802 research outputs found
Spinarc gas tungsten arc torch holder
Semiautomatic welding torch enables operator to control arc length, torch angle, and spring tension when welding small diameter aluminum tubing. Tungsten is preset for the weld to make arc initiation easier and to eliminate searching for the joint through a dark welding lens
Seiberg-Witten maps and noncommutative Yang-Mills theories for arbitrary gauge groups
Seiberg-Witten maps and a recently proposed construction of noncommutative
Yang-Mills theories (with matter fields) for arbitrary gauge groups are
reformulated so that their existence to all orders is manifest. The ambiguities
of the construction which originate from the freedom in the Seiberg-Witten map
are discussed with regard to the question whether they can lead to inequivalent
models, i.e., models not related by field redefinitions.Comment: 12 pages; references added, minor misprints correcte
A Cohomological Approach to the Non-Abelian Seiberg-Witten Map
We present a cohomological method for obtaining the non-Abelian
Seiberg-Witten map for any gauge group and to any order in theta. By
introducing a ghost field, we are able to express the equations defining the
Seiberg-Witten map through a coboundary operator, so that they can be solved by
constructing a corresponding homotopy operator.Comment: 18 pages. References added and some misprints correcte
Dynamics and stability of the Godel universe
We use covariant techniques to describe the properties of the Godel universe
and then consider its linear response to a variety of perturbations. Against
matter aggregations, we find that the stability of the Godel model depends
primarily upon the presence of gradients in the centrifugal energy, and
secondarily on the equation of state of the fluid. The latter dictates the
behaviour of the model when dealing with homogeneous perturbations. The
vorticity of the perturbed Godel model is found to evolve as in almost-FRW
spacetimes, with some additional directional effects due to shape distortions.
We also consider gravitational-wave perturbations by investigating the
evolution of the magnetic Weyl component. This tensor obeys a simple plane-wave
equation, which argues for the neutral stability of the Godel model against
linear gravity-wave distortions. The implications of the background rotation
for scalar-field Godel cosmologies are also discussed.Comment: Revised version, to match paper published in Class. Quantum Gra
Nightside ionosphere of Mars studied with local electron densities: A general overview and electron density depressions
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95308/1/jgra21490.pd
Noncommutative QCD, first-order-in-theta-deformed instantons and 't Hooft vertices
For commutative Euclidean time, we study the existence of field
configurations that {\it a)} are formal power series expansions in
h\theta^{\m\n}, {\it b)} go to ordinary (anti-)instantons as
h\theta^{\m\n}\to 0, and {\it c)} render stationary the classical action of
Euclidean noncommutative SU(3) Yang-Mills theory. We show that the
noncommutative (anti-)self-duality equations have no solutions of this type at
any order in h\theta^{\m\n}. However, we obtain all the deformations --called
first-order-in--deformed instantons-- of the ordinary instanton that,
at first order in h\theta^{\m\n}, satisfy the equations of motion of
Euclidean noncommutative SU(3) Yang-Mills theory. We analyze the quantum
effects that these field configurations give rise to in noncommutative SU(3)
with one, two and three nearly massless flavours and compute the corresponding
't Hooft vertices, also, at first order in h\theta^{\m\n}. Other issues
analyzed in this paper are the existence at higher orders in h\theta^{\m\n}
of topologically nontrivial solutions of the type mentioned above and the
classification of the classical vacua of noncommutative SU(N) Yang-Mills theory
that are power series in h\theta^{\m\n}.Comment: Latex. Some macros. No figures. 42 pages. Typos correcte
Matrix Compactification On Orientifolds
Generalizing previous results for orbifolds, in this paper we describe the
compactification of Matrix model on an orientifold which is a quotient space as
a Yang-Mills theory living on a quantum space. The information of the
compactification is encoded in the action of the discrete symmetry group G on
Euclidean space and a projective representation U of G. The choice of Hilbert
space on which the algebra of U is realized as an operator algebra corresponds
to the choice of a physical background for the compactification. All these data
are summarized in the spectral triple of the quantum space.Comment: 28 pages, late
Acoustic radiation controls friction: Evidence from a spring-block experiment
Brittle failures of materials and earthquakes generate acoustic/seismic waves
which lead to radiation damping feedbacks that should be introduced in the
dynamical equations of crack motion. We present direct experimental evidence of
the importance of this feedback on the acoustic noise spectrum of
well-controlled spring-block sliding experiments performed on a variety of
smooth surfaces. The full noise spectrum is quantitatively explained by a
simple noisy harmonic oscillator equation with a radiation damping force
proportional to the derivative of the acceleration, added to a standard viscous
term.Comment: 4 pages including 3 figures. Replaced with version accepted in PR
Tachyon Condensation on Noncommutative Torus
We discuss noncommutative solitons on a noncommutative torus and their
application to tachyon condensation. In the large B limit, they can be exactly
described by the Powers-Rieffel projection operators known in the mathematical
literature. The resulting soliton spectrum is consistent with T-duality and is
surprisingly interesting. It is shown that an instability arises for any
D-branes, leading to the decay into many smaller D-branes. This phenomenon is
the consequence of the fact that K-homology for type II von Neumann factor is
labeled by R.Comment: LaTeX, 17 pages, 1 figur
Evidence for unusually high densities of plasma in the Venusian ionosheath
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94972/1/grl5268.pd
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