131 research outputs found
Tangent unit-vector fields: nonabelian homotopy invariants and the Dirichlet energy
Let O be a closed geodesic polygon in S 2 . Maps from O into S 2 are said to satisfy tangent boundary conditions if the edges of O are mapped into the geodesics which contain them. Taking O to be an octant of S 2 , we compute the infimum Dirichlet energy, E(H), for continuous maps satisfying tangent boundary conditions of arbitrary homotopy type H. The expression for E(H) involves a topological invariant – the spelling length – associated with the (nonabelian) fundamental group of the n-times punctured two-sphere, π1 (S 2 − {s1 , . . . , sn }, ∗). The lower bound for E(H) is obtained from combinatorial group theory arguments, while the upper bound is obtained by constructing explicit representatives which, on all but an arbitrarily small subset of O, are alternatively locally conformal or anticonformal. For conformal and anticonformal classes (classes containing wholly conformal and anticonformal representatives respectively), the expression for E(H) reduces to a previous result involving the degrees of a set of regular values s1 , . . . , sn in the target S 2 space. These degrees may be viewed as invariants associated with the abelianization of π1 (S 2 − {s1 , . . . , sn }, ∗). For nonconformal classes, however, E(H) may be strictly greater than the abelian bound. This stems from the fact that, for nonconformal maps, the number of preimages of certain regular values may necessarily be strictly greater than the absolute value of their degrees.\ud
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This work is motivated by the theoretical modelling of nematic liquid crystals in confined polyhedral geometries. The results imply new lower and upper bounds for the Dirichlet energy (one-constant Oseen-Frank energy) of reflection-symmetric tangent unit-vector fields in a rectangular prism
Wilson loops in heavy ion collisions and their calculation in AdS/CFT
Expectation values of Wilson loops define the nonperturbative properties of
the hot medium produced in heavy ion collisions that arise in the analysis of
both radiative parton energy loss and quarkonium suppression. We use the
AdS/CFT correspondence to calculate the expectation values of such Wilson loops
in the strongly coupled plasma of N=4 super Yang-Mills (SYM) theory, allowing
for the possibility that the plasma may be moving with some collective flow
velocity as is the case in heavy ion collisions. We obtain the N=4 SYM values
of the jet quenching parameter , which describes the energy loss of a
hard parton in QCD, and of the velocity-dependence of the quark-antiquark
screening length for a moving dipole as a function of the angle between its
velocity and its orientation. We show that if the quark-gluon plasma is flowing
with velocity v_f at an angle theta with respect to the trajectory of a hard
parton, the jet quenching parameter is modified by a factor
gamma_f(1-v_f cos theta), and show that this result applies in QCD as in N=4
SYM. We discuss the relevance of the lessons we are learning from all these
calculations to heavy ion collisions at RHIC and at the LHC. Furthermore, we
discuss the relation between our results and those obtained in other theories
with gravity duals, showing in particular that the ratio between in
any two conformal theories with gravity duals is the square root of the ratio
of their central charges. This leads us to conjecture that in nonconformal
theories defines a quantity that always decreases along
renormalization group trajectories and allows us to use our calculation of
in N=4 SYM to make a conjecture for its value in QCD.Comment: 61 pages, 8 figures. Note added discussing relation between our work
and that in several papers that have appeared recently. References adde
Spacetime in String Theory
We give a brief overview of the nature of spacetime emerging from string
theory. This is radically different from the familiar spacetime of Einstein's
relativity. At a perturbative level, the spacetime metric appears as ``coupling
constants" in a two dimensional quantum field theory. Nonperturbatively (with
certain boundary conditions), spacetime is not fundamental but must be
reconstructed from a holographic, dual theory.Comment: 20 pages; references adde
On Dynamics of Cubic Siegel Polynomials
Motivated by the work of Douady, Ghys, Herman and Shishikura on Siegel
quadratic polynomials, we study the one-dimensional slice of the cubic
polynomials which have a fixed Siegel disk of rotation number theta, with theta
being a given irrational number of Brjuno type. Our main goal is to prove that
when theta is of bounded type, the boundary of the Siegel disk is a quasicircle
which contains one or both critical points of the cubic polynomial. We also
prove that the locus of all cubics with both critical points on the boundary of
their Siegel disk is a Jordan curve, which is in some sense parametrized by the
angle between the two critical points. A main tool in the bounded type case is
a related space of degree 5 Blaschke products which serve as models for our
cubics. Along the way, we prove several results about the connectedness locus
of these cubic polynomials.Comment: 58 pages. 20 PostScript figure
The solution of the Elrod algorithm for a dynamically loaded journal bearing using multigrid techniques
A numerical solution to a theoretical model of vapor cavitation in a dynamically loaded journal bearing is developed utilizing a multigrid iteration technique. The method is compared with a noniterative approach in terms of computational time and accuracy. The computational model is based on the Elrod algorithm, a control volume approach to the Reynolds equation which mimics the Jakobsson-Floberg and Olsson cavitation theory. Besides accounting for a moving cavitation boundary and conservation of mass at the boundary, it also conserves mass within the cavitated region via a smeared mass or striated flow extending to both surfaces in the film gap. The mixed nature of the equations (parabolic in the full film zone and hyperbolic in the cavitated zone) coupled with the dynamic aspects of the problem create interesting difficulties for the present solution approach. Emphasis is placed on the methods found to eliminate solution instabilities. Excellent results are obtained for both accuracy and reduction of computational time
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