171 research outputs found
Analogue Wormholes and Black Hole LASER Effect in Hydrodynamics
We numerically study water wave packets on a spatially varying
counter-current in the presence of surface tension. Depending on the details of
the velocity profile, we show that traversable and bi-directional analogue
wormholes exist in fluid mechanics. The limitations on traversability of
wormholes in general relativity are absent here because of the dispersion of
water waves and the ability to form flow profiles that are not solutions of
Einstein's equations. We observe that negative energy can be trapped between
analogue horizons forming a LASER-like cavity. Six horizons are involved in the
trapping cavity because of the existence of two dispersive scales, in contrast
to previous treatments which considered two horizons and one dispersive scale.Comment: 11 page
Hawking tunneling and boomerang behaviour of massive particles with E < m
Copyright © 2012 American Institute of PhysicsTowards New Paradigms: Proceeding of the Spanish Relativity Meeting 2011 (ERE2011), 29 Augustâ2 September 2011, Madrid, SpainMassive particles are radiated from black holes through the Hawking mechanism together with the more familiar radiation of massless particles. For E â„ m, the emission rate is identical to the massless case. But E < m particles can also tunnel across the horizon. A study of the dispersion relation and wave packet simulations show that their classical trajectory is similar to that of a boomerang. The tunneling formalism is used to calculate the probability for detecting such E < m particles, for a Schwarzschild black hole of astrophysical size or in an analogue gravity experiment, as a function of the distance from the horizon and the energy of the particle
Hawking radiation and the boomerang behaviour of massive modes near a horizon
Copyright © 2011 American Physical SocietyWe discuss the behaviour of massive modes near a horizon based on a study of the dispersion relation and wave packet simulations of the Klein-Gordon equation. We point out an apparent paradox between two (in principle equivalent) pictures of black hole evaporation through Hawking radiation. In the picture in which the evaporation is due to the emission of positive-energy modes, one immediately obtains a threshold for the emission of massive particles. In the picture in which the evaporation is due to the absorption of negative-energy modes, such a threshold apparently does not exist. We resolve this paradox by tracing the evolution of the positive-energy massive modes with an energy below the threshold. These are seen to be emitted and move away from the black hole horizon, but they bounce back at a "red horizon" and are re-absorbed by the black hole, thus compensating exactly for the difference between the two pictures. For astrophysical black holes, the consequences are curious but do not affect the terrestrial constraints on observing Hawking radiation. For analogue gravity systems with massive modes, however, the consequences are crucial and rather surprising
Horizon effects with surface waves on moving water
Surface waves on a stationary flow of water are considered, in a linear model
that includes the surface tension of the fluid. The resulting gravity-capillary
waves experience a rich array of horizon effects when propagating against the
flow. In some cases three horizons (points where the group velocity of the wave
reverses) exist for waves with a single laboratory frequency. Some of these
effects are familiar in fluid mechanics under the name of wave blocking, but
other aspects, in particular waves with negative co-moving frequency and the
Hawking effect, were overlooked until surface waves were investigated as
examples of analogue gravity [Sch\"utzhold R and Unruh W G 2002 Phys. Rev. D 66
044019]. A comprehensive presentation of the various horizon effects for
gravity-capillary waves is given, with emphasis on the deep water/short
wavelength case kh>>1 where many analytical results can be derived. A
similarity of the state space of the waves to that of a thermodynamic system is
pointed out.Comment: 30 pages, 15 figures. Minor change
Modeling atmospheric effects of the September 1859 Solar Flare
We have modeled atmospheric effects, especially ozone depletion, due to a
solar proton event which probably accompanied the extreme magnetic storm of 1-2
September 1859. We use an inferred proton fluence for this event as estimated
from nitrate levels in Greenland ice cores. We present results showing
production of odd nitrogen compounds and their impact on ozone. We also compute
rainout of nitrate in our model and compare to values from ice core data.Comment: Revised version including improved figures; Accepted for publication
in Geophys. Res. Lett, chosen to be highlighted by AG
The Basics of Water Waves Theory for Analogue Gravity
This chapter gives an introduction to the connection between the physics of
water waves and analogue gravity. Only a basic knowledge of fluid mechanics is
assumed as a prerequisite.Comment: 36 pages. Lecture Notes for the IX SIGRAV School on "Analogue
Gravity", Como (Italy), May 201
Earthworm management in tropical agroecosystems
Collaborative research in the Macrofauna project has enabled development of some techniques that presently are at different stages of advancement, from promising pilot experiments (tomato production and inoculation in plant nursery bags at Yurimaguas and in India) to the fully developed technique of massive worm production and biofertilization of tea gardens in Tamil Nadu (India) (patent deposited). Failures have also helped to gain better insight into the potential feasibility of techniques that had been considered in the objectives of this project. Endogeic earthworms (#Pontoscolex corethrurus$) may be produced in large quantities, i.e.about 12000 worms (1.6-2.8 kg live wt)/m2/year in specific culture beds using either sawdust (Yurimaguas, Peru) or a mixture of high and low quality materials (Tamil Nadu, India) mixed into soil as substrates. Cost of production of 1 kg of earthworm biomass through bed culture is about 3.6 Euro, much lower than the cost of hand collection of worms from pastures/grasslands where these species are abundant (6-125 Euro depending on the cost of labour and earthworm density). The theorical value of an active earthworm community with an average biomass of 400 kg live wt has been estimated at 1400 Euro, the price that it would cost to reintroduce an equivalent biomass produced in our culture units, indicating the cost of land restoration. Direct inoculation of earthworms in the field to improve production may only affect plant growth positively if a large biomass (greater than 30 g live wt/m2) is inoculated from the beginning. An alternative may be to concentrate the inoculum in small areas regularly distributed across the field... (D'aprÚs résumé d'auteur
Ocean carbon from space: Current status and priorities for the next decade
The ocean plays a central role in modulating the Earth\u27s carbon cycle. Monitoring how the ocean carbon cycle is changing is fundamental to managing climate change. Satellite remote sensing is currently our best tool for viewing the ocean surface globally and systematically, at high spatial and temporal resolutions, and the past few decades have seen an exponential growth in studies utilising satellite data for ocean carbon research. Satellite-based observations must be combined with in-situ observations and models, to obtain a comprehensive view of ocean carbon pools and fluxes. To help prioritise future research in this area, a workshop was organised that assembled leading experts working on the topic, from around the world, including remote-sensing scientists, field scientists and modellers, with the goal to articulate a collective view of the current status of ocean carbon research, identify gaps in knowledge, and formulate a scientific roadmap for the next decade, with an emphasis on evaluating where satellite remote sensing may contribute. A total of 449 scientists and stakeholders participated (with balanced gender representation), from North and South America, Europe, Asia, Africa, and Oceania. Sessions targeted both inorganic and organic pools of carbon in the ocean, in both dissolved and particulate form, as well as major fluxes of carbon between reservoirs (e.g., primary production) and at interfaces (e.g., air-sea and landâocean). Extreme events, blue carbon and carbon budgeting were also key topics discussed. Emerging priorities identified include: expanding the networks and quality of in-situ observations; improved satellite retrievals; improved uncertainty quantification; improved understanding of vertical distributions; integration with models; improved techniques to bridge spatial and temporal scales of the different data sources; and improved fundamental understanding of the ocean carbon cycle, and of the interactions among pools of carbon and light. We also report on priorities for the specific pools and fluxes studied, and highlight issues and concerns that arose during discussions, such as the need to consider the environmental impact of satellites or space activities; the role satellites can play in monitoring ocean carbon dioxide removal approaches; economic valuation of the satellite based information; to consider how satellites can contribute to monitoring cycles of other important climatically-relevant compounds and elements; to promote diversity and inclusivity in ocean carbon research; to bring together communities working on different aspects of planetary carbon; maximising use of international bodies; to follow an open science approach; to explore new and innovative ways to remotely monitor ocean carbon; and to harness quantum computing. Overall, this paper provides a comprehensive scientific roadmap for the next decade on how satellite remote sensing could help monitor the ocean carbon cycle, and its links to the other domains, such as terrestrial and atmosphere
: Recueil de fiches peÌdagogiques du reÌseau MAPS
DoctoralLe reÌseau theÌmatique MAPS «ModeÌlisation multi-Agent appliqueÌe aux PheÌnomeÌnes SpatialiseÌs » propose depuis 2009 des eÌveÌnements scientifiques ayant pour but de diffuser les pratiques de modeÌlisations multi-agents au sein des Sciences de lâHomme et de la SocieÌteÌ (SHS). Ce collectif pluridisciplinaire de chercheurs, dâenseignants-chercheurs et de doctorants est labelliseÌ en tant que âȘ reÌseau theÌmatique » par le ReÌseau National des SysteÌmes Complexes (GIS RNSC) et beÌneÌficie du soutien du CNRS au titre de la Formation Permanente. Depuis 2009, plusieurs modĂšles ont Ă©tĂ© dĂ©veloppĂ©s au cours d'Ă©vĂ©nements MAPS. Ces modĂšles ont fait l'objet de fiches pĂ©dagogiques dĂ©taillĂ©es destineÌes aux communauteÌs eÌducatives et universitaires et en particulier aux enseignants qui souhaiteraient faire deÌcouvrir la modeÌlisation aÌ leurs eÌtudiants, mais aussi aÌ ceux qui envisagent dâapprofondir certains aspects avec un public plus averti. Elles sont eÌgalement destineÌes aÌ tous les curieux qui souhaiteraient deÌcouvrir ce que la modeÌlisation apporte aux SHS, du point de vue heuristique et du point de vue opeÌrationnel. Enfin, elles sont aussi des supports pour toutes les personnes qui souhaiteraient diffuser les reÌflexions scientifiques sur la modeÌlisation et la simulation qui ont preÌsideÌ aÌ la reÌdaction de ces fiches
Observation of negative-frequency waves in a water tank: A classical analogue to the Hawking effect?
The conversion of positive-frequency waves into negative-frequency waves at
the event horizon is the mechanism at the heart of the Hawking radiation of
black holes. In black-hole analogues, horizons are formed for waves propagating
in a medium against the current when and where the flow exceeds the wave
velocity. We report on the first direct observation of negative-frequency waves
converted from positive-frequency waves in a moving medium. The measured degree
of mode conversion is significantly higher than expected from theory
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