14,585 research outputs found
An Intrinsic Approach to Forces in Magnetoelectric Media
This paper offers a conceptually straightforward method for the calculation
of stresses in polarisable media based on the notion of a drive form and its
property of being closed in spacetimes with symmetry. After an outline of the
notation required to exploit the powerful exterior calculus of differential
forms, a discussion of the relation between Killing isometries and conservation
laws for smooth and distributional drive forms is given. Instantaneous forces
on isolated spacetime domains and regions with interfaces are defined, based on
manifestly covariant equations of motion. The remaining sections apply these
notions to media that sustain electromagnetic stresses, with emphasis on
homogeneous magnetoelectric material. An explicit calculation of the average
pressure exerted by a monochromatic wave normally incident on a homogeneous,
magnetoelectric slab in vacuo is presented and the concluding section
summarizes how this pressure depends on the parameters in the magnetoelectric
tensors for the medium.Comment: 25 pages, 3 figures, to appear in Il Nuovo Cimento B, proceedings of
GCM8, Catania (Oct 2008) - References added, minor corrections mad
Fast autotuning of a hydrogen maser by cavity Q modulation
A new fast autotuner for the hydrogen maser was implemented. By modulating the cavity, a phase shift in the maser output signal is induced which is proportional to the cavity tuning error. The phase shift is detected and fed back to a varactor tuner to stabilize the cavity against long-term drifts. Also, a PIN-diode cavity modulator which gives no incidental frequency shift over a very wide range of operation was developed. Modulated at over 200 Hz, it allows variations in maser cavity frequency to be compensated with a loop gain greater than 1000. Compensation of incidental amplitude modulation of the output was demonstrated
Non-Riemannian Gravity and the Einstein-Proca System
We argue that all Einstein-Maxwell or Einstein-Proca solutions to general
relativity may be used to construct a large class of solutions (involving
torsion and non-metricity) to theories of non-Riemannian gravitation that have
been recently discussed in the literature.Comment: 9 pages Plain Tex (No Figures), Letter to Editor Classical and
Quantum Gravit
The Construction of Spinor Fields on Manifolds with Smooth Degenerate Metrics
We examine some of the subtleties inherent in formulating a theory of spinors
on a manifold with a smooth degenerate metric. We concentrate on the case where
the metric is singular on a hypersurface that partitions the manifold into
Lorentzian and Euclidean domains. We introduce the notion of a complex spinor
fibration to make precise the meaning of continuity of a spinor field and give
an expression for the components of a local spinor connection that is valid in
the absence of a frame of local orthonormal vectors. These considerations
enable one to construct a Dirac equation for the discussion of the behavior of
spinors in the vicinity of the metric degeneracy. We conclude that the theory
contains more freedom than the spacetime Dirac theory and we discuss some of
the implications of this for the continuity of conserved currents.Comment: 24 pages, LaTeX (RevTeX 3.0, no figures), To appear in J. Math. Phy
The Transition State in a Noisy Environment
Transition State Theory overestimates reaction rates in solution because
conventional dividing surfaces between reagents and products are crossed many
times by the same reactive trajectory. We describe a recipe for constructing a
time-dependent dividing surface free of such recrossings in the presence of
noise. The no-recrossing limit of Transition State Theory thus becomes
generally available for the description of reactions in a fluctuating
environment
Transformation of Rhodopseudomonas sphaeroides with deoxyribonucleic acid isolated from bacteriophage Rφ6P
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