115 research outputs found
``Good Propagation'' Constraints on Dual Invariant Actions in Electrodynamics and on Massless Fields
We present some consequences of non-anomalous propagation requirements on
various massless fields. Among the models of nonlinear electrodynamics we show
that only Maxwell and Born-Infeld also obey duality invariance. Separately we
show that, for actions depending only on the F_\mn^2 invariant, the permitted
models have . We also characterize acceptable
vector-scalar systems. Finally we find that wide classes of gravity models
share with Einstein the null nature of their characteristic surfaces.Comment: 11 pages, LaTeX, no figure
Hamiltonian structure for dispersive and dissipative dynamical systems
We develop a Hamiltonian theory of a time dispersive and dissipative
inhomogeneous medium, as described by a linear response equation respecting
causality and power dissipation. The proposed Hamiltonian couples the given
system to auxiliary fields, in the universal form of a so-called canonical heat
bath. After integrating out the heat bath the original dissipative evolution is
exactly reproduced. Furthermore, we show that the dynamics associated to a
minimal Hamiltonian are essentially unique, up to a natural class of
isomorphisms. Using this formalism, we obtain closed form expressions for the
energy density, energy flux, momentum density, and stress tensor involving the
auxiliary fields, from which we derive an approximate, ``Brillouin-type,''
formula for the time averaged energy density and stress tensor associated to an
almost mono-chromatic wave.Comment: 68 pages, 1 figure; introduction revised, typos correcte
Conditional symmetries and Riemann invariants for inhomogeneous hydrodynamic-type systems
A new approach to the solution of quasilinear nonelliptic first-order systems
of inhomogeneous PDEs in many dimensions is presented. It is based on a version
of the conditional symmetry and Riemann invariant methods. We discuss in detail
the necessary and sufficient conditions for the existence of rank-2 and rank-3
solutions expressible in terms of Riemann invariants. We perform the analysis
using the Cayley-Hamilton theorem for a certain algebraic system associated
with the initial system. The problem of finding such solutions has been reduced
to expanding a set of trace conditions on wave vectors and their profiles which
are expressible in terms of Riemann invariants. A couple of theorems useful for
the construction of such solutions are given. These theoretical considerations
are illustrated by the example of inhomogeneous equations of fluid dynamics
which describe motion of an ideal fluid subjected to gravitational and Coriolis
forces. Several new rank-2 solutions are obtained. Some physical interpretation
of these results is given.Comment: 19 page
Do we need to rethink our waterways? Values of ageing waterways in current and future society
In the past canals were developed, and some rivers were heavily altered, driven by the need for good transportation infrastructure. Major investments were made in navigation locks, weirs and artificial embankments, and many of these assets are now reaching the end of their technical lifetime. Since then the concept of integrated water resource management (IWRM) emerged as a concept to manage and develop water-bodies in general. Two pressing problems arise from these developments: (1) major reinvestment is needed in order to maintain the transportation function of these waterways, and (2), it is not clear how the implementation of the concept of IWRM can be brought into harmony with such reinvestment. This paper aims to illustrate the problems in capital-intensive parts of waterway systems, and argues for exploring value-driven solutions that rely on the inclusion of multiple values, thus solving both funding problems and stakeholder conflicts. The focus on value in cooperative strategies is key to defining viable implementation strategies for waterway projects
A communal catalogue reveals Earth’s multiscale microbial diversity
Our growing awareness of the microbial world’s importance and diversity contrasts starkly with our limited understanding of its fundamental structure. Despite recent advances in DNA sequencing, a lack of standardized protocols and common analytical frameworks impedes comparisons among studies, hindering the development of global inferences about microbial life on Earth. Here we present a meta-analysis of microbial community samples collected by hundreds of researchers for the Earth Microbiome Project. Coordinated protocols and new analytical methods, particularly the use of exact sequences instead of clustered operational taxonomic units, enable bacterial and archaeal ribosomal RNA gene sequences to be followed across multiple studies and allow us to explore patterns of diversity at an unprecedented scale. The result is both a reference database giving global context to DNA sequence data and a framework for incorporating data from future studies, fostering increasingly complete characterization of Earth’s microbial diversity
A communal catalogue reveals Earth's multiscale microbial diversity
Our growing awareness of the microbial world's importance and diversity contrasts starkly with our limited understanding of its fundamental structure. Despite recent advances in DNA sequencing, a lack of standardized protocols and common analytical frameworks impedes comparisons among studies, hindering the development of global inferences about microbial life on Earth. Here we present a meta-analysis of microbial community samples collected by hundreds of researchers for the Earth Microbiome Project. Coordinated protocols and new analytical methods, particularly the use of exact sequences instead of clustered operational taxonomic units, enable bacterial and archaeal ribosomal RNA gene sequences to be followed across multiple studies and allow us to explore patterns of diversity at an unprecedented scale. The result is both a reference database giving global context to DNA sequence data and a framework for incorporating data from future studies, fostering increasingly complete characterization of Earth's microbial diversity.Peer reviewe
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