1,511 research outputs found
Wilson line approach to gravity in the high energy limit
We examine the high energy (Regge) limit of gravitational scattering using a
Wilson line approach previously used in the context of non-Abelian gauge
theories. Our aim is to clarify the nature of the Reggeization of the graviton
and the interplay between this Reggeization and the so-called eikonal phase
which determines the spectrum of gravitational bound states. Furthermore, we
discuss finite corrections to this picture. Our results are of relevance to
various supergravity theories, and also help to clarify the relationship
between gauge and gravity theories.Comment: 33 pages, 5 figure
See a Black Hole on a Shoestring
The modes of vibration of hanging and partially supported strings provide
useful analogies to scalar fields travelling through spacetimes that admit
conformally flat spatial sections. This wide class of spacetimes includes
static, spherically symmetric spacetimes. The modes of a spacetime where the
scale factor depends as a power-law on one of the coordinates provide a useful
starting point and yield a new classification of these spacetimes on the basis
of the shape of the string analogue. The family of corresponding strings follow
a family of curves related to the cycloid, denoted here as hypercycloids (for
reasons that will become apparent). Like the spacetimes that they emulate these
strings exhibit horizons, typically at their bottommost points where the string
tension vanishes; therefore, hanging strings may provide a new avenue for the
exploration of the quantum mechanics of horizons.Comment: 5 pages, 1 figure, extensive changes to refect version accepted to
PR
Quantum corrections to generic branes: DBI, NLSM, and more
We study quantum corrections to hypersurfaces of dimension embedded
in generic higher-dimensional spacetimes. Manifest covariance is maintained
throughout the analysis and our methods are valid for arbitrary co-dimension
and arbitrary bulk metric. A variety of theories which are prominent in the
modern amplitude literature arise as special limits: the scalar sector of
Dirac-Born-Infeld theories and their multi-field variants, as well as generic
non-linear sigma models and extensions thereof. Our explicit one-loop results
unite the leading corrections of all such models under a single umbrella. In
contrast to naive computations which generate effective actions that appear to
violate the non-linear symmetries of their classical counterparts, our
efficient methods maintain manifest covariance at all stages and make the
symmetry properties of the quantum action clear. We provide an explicit
comparison between our compact construction and other approaches and
demonstrate the ultimate physical equivalence between the superficially
different results
Brazil, Ethiopia, and New Zealand lead the way on climate-smart agriculture.
As countries around the world face urgent agricultural challenges, the concept of ?climate-smart? agriculture (CSA) has been put forward to achieve climate change adaptation, mitigation, and food security synergistically. A new report explores how three countries are using integrated policy approaches to CSA and offers insights for how other countries can build CSA into their policy mix. Brazil has invested in research to support sustainable intensification while creating legal and enforcement mechanisms to protect forest areas as a response to unrestrained agricultural expansion driven by market demand. Ethiopia initiated innovative participatory watershed development programs, in partnership with numerous international institutions, which helped smallholder farmers to rehabilitate marginal land and break out of a poverty cycle. New Zealand has removed agricultural subsidies while partnering on research and development with the private sector as a way to ensure efficiency and resilience in an agricultural sector influenced by climate change and international trade dynamics. To assemble an integrated set of national policies that fosters CSA, governments will need context-specific assessments, strong multi-stakeholder institutions, coordination frameworks, and multi-scale information systems. Governments can select from an array of policy instruments ranging from regulatory mechanisms and economic incentives to public investments and educational campaigns. Many existing national policy goals and public programs designed to increase agricultural production, improve livelihoods, and reduce environmental risks can become important pillars of a national CSA strategy. Countries have obvious interests in fostering an agriculture sector that is climate-resilient, provides national needs for food, fiber, and fuel, and supports farm livelihoods. However, the incentives for national-level action toward reducing global greenhouse gas (GHG) levels are less clear in the absence of serious and shared international commitment. Integrated national CSA policies will be encouraged by clear, consistent signals from multilateral agencies, global donors, and international conventions and trade agreements that promote agriculture as a pathway for poverty reduction and food security
3D integrated superconducting qubits
As the field of superconducting quantum computing advances from the few-qubit
stage to larger-scale processors, qubit addressability and extensibility will
necessitate the use of 3D integration and packaging. While 3D integration is
well-developed for commercial electronics, relatively little work has been
performed to determine its compatibility with high-coherence solid-state
qubits. Of particular concern, qubit coherence times can be suppressed by the
requisite processing steps and close proximity of another chip. In this work,
we use a flip-chip process to bond a chip with superconducting flux qubits to
another chip containing structures for qubit readout and control. We
demonstrate that high qubit coherence (, s) is
maintained in a flip-chip geometry in the presence of galvanic, capacitive, and
inductive coupling between the chips
Fermi Surface as the Driving Mechanism for Helical Antiferromagnetic Ordering in Gd-Y Alloys
The first direct experimental evidence for the Fermi surface (FS) driving the
helical antiferromagnetic ordering in a gadolinium-yttrium alloy is reported.
The presence of a FS sheet capable of nesting is revealed, and the nesting
vector associated with the sheet is found to be in excellent agreement with the
periodicity of the helical ordering.Comment: 4 pages, 4 figure
Magnetic phase transitions in Gd64Sc36 studied using non-contact ultrasonics
The speed and attenuation of ultrasound propagation can be used to determine material properties and identify phase transitions. Standard ultrasonic contact techniques are not always convenient due to the necessity of using couplant; however, recently reliable non-contact ultrasonic techniques involving electromagnetic generation and detection of ultrasound with electromagnetic acoustic transducers (EMATs) have been developed for use on electrically conducting and/or magnetic materials. We present a detailed study of magnetic phase transitions in a single crystal sample of Gd64Sc36 magnetic alloy using contact and non-contact ultrasonic techniques for two orientations of external magnetic field. Phase diagrams are constructed based on measurements of elastic constant C33, the attenuation and the efficiency of generation when using an EMAT. The EMATs are shown to provide additional information related to the magnetic phase transitions in the studied sample, and results identify a conical helix phase in Gd64Sc36 in the magnetic field orientation
Challenges in context-aware mobile language learning: the MASELTOV approach
Smartphones, as highly portable networked computing devices with embedded sensors including GPS receivers, are ideal platforms to support context-aware language learning. They can enable learning when the user is en-gaged in everyday activities while out and about, complementing formal language classes. A significant challenge, however, has been the practical implementation of services that can accurately identify and make use of context, particularly location, to offer meaningful language learning recommendations to users. In this paper we review a range of approaches to identifying context to support mobile language learning. We consider how dynamically changing aspects of context may influence the quality of recommendations presented to a user. We introduce the MASELTOV project’s use of context awareness combined with a rules-based recommendation engine to present suitable learning content to recent immigrants in urban areas; a group that may benefit from contextual support and can use the city as a learning environment
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