113 research outputs found
Reduzierte Bodenbearbeitung im Ökologischen Landbau: Einfluss auf Leistung und Struktur der Bodenmikroorganismengemeinschaft
Problemstellung/Ziele: Im Projekt ‚Ökologische Bodenbewirtschaftung’ (PÖB) der Stiftung Ökologie und Landbau, Bad Dürckheim, wird seit 1995 am Standort Rommersheim, Rheinhessen, eine differenzierte Grundbodenbearbeitung mit den Varianten Pflug (P), Zweischichtenpflug (LP) und Schichtengrubber (LC) durchgeführt. Ziel der Untersuchungen war es, vertiefende Einsichten in die Reaktion der mikrobiellen Biomasse auf die differenzierte Bodenbearbeitung unter den besonderen Bedingungen des Ökologischen Landbaus zu erhalten.
Hypothesen: Eine reduzierte (LP) und konservierende (LC) Bodenbearbeitung führt im Vergleich zum Pflug (P) zu einer Anreicherung und Sequestrierung sowie einer qualitativen Modifikation von organischer Bodensubstanz und mikrobieller Biomasse. Die funktionelle und strukturelle Diversität der Bodenmikroorganismen-Gemeinschaft wird hierdurch ebenfalls modifiziert.
Methoden: Im Frühjahr 2001 wurden Bodenproben aus Grünbrache-Parzellen in vierfacher Wiederholung je Bodenbearbeitungsvariante differenziert nach Ober- (0-15cm) und Unterkrume (15-25cm) entnommen und hinsichtlicher der Gehalte an organischer Substanz (trockene Veraschung), mikrobieller Biomasse (CFE-C) und Aktivität (Infrarotgasanalysator) sowie der funktionellen Diversität (community level substrate utilization profiles – BIOLOG GN2) untersucht. Die strukturelle Diversität wurde mittels Phospholipid-Fettsäure (PLFA) und Phospholipid-Etherlipide (PLEL) –Muster analysiert. Ergänzend wurde die Qualität der organischen Bodensubstanz durch eine Kaltwasser-Extraktion und der spektroskopischen Eigenschaften untersucht.
Fazit: Reduzierte und konservierende Bodenbearbeitung modifiziert die Organische Bodensubstanz, die Leistung sowie die funktionelle und strukturelle Diversität von Bodenmikroorganismen-Gemeinschaften
Complex lapse, complex action and path integrals
Imaginary time is often used in quantum tunnelling calculations. This article
advocates a conceptually sounder alternative: complex lapse. In the ``3+1''
action for the Einstein gravitational field minimally coupled to a Klein-Gordon
field, allowing the lapse function to be complex yields a complex action which
generates both the usual Lorentzian theory and its Riemannian analogue, and in
particular allows a change of signature between the two. The action and
variational equations are manifestly well defined in the Hamiltonian
representation, with the momentum fields consequently being complex. The
complex action interpolates between the Lorentzian and Riemannian actions as
they appear formally in the respective path integrals. Thus the complex-lapse
theory provides a unified basis for a path-integral quantum theory of gravity
involving both Lorentzian and Riemannian aspects. A major motivation is the
quantum-tunnelling scenario for the origin of the universe. Taken as an
explanation for the observed quantum tunnelling of particles, the complex-lapse
theory determines that the argument of the lapse for the universe now is
extremely small but negative.Comment: 12 pages, Te
The trace left by signature-change-induced compactification
Recently, it has been shown that an infinite succession of classical
signature changes (''signature oscillations'') can compactify and stabilize
internal dimensions, and simultaneously leads, after a coarse graining type of
average procedure, to an effective (''physical'') space-time geometry
displaying the usual Lorentzian metric signature. Here, we consider a minimally
coupled scalar field on such an oscillating background and study its effective
dynamics. It turns out that the resulting field equation in four dimensions
contains a coupling to some non-metric structure, the imprint of the
''microscopic'' signature oscillations on the effective properties of matter.
In a multidimensional FRW model, this structure is identical to a massive
scalar field evolving in its homogeneous mode.Comment: 15 pages, LaTeX, no figure
Comment on `Smooth and Discontinuous Signature Type Change in General Relativity'
Kossowski and Kriele derived boundary conditions on the metric at a surface
of signature change. We point out that their derivation is based not only on
certain smoothness assumptions but also on a postulated form of the Einstein
field equations. Since there is no canonical form of the field equations at a
change of signature, their conclusions are not inescapable. We show here that a
weaker formulation is possible, in which less restrictive smoothness
assumptions are made, and (a slightly different form of) the Einstein field
equations are satisfied. In particular, in this formulation it is possible to
have a bounded energy-momentum tensor at a change of signature without
satisfying their condition that the extrinsic curvature vanish.Comment: Plain TeX, 6 pages; Comment on Kossowski and Kriele: Class. Quantum
Grav. 10, 2363 (1993); Reply by Kriele: Gen. Rel. Grav. 28, 1409-1413 (1996
BOUNDARY CONDITIONS FOR THE SCALAR FIELD IN THE PRESENCE OF SIGNATURE CHANGE
We show that, contrary to recent criticism, our previous work yields a
reasonable class of solutions for the massless scalar field in the presence of
signature change.Comment: 11 pages, Plain Tex, no figure
Closed Strings with Low Harmonics and Kinks
Low-harmonic formulas for closed relativistic strings are given. General
parametrizations are presented for the addition of second- and third-harmonic
waves to the fundamental wave. The method of determination of the
parametrizations is based upon a product representation found for the finite
Fourier series of string motion in which the constraints are automatically
satisfied. The construction of strings with kinks is discussed, including
examples. A procedure is laid out for the representation of kinks that arise
from self-intersection, and subsequent intercommutation, for harmonically
parametrized cosmic strings.Comment: 39, CWRUTH-93-
Actions for signature change
This is a contribution on the controversy about junction conditions for
classical signature change. The central issue in this debate is whether the
extrinsic curvature on slices near the hypersurface of signature change has to
be continuous ({\it weak} signature change) or to vanish ({\it strong}
signature change). Led by a Lagrangian point of view, we write down eight
candidate action functionals ,\dots as possible generalizations of
general relativity and investigate to what extent each of these defines a
sensible variational problem, and which junction condition is implied. Four of
the actions involve an integration over the total manifold. A particular
subtlety arises from the precise definition of the Einstein-Hilbert Lagrangian
density . The other four actions are constructed as sums of
integrals over singe-signature domains. The result is that {\it both} types of
junction conditions occur in different models, i.e. are based on different
first principles, none of which can be claimed to represent the ''correct''
one, unless physical predictions are taken into account. From a point of view
of naturality dictated by the variational formalism, {\it weak} signature
change is slightly favoured over {\it strong} one, because it requires less
{\it \`a priori} restrictions for the class of off-shell metrics. In addition,
a proposal for the use of the Lagrangian framework in cosmology is made.Comment: 36 pages, LaTeX, no figures; some corrections have been made, several
Comments and further references are included and a note has been added
Gravity and Signature Change
The use of proper ``time'' to describe classical ``spacetimes'' which contain
both Euclidean and Lorentzian regions permits the introduction of smooth
(generalized) orthonormal frames. This remarkable fact permits one to describe
both a variational treatment of Einstein's equations and distribution theory
using straightforward generalizations of the standard treatments for constant
signature.Comment: Plain TeX, 6 pages; to appear in GR
Evolution of cosmic string configurations
We extend and develop our previous work on the evolution of a network of
cosmic strings. The new treatment is based on an analysis of the probability
distribution of the end-to-end distance of a randomly chosen segment of
left-moving string of given length. The description involves three distinct
length scales: , related to the overall string density, , the
persistence length along the string, and , describing the small-scale
structure, which is an important feature of the numerical simulations that have
been done of this problem. An evolution equation is derived describing how the
distribution develops in time due to the combined effects of the universal
expansion, of intercommuting and loop formation, and of gravitational
radiation. With plausible assumptions about the unknown parameters in the
model, we confirm the conclusions of our previous study, that if gravitational
radiation and small-scale structure effects are neglected, the two dominant
length scales both scale in proportion to the horizon size. When the extra
effects are included, we find that while and grow,
initially does not. Eventually, however, it does appear to scale, at a much
lower level, due to the effects of gravitational back-reaction.Comment: 61 pages, requires RevTex v3.0, SUSSEX-TH-93/3-4,
IMPERIAL/TP/92-93/4
Postmodern String Theory: Stochastic Formulation
In this paper we study the dynamics of a statistical ensemble of strings,
building on a recently proposed gauge theory of the string geodesic field. We
show that this stochastic approach is equivalent to the Carath\'eodory
formulation of the Nambu-Goto action, supplemented by an averaging procedure
over the family of classical string world-sheets which are solutions of the
equation of motion. In this new framework, the string geodesic field is
reinterpreted as the Gibbs current density associated with the string
statistical ensemble. Next, we show that the classical field equations derived
from the string gauge action, can be obtained as the semi-classical limit of
the string functional wave equation. For closed strings, the wave equation
itself is completely analogous to the Wheeler-DeWitt equation used in quantum
cosmology. Thus, in the string case, the wave function has support on the space
of all possible spatial loop configurations. Finally, we show that the string
distribution induces a multi-phase, or {\it cellular} structure on the
spacetime manifold characterized by domains with a purely Riemannian geometry
separated by domain walls over which there exists a predominantly Weyl
geometry.Comment: 24pages, ReVTe
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