458 research outputs found
Torsion nonminimally coupled to the electromagnetic field and birefringence
In conventional Maxwell--Lorentz electrodynamics, the propagation of light is
influenced by the metric, not, however, by the possible presence of a torsion
T. Still the light can feel torsion if the latter is coupled nonminimally to
the electromagnetic field F by means of a supplementary Lagrangian of the type
l^2 T^2 F^2 (l = coupling constant). Recently Preuss suggested a specific
nonminimal term of this nature. We evaluate the spacetime relation of Preuss in
the background of a general O(3)-symmetric torsion field and prove by
specifying the optical metric of spacetime that this can yield birefringence in
vacuum. Moreover, we show that the nonminimally coupled homogeneous and
isotropic torsion field in a Friedmann cosmos affects the speed of light.Comment: Revtex, 12 pages, no figure
EUV Sunspot Plumes Observed with SOHO
Bright EUV sunspot plumes have been observed in five out of nine sunspot
regions with the Coronal Diagnostic Spectrometer -- CDS on SOHO. In the other
four regions the brightest line emissions may appear inside the sunspot but are
mainly concentrated in small regions outside the sunspot areas. These results
are in contrast to those obtained during the Solar Maximum Mission, but are
compatible with the Skylab mission results. The present observations show that
sunspot plumes are formed in the upper part of the transition region, occur
both in magnetic unipolar-- and bipolar regions, and may extend from the umbra
into the penumbra.Comment: 8 pages, 3 figures, to be published in ApJ Letter
Solar constraints on new couplings between electromagnetism and gravity
The unification of quantum field theory and general relativity is a fundamental goal of modern physics. In many cases, theoretical efforts to achieve this goal introduce auxiliary gravitational fields, ones in addition to the familiar symmetric second-rank tensor potential of general relativity, and lead to nonmetric theories because of direct couplings between these auxiliary fields and matter. Here, we consider an example of a metric-affine gauge theory of gravity in which torsion couples nonminimally to the electromagnetic field. This coupling causes a phase difference to accumulate between different polarization states of light as they propagate through the metric-affine gravitational field. Solar spectropolarimetric observations are reported and used to set strong constraints on the relevant coupling constant k:k(2)\u3c (2.5 km)(2)
Dirac theory within the Standard-Model Extension
The modified Dirac equation in the Lorentz-violating Standard-Model Extension
(SME) is considered. Within this framework, the construction of a hermitian
Hamiltonian to all orders in the Lorentz-breaking parameters is investigated,
discrete symmetries and the first-order roots of the dispersion relation are
determined, and various properties of the eigenspinors are discussed.Comment: 11 pages REVTe
Effective dissipative dynamics for polarized photons
In the framework of open quantum systems, the propagation of polarized
photons can be effectively described using quantum dynamical semigroups. These
extended time-evolutions induce irreversibility and dissipation. Planned, high
sensitive experiments, both in the laboratory and in space, will be able to put
stringent bounds on these non-standard effects.Comment: 15 pages, plain-TeX, no figure
Gravity, Lorentz Violation, and the Standard Model
The role of the gravitational sector in the Lorentz- and CPT-violating
Standard-Model Extension (SME) is studied. A framework is developed for
addressing this topic in the context of Riemann-Cartan spacetimes, which
include as limiting cases the usual Riemann and Minkowski geometries. The
methodology is first illustrated in the context of the QED extension in a
Riemann-Cartan background. The full SME in this background is then considered,
and the leading-order terms in the SME action involving operators of mass
dimension three and four are constructed. The incorporation of arbitrary
Lorentz and CPT violation into general relativity and other theories of gravity
based on Riemann-Cartan geometries is discussed. The dominant terms in the
effective low-energy action for the gravitational sector are provided, thereby
completing the formulation of the leading-order terms in the SME with gravity.
Explicit Lorentz symmetry breaking is found to be incompatible with generic
Riemann-Cartan geometries, but spontaneous Lorentz breaking evades this
difficulty.Comment: 21 pages REVTeX, references added, accepted in Physical Review
Signals for Lorentz Violation in Electrodynamics
An investigation is performed of the Lorentz-violating electrodynamics
extracted from the renormalizable sector of the general Lorentz- and
CPT-violating standard-model extension. Among the unconventional properties of
radiation arising from Lorentz violation is birefringence of the vacuum. Limits
on the dispersion of light produced by galactic and extragalactic objects
provide bounds of 3 x 10^{-16} on certain coefficients for Lorentz violation in
the photon sector. The comparative spectral polarimetry of light from
cosmologically distant sources yields stringent constraints of 2 x 10^{-32}.
All remaining coefficients in the photon sector are measurable in
high-sensitivity tests involving cavity-stabilized oscillators. Experimental
configurations in Earth- and space-based laboratories are considered that
involve optical or microwave cavities and that could be implemented using
existing technology.Comment: 23 pages REVTe
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