4,048 research outputs found
Wedges, Cones, Cosmic Strings, and the Reality of Vacuum Energy
One of J. Stuart Dowker's most significant achievements has been to observe
that the theory of diffraction by wedges developed a century ago by Sommerfeld
and others provided the key to solving two problems of great interest in
general-relativistic quantum field theory during the last quarter of the
twentieth century: the vacuum energy associated with an infinitely thin,
straight cosmic string, and (after an interchange of time with a space
coordinate) the apparent vacuum energy of empty space as viewed by an
accelerating observer. In a sense the string problem is more elementary than
the wedge, since Sommerfeld's technique was to relate the wedge problem to that
of a conical manifold by the method of images. Indeed, Minkowski space, as well
as all cone and wedge problems, are related by images to an infinitely sheeted
master manifold, which we call Dowker space. We review the research in this
area and exhibit in detail the vacuum expectation values of the energy density
and pressure of a scalar field in Dowker space and the cone and wedge spaces
that result from it. We point out that the (vanishing) vacuum energy of
Minkowski space results, from the point of view of Dowker space, from the
quantization of angular modes, in precisely the way that the Casimir energy of
a toroidal closed universe results from the quantization of Fourier modes; we
hope that this understanding dispels any lingering doubts about the reality of
cosmological vacuum energy.Comment: 28 pages, 16 figures. Special volume in honor of J. S. Dowke
Design and Development of Personal GeoServices for Universities
Personal GeoServices are emerging as an interaction paradigm linking users to information rich environments like a university campus or to Big Data sources like the Internet of Things by delivering spatially intelligent web-services. OpenStreetMap (OSM) constitutes a valuable source of spatial base-data that can be extracted, integrated, and utilised with such heterogeneous data sources for free. In this paper, we present a Personal GeoServices application built on OSM spatial data and university-specific business data for staff, faculty, and students. While generic products such as Google Maps and Google Earth enable basic forms of spatial exploration, the domain of a university campus presents specific business information needs, such as “What classes are scheduled in that room over there?” and “How can I get to Prof. Murray’s office from here?” Within the framework of the StratAG project (www.StratAG.ie), an eCampus Demonstrator was developed for the National University of Ireland Maynooth (NUIM) to assist university users in exploring and analysing their surroundings within a detailed data environment. This work describes this system in detail, discussing the usage of OSM vector data, and providing insights for developers of spatial information systems for personalised visual exploration of an area
Crossing Symmetry Violation of Unitarized Pion-Pion Amplitude in the Resonance Region
Pion-pion scattering amplitude obtained from one-loop Chiral Perturbation
Theory (ChPT) is crossing symmetric, however the corresponding partial wave
amplitudes do not respect exact unitarity relation. There are different
approaches to get unitarized partial wave amplitudes from ChPT. Here we
consider the inverse amplitude method (IAM) that is often used to fit pion-pion
phase shifts to experimental data, by adjusting free parameters. We measure the
amount of crossing symmetry violation (CSV) in this case and we show that
crossing symmetry is badly violated by the IAM unitarized ChPT amplitude in the
resonance region. Important CSV also occurs when all free parameters are set
equal to zero.Comment: 6 pages, 4 figure
Hybrid squeezing of solitonic resonant radiation in photonic crystal fibers
We report on the existence of a novel kind of squeezing in photonic crystal
fibers which is conceptually intermediate between the four-wave mixing induced
squeezing, in which all the participant waves are monochromatic waves, and the
self-phase modulation induced squeezing for a single pulse in a coherent state.
This hybrid squeezing occurs when an arbitrary short soliton emits
quasi-monochromatic resonant radiation near a zero group velocity dispersion
point of the fiber. Photons around the resonant frequency become strongly
correlated due to the presence of the classical soliton, and a reduction of the
quantum noise below the shot noise level is predicted.Comment: 5 pages, 2 figure
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Some Case Example Exact Solutions for Quadratically Nonlinear Optical Media with PT-Symmetric Potentials
In the present paper we consider an optical system with a χ (2)-type nonlinearity and unspecified PT -symmetric potential functions. Considering this as an inverse problem and positing a family of exact solutions in terms of cnoidal functions, we solve for the resulting potential functions in a way that ensures the potentials obey the requirements of PT -symmetry. We then focus on case examples of soliton and periodic solutions for which we present a stability analysis as a function of their amplitude parameters. Finally, we numerically explore the nonlinear dynamics of the associated waveforms to identify the outcome of the relevant dynamical instabilities of localized and extended states
Wavelength-Orthogonal Stiffening of Hydrogel Networks with Visible Light
Herein, we introduce the wavelength-orthogonal crosslinking of hydrogel networks using two red-shifted chromophores, i.e. acrylpyerene (AP, λ=410–490 nm) and styrylpyrido[2,3-b]pyrazine (SPP, λ=400–550 nm), able to undergo [2+2] photocycloaddition in the visible-light regime. The photoreactivity of the SPP moiety is pH-dependent, whereby an acidic environment inhibits the cycloaddition. By employing a spiropyran-based photoacid generator with suitable absorption wavelength, we are able to restrict the activation wavelength of the SPP moiety to the green light region (λ=520–550 nm), enabling wavelength-orthogonal activation of the AP group. Our wavelength-orthogonal photochemical system was successfully applied in the design of hydrogels whose stiffness can be tuned independently by either green or blue light
Unitarized pion-nucleon scattering within Heavy Baryon Chiral Perturbation Theory
By means of the Inverse Amplitude Method we unitarize the elastic
pion-nucleon scattering amplitudes obtained from Heavy Baryon Chiral
Perturbation Theory to O(q^3). Within this approach we can enlarge their
applicability range and generate the Delta(1232) resonance. We can find a
reasonable description of the pion nucleon phase shifts with (q^2) parameters
in agreement with the resonance saturation hypothesis. However, the
uncertainties in the analysis of the low energy data as well as the large
number of chiral parameters, which can have strong correlations, allow us to
obtain very good fits with rather different sets of chiral constants.Comment: Shortened version to appear in Phys. Rev. D. Brief Report
K_S\rightarrow \gamma\gamma , K_L\rightarrow\pi^0\gamma\gamma$ and Unitarity
Agreement between the experimental value and the number predicted via a one-loop chiral perturbation
theory calculation has been cited as a success for the latter. On the other
hand the one-loop prediction for the closely related process has been found to be a factor three below the experimental
value. Using the inputs of unitarity and dispersion relations, we demonstrate
the importance of higher order loop effects to both of these processes.Comment: 20 pages (4 figures available on request), UMHEP-39
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