440 research outputs found
Исследование пластифицирующих свойств некоторых производных карбазола
During the assembly of customized miniaturized solid-state lasers, robot based operations can significantly reduce cycle times and improve performance and quality. Automation of the resistance soldering technique for optical components based on planar technology described in the previous work of Dolkemeyer et. al can be self-optimized to compensate component and process tolerances, cope with different element geometries and therefore reduce planning efforts and facilitate the operator's work.solder pads. Planar adjustment of an optical element and exemplary resistance measurement during soldering process For the automation of the soldering process itself the modification of the system's electrical resistance is measured since this value can be calculated by evaluating the process parameters without supplementary equipment. In addition, it can be used as a documented quality control. The increase of the resistance at the solder's melting point is related to the length of the molten zone and is always continuous even if current and voltage are discontinuous. Presented are the technique for planar adjustment and current results of the automation and self-optimization process
An Exact String Theory Model of Closed Time-Like Curves and Cosmological Singularities
We study an exact model of string theory propagating in a space-time
containing regions with closed time-like curves (CTCs) separated from a finite
cosmological region bounded by a Big Bang and a Big Crunch. The model is an
non-trivial embedding of the Taub-NUT geometry into heterotic string theory
with a full conformal field theory (CFT) definition, discovered over a decade
ago as a heterotic coset model. Having a CFT definition makes this an excellent
laboratory for the study of the stringy fate of CTCs, the Taub cosmology, and
the Milne/Misner-type chronology horizon which separates them. In an effort to
uncover the role of stringy corrections to such geometries, we calculate the
complete set of alpha' corrections to the geometry. We observe that the key
features of Taub-NUT persist in the exact theory, together with the emergence
of a region of space with Euclidean signature bounded by time-like curvature
singularities. Although such remarks are premature, their persistence in the
exact geometry is suggestive that string theory theory is able to make physical
sense of the Milne/Misner singularities and the CTCs, despite their
pathological character in General Relativity. This may also support the
possibility that CTCs may be viable in some physical situations, and may be a
natural ingredient in pre-Big-Bang cosmological scenarios.Comment: 37 pages, 4 figures. V2: discussion of computation of metric refined,
references adde
Classical glueballs in non-Abelian Born-Infeld theory
It is shown that the Born-Infeld-type modification of the quadratic
Yang-Mills action suggested by the superstring theory gives rise to classical
particle-like solutions prohibited in the standard Yang-Mills theory. This
becomes possible due to the scale invariance breaking by the Born-Infeld
non-linearity. New classical glueballs are sphaleronic in nature and exhibit a
striking similarity with the Bartnik-McKinnon solutions of the Yang-Mills
theory coupled to gravity.Comment: Revtex, 4 pages, 2 eps figure
Black holes in Goedel-type universes with a cosmological constant
We discuss supersymmetric black holes embedded in a Goedel-type universe with
cosmological constant in five dimensions. The spacetime is a fibration over a
four-dimensional Kaehler base manifold, and generically has closed timelike
curves. Asymptotically the space approaches a deformation of AdS_5, which
suggests that the appearance of closed timelike curves should have an
interpretation in some deformation of D=4, N=4 super-Yang-Mills theory.
Finally, a Goedel-de Sitter universe is also presented and its causal structure
is discussed.Comment: 25 pages, Latex, no figures, references updated, physical discussion
of the solutions considerably expanded, holographic stress tensor and
conserved charges of Goedel-AdS(5) solution compute
Typicality versus thermality: An analytic distinction
In systems with a large degeneracy of states such as black holes, one expects
that the average value of probe correlation functions will be well approximated
by the thermal ensemble. To understand how correlation functions in individual
microstates differ from the canonical ensemble average and from each other, we
study the variances in correlators. Using general statistical considerations,
we show that the variance between microstates will be exponentially suppressed
in the entropy. However, by exploiting the analytic properties of correlation
functions we argue that these variances are amplified in imaginary time,
thereby distinguishing pure states from the thermal density matrix. We
demonstrate our general results in specific examples and argue that our results
apply to the microstates of black holes.Comment: 22 pages + appendices, 3 eps figure
Spectrometric method to detect exoplanets as another test to verify the invariance of the velocity of light
Hypothetical influences of variability of light velocity due to the
parameters of the source of radiation, for the results of spectral measurements
of stars to search for exoplanets are considered. Accounting accelerations of
stars relative to the barycenter of the star - a planet (the planets) was
carried out. The dependence of the velocity of light from the barycentric
radial velocity and barycentric radial acceleration component of the star
should lead to a substantial increase (up to degree of magnitude) semi-major
axes of orbits detected candidate to extrasolar planets. Consequently, the
correct comparison of the results of spectral method with results of other
well-known modern methods of detecting extrasolar planets can regard the
results obtained in this paper as a reliable test for testing the invariance of
the velocity of light.Comment: 11 pages, 5 figure
A deformation of AdS_5 x S^5
We analyse a one parameter family of supersymmetric solutions of type IIB
supergravity that includes AdS_5 x S^5. For small values of the parameter the
solutions are causally well-behaved, but beyond a critical value closed
timelike curves (CTC's) appear. The solutions are holographically dual to N=4
supersymmetric Yang-Mills theory on a non-conformally flat background with
non-vanishing R-currents. We compute the holographic energy-momentum tensor for
the spacetime and show that it remains finite even when the CTC's appear. The
solutions, as well as the uplift of some recently discovered AdS_5 black hole
solutions, are shown to preserve precisely two supersymmetries.Comment: 16 pages, v2: typos corrected and references adde
Deformations of calibrated D-branes in flux generalized complex manifolds
We study massless deformations of generalized calibrated cycles, which
describe, in the language of generalized complex geometry, supersymmetric
D-branes in N=1 supersymmetric compactifications with fluxes. We find that the
deformations are classified by the first cohomology group of a Lie algebroid
canonically associated to the generalized calibrated cycle, seen as a
generalized complex submanifold with respect to the integrable generalized
complex structure of the bulk. We provide examples in the SU(3) structure case
and in a `genuine' generalized complex structure case. We discuss cases of
lifting of massless modes due to world-volume fluxes, background fluxes and a
generalized complex structure that changes type.Comment: 52 pages, added references, added comment on ellipticity in appendix
B, made minor changes according to instructions referee JHE
Yang-Mills Instantons Sitting on a Ricci-flat Worldspace of Double D4-brane
Thus far, there seem to be no complete criteria that can settle the issue as
to what the correct generalization of the Dirac-Born-Infeld (DBI) action,
describing the low-energy dynamics of the D-branes, to the non-abelian case
would be. According to recent suggestions, one might pass the issue of
worldvolume solitons from abelian to non-abelian setting by considering the
stack of multiple, coincident D-branes and use it as a guideline to construct
or censor the relevant non-abelian version of the DBI action. In this spirit,
here we are interested in the explicit construction of SU(2) Yang-Mills (YM)
instanton solutions in the background geometry of two coincident probe D4-brane
worldspaces particularly when the metric of target spacetime in which the probe
branes are embedded is given by the Ricci-flat, magnetic extremal 4-brane
solution in type IIA supergravity theory with its worldspace metric being given
by that of Taub-NUT and Eguchi-Hanson solutions, the two best-known
gravitational instantons. And then we demonstrate that with this YM instanton-
gravitational instanton configuration on the probe D4-brane worldvolume, the
energy of the probe branes attains its minimum value and hence enjoys stable
state provided one employs the Tseytlin's non-abelian DBI action for the
description of multiple probe D-branes. In this way, we support the arguments
in the literature in favor of Tseytlin's proposal for the non-abelian DBI
action.Comment: 39 pages, Revtex, some more comments adde
Goedel-type Universes and the Landau Problem
We point out a close relation between a family of Goedel-type solutions of
3+1 General Relativity and the Landau problem in S^2, R^2 and H_2; in
particular, the classical geodesics correspond to Larmor orbits in the Landau
problem. We discuss the extent of this relation, by analyzing the solutions of
the Klein-Gordon equation in these backgrounds. For the R^2 case, this relation
was independently noticed in hep-th/0306148. Guided by the analogy with the
Landau problem, we speculate on the possible holographic description of a
single chronologically safe region.Comment: Latex, 21 pages, 1 figure. v2 missing references to previous work on
the subject adde
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