44 research outputs found
Prospects for Measuring Planetary Spin and Frame-Dragging in Spacecraft Timing Signals
Satellite tracking involves sending electromagnetic signals to Earth. Both
the orbit of the spacecraft and the electromagnetic signals themselves are
affected by the curvature of spacetime. The arrival time of the pulses is
compared to the ticks of local clocks to reconstruct the orbital path of the
satellite to high accuracy, and to implicitly measure general relativistic
effects. In particular, Schwarzschild space curvature (static) and
frame-dragging (stationary) due to the planet's spin affect the satellite's
orbit. The dominant relativistic effect on the path of the signal photons is
Shapiro delay due to static space curvature. We compute these effects for some
current and proposed space missions, using a Hamiltonian formulation in four
dimensions. For highly eccentric orbits, such as in the Juno mission and in the
Cassini Grand Finale, the relativistic effects have a kick-like nature, which
could be advantageous for detecting them if their signatures are properly
modeled as functions of time. Frame-dragging appears, in principle, measurable
by Juno and Cassini, though not by Galileo 5 and 6. Practical measurement would
require disentangling frame-dragging from the Newtonian 'foreground' such as
the gravitational quadrupole which has an impact on both the spacecraft's orbit
and the signal propagation. The foreground problem remains to be solved.Comment: 10 pages, 6 figures, provisionally accepted for publication in
Frontiers in Astronomy and Space Sciences, section Fundamental Astronom
Atomic clocks as a tool to monitor vertical surface motion
Atomic clock technology is advancing rapidly, now reaching stabilities of
, which corresponds to resolving cm in equivalent
geoid height over an integration timescale of about 7 hours. At this level of
performance, ground-based atomic clock networks emerge as a tool for monitoring
a variety of geophysical processes by directly measuring changes in the
gravitational potential. Vertical changes of the clock's position due to
magmatic, volcanic, post-seismic or tidal deformations can result in measurable
variations in the clock tick rate. As an example, we discuss the geopotential
change arising due to an inflating point source (Mogi model), and apply it to
the Etna volcano. Its effect on an observer on the Earth's surface can be
divided into two different terms: one purely due to uplift and one due to the
redistribution of matter. Thus, with the centimetre-level precision of current
clocks it is already possible to monitor volcanoes. The matter redistribution
term is estimated to be 2-3 orders of magnitude smaller than the uplift term,
and should be resolvable when clocks improve their stability to the
sub-millimetre level. Additionally, clocks can be compared over distances of
thousands of kilometres on a short-term basis (e.g. hourly). These clock
networks will improve our ability to monitor periodic effects with
long-wavelength like the solid Earth tide.Comment: 11 pages, 5 figures, accepted as express letter in the Geophysical
Journal Internationa
Gegen den Feuerbrand mit intensiver Forschung
ZusÀtzlich zur Beseitigung von Infektionsherden erhalten Hygiene und Sortenwahl sowie die direkte BekÀmpfung mit Pflanzenschutzmitteln Bedeutung
Testing scalar-tensor theories and PPN parameters in Earth orbit
We compute the PPN parameters and for general scalar-tensor
theories in the Einstein frame, which we compare to the existing PPN
formulation in the Jordan frame for alternative theories of gravity. This
computation is important for scalar-tensor theories that are expressed in the
Einstein frame, such as chameleon and symmetron theories, which can incorporate
hiding mechanisms that predict environment-dependent PPN parameters. We
introduce a general formalism for scalar-tensor theories and constrain it using
the limit on given by the Cassini experiment. In particular we discuss
massive Brans-Dicke scalar fields for extended sources. Next, using a recently
proposed Earth satellite experiment, in which atomic clocks are used for
spacecraft tracking, we compute the observable perturbations in the redshift
induced by PPN parameters deviating from their general relativistic values. Our
estimates suggest that may be
detectable by a satellite that carries a clock with fractional frequency
uncertainty in an eccentric orbit around the Earth.
Such space experiments are within reach of existing atomic clock technology. We
discuss further the requirements necessary for such a mission to detect
deviations from Einstein relativity.Comment: 17 pages, 6 figures, accepted for publication in Phys. Rev.
Testing General Relativity and Alternative Theories of Gravity with Space-based Atomic Clocks and Atom Interferometers
The successful miniaturisation of extremely accurate atomic clocks and atom
interferometers invites prospects for satellite missions to perform precision
experiments. We discuss the effects predicted by general relativity and
alternative theories of gravity that can be detected by a clock, which orbits
the Earth. Our experiment relies on the precise tracking of the spacecraft
using its observed tick-rate. The spacecraft's reconstructed four-dimensional
trajectory will reveal the nature of gravitational perturbations in Earth's
gravitational field, potentially differentiating between different theories of
gravity. This mission can measure multiple relativistic effects all during the
course of a single experiment, and constrain the Parametrized Post-Newtonian
Parameters around the Earth. A satellite carrying a clock of fractional timing
inaccuracy of in an elliptic orbit around the Earth
would constrain the PPN parameters .
We also briefly review potential constraints by atom interferometers on scalar
tensor theories and in particular on Chameleon and dilaton models.Comment: 12 pages, 4 figures, 2 tables. Proceeding for ICNFP 201
Ground-based optical atomic clocks as a tool to monitor vertical surface motion
According to general relativity, a clock experiencing a shift in the gravitational potential ÎU will measure a frequency change given by Îf/fâÎU/c2. The best clocks are optical clocks. After about 7 hr of integration they reach stabilities of Îf/fâŒ10â18 and can be used to detect changes in the gravitational potential that correspond to vertical displacements of the centimetre level. At this level of performance, ground-based atomic clock networks emerge as a tool that is complementary to existing technology for monitoring a wide range of geophysical processes by directly measuring changes in the gravitational potential. Vertical changes of the clock's position due to magmatic, post-seismic or tidal deformations can result in measurable variations in the clock tick rate. We illustrate the geopotential change arising due to an inflating magma chamber using the Mogi model and apply it to the Etna volcano. Its effect on an observer on the Earth's surface can be divided into two different terms: one purely due to uplift (free-air gradient) and one due to the redistribution of matter. Thus, with the centimetre-level precision of current clocks it is already possible to monitor volcanoes. The matter redistribution term is estimated to be 3 orders of magnitude smaller than the uplift term. Additionally, clocks can be compared over distances of thousands of kilometres over short periods of time, which improves our ability to monitor periodic effects with long wavelength like the solid Earth tid
Die kombinierte Wirkung von spezialisierten wurzelbohrenden Insekten und Pflanzenkonkurrenz reduziert das Wachstum von Rumex obtusifolius
Die kombinierte Wirkungen von wurzelbohrenden Larven des europĂ€ische Schmetterling Pyropteron chrysidiforme (Massenausbringung) auf dem StumpfblĂ€ttrigen Ampfer wurden fĂŒr zwei Gruppen von anfĂ€nglich kleinen und groĂen Pflanzen mit oder ohne Konkurrenz von dem Englischen Raygras bewertet
Anthropogenic disturbance as a driver of microspatial and microhabitat segregation of cytotypes of Centaurea stoebe and cytotype interactions in secondary contact zones
Background and Aims In a mixed-ploidy population, strong frequency-dependent mating will lead to the elimination of the less common cytotype, unless prezygotic barriers enhance assortative mating. However, such barriers favouring cytotype coexistence have only rarely been explored. Here, an assessment is made of the mechanisms involved in formation of mixed-ploidy populations and coexistence of diploid plants and their closely related allotetraploid derivates from the Centaurea stoebe complex (Asteraceae). Methods An investigation was made of microspatial and microhabitat distribution, life-history and fitness traits, flowering phenology, genetic relatedness of cytotypes and intercytotype gene flow (cpDNA and microsatellites) in six mixed-ploidy populations in Central Europe. Key Results Diploids and tetraploids were genetically differentiated, thus corroborating the secondary origin of contact zones. The cytotypes were spatially segregated at all sites studied, with tetraploids colonizing preferentially drier and open microhabitats created by human-induced disturbances. Conversely, they were rare in more natural microsites and microsites with denser vegetation despite their superior persistence ability (polycarpic life cycle). The seed set of tetraploid plants was strongly influenced by their frequency in mixed-ploidy populations. Triploid hybrids originated from bidirectional hybridizations were extremely rare and almost completely sterile, indicating a strong postzygotic barrier between cytotypes. Conclusions The findings suggest that tetraploids are later immigrants into already established diploid populations and that anthropogenic activities creating open niches favouring propagule introductions were the major factor shaping the non-random distribution and habitat segregation of cytotypes at fine spatial scale. Establishment and spread of tetraploids was further facilitated by their superior persistence through the perennial life cycle. The results highlight the importance of non-adaptive spatio-temporal processes in explaining microhabitat and microspatial segregation of cytotype