9,515 research outputs found
An accurate formula for the period of a simple pendulum oscillating beyond the small-angle regime
A simple approximation formula is derived here for the dependence of the
period of a simple pendulum on amplitude that only requires a pocket calculator
and furnishes an error of less than 0.25% with respect to the exact period. It
is shown that this formula describes the increase of the pendulum period with
amplitude better than other simple formulas found in literature. A good
agreement with experimental data for a low air-resistance pendulum is also
verified and it suggests, together with the current availability/precision of
timers and detectors, that the proposed formula is useful for extending the
pendulum experiment beyond the usual small-angle oscillations.Comment: 15 pages and 4 figures. to appear in American Journal of Physic
Molecular Phylogenetics of Cypripedium L. (Cypripedioideae: Orchidaceae) Based on Plastid and Nuclear DNA Sequences
Topology Change and Causal Continuity
The result that, for a scalar quantum field propagating on a ``trousers''
topology in 1+1 dimensions, the crotch singularity is a source for an infinite
burst of energy has been used to argue against the occurrence of topology
change in quantum gravity. We draw attention to a conjecture due to Sorkin that
it may be the particular type of topology change involved in the trousers
transition that is problematic and that other topology changes may not cause
the same difficulties. The conjecture links the singular behaviour to the
existence of ``causal discontinuities'' in the spacetime and relies on a
classification of topology changes using Morse theory. We investigate various
topology changing transitions, including the pair production of black holes and
of topological geons, in the light of these ideas.Comment: Latex, 28 pages, 10 figures, small changes in text (one figure
removed), conclusions remain unchanged. Accepted for publication in Physical
Review
Notes on Euclidean Wilson loops and Riemann Theta functions
The AdS/CFT correspondence relates Wilson loops in N=4 SYM theory to minimal
area surfaces in AdS5 space. In this paper we consider the case of Euclidean
flat Wilson loops which are related to minimal area surfaces in Euclidean AdS3
space. Using known mathematical results for such minimal area surfaces we
describe an infinite parameter family of analytic solutions for closed Wilson
loops. The solutions are given in terms of Riemann theta functions and the
validity of the equations of motion is proven based on the trisecant identity.
The world-sheet has the topology of a disk and the renormalized area is written
as a finite, one-dimensional contour integral over the world-sheet boundary. An
example is discussed in detail with plots of the corresponding surfaces.
Further, for each Wilson loops we explicitly construct a one parameter family
of deformations that preserve the area. The parameter is the so called spectral
parameter. Finally, for genus three we find a map between these Wilson loops
and closed curves inside the Riemann surface.Comment: 35 pages, 7 figures, pdflatex. V2: References added. Typos corrected.
Some points clarifie
On Charge-3 Cyclic Monopoles
We determine the spectral curve of charge 3 BPS su(2) monopoles with C_3
cyclic symmetry. The symmetry means that the genus 4 spectral curve covers a
(Toda) spectral curve of genus 2. A well adapted homology basis is presented
enabling the theta functions and monopole data of the genus 4 curve to be given
in terms of genus 2 data. The Richelot correspondence, a generalization of the
arithmetic mean, is used to solve for this genus 2 curve. Results of other
approaches are compared.Comment: 34 pages, 16 figures. Revision: Abstract added and a few small
change
Nonadiabatic Pauli susceptibility in fullerene compounds
Pauli paramagnetic susceptibility is unaffected by the electron-phonon
interaction in the Migdal-Eliashberg context. Fullerene compounds however do
not fulfill the adiabatic assumption of Migdal's theorem and nonadiabatic
effects are expected to be relevant in these materials. In this paper we
investigate the Pauli spin susceptibility in nonadiabatic regime by following a
conserving approach based on Ward's identity. We find that a sizable
renormalization of due to electron-phonon coupling appears when
nonadiabatic effects are taken into account. The intrinsic dependence of
on the electron-phonon interaction gives rise to a finite and negative isotope
effect which could be experimentally detected in fullerides. In addition, we
find an enhancement of the spin susceptibility with temperature increasing, in
agreement with the temperature dependence of observed in fullerene
compounds. The role of electronic correlation is also discussed.Comment: Revtex, 10 pages, 8 figures include
G3-homogeneous gravitational instantons
We provide an exhaustive classification of self-dual four-dimensional
gravitational instantons foliated with three-dimensional homogeneous spaces,
i.e. homogeneous self-dual metrics on four-dimensional Euclidean spaces
admitting a Bianchi simply transitive isometry group. The classification
pattern is based on the algebra homomorphisms relating the Bianchi group and
the duality group SO(3). New and general solutions are found for Bianchi III.Comment: 24 pages, few correction
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