18,389 research outputs found
Time (in)dependence in general relativity
We clarify the conditions for Birkhoff's theorem, that is, time-independence
in general relativity. We work primarily at the linearized level where guidance
from electrodynamics is particularly useful. As a bonus, we also derive the
equivalence principle. The basic time-independent solutions due to
Schwarzschild and Kerr provide concrete illustrations of the theorem. Only
familiarity with Maxwell's equations and tensor analysis is required.Comment: Revised version of originally titled "Kinder Kerr", to appear in
American Journal of Physic
The design of low cost structures for extensive ground arrays
The development of conceptual designs of solar array support structures and their foundations including considerations of the use of concrete, steel, aluminum, or timber are reported. Some cost trends were examined by varying selected parameters to determine optimum configurations. Detailed civil/structural design criteria were developed. Using these criteria, eight detailed designs for support structures and foundations were developed and cost estimates were made. As a result of the study wind was identified as the major loading experienced by these low height structures, whose arrays are likely to extend over large tracts of land. Proper wind load estimating is considered essential to developing realistic structural designs and achieving minimum cost support structures. Wind tunnel testing of a conceptual array field was undertaken and some of the resulting wind design criteria are presented. The SPS rectenna system designs may be less sensitive to wind load estimates, but consistent design criteria remain important
Shortcuts to Spherically Symmetric Solutions: A Cautionary Note
Spherically symmetric solutions of generic gravitational models are
optimally, and legitimately, obtained by expressing the action in terms of the
two surviving metric components. This shortcut is not to be overdone, however:
a one-function ansatz invalidates it, as illustrated by the incorrect solutions
of [1].Comment: 2 pages. Amplified derivation, accepted for publication in Class
Quant Gra
The decline of traditional banking: implications for financial stability and regulatory policy
In recent years, the traditional business of banks--making long-term loans and funding them by issuing short--dated deposits-has declined. This development has raised concerns that more banks will fail or be forced to assume greater risk to remain profitable. This article first examines the economic forces responsible for banks' reduced role in financial intermediation. The authors then consider whether banks may be jeopardizing the stability of the financial system by extending riskier loans or engaging in derivatives dealing and other "nontraditional" financial activities that bring higher returns but could carry greater risk. The authors conclude that because most nontraditional activities expose banks to risks and moral hazard problems similar to those associated with banks' traditional activities, the new activities can be regulated as effectively as the old.Banks and banking ; Financial services industry ; Bank supervision
The Decline of Traditional Banking: Implications for Financial Stabilityand Regulatory Policy
This paper outlines the fundamental economic forces that have led to the decline in traditional banking, that is the process of making loans and funding them by issuing short-dated deposits. The declining competitiveness of traditional banking may threaten financial stability by increasing bank failures and by increasing the incentives for banks to take on more risk, either by making more risky loans or by engaging in 'nontraditional' financial activities that promise higher returns but greater risk. This paper argues that most nontraditional activities, such as banks acting as derivatives dealers, expose banks to risks and moral hazard problems that are similar to those associated with banks' traditional activities, and that these activities can be regulated as effectively as can traditional activities. One regulatory approach to maintain financial stability and strengthen the banking system is to adopt a system of structured bank capital requirements with early corrective action by regulators. An important element in this approach is that market- value accounting principles would be applied to banks and there would be increased public disclosure by banks of the risks associated with their trading activities. With this regulatory structure in place, banks could be permitted greater freedom to expand into nontraditional activities.
Birkhoff for Lovelock Redux
We show succinctly that all metric theories with second order field equations
obey Birkhoff's theorem: their spherically symmetric solutions are static.Comment: Submitted to CQ
Plasmas generated by ultra-violet light rather than electron impact
We analyze, in both plane and cylindrical geometries, a collisionless plasma
consisting of an inner region where generation occurs by UV illumination, and
an un-illuminated outer region with no generation. Ions generated in the inner
region flow outwards through the outer region and into a wall. We solve for
this system's steady state, first in the quasi-neutral regime (where the Debye
length vanishes and analytic solutions exist) and then in the
general case, which we solve numerically. In the general case a double layer
forms where the illuminated and un-illuminated regions meet, and an
approximately quasi-neutral plasma connects the double layer to the wall
sheath; in plane geometry the ions coast through the quasi-neutral section at
slightly more than the Bohm speed . The system, although simple, therefore
has two novel features: a double layer that does not require counter-streaming
ions and electrons, and a quasi-neutral plasma where ions travel in straight
lines with at least the Bohm speed. We close with a pr\'{e}cis of our
asymptotic solutions of this system, and suggest how our theoretical
conclusions might be extended and tested in the laboratory.Comment: 10 pages, 3 figures, accepted by Physics of Plasma
Extraordinary sensitivity of the electronic structure and properties of single-walled carbon nanotubes to molecular charge-transfer
Interaction of single-walled carbon nanotubes with electron donor and
acceptor molecules causes significant changes in the electronic and Raman
spectra, the relative proportion of the metallic species increasing on electron
donation through molecular charge transfer, as also verified by electrical
resistivity measurements.Comment: 15 pages, 5 figurre
Survival and Growth of American Alligator (Alligator mississippiensis) hatchlings after artificial incubation and repatriation
Hatchling American Alligators (Alligator mississippiensis) produced from artificially incubated
wild eggs were returned to their natal areas (repatriated). We compared artificially incubated and
repatriated hatchlings released within and outside the maternal alligator’s home range with naturally
incubated hatchlings captured and released within the maternal alligator’s home range on Lake Apopka,
Lake Griffin, and Orange Lake in Florida. We used probability of recapture and total length at approximately
nine months after hatching as indices of survival and growth rates. Artificially incubated hatchlings released
outside of the maternal alligator’s home range had lower recapture probabilities than either naturally
incubated hatchlings or artificially incubated hatchlings released near the original nest site. Recapture
probabilities of other treatments did not differ significantly. Artificially incubated hatchlings were
approximately 6% shorter than naturally incubated hatchlings at approximately nine months after hatching.
We concluded that repatriation of hatchlings probably would not have long-term effects on populations
because of the resiliency of alligator populations to alterations of early age-class survival and growth rates of
the magnitude that we observed. Repatriation of hatchlings may be an economical alternative to repatriation
of older juveniles for population restoration. However, the location of release may affect subsequent survival
and growth
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