1,255 research outputs found
Particle Content in Topological Field Theories
By demanding that the path integral measure of topological field theories be
metric independent, we can derive powerful constraints on the particle content
of a topological field theory as well as on the dimensionality of space-time.Comment: 9p, OUTP-92-30
Quantum Einstein Gravity as a Topological Field Theory
General covariance in quantum gravity is seen once one integrates over all
possible metrics. In recent years topological field theories have given us a
different route to general covariance without integrating over all possible
metrics. Here we argue that Einstein quantum gravity may be viewed as a
topological field theory provided a certain constrant from the path integral
measure is satisfied.Comment: 10 pages, LaTe
Topology, Quantum Gravity and Particle Physics
It is argued that quantum gravity has an interpretation as a topological
field theory provided a certain constraint from the path intergral measure is
respected. The constraint forces us to couple gauge and matter fields to
gravity for space - time dimensions different from 3. We then discuss possible
models which may be relevant to our universe.Comment: 18 pages, LaTeX. Replaced version corrects typos and has additional
reference
Epistemology as fiction
This is the author accepted manuscript. The final version is available from Routledge via the link in this record
Nest predation in Afrotropical forest fragments shaped by inverse edge effects, timing of nest initiation and vegetation structure
High levels of nest predation influence the population dynamics of many tropical birds, especially when deforestation alters nest predator communities. The consequences of tropical forest fragmentation on nest predation, however, remain poorly understood, as natural predation patterns have only been well documented in a handful of tropical forests. Here, we show the results of an extensive study of predation on natural nests of Cabanis's Greenbul (Phyllastrephus cabanisi) during 3 years in a highly fragmented cloud forest in SE Kenya. Overall predation rates derived from 228 scrub nests averaged 69 %, matching the typical high predation level on tropical bird species. However, predation rates strongly varied in space and time, and a model that combined timing effects of fragment, edge, concealment, year and nest was best supported by our data. Nest predation rates consistently increased from forest edge to interior, opposing the classic edge effect on nest predation, and supporting the idea that classic edge effects are much rarer in Afrotropical forests than elsewhere. Nest concealment also affected predation rates, but the strength and direction of the relationship varied across breeding seasons and fragments. Apart from spatial variation, predation rates declined during the breeding season, although the strength of this pattern varied among breeding seasons. Complex and variable relationships with nest predation, such as those demonstrated here, suggest that several underlying mechanisms interact and imply that fixed nesting strategies may have variable-even opposing-fitness effects between years, sites and habitats
Angle-resolved cathodoluminescence imaging polarimetry
Cathodoluminescence spectroscopy (CL) allows characterizing light emission in
bulk and nanostructured materials and is a key tool in fields ranging from
materials science to nanophotonics. Previously, CL measurements focused on the
spectral content and angular distribution of emission, while the polarization
was not fully determined. Here we demonstrate a technique to access the full
polarization state of the cathodoluminescence emission, that is the Stokes
parameters as a function of the emission angle. Using this technique, we
measure the emission of metallic bullseye nanostructures and show that the
handedness of the structure as well as nanoscale changes in excitation position
induce large changes in polarization ellipticity and helicity. Furthermore, by
exploiting the ability of polarimetry to distinguish polarized from unpolarized
light, we quantify the contributions of different types of coherent and
incoherent radiation to the emission of a gold surface, silicon and gallium
arsenide bulk semiconductors. This technique paves the way for in-depth
analysis of the emission mechanisms of nanostructured devices as well as
macroscopic media.Comment: 8 figures. Includes supplementary informatio
Nitrogen Incorporation in CH_4-N_2 Photochemical Aerosol Produced by Far Ultraviolet Irradiation
Nitrile incorporation into Titan aerosol accompanying hydrocarbon chemistry is thought to be driven by extreme UV wavelengths (λ120 nm is presently unaccounted for in atmospheric photochemical models. We suggest that reaction with CH radicals produced from CH_4 photolysis may provide a mechanism for incorporating N into the molecular structure of the aerosol. Further work is needed to understand the chemistry involved, as these processes may have significant implications for how we view prebiotic chemistry on early Earth and similar planets. Key Words: Titan—Photochemical aerosol—CH_4-N_2 photolysis—Far UV—Nitrogen activation
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