVE simulation; Tetragonal ferroelectrics; Polarization patterns; Polarization vortex; Multiphysics simulation
The Infrared Extinction Law at Extreme Depth in a Dark Cloud Core
We combined sensitive near-infrared data obtained with ground-based imagers
on the ESO NTT and VLT telescopes with space mid-infrared data acquired with
the IRAC imager on the Spitzer Space Telescope to calculate the extinction law
A_\lambda/A_K as a function of \lambda between 1.25 and 7.76 micron to an
unprecedented depth in Barnard 59, a star forming, dense core located in the
Pipe Nebula. The ratios A_\lambda/A_K were calculated from the slopes of the
distributions of sources in color-color diagrams \lambda-K vs. H-K. The
distributions in the color-color diagrams are fit well with single slopes to
extinction levels of A_K ~ 7 (A_V ~ 59 mag). Consequently, there appears to be
no significant variation of the extinction law with depth through the B59 line
of sight. However, when slopes are translated into the relative extinction
coefficients A_\lambda/A_K, we find an extinction law which departs from the
simple extrapolation of the near-infrared power law extinction curve, and
agrees more closely with a dust extinction model for a cloud with a total to
selective absorption R_V=5.5 and a grain size distribution favoring larger
grains than those in the diffuse ISM. Thus, the difference we observe could be
possibly due to the effect of grain growth in denser regions. Finally, the
slopes in our diagrams are somewhat less steep than those from the study of
Indebetouw et al. (2005) for clouds with lower column densities, and this
indicates that the extinction law between 3 and 8 micron might vary slightly as
a function of environment.Comment: 22 pages manuscript, 4 figures (2 multipart), 1 tabl
Substrate polyspecificity and conformational relevance in ABC transporters: new insights from structural studies
Transport of molecules and ions across biological membranes is an essential process in all organisms. It is carried out by a range of evolutionarily conserved primary and secondary transporters. A significant portion of the primary transporters belong to the ATP-binding cassette (ABC) superfamily, which utilise the free-energy from ATP hydrolysis to shuttle many different substrates across various biological membranes, and consequently, are involved in both normal and abnormal physiology. In humans, ABC transporter-associated pathologies are perhaps best exemplified by multidrug-resistance transporters that efflux many xenobiotic compounds due to their remarkable substrate polyspecificity. Accordingly, understanding the transport mechanism(s) is of great significance, and indeed, much progress has been made in recent years, particularly from structural studies on ABC exporters. Consequently, the general mechanism of 'alternate access' has been modified to describe individual transporter nuances, though some aspects of the transport process remain unclear. Moreover, as new information has emerged, the physiological relevance of the 'open-apo' conformation of MsbA (a bacterial exporter) has been questioned and, by extension, its contribution to mechanistic models. We present here a comprehensive overview of the most recently solved structures of ABC exporters, focusing on new insights regarding the nature of substrate polyspecificity and the physiological relevance of the 'open-apo' conformation. This review evaluates the claim that the latter may be an artefact of detergent solubilisation, and we hypothesise that the biophysical properties of the membrane play a key role in the function of ABC exporters allowing them to behave like a 'spring-hinge' during their transport cycle
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