418 research outputs found
Fine structure of Vavilov-Cherenkov radiation near the Cherenkov threshold
We analyze the Vavilov-Cherenkov radiation (VCR) in a dispersive
nontransparent dielectric air-like medium both below and above the Cherenkov
threshold, in the framework of classical electrodynamics. It is shown that the
transition to the subthreshold energies leads to the destruction of
electromagnetic shock waves and to the sharp reduction of the frequency domain
where VCR is emitted. The fine wake-like structure of the Vavilov-Cherenkov
radiation survives and manifests the existence of the subthreshold radiation in
the domain of anomalous dispersion. These domains can approximately be defined
by the two phenomenological parameters of the medium, namely, the effective
frequency of oscillators and the damping describing an interaction with the
other degrees of freedom.Comment: 9 pages, 6 figure
M-theory on eight-manifolds revisited: N=1 supersymmetry and generalized Spin(7) structures
The requirement of supersymmetry for M-theory backgrounds of the
form of a warped product , where is an eight-manifold
and is three-dimensional Minkowski or AdS space, implies the
existence of a nowhere-vanishing Majorana spinor on . lifts to a
nowhere-vanishing spinor on the auxiliary nine-manifold , where
is a circle of constant radius, implying the reduction of the structure
group of to . In general, however, there is no reduction of the
structure group of itself. This situation can be described in the language
of generalized structures, defined in terms of certain spinors of
. We express the condition for supersymmetry
in terms of differential equations for these spinors. In an equivalent
formulation, working locally in the vicinity of any point in in terms of a
`preferred' structure, we show that the requirement of
supersymmetry amounts to solving for the intrinsic torsion and all irreducible
flux components, except for the one lying in the of , in
terms of the warp factor and a one-form on (not necessarily
nowhere-vanishing) constructed as a bilinear; in addition, is
constrained to satisfy a pair of differential equations. The formalism based on
the group is the most suitable language in which to describe
supersymmetric compactifications on eight-manifolds of structure,
and/or small-flux perturbations around supersymmetric compactifications on
manifolds of holonomy.Comment: 24 pages. V2: introduction slightly extended, typos corrected in the
text, references added. V3: the role of Spin(7) clarified, erroneous
statements thereof corrected. New material on generalized Spin(7) structures
in nine dimensions. To appear in JHE
Liquid racism and the Danish Prophet Muhammad cartoons
This is the author's accepted manuscript. The final published article is available from the link below. Copyright @ 2010 The Author.This article examines reactions to the October 2005 publication of the Prophet Muhammad cartoons in the Danish newspaper Jyllands-Posten. It does so by using the concept of ‘liquid racism’. While the controversy arose because it is considered blasphemous by many Muslims to create images of the Prophet Muhammad, the article argues that the meaning of the cartoons is multidimensional, that their analysis is significantly more complex than most commentators acknowledge, and that this complexity can best be addressed via the concept of liquid racism. The article examines the liquidity of the cartoons in relation to four readings. These see the cartoons as: (1) a criticism of Islamic fundamentalism; (2) blasphemous images; (3) Islamophobic and racist; and (4) satire and a defence of freedom of speech. Finally, the relationship between postmodernity and the rise of fundamentalism is discussed because the cartoons, reactions to them, and Islamic fundamentalism, all contain an important postmodern dimension.ESR
Semigroup Closures of Finite Rank Symmetric Inverse Semigroups
We introduce the notion of semigroup with a tight ideal series and
investigate their closures in semitopological semigroups, particularly inverse
semigroups with continuous inversion. As a corollary we show that the symmetric
inverse semigroup of finite transformations of the rank
is algebraically closed in the class of (semi)topological inverse
semigroups with continuous inversion. We also derive related results about the
nonexistence of (partial) compactifications of classes of semigroups that we
consider.Comment: With the participation of the new coauthor - Jimmie Lawson - the
manuscript has been substantially revised and expanded. Accordingly, we have
also changed the manuscript titl
Foreground removal from CMB temperature maps using an MLP neural network
One of the main obstacles in extracting the Cosmic Microwave Background (CMB)
signal from observations in the mm-submm range is the foreground contamination
by emission from galactic components: mainly synchrotron, free-free and thermal
dust emission. Due to the statistical nature of the intrinsic CMB signal it is
essential to minimize the systematic errors in the CMB temperature
determinations. Following the available knowledge of the spectral behavior of
the galactic foregrounds simple, power law-like spectra have been assumed. The
feasibility of using a simple neural network for extracting the CMB temperature
signal from the combined CMB and foreground signals has been investigated. As a
specific example, we have analysed simulated data, like that expected from the
ESA Planck Surveyor mission. A simple multilayer perceptron neural network with
2 hidden layers can provide temperature estimates, over more than 80 percent of
the sky, that are to a high degree uncorrelated with the foreground signals. A
single network will be able to cover the dynamic range of the Planck noise
level over the entire sky.Comment: Accepted for publication in Astrophysics and Space Scienc
Schwinger Terms and Cohomology of Pseudodifferential Operators
We study the cohomology of the Schwinger term arising in second quantization
of the class of observables belonging to the restricted general linear algebra.
We prove that, for all pseudodifferential operators in 3+1 dimensions of this
type, the Schwinger term is equivalent to the ``twisted'' Radul cocycle, a
modified version of the Radul cocycle arising in non-commutative differential
geometry. In the process we also show how the ordinary Radul cocycle for any
pair of pseudodifferential operators in any dimension can be written as the
phase space integral of the star commutator of their symbols projected to the
appropriate asymptotic component.Comment: 19 pages, plain te
De Sitter and Schwarzschild-De Sitter According to Schwarzschild and De Sitter
When de Sitter first introduced his celebrated spacetime, he claimed,
following Schwarzschild, that its spatial sections have the topology of the
real projective space RP^3 (that is, the topology of the group manifold SO(3))
rather than, as is almost universally assumed today, that of the sphere S^3.
(In modern language, Schwarzschild was disturbed by the non-local correlations
enforced by S^3 geometry.) Thus, what we today call "de Sitter space" would not
have been accepted as such by de Sitter. There is no real basis within
classical cosmology for preferring S^3 to RP^3, but the general feeling appears
to be that the distinction is in any case of little importance. We wish to
argue that, in the light of current concerns about the nature of de Sitter
space, this is a mistake. In particular, we argue that the difference between
"dS(S^3)" and "dS(RP^3)" may be very important in attacking the problem of
understanding horizon entropies. In the approach to de Sitter entropy via
Schwarzschild-de Sitter spacetime, we find that the apparently trivial
difference between RP^3 and S^3 actually leads to very different perspectives
on this major question of quantum cosmology.Comment: 26 pages, 8 figures, typos fixed, references added, equation numbers
finally fixed, JHEP versio
A measurement of the tau mass and the first CPT test with tau leptons
We measure the mass of the tau lepton to be 1775.1+-1.6(stat)+-1.0(syst.) MeV
using tau pairs from Z0 decays. To test CPT invariance we compare the masses of
the positively and negatively charged tau leptons. The relative mass difference
is found to be smaller than 3.0 10^-3 at the 90% confidence level.Comment: 10 pages, 4 figures, Submitted to Phys. Letts.
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