12,247 research outputs found
Probing the gravitational geon
The Brill-Hartle gravitational geon construct as a spherical shell of small
amplitude, high frequency gravitational waves is reviewed and critically
analyzed. The Regge-Wheeler formalism is used to represent gravitational wave
perturbations of the spherical background as a superposition of tensor
spherical harmonics and an attempt is made to build a non-singular solution to
meet the requirements of a gravitational geon. High-frequency waves are seen to
be a necessary condition for the geon and the field equations are decomposed
accordingly. It is shown that this leads to the impossibility of forming a
spherical gravitational geon. The attempted constructs of gravitational and
electromagnetic geons are contrasted. The spherical shell in the proposed
Brill-Hartle geon does not meet the regularity conditions required for a
non-singular source and hence cannot be regarded as an adequate geon construct.
Since it is the high frequency attribute which is the essential cause of the
geon non-viability, it is argued that a geon with less symmetry is an unlikely
prospect. The broader implications of the result are discussed with particular
reference to the problem of gravitational energy.Comment: Replaced with revised version (substantial changes and additions,
conclusions unchanged), 36 pages, LaTex, 3 figures available from the author
Stability of Gravitational and Electromagnetic Geons
Recent work on gravitational geons is extended to examine the stability
properties of gravitational and electromagnetic geon constructs. All types of
geons must possess the property of regularity, self-consistency and
quasi-stability on a time-scale much longer than the period of the comprising
waves. Standard perturbation theory, modified to accommodate time-averaged
fields, is used to test the requirement of quasi-stability. It is found that
the modified perturbation theory results in an internal inconsistency. The
time-scale of evolution is found to be of the same order in magnitude as the
period of the comprising waves. This contradicts the requirement of slow
evolution. Thus not all of the requirements for the existence of
electromagnetic or gravitational geons are met though perturbation theory. From
this result it cannot be concluded that an electromagnetic or a gravitational
geon is a viable entity. The broader implications of the result are discussed
with particular reference to the problem of gravitational energy.Comment: 40 pages, 5 EPS figures, uses overcite.st
Perturbative Tamm-Dancoff Renormalization
A new two-step renormalization procedure is proposed. In the first step, the
effects of high-energy states are considered in the conventional (Feynman)
perturbation theory. In the second step, the coupling to many-body states is
eliminated by a similarity transformation. The resultant effective Hamiltonian
contains only interactions which do not change particle number. It is subject
to numerical diagonalization. We apply the general procedure to a simple
example for the purpose of illustration.Comment: 20 pages, RevTeX, 10 figure
Nonperturbative renormalization group in a light-front three-dimensional real scalar model
The three-dimensional real scalar model, in which the symmetry
spontaneously breaks, is renormalized in a nonperturbative manner based on the
Tamm-Dancoff truncation of the Fock space. A critical line is calculated by
diagonalizing the Hamiltonian regularized with basis functions. The marginal
() coupling dependence of the critical line is weak. In the broken
phase the canonical Hamiltonian is tachyonic, so the field is shifted as
. The shifted value is determined as a function of
running mass and coupling so that the mass of the ground state vanishes.Comment: 23 pages, LaTeX, 6 Postscript figures, uses revTeX and epsbox.sty. A
slight revision of statements made, some references added, typos correcte
Faddeev eigenfunctions for two-dimensional Schrodinger operators via the Moutard transformation
We demonstrate how the Moutard transformation of two-dimensional Schrodinger
operators acts on the Faddeev eigenfunctions on the zero energy level and
present some explicitly computed examples of such eigenfunctions for smooth
fast decaying potentials of operators with non-trivial kernel and for deformed
potentials which correspond to blowing up solutions of the Novikov-Veselov
equation.Comment: 11 pages, final remarks are adde
In-situ temperature calibration procedure for temperature and strain fibre Bragg grating sensors for monitoring pre-stressing strands
In this work, we demonstrate active and passive methods for in-situ temperature calibration of fibre Bragg grating strain and temperature sensors. The method is suitable for characterizing sensors which are already attached to the steel reinforcements of civil structures. The proposed method, which involves the use of active induction heating or passive room temperature fluctuations, can be implemented using portable equipment, is time efficient, and can be used to calibrate attached sensors on-site, rather than in lab conditions. Preliminary results of the induction heating calibration show good agreement with pre-calibrated temperature sensors. In-situ calibration of fibre strain sensors, attached to a prestressing strand is also successfully carried out
Metal-Packaged fibre Bragg grating strain sensors for surface mounting onto spalled concrete wind turbine foundations
In this work, we demonstrate preliminary results for a hermetically sealed, metal-packaged fibre Bragg grating strain sensor for monitoring existing concrete wind turbine foundations. As the sensor is bolted to the sub-surface of the concrete, it is suitable for mounting onto uneven, wet and degraded surfaces, which may be found in buried foundations. The sensor was able to provide reliable measurements of concrete beam strain during cyclic three- And four- point bend tests. The strain sensitivity of the prototype sensor is currently 10 % of that of commercial, epoxied fibre strain sensors
Conductivity, weak ferromagnetism and charge instability in single crystal
The temperature dependence of resistivity, magnetization and electron-spin
resonance of the single crystal were measured in temperature
range of . Magnetization hysteresis in applied magnetic field
up to 0.7 T at , irreversible temperature behavior of
magnetization and resistivity were found . The obtained data were explained in
terms of degenerate tight binding model using random phase approximation. The
contribution of holes in and bands of manganese ions to the
conductivity, optical absorbtion spectra and charge instability in were studied. Charge susceptibility maxima resulted from the competition of
the on-site Coulomb interaction between the holes in different orbitals and
small hybridization of sub-bands were calculated at .Comment: 6 pages, 12 figure
Gravitational Geons in 1+1 Dimensions
It is well known that general relativity does not admit gravitational geons
that are stationary, asymptotically flat, singularity free and topologically
trivial. However, it is likely that general relativity will receive corrections
at large curvatures and the modified field equations may admit solutions
corresponding to this type of geon. If geons are produced in the early universe
and survive until today they could account for some of the dark matter that has
been "observed" in galaxies and galactic clusters.
In this paper I consider gravitational geons in 1+1 dimensional theories of
gravity. I show that the Jackiw-Teitelboim theory with corrections proportional
to and admits gravitational geons. I also show that
gravitational geons exist in a class of theories that includes Lagrangians
proportional to .Comment: 8 pages, a comment added, two references corrected, to appear in
Classical and Quantum Gravit
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