266 research outputs found
Field theory of the photon self-energy in a medium with a magnetic field and the Faraday effect
A convenient and general decomposition of the photon self-energy in a
magnetized, but otherwise isotropic, medium is given in terms of the minimal
set of tensors consistent with the transversality condition. As we show, the
self-energy in such a medium is completely parametrized in terms of nine
independent form factors, and they reduce to three in the long wavelength
limit. We consider in detail an electron gas with a background magnetic field,
and using finite temperature field theory methods, we obtain the one-loop
formulas for the form factors, which are exact to all orders in the magnetic
field. Explicit results are derived for a variety of physical conditions. In
the appropriate limits, we recover the well-known semi-classical results for
the photon dispersion relations and the Faraday effect. In more general cases,
where the semi-classical treatment or the linear approximation (weak field
limit) are not applicable, our formulas provide a consistent and systematic way
for computing the self-energy form factors and, from them, the photon
dispersion relations.Comment: Revtex, 27 page
Properties and microstructure of alkali-activated red clay brick waste
Sintered red clay ceramic is used to produce hollow bricks which are manufactured in enormous quantities in Spain. They also constitute a major fraction of construction and demolition waste. The aim of this research was to investigate the properties and microstructure of alkali-activated cement pastes and mortars produced using red clay brick waste. The work shows that the type and concentration of alkali activator can be optimised to produce mortar samples with compressive strengths up to 50 MPa after curing for 7 days at 65 C. This demonstrates a new potential added value reuse application for this important waste material.The authors are grateful to the Spanish Ministry of Science and Innovation for supporting this study through Project GEOCEDEM BIA 2011-26947, and to FEDER funding. They also thank the Institute for Science and Technology of Concrete - ICITECH, for providing the means to carry out this investigation; and Universitat Jaume I, for supporting this research through the research stay granted.Reig Cerdá, L.; Tashima, MM.; Borrachero Rosado, MV.; Monzó Balbuena, JM.; Cheeseman, C.; Paya Bernabeu, JJ. (2013). Properties and microstructure of alkali-activated red clay brick waste. Construction and Building Materials. 43:98-106. doi:10.1016/j.conbuildmat.2013.01.031S981064
Gamma Ray Bursts as Probes of Quantum Gravity
Gamma ray bursts (GRBs) are short and intense pulses of -rays
arriving from random directions in the sky. Several years ago Amelino-Camelia
et al. pointed out that a comparison of time of arrival of photons at different
energies from a GRB could be used to measure (or obtain a limit on) possible
deviations from a constant speed of light at high photons energies. I review
here our current understanding of GRBs and reconsider the possibility of
performing these observations.Comment: Lectures given at the 40th winter school of theretical physics:
Quantum Gravity and Phenomenology, Feb. 2004 Polan
Computing gravitational waves from slightly nonspherical stellar collapse to black hole: Odd-parity perturbation
Nonspherical stellar collapse to a black hole is one of the most promising
gravitational wave sources for gravitational wave detectors. We numerically
study gravitational waves from a slightly nonspherical stellar collapse to a
black hole in linearized Einstein theory. We adopt a spherically collapsing
star as the zeroth-order solution and gravitational waves are computed using
perturbation theory on the spherical background. In this paper we focus on the
perturbation of odd-parity modes. Using the polytropic equations of state with
polytropic indices and 3, we qualitatively study gravitational waves
emitted during the collapse of neutron stars and supermassive stars to black
holes from a marginally stable equilibrium configuration. Since the matter
perturbation profiles can be chosen arbitrarily, we provide a few types for
them. For , the gravitational waveforms are mainly characterized by a
black hole quasinormal mode ringing, irrespective of perturbation profiles
given initially. However, for , the waveforms depend strongly on the
initial perturbation profiles. In other words, the gravitational waveforms
strongly depend on the stellar configuration and, in turn, on the ad hoc choice
of the functional form of the perturbation in the case of supermassive stars.Comment: 31 pages, accepted for publication in Phys. Rev. D, typos and minor
errors correcte
Magnetic Reconnection in Extreme Astrophysical Environments
Magnetic reconnection is a basic plasma process of dramatic rearrangement of
magnetic topology, often leading to a violent release of magnetic energy. It is
important in magnetic fusion and in space and solar physics --- areas that have
so far provided the context for most of reconnection research. Importantly,
these environments consist just of electrons and ions and the dissipated energy
always stays with the plasma. In contrast, in this paper I introduce a new
direction of research, motivated by several important problems in high-energy
astrophysics --- reconnection in high energy density (HED) radiative plasmas,
where radiation pressure and radiative cooling become dominant factors in the
pressure and energy balance. I identify the key processes distinguishing HED
reconnection: special-relativistic effects; radiative effects (radiative
cooling, radiation pressure, and Compton resistivity); and, at the most extreme
end, QED effects, including pair creation. I then discuss the main
astrophysical applications --- situations with magnetar-strength fields
(exceeding the quantum critical field of about 4 x 10^13 G): giant SGR flares
and magnetically-powered central engines and jets of GRBs. Here, magnetic
energy density is so high that its dissipation heats the plasma to MeV
temperatures. Electron-positron pairs are then copiously produced, making the
reconnection layer highly collisional and dressing it in a thick pair coat that
traps radiation. The pressure is dominated by radiation and pairs. Yet,
radiation diffusion across the layer may be faster than the global Alfv\'en
transit time; then, radiative cooling governs the thermodynamics and
reconnection becomes a radiative transfer problem, greatly affected by the
ultra-strong magnetic field. This overall picture is very different from our
traditional picture of reconnection and thus represents a new frontier in
reconnection research.Comment: Accepted to Space Science Reviews (special issue on magnetic
reconnection). Article is based on an invited review talk at the
Yosemite-2010 Workshop on Magnetic Reconnection (Yosemite NP, CA, USA;
February 8-12, 2010). 30 pages, no figure
The Mathematical Universe
I explore physics implications of the External Reality Hypothesis (ERH) that
there exists an external physical reality completely independent of us humans.
I argue that with a sufficiently broad definition of mathematics, it implies
the Mathematical Universe Hypothesis (MUH) that our physical world is an
abstract mathematical structure. I discuss various implications of the ERH and
MUH, ranging from standard physics topics like symmetries, irreducible
representations, units, free parameters, randomness and initial conditions to
broader issues like consciousness, parallel universes and Godel incompleteness.
I hypothesize that only computable and decidable (in Godel's sense) structures
exist, which alleviates the cosmological measure problem and help explain why
our physical laws appear so simple. I also comment on the intimate relation
between mathematical structures, computations, simulations and physical
systems.Comment: Replaced to match accepted Found. Phys. version, 31 pages, 5 figs;
more details at http://space.mit.edu/home/tegmark/toe.htm
Підручник зі слов’янського фольклору в Канаді
Рецензія на підручник: Kononenko N. Slavic Folklore: a Handbook. - Westport (Connecticut); London: Greenwood Press, 2007. - 209 pp
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