3,260 research outputs found
Symmetries and charges of general relativity at null boundaries
We study general relativity at a null boundary using the covariant phase
space formalism. We define a covariant phase space and compute the algebra of
symmetries at the null boundary by considering the boundary-preserving
diffeomorphisms that preserve this phase space. This algebra is the semi-direct
sum of diffeomorphisms on the two sphere and a nonabelian algebra of
supertranslations that has some similarities to supertranslations at null
infinity. By using the general prescription developed by Wald and Zoupas, we
derive the localized charges of this algebra at cross sections of the null
surface as well as the associated fluxes. Our analysis is covariant and applies
to general non-stationary null surfaces. We also derive the global charges that
generate the symmetries for event horizons, and show that these obey the same
algebra as the linearized diffeomorphisms, without any central extension. Our
results show that supertranslations play an important role not just at null
infinity but at all null boundaries, including non-stationary event horizons.
They should facilitate further investigations of whether horizon symmetries and
conservation laws in black hole spacetimes play a role in the information loss
problem, as suggested by Hawking, Perry, and Strominger.Comment: v2: added appendices on trivial diffeomorphisms and relation to
1810.01847; v1: 59 pages + appendices, 2 figure
Mechanically-Deployed Hypersonic Decelerator and Conformal Ablator Technologies for Mars Missions
The concept of a mechanically deployable hypersonic decelerator, developed initially for high mass (approximately 40 MT) human Mars missions, is currently funded by OCT for technology maturation. The ADEPT (Adaptive, Deployable Entry and Placement Technology) project has broad, game-changing applicability to in situ science missions to Venus, Mars, and the Outer Planets. Combined with maturation of conformal ablator technology (another current OCT investment), the two technologies provide unique low-mass mission enabling capabilities otherwise not achievable by current rigid aeroshell or by inflatables. If this abstract is accepted, we will present results that illustrate the mission enabling capabilities of the mechanically deployable architecture for: (1) robotic Mars (Discovery or New Frontiers class) in the near term (2) alternate approaches to landing MSL-class payloads, without the need for supersonic parachute or lifting entry, in the mid-term and (3) Heavy mass and human missions to Mars in the long term
Collective multipole-like signatures of entanglement in symmetric N-qubit systems
A cogent theory of collective multipole-like quantum correlations in
symmetric multiqubit states is presented by employing SO(3) irreducible
spherical tensor representation. An arbitrary bipartite division of this system
leads to a family of inequalities to detect entanglement involving averages of
these tensors expressed in terms of the total system angular momentum operator.
Implications of this theory to the quantum nature of multipole-like
correlations of all orders in the Dicke states are deduced. A selected set of
examples illustrate these collective tests. Such tests detect entanglement in
macroscopic atomic ensembles, where individual atoms are not accessible.Comment: REVTEX, 4 pages with 1 figure; To appear in Phys. Rev.
Chemical Compositions of Four Metal-poor Giants
We present the chemical compositions of four K giants CS 22877-1, CS
22166-16, CS22169-35 and BS 16085 - 0050 that have [Fe/H] in the range -2.4 to
-3.1. Metal-poor stars with [Fe/H] < -2.5 are known to exhibit considerable
star - to - star variations of many elements. This quartet confirms this
conclusion. CS 22877-1 and CS 22166-16 are carbon-rich. There is significant
spread for [/Fe] within our sample where [/Fe] is computed from
the mean of the [Mg/Fe], and [Ca/Fe] ratios. BS 16085 - 0050 is remarkably
enriched with a mean [/Fe] of 0.7 but CS 22169-35 is
-poor. The aluminium abundance also shows a significant variation over
the sample. A parallel and unsuccessful search among high-velocity late-type
stars for metal-poor stars is described.Comment: 14 pages (text), 4 tables, 3 figures, Accepted for publication in
PAS
Stagnation Point Radiative Heating Relations for Venus Entry
Improved analytic expressions for calculating the stagnation point radiative heating during entry into the atmosphere of Venus have been developed. These analytic expressions can be incorporated into entry trajectory simulation codes. Together with analytical expressions for convective heating at the stagnation point, the time-integrated total heat load at the stagnation point is used in determining the thickness of protective material required, and hence the mass of the fore body heatshield of uniform thickness
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Solar Trough Organic Rankine Electricity System (STORES) Stage 1: Power Plant Optimization and Economics; November 2000 -- May 2005
Report regarding a Stage 1 Study to further develop the concept of the Solar Trough Organic Rankine Cycle Electricity Systems (STORES)
Imaging Flaws in Thin Metal Plates Using a Magneto-Optic Device
The ability to quickly detect flaws in thin aluminum alloy sheets is of critical importance to the commercial airline fleets. Current eddy-current methods, although very effective at exposing flawed regions, can be very time consuming when applied to airframe structures. The need for a fast, effective means of detecting critical flaws needs to be met. The purpose of this paper is to present preliminary results describing some capabilities of a new instrument, The Magneto-Optic/Eddy Current Imager, as an NDE tool for use in the Airframe Structural Integrity Program
Enabling Entry Technologies for Ice Giant Missions
The highest priority science goals for Ice Giant missions are: 1) Interior structure of the Planet, and 2) Bulk composition that includes isotopes and noble gases. The interaction between the planetary interior and the atmosphere requires sustained global measurements. Noble gas and Isotope measurements require in situ measurement. Drag modulated aerocapture utilizing ADEPT offers more mass delivered to the Ice Giants than with propulsive orbit insertion. The Galileo Probe entered at a hot spot which created interpretation challenges. Juno is providing valuable orbital measurements, but without in situ measurements the story is incomplete. Planetary scientists interested in Ice Giant missions should perform mission design studies with these new Entry System technologies to assess the feasibility within the context of the international collaboration framework. A mission architecture that includes probe(s) along with an orbiting spacecraft can deploy the probes at the desired location while taking simultaneous measurements from orbit to provide invaluable data that can correlate both global and local measurements. Entry System Technologies currently being developed by NASA are poised to enable missions that position the Orbiter & Probes through drag modulated aerocapture (ADEPT), and HEEET enables the Probes to survive the extreme environments encountered for entry into the atmospheric interior
Fabrication of Microfluidic Devices with Integrated Electronic Components via Dual Extrusion-Based 3D Printing
Microfluidics channels as small as 50 μm in width were produced via extrusionbased 3D printing. Multi-layered, multi-electrode devices were fabricated with inlaid conductive filament. Electrodes were used for simple electroanalysis of proteins. Prototyping of microfluidics devices via extrusion-based 3D printing may ultimately allow for an easy, cost-effective, high-throughput, high-resolution, and single-step method of rapidly prototyping electronic microfluidic devices, principally suited for use in diagnostics applications
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