X-rays From Quasars and Active Galaxies

Features of quasars and active galactic nuclei are discussed and include: the nature of the power source, the radiation processes, and the mechanism for the formation and collimation of long-lived jets of matter observed to emanate from the center of these of these objects. The phenomena that produce X-rays are highlighted

El físico como novelista

Este trabajo trata sobre las diferencias y semejanzas entre la ciencia y la literatura. Basándome en mi experiencia como físico y como novelista, intento descifrar las características de estas dos pasiones que tengo en la vida. Aunque existen diferencias notables tanto en la naturaleza de lo que se escribe como en las normas que rigen estos dos mundos, en ambos casos hay una experiencia creativa y una necesidad de contraste con nuestras experiencias vividas

Foundations for a theory of gravitation theories

A foundation is laid for future analyses of gravitation theories. This foundation is applicable to any theory formulated in terms of geometric objects defined on a 4-dimensional spacetime manifold. The foundation consists of (i) a glossary of fundamental concepts; (ii) a theorem that delineates the overlap between Lagrangian-based theories and metric theories; (iii) a conjecture (due to Schiff) that the weak equivalence principle implies the Einstein equivalence principle; and (iv) a plausibility argument supporting this conjecture for the special case of relativistic, Lagrangian-based theories

Adventures in Friedmann Cosmology: An Educationally Detailed Expansion of the Cosmological Friedmann Equations

The general relativistic cosmological Friedmann equations which describe how the scale factor of the universe evolves are expanded explicitly to include energy forms not usually seen. The evolution of the universe as predicted by the Friedmann equations when dominated by a single, isotropic, stable, static, perfect-fluid energy form is discussed for different values of its gravitational pressure to density ratio $w$. These energy forms include phantom energy ($w<-1$), cosmological constant ($w=-1$), domain walls ($w = -2/3$), cosmic strings ($w = -1/3$), normal matter ($w = 0$), radiation and relativistic matter ($w = 1/3$), and a previously little-discussed form of energy called "ultralight" ($w>1/3$). A brief history and possible futures of Friedmann universes dominated by a single energy form are discussed.Comment: 32 pages, 4 tables; modified version accepted for publication in the American Journal of Physic

Analysis of the Belinfante-Swihart Theory of Gravity

We show that the Belinfante-Swihart (BS) theory can be reformulated in a representation in which uncharged matter responds to gravity in the same way as in metric theories. The BS gravitationally modified Maxwell equations can also be put into metric form to first order in the deviations of the physical metric from flat space, but not to second order; consequently, the theory is nonmetric except in first order. We also show that the theory violates the high-precision Eötvös-Dicke experiment, but cannot be ruled out by the gravitational precession of gyroscopes

Constraints on Lorentz violation from clock-comparison experiments

Constraints from clock-comparison experiments on violations of Lorentz and CPT symmetry are investigated in the context of a general Lorentz-violating extension of the standard model. The experimental signals are shown to depend on the atomic and ionic species used as clocks. Certain experiments usually regarded as establishing comparable bounds are in this context sensitive to different types of Lorentz violation. Some considerations relevant to possible future measurements are presented. All these experiments are potentially sensitive to Lorentz-violating physics at the Planck scale.Comment: accepted for publication in Physical Review D; scheduled for issue of December 1, 199

Interobserver Agreement Among Uveitis Experts on Uveitic Diagnoses:The Standardization of Uveitis Nomenclature Experience

• PURPOSE: To evaluate the interobserver agreement among uveitis experts on the diagnosis of the specific uveitic disease. • DESIGN: Interobserver agreement analysis. • METHODS: Five committees, each comprised of 9 individuals and working in parallel, reviewed cases from a preliminary database of 25 uveitic diseases, collected by disease, and voted independently online whether the case was the disease in question or not. The agreement statistic, κ, was calculated for the 36 pairwise comparisons for each disease, and a mean κ was calculated for each disease. After the independent online voting, committee consensus conference calls, using nominal group techniques, reviewed all cases not achieving supermajority agreement (> 75%) on the diagnosis in the online voting to attempt to arrive at a supermajority agreement. • RESULTS: A total of 5766 cases for the 25 diseases were evaluated. The overall mean κ for the entire project was 0.39, with disease-specific variation ranging from 0.23 to 0.79. After the formalized consensus conference calls to address cases that did not achieve supermajority agreement in the online voting, supermajority agreement overall was reached on approximately 99% of cases, with disease-specific variation ranging from 96% to 100%. • CONCLUSIONS: Agreement among uveitis experts on diagnosis is moderate at best but can be improved by discussion among them. These data suggest the need for validated and widely used classification criteria in the field of uveitis

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

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