Mechanical Evidence of the Orbital Angular Momentum to Energy Ratio of Vortex Beams

We measure, in a single experiment, both the radiation pressure and the torque due to a wide variety of propagating acoustic vortex beams. The results validate, for the first time directly, the theoretically predicted ratio of the orbital angular momentum to linear momentum in a propagating beam. We experimentally determine this ratio using simultaneous measurements of both the levitation force and the torque on an acoustic absorber exerted by a broad range of helical ultrasonic beams produced by a 1000-element matrix transducer array. In general, beams with helical phase fronts have been shown to contain orbital angular momentum as the result of the azimuthal component of the Poynting vector around the propagation axis. Theory predicts that for both optical and acoustic helical beams the ratio of the angular momentum current of the beam to the power should be given by the ratio of the beam’s topological charge to its angular frequency. This direct experimental observation that the ratio of the torque to power does convincingly match the expected value (given by the topological charge to angular frequency ratio of the beam) is a fundamental result

Review Essay—Official History, Not “Instant Analysis”

Shield and Sword: The United States Navy and the Persian Gulf Wa

On the tidal evolution of the orbits of low-mass satellites around black holes

Low-mass satellites, like asteroids and comets, are expected to be present around the black hole at the Galactic center. We consider small bodies orbiting a black hole, and we study the evolution of their orbits due to tidal interaction with the black hole. In this paper we investigate the consequences of the existence of plunging orbits when a black hole is present. We are interested in finding the conditions that exist when capture occurs. The main difference between the Keplerian and black hole cases is in the existence of plunging orbits. Orbital evolution, leading from bound to plunging orbits, goes through a final unstable circular orbit. On this orbit, tidal energy is released on a characteristic black hole timescale. This process may be relevant for explaining how small, compact clumps of material can be brought onto plunging orbits, where they may produce individual short duration accretion events. The available energy and the characteristic timescale are consistent with energy released and the timescale typical of Galactic flares.Comment: 7 pages, 6 figure

Tidal effects on small bodies by massive black holes

The compact radio source Sagittarius A (Sgr A) at the centre of our Galaxy harbours a supermassive black hole, whose mass has been measured from stellar orbital motions. Sgr A is therefore the nearest laboratory where super-massive black hole astrophysics can be tested, and the environment of black holes can be investigated. Since it is not an active galactic nucleus, it also offers the possibility of observing the capture of small objects that may orbit the central black hole. We study the effects of the strong gravitational field of the black hole on small objects, such as a comet or an asteroid. We also explore the idea that the flares detected in Sgr A might be produced by the final accretion of single, dense objects with mass of the order of 10^20 g, and that their timing is not a characteristic of the sources, but rather of the space-time of the central galactic black hole in which they are moving. We find that tidal effects are strong enough to melt the solid object, and present calculations of the temporal evolution of the light curve of infalling objects as a function of various parameters. Our modelling of tidal disruption suggests that during tidal squeezing, the conditions for synchrotron radiation can be met. We show that the light curve of a flare can be deduced from dynamical properties of geodesic orbits around black holes and that it depends only weakly on the physical properties of the source.Comment: 10 pages, 14 figures, A&A accepte

Glacial controls on redox-sensitive trace element cycling in Arctic fjord sediments (Spitsbergen, Svalbard)

Glacial meltwater is an important source of bioessential trace elements to high latitude oceans. Upon delivery to coastal waters, glacially sourced particulate trace elements are processed during early diagenesis in sediments and may be sequestered or recycled back to the water column depending on local biogeochemical conditions. In the glaciated fjords of Svalbard, large amounts of reactive Fe and Mn (oxyhydr)oxides are delivered to the sediment by glacial discharge, resulting in pronounced Fe and Mn cycling concurrent with microbial sulfate reduction. In order to investigate the diagenetic cycling of selected trace elements (As, Co, Cu, Mo, Ni, and U) in this system, we collected sediment cores from two Svalbard fjords, Van Keulenfjorden and Van Mijenfjorden, in a transect along the head-to-mouth fjord axis and analyzed aqueous and solid phase geochemistry with respect to trace elements, sulfur, and carbon along with sulfate reduction rates. We found that Co and Ni associate with Fe and Mn (oxyhydr)oxides and enter the pore water upon reductive metal oxide dissolution. Copper is enriched in the solid phase where sulfate reduction rates are high, likely due to reactions with H2S and the formation of sulfide minerals. Uranium accumulates in the solid phase likely following reduction by both Fe- and sulfate-reducing bacteria, while Mo adsorbs to Fe and Mn (oxyhydr)oxides in the surface sediment and is removed from the pore water at depth where sulfidization makes it particle-reactive. Arsenic is tightly coupled to Fe redox cycling and its partitioning between solid and dissolved phases is influenced by competition with FeS for adsorption sites on crystalline Fe oxides. Differences in trace element cycling between the two fjords suggest delivery of varying amount and composition of tidewater glacier (Van Keulenfjorden) and meltwater stream (Van Mijenfjorden) material, likely related to oxidative processes occurring in meltwater streams. This processing produces a partially weathered, more reactive sediment that is subject to stronger redox cycling of Fe, Mn, S, and associated trace elements upon delivery to Van Mijenfjorden. With climate warming, the patterns of trace element cycling observed in Van Mijenfjorden may also become more prevalent in other Svalbard fjords as tidewater glaciers retreat into meltwater stream valleys

Extrasolar planet population synthesis IV. Correlations with disk metallicity, mass and lifetime

Context. This is the fourth paper in a series showing the results of planet population synthesis calculations. Aims. Our goal in this paper is to systematically study the effects of important disk properties, namely disk metallicity, mass and lifetime on fundamental planetary properties. Methods. For a large number of protoplanetary disks we calculate a population of planets with our core accretion formation model including planet migration and disk evolution. Results. We find a large number of correlations: Regarding the planetary initial mass function, metallicity, disk mass and disk lifetime have different roles: For high [Fe/H], giant planets are more frequent. For high disk masses, giant planets are more massive. For long disk lifetimes, giant planets are both more frequent and massive. At low metallicities, very massive giant planets cannot form, but otherwise giant planet mass and metallicity are uncorrelated. In contrast, planet masses and disk gas masses are correlated. The sweet spot for giant planet formation is at 5 AU. In- and outside this distance, higher planetesimals surface densities are necessary. Low metallicities can be compensated by high disk masses, and vice versa, but not ad infinitum. At low metallicities, giant planets only form outside the ice line, while at high metallicities, giant planet formation occurs throughout the disk. The extent of migration increases with disk mass and lifetime and usually decreases with metallicity. No clear correlation of metallicity and the semimajor axis of giant planets exists because in low [Fe/H] disks, planets start further out, but migrate more, whereas for high [Fe/H] they start further in, but migrate less. Close-in low mass planets have a lower mean metallicity than Hot Jupiters. Conclusions. The properties of protoplanetary disks are decisive for the properties of planets, and leave many imprints.Comment: 23 pages, 16 figures. Accepted for A&

Me, My Girls, and the Ideal Hotel: Segmenting Motivations of the Girlfriend Getaway Market Using Fuzzy C-Medoids for Fuzzy Data.

Segmenting the motivation of travelers using the push and pull framework remains ubiquitous in tourism. This study segments the girlfriend getaway (GGA) market on motivation (push) and accommodation (pull) attributes and identifies relationships between these factors. Using a relatively novel clustering algorithm, the Fuzzy C-Medoids clustering for fuzzy data (FCM-FD), on a sample of 749 women travelers, three segments (Socializers, Enjoyers, and Rejoicers) are uncovered. The results of a multinomial fractional model show relationships between the clusters of motivation and accommodation attributes as well as sociodemographic characteristics. The research highlights the importance of using a gendered perspective in applying well established motivation models such as the push and pull framework. The findings have implications for both destination and accommodation management

Attributing scientific and technical progress: the case of holography

Holography, the three-dimensional imaging technology, was portrayed widely as a paradigm of progress during its decade of explosive expansion 1964–73, and during its subsequent consolidation for commercial and artistic uses up to the mid 1980s. An unusually seductive and prolific subject, holography successively spawned scientific insights, putative applications and new constituencies of practitioners and consumers. Waves of forecasts, associated with different sponsors and user communities, cast holography as a field on the verge of success—but with the dimensions of success repeatedly refashioned. This retargeting of the subject represented a degree of cynical marketeering, but was underpinned by implicit confidence in philosophical positivism and faith in technological progressivism. Each of its communities defined success in terms of expansion, and anticipated continual progressive increase. This paper discusses the contrasting definitions of progress in holography, and how they were fashioned in changing contexts. Focusing equally on reputed ‘failures’ of some aspects of the subject, it explores the varied attributes by which success and failure were linked with progress by different technical communities. This important case illuminates the peculiar post-World War II environment that melded the military, commercial and popular engagement with scientific and technological subjects, and the competing criteria by which they assessed the products of science

The planarity of the stickface motion in the field hockey hit

The field hockey hit is an important but poorly understood stroke. This study investigated the planarity of the stickface motion during the downswing, in order to better characterise the kinematics and to assess the suitability of planar pendulum models for simulating the hit. Thirteen experienced female field hockey players were filmed executing hits with a single approach step, and the kinematics of the centre of the stickface were measured. A method was developed for identifying how far back from impact the stickface motion was planar. Orthogonal least-squares regression was used to determine best-fit planes for sections of the stickface path of varying length, each of which ended at impact, and these sections were considered planar if the mean residual between the stickface path and the fitted plane was less than 0.25% of the distance traveled by the stickface during that period. On average the stickface motion was planar for the last 83±12% of its downswing path, with the length of the planar section ranging from 1.85 m to 2.70 m. The suitability of a planar model for the stickface motion was supported, but further investigation of the stick and arm kinematics is warranted