47,515 research outputs found
Quantum Field Theory Constrains Traversable Wormhole Geometries
Recently a bound on negative energy densities in four-dimensional Minkowski
spacetime was derived for a minimally coupled, quantized, massless, scalar
field in an arbitrary quantum state. The bound has the form of an uncertainty
principle-type constraint on the magnitude and duration of the negative energy
density seen by a timelike geodesic observer. When spacetime is curved and/or
has boundaries, we argue that the bound should hold in regions small compared
to the minimum local characteristic radius of curvature or the distance to any
boundaries, since spacetime can be considered approximately Minkowski on these
scales. We apply the bound to the stress-energy of static traversable wormhole
spacetimes. Our analysis implies that either the wormhole must be only a little
larger than Planck size or that there is a large discrepancy in the length
scales which characterize the wormhole. In the latter case, the negative energy
must typically be concentrated in a thin band many orders of magnitude smaller
than the throat size. These results would seem to make the existence of
macroscopic traversable wormholes very improbable.Comment: 26 pages, plain LaTe
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The pricing journey – The evolution of approach and execution as organisational pricing capability develops
This is an author version of an article which has been published in its definitive form as a Margin2 / Brunel Pricing Forum white paper and has been posted by permission of the Brunel Pricing Forum for personal use, not for redistribution. The article was published online by Margin2 and the original version can be accessed at the link below.An organisation’s approach to Pricing can be seen as a competency in the sense that it is a combination of skills, behaviours and the application of knowledge. As the efficiency and effectiveness of pricing decisions improve, then a range of indicators highlight a more thorough, considered and generally more successful approach. Fewer mistakes are made, results are more visibility linked to previous actions, and implementation becomes quicker and more comprehensive
Effect of an External Field on Decoherence
"Decoherence of quantum superpositions through coupling to engineered
reservoirs" is the topic of a recent article by Myatt et al. [Nature
{\underline{403}}, 269 (2000)] which has attracted much interest because of its
relevance to current research in fundamental quantum theory, quantum
computation, teleportation, entanglement and the quantum-classical interface.
However, the preponderance of theoretical work on decoherence does not consider
the effect of an {\underline{external field}}. Here, we present an analysis of
such an effect in the case of the random delta-correlated force discussed by
Myatt et al
Note on the derivative of the hyperbolic cotangent
In a letter to Nature (Ford G W and O'Connell R F 1996 Nature 380 113) we
presented a formula for the derivative of the hyperbolic cotangent that differs
from the standard one in the literature by an additional term proportional to
the Dirac delta function. Since our letter was necessarily brief, shortly after
its appearance we prepared a more extensive unpublished note giving a detailed
explanation of our argument. Since this note has been referenced in a recent
article (Estrada R and Fulling S A 2002 J. Phys. A: Math. Gen. 35 3079) we
think it appropriate that it now appear in print. We have made no alteration to
the original note
Bounds on negative energy densities in flat spacetime
We generalise results of Ford and Roman which place lower bounds -- known as
quantum inequalities -- on the renormalised energy density of a quantum field
averaged against a choice of sampling function. Ford and Roman derived their
results for a specific non-compactly supported sampling function; here we use a
different argument to obtain quantum inequalities for a class of smooth, even
and non-negative sampling functions which are either compactly supported or
decay rapidly at infinity. Our results hold in -dimensional Minkowski space
() for the free real scalar field of mass . We discuss various
features of our bounds in 2 and 4 dimensions. In particular, for massless field
theory in 2-dimensional Minkowski space, we show that our quantum inequality is
weaker than Flanagan's optimal bound by a factor of 3/2.Comment: REVTeX, 13 pages and 2 figures. Minor typos corrected, one reference
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Operative mortality in resource-limited settings: the experience of Medecins Sans Frontieres in 13 countries.
OBJECTIVE: To determine operative mortality in surgical programs from resource-limited settings. DESIGN, SETTING, AND PARTICIPANTS: A retrospective cohort study of 17 surgical programs in 13 developing countries by 1 humanitarian organization, Médecins Sans Frontières, was performed between January 1, 2001, and December 31, 2008. Participants included patients undergoing surgical procedures. MAIN OUTCOME MEASURE: Operative mortality. Determinants of mortality were modeled using logistic regression. RESULTS: Between 2001 and 2008, 19,643 procedures were performed on 18,653 patients. Among these, 8329 procedures (42%) were emergent; 7933 (40%) were for obstetric-related pathology procedures and 2767 (14%) were trauma related. Operative mortality was 0.2% (31 deaths) and was associated with programs in conflict settings (adjusted odds ratio [AOR] = 4.6; P = .001), procedures performed under emergency conditions (AOR = 20.1; P = .004), abdominal surgical procedures (AOR = 3.4; P = .003), hysterectomy (AOR = 12.3; P = .001), and American Society of Anesthesiologists classifications of 3 to 5 (AOR = 20.2; P < .001). CONCLUSIONS: Surgical care can be provided safely in resource-limited settings with appropriate minimum standards and protocols. Studies on the burden of surgical disease in these populations are needed to improve service planning and delivery. Quality improvement programs are needed for the various stakeholders involved in surgical delivery in these settings
Detection of negative energy: 4-dimensional examples
We study the response of switched particle detectors to static negative
energy densities and negative energy fluxes. It is demonstrated how the
switching leads to excitation even in the vacuum and how negative energy can
lead to a suppression of this excitation. We obtain quantum inequalities on the
detection similar to those obtained for the energy density by Ford and
co-workers and in an `operational' context by Helfer. We revisit the question
`Is there a quantum equivalence principle?' in terms of our model. Finally, we
briefly address the issue of negative energy and the second law of
thermodynamics.Comment: 10 pages, 7 figure
Two-loop Renormalization Group Equations in the Standard Model
Two-loop renormalization group equations in the standard model are
re-calculated. A new coefficient is found in the beta-function of the quartic
coupling and a class of gauge invariants are found to be absent in the
beta-functions of hadronic Yukawa couplings. The two-loop beta-function of the
Higgs mass parameter is presented in complete form.Comment: 4 pages, RevTe
The Effects of Stress Tensor Fluctuations upon Focusing
We treat the gravitational effects of quantum stress tensor fluctuations. An
operational approach is adopted in which these fluctuations produce
fluctuations in the focusing of a bundle of geodesics. This can be calculated
explicitly using the Raychaudhuri equation as a Langevin equation. The physical
manifestation of these fluctuations are angular blurring and luminosity
fluctuations of the images of distant sources. We give explicit results for the
case of a scalar field on a flat background in a thermal state.Comment: 26 pages, 1 figure, new material added in Sect. III and in Appendices
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