1,961 research outputs found
Simulation study of intracity helicopter operations under instrument conditions to category 1 minimums
A fixed-base simulator study was conducted to define pilot workload and task performance associated with instrument flight operations for an intracity helicopter passenger service. Displays considered necessary to provide a minimal capability under Instrument Flight Rules conditions were used to fly a representative commercial helicopter route structure in the New York area, with each terminal assumed to be equipped with a precision approach guidance system. A cross section of pilots participated as test subjects, and despite the high workload level, the results indicated that for the assumptions employed, minimums of 61 m (200 ft) ceiling and 805 m (0.5 mile) visibility were feasible
A grid-based infrastructure for distributed retrieval
In large-scale distributed retrieval, challenges of latency, heterogeneity, and dynamicity emphasise the importance of infrastructural support in reducing the development costs of state-of-the-art solutions. We present a service-based infrastructure for distributed retrieval which blends middleware facilities and a design framework to âliftâ the resource sharing approach and the computational services of a European Grid platform into the domain of e-Science applications. In this paper, we give an overview of the DILIGENT Search Framework and illustrate its exploitation in the ïŹeld of Earth Science
Information Loss and Anomalous Scattering
The approach of 't Hooft to the puzzles of black hole evaporation can be
applied to a simpler system with analogous features. The system is
dimensional electrodynamics in a linear dilaton background. Analogues of black
holes, Hawking radiation and evaporation exist in this system. In perturbation
theory there appears to be an information paradox but this gets resolved in the
full quantum theory and there exists an exact -matrix, which is fully
unitary and information conserving. 't Hooft's method gives the leading terms
in a systematic approximation to the exact result.Comment: 18 pages, 3 figures (postscript files available soon on request),
(earlier version got corrupted by mail system
Dynamics of Extremal Black Holes
Particle scattering and radiation by a magnetically charged, dilatonic black
hole is investigated near the extremal limit at which the mass is a constant
times the charge. Near this limit a neighborhood of the horizon of the black
hole is closely approximated by a trivial product of a two-dimensional black
hole with a sphere. This is shown to imply that the scattering of
long-wavelength particles can be described by a (previously analyzed)
two-dimensional effective field theory, and is related to the
formation/evaporation of two-dimensional black holes. The scattering proceeds
via particle capture followed by Hawking re-emission, and naively appears to
violate unitarity. However this conclusion can be altered when the effects of
backreaction are included. Particle-hole scattering is discussed in the light
of a recent analysis of the two-dimensional backreaction problem. It is argued
that the quantum mechanical possibility of scattering off of extremal black
holes implies the potential existence of additional quantum numbers - referred
to as ``quantum whiskers'' - characterizing the black hole.Comment: 31 page
SO(10) Cosmic Strings and Baryon Number Violation
SO(10) cosmic strings formed during the phase transition Spin(10)
SU(5) are studied. Two types of strings ---
one effectively Abelian and one non-Abelian --- are constructed and the string
solutions are calculated numerically. The non-Abelian string can catalyze
baryon number violation via the ``twisting'' of the scalar field which causes
mixing of leptons and quarks in the fermion multiplet. The non-Abelian string
is also found to have the lower energy possibly for the entire range of the
parameters in the theory. Scattering of fermions in the fields of the strings
is analyzed, and the baryon number violation cross section is calculated. The
role of the self-adjoint parameters is discussed and the values are computed.Comment: LaTex (RevTex), 36 pages, 6 figures (available upon request),
MIT-CTP#215
Born-Regulated Gravity in Four Dimensions
Previous work involving Born-regulated gravity theories in two dimensions is
extended to four dimensions. The action we consider has two dimensionful
parameters. Black hole solutions are studied for typical values of these
parameters. For masses above a critical value determined in terms of these
parameters, the event horizon persists. For masses below this critical value,
the event horizon disappears, leaving a ``bare mass'', though of course no
singularity.Comment: LaTeX, 15 pages, 2 figure
Automatic Classification of Text Databases through Query Probing
Many text databases on the web are "hidden" behind search interfaces, and
their documents are only accessible through querying. Search engines typically
ignore the contents of such search-only databases. Recently, Yahoo-like
directories have started to manually organize these databases into categories
that users can browse to find these valuable resources. We propose a novel
strategy to automate the classification of search-only text databases. Our
technique starts by training a rule-based document classifier, and then uses
the classifier's rules to generate probing queries. The queries are sent to the
text databases, which are then classified based on the number of matches that
they produce for each query. We report some initial exploratory experiments
that show that our approach is promising to automatically characterize the
contents of text databases accessible on the web.Comment: 7 pages, 1 figur
Spacetime Embedding Diagrams for Black Holes
We show that the 1+1 dimensional reduction (i.e., the radial plane) of the
Kruskal black hole can be embedded in 2+1 Minkowski spacetime and discuss how
features of this spacetime can be seen from the embedding diagram. The purpose
of this work is educational: The associated embedding diagrams may be useful
for explaining aspects of black holes to students who are familiar with special
relativity, but not general relativity.Comment: 22 pages, 21 figures, RevTex. To be submitted to the American Journal
of Physics. Experts will wish only to skim appendix A and to look at the
pictures. Suggested Maple code is now compatible with MapleV4r
Interaction potential in compact three-dimensional QED with mixed action
We use a variational wave function to calculate the energy of the interaction
between external charges in the compact Abelian gauge theory in 2+1 dimensions
with mixed action. Our variational wave functions preserve the compact gauge
invariance of the theory both in the vacuum and in the charged sectors. We find
that a good estimate of the interaction energy is obtained only when we allow
more variational parameters in the charged sector than in the vacuum sector.
These extra parameters are the profile of an induced electric field. We find
that the theory has a two-phase structure: When the charge-2 coupling is large
and negative there is no mass gap in the theory and no confinement, while
otherwise a mass gap is generated dynamically and the theory confines charges.
The pure Wilson theory is in the confining phase.Comment: 22 pages, Latex -- final version, minor changes from first versio
Renewing the Promise: State Policies to Improve Higher Education Performance
As the need for a highly knowledgeable citizenry grows, fewer Americans are accessing training and education beyond high school. The failure to attain postsecondary degrees and workforce certificates is particularly pervasive among low-income and minority populations. An undereducated citizenry leaves the country at a competitive disadvantage, diminishes the middle class, and lowers the standard of living for more and more people. Although the federal government plays an important role in higher education, states bear the primary responsibility for developing their own public higher education systems, including policies for funding and governing higher education and for connecting higher education with public schools.
Renewing the Promise: State Policies to Improve Higher Education Performance summarizes the findings from a study that sought to understand how public policy explains the collective performance of higher education institutions in five statesâ Georgia, Illinois, Maryland, Texas, and Washingtonâthat have similar challenges as other states, such as the need to increase educational attainment and close persistent gaps in opportunity by race, ethnicity, income, and geography. The study reviewed state higher education performance and policies from the early 1990s through approximately 2010, including policies and statutes related to higher education finance, accountability, structure, and governance. We augmented our review of state policies and data by interviewing political, business, and higher education leaders in each state
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