20,630 research outputs found

### State Vector Reduction as a Shadow of a Noncommutative Dynamics

A model, based on a noncommutative geometry, unifying general relativity with
quantum mechanics, is further develped. It is shown that the dynamics in this
model can be described in terms of one-parameter groups of random operators. It
is striking that the noncommutative counterparts of the concept of state and
that of probability measure coincide. We also demonstrate that the equation
describing noncommutative dynamics in the quantum gravitational approximation
gives the standard unitary evolution of observables, and in the "space-time
limit" it leads to the state vector reduction. The cases of the spin and
position operators are discussed in details.Comment: 20 pages, LaTex, no figure

### Detection and Characterization of Stress Symptoms in Forest Vegetation

Techniques used at the Pacific Southwest Forest and Range Experiment Station to detect advanced and previsual symptoms of vegetative stress are discussed. Stresses caused by bark beetles in coniferous stands of timber are emphasized because beetles induce stress more rapidly than most other destructive agents. Bark beetles are also the most damaging forest insects in the United States. In the work on stress symptoms, there are two primary objectives: (1) to learn the best combination of films, scales, and filters to detect and locate injured trees from aircraft and spacecraft, and (2) to learn if stressed trees can be detected before visual symptoms of decline occur. Equipment and techniques used in a study of the epidemic of the Black Hills bark beetle are described

### Remote sensing in forestry: Promises and problems

There are no author-identified significant results in this report

### Semiclassical instanton formulation of Marcus-Levich-Jortner theory

Marcus-Levich-Jortner (MLJ) theory is one of the most commonly used methods
for including nuclear quantum effects into the calculation of electron-transfer
rates and for interpreting experimental data. It divides the molecular problem
into a subsystem treated quantum-mechanically by Fermi's golden rule and a
solvent bath treated by classical Marcus theory. As an extension of this idea,
we here present a "reduced" semiclassical instanton theory, which is a
multiscale method for simulating quantum tunnelling of the subsystem in
molecular detail in the presence of a harmonic bath. We demonstrate that
instanton theory is typically significantly more accurate than the cumulant
expansion or the semiclassical Franck-Condon sum, which can give
orders-of-magnitude errors and in general do not obey detailed balance. As
opposed to MLJ theory, which is based on wavefunctions, instanton theory is
based on path integrals and thus does not require solutions of the
Schr\"odinger equation, nor even global knowledge of the ground- and
excited-state potentials within the subsystem. It can thus be efficiently
applied to complex, anharmonic multidimensional subsystems without making
further approximations. In addition to predicting accurate rates, instanton
theory gives a high level of insight into the reaction mechanism by locating
the dominant tunnelling pathway as well as providing information on the
reactant and product vibrational states involved in the reaction and the
activation energy in the bath similarly to what would be found with MLJ theory.Comment: 21 pages, 4 figure

### Conceptual Unification of Gravity and Quanta

We present a model unifying general relativity and quantum mechanics. The
model is based on the (noncommutative) algebra \mbox{{\cal A}} on the groupoid
\Gamma = E \times G where E is the total space of the frame bundle over
spacetime, and G the Lorentz group. The differential geometry, based on
derivations of \mbox{{\cal A}}, is constructed. The eigenvalue equation for the
Einstein operator plays the role of the generalized Einstein's equation. The
algebra \mbox{{\cal A}}, when suitably represented in a bundle of Hilbert
spaces, is a von Neumann algebra \mathcal{M} of random operators representing
the quantum sector of the model. The Tomita-Takesaki theorem allows us to
define the dynamics of random operators which depends on the state \phi . The
same state defines the noncommutative probability measure (in the sense of
Voiculescu's free probability theory). Moreover, the state \phi satisfies the
Kubo-Martin-Schwinger (KMS) condition, and can be interpreted as describing a
generalized equilibrium state. By suitably averaging elements of the algebra
\mbox{{\cal A}}, one recovers the standard geometry of spacetime. We show that
any act of measurement, performed at a given spacetime point, makes the model
to collapse to the standard quantum mechanics (on the group G). As an example
we compute the noncommutative version of the closed Friedman world model.
Generalized eigenvalues of the Einstein operator produce the correct components
of the energy-momentum tensor. Dynamics of random operators does not ``feel''
singularities.Comment: 28 LaTex pages. Substantially enlarged version. Improved definition
of generalized Einstein's field equation

### Spectral analysis of 636 white dwarf - M star binaries from the Sloan Digital Sky Survey

We present a catalog of 857 white dwarf (WD)-M binaries from the sixth data
release (DR6) of the Sloan Digital Sky Survey (SDSS), most of which were
previously identified. For 636 of them, we complete a spectral analysis and
derive the basic parameters of their stellar constituents and their distances
from Earth. We attempt to measure fundamental parameters of these systems by
completing spectral analyses. We use a Chi^2 minimization technique to
decompose each combined spectrum and derive independent parameter estimates for
its components. Forty-one of the stellar duets in our spectroscopic sample are
optically resolved in their respective SDSS images. For these systems, we also
derive a minimum true spatial separation and a lower limit to their orbital
periods, typically which are some 10^4 yr. Spectra of 167 stellar duets show
significant hydrogen emission and in most cases no additional He i or He ii
features. We also find that 20 of the 636 WDs are fitted to be DOs, with 16
measured to have T_eff around 40,000 K. Furthermore, we identify 70 very
low-mass objects, which are secondaries of masses smaller than about 0.1 solar
masses, to be candidate substellar companions. Although various selection
effects may play a role, the fraction 6.4 % of WD-M star binaries with orbital
separations of around 500 AU is a criterion for evolutionary models of stellar
binary systems. Active M dwarfs are likely present in 155 Balmer-emitting
systems, corresponding to a fraction of 24.4 %. The excess of cool DOs is most
likely due to additional WDs in the DB-DO T_eff range, for which no detailed
fitting was completed. The trend of the M stars being closer to Earth than the
WD component is probably due to an underestimation of the theoretical M star
radii.Comment: accepted by A&A October 3, 2008, 15 pages, 16 figures, 3 tables; v2,
minor grammatical changes, essential changes in Sect. 5.

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