84 research outputs found
Quantum-classical crossover in electrodynamics
A classical field theory is proposed for the electric current and the
electromagnetic field interpolating between microscopic and macroscopic
domains. It represents a generalization of the density functional for the
dynamics of the current and the electromagnetic field in the quantum side of
the crossover and reproduces standard classical electrodynamics on the other
side. The effective action derived in the closed time path formalism and the
equations of motion follow from the variational principle. The polarization of
the Dirac-see can be taken into account in the quadratic approximation of the
action by the introduction of the deplacement field strengths as in
conventional classical electrodynamics. Decoherence appears naturally as a
simple one-loop effect in this formalism. It is argued that the radiation time
arrow is generated from the quantum boundary conditions in time by decoherence
at the quantum-classical crossover and the Abraham-Lorentz force arises from
the accelerating charge or from other charges in the macroscopic or the
microscopic side, respectively. The functional form of quantum renormalization
group, the generalization of the renormalization group method for the density
matrix, is proposed to follow the scale dependence through the
quantum-classical crossover in a systematical manner.Comment: new references added, few sign errors fixed, to appear in Physical
Review
Stochastic Gravity
Gravity is treated as a stochastic phenomenon based on fluctuations of the
metric tensor of general relativity. By using a (3+1) slicing of spacetime, a
Langevin equation for the dynamical conjugate momentum and a Fokker-Planck
equation for its probability distribution are derived. The Raychaudhuri
equation for a congruence of timelike or null geodesics leads to a stochastic
differential equation for the expansion parameter in terms of the
proper time . For sufficiently strong metric fluctuations, it is shown that
caustic singularities in spacetime can be avoided for converging geodesics. The
formalism is applied to the gravitational collapse of a star and the
Friedmann-Robertson-Walker cosmological model. It is found that owing to the
stochastic behavior of the geometry, the singularity in gravitational collapse
and the big-bang have a zero probability of occurring. Moreover, as a star
collapses the probability of a distant observer seeing an infinite red shift at
the Schwarzschild radius of the star is zero. Therefore, there is a vanishing
probability of a Schwarzschild black hole event horizon forming during
gravitational collapse.Comment: Revised version. Eq. (108) has been modified. Additional comments
have been added to text. Revtex 39 page
Gyroscope precession in cylindrically symmetric spacetimes
We present calculations of gyroscope precession in spacetimes described by
Levi-Civita and Lewis metrics, under different circumstances. By doing so we
are able to establish a link between the parameters of the metrics and
observable quantities, providing thereby a physical interpretation for those
parameters, without specifying the source of the field.Comment: 13 pages, Latex. To appear in Class.Q.Gra
A relativistic action-at-a-distance description of gravitational interactions?
It is shown that certain aspects of gravitation may be described using a
relativistic action-at-a-distance formulation. The equations of motion of the
model presented are invariant under Lorentz transformations and agree with the
equations of Einstein's theory of General Relativity, at the first
Post-Newtonian approximation, for any number of interacting point masses
ADM canonical formalism for gravitating spinning objects
In general relativity, systems of spinning classical particles are
implemented into the canonical formalism of Arnowitt, Deser, and Misner [1].
The implementation is made with the aid of a symmetric stress-energy tensor and
not a 4-dimensional covariant action functional. The formalism is valid to
terms linear in the single spin variables and up to and including the
next-to-leading order approximation in the gravitational spin-interaction part.
The field-source terms for the spinning particles occurring in the Hamiltonian
are obtained from their expressions in Minkowski space with canonical variables
through 3-dimensional covariant generalizations as well as from a suitable
shift of projections of the curved spacetime stress-energy tensor originally
given within covariant spin supplementary conditions. The applied coordinate
conditions are the generalized isotropic ones introduced by Arnowitt, Deser,
and Misner. As applications, the Hamiltonian of two spinning compact bodies
with next-to-leading order gravitational spin-orbit coupling, recently obtained
by Damour, Jaranowski, and Schaefer [2], is rederived and the derivation of the
next-to-leading order gravitational spin(1)-spin(2) Hamiltonian, shown for the
first time in [3], is presented.Comment: REVTeX4, 18 pages. v1: published version. v2: corrected misprints in
(8.4) and (9.3), updated reference
Stochastic thermodynamics of holonomic systems
International audienc
Research at the University of Sheffield postgraduate school of librarianship and information science
Stimulated emission of phonons in a ruby fiber
Avalanches of resonant phonons generated by stimulated emission within the optically excited Ä’(2E) doublet are observed in a 400-ÎĽm thick fiber of single-crystalline ruby. In comparison with bulk ruby, the avalanches develop fully in a shorter time because the phonons are confined to the active medium by reflection from the fiber boundaries. Position-independent rate equations adequately describe the dynamics
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