4,293 research outputs found
Feasibility of an onboard wake vortex avoidance system
It was determined that an onboard vortex wake detection system using existing, proven instrumentation is technically feasible. This system might be incorporated into existing onboard systems such as a wind shear detection system, and might provide the pilot with the location of a vortex wake, as well as an evasive maneuver so that the landing separations may be reduced. It is suggested that this system might be introduced into our nation's commuter aircraft fleet and major air carrier fleet and permit a reduction of current landing separation standards, thereby reducing takeoff and departure delays
Identity of the van der Waals Force and the Casimir Effect and the Irrelevance of these Phenomena to Sonoluminescence
We show that the Casimir, or zero-point, energy of a dilute dielectric ball,
or of a spherical bubble in a dielectric medium, coincides with the sum of the
van der Waals energies between the molecules that make up the medium. That
energy, which is finite and repulsive when self-energy and surface effects are
removed, may be unambiguously calculated by either dimensional continuation or
by zeta function regularization. This physical interpretation of the Casimir
energy seems unambiguous evidence that the bulk self-energy cannot be relevant
to sonoluminescence.Comment: 7 pages, no figures, REVTe
Sonoluminescence as a QED vacuum effect: Probing Schwinger's proposal
Several years ago Schwinger proposed a physical mechanism for
sonoluminescence in terms of photon production due to changes in the properties
of the quantum-electrodynamic (QED) vacuum arising from a collapsing dielectric
bubble. This mechanism can be re-phrased in terms of the Casimir effect and has
recently been the subject of considerable controversy. The present paper probes
Schwinger's suggestion in detail: Using the sudden approximation we calculate
Bogolubov coefficients relating the QED vacuum in the presence of the expanded
bubble to that in the presence of the collapsed bubble. In this way we derive
an estimate for the spectrum and total energy emitted. We verify that in the
sudden approximation there is an efficient production of photons, and further
that the main contribution to this dynamic Casimir effect comes from a volume
term, as per Schwinger's original calculation. However, we also demonstrate
that the timescales required to implement Schwinger's original suggestion are
not physically relevant to sonoluminescence. Although Schwinger was correct in
his assertion that changes in the zero-point energy lead to photon production,
nevertheless his original model is not appropriate for sonoluminescence. In
other works (see quant-ph/9805023, quant-ph/9904013, quant-ph/9904018,
quant-ph/9905034) we have developed a variant of Schwinger's model that is
compatible with the physically required timescales.Comment: 18 pages, ReV_TeX 3.2, 9 figures. Major revisions: This document is
now limited to providing a probe of Schwinger's original suggestion for
sonoluminescence. For details on our own variant of Schwinger's ideas see
quant-ph/9805023, quant-ph/9904013, quant-ph/9904018, quant-ph/990503
Casimir bag energy in the stochastic approximation to the pure QCD vacuum
We study the Casimir contribution to the bag energy coming from gluon field
fluctuations, within the context of the stochastic vacuum model (SVM) of pure
QCD. After formulating the problem in terms of the generating functional of
field strength cumulants, we argue that the resulting predictions about the
Casimir energy are compatible with the phenomenologically required bag energy
term.Comment: 16 page
How do ecological perspectives help understand schools as sites for teacher learning?
Schools are sites of teachers’ professional learning for both new entrants and experienced practitioners. In this paper, schools are conceptualised as complex, multidimensional ecologies that are constituted by the relations that exist between school leaders, teachers, mentors and all members of the school community. As relational environments, the conditions affecting professional learning – both formal and informal – are constantly dynamic, with multiple and simultaneous interactions taking place between these stakeholders. Interactions are also multi-layered – between the school system, individuals, classrooms, the community and the policy environment. School leaders are a major influence on these dynamics and affect how schools act as sites of professional formation, mediating external policy as well as affecting micro-dynamics within individual school systems. The challenge of realising professional learning within these relational contexts can be viewed as a ‘wicked problem’, a feature of complex systems that resists simplified solutions. In conceptualising a complex ecology at work, we illuminate the relational dynamics with a focus, for all stakeholders within schools, including leaders, on the need to recognise and value the importance of ‘emergence’ in professional learning. This means embracing inevitable uncertainty as a feature of schools as complex systems
Optical BCS conductivity at imaginary frequencies and dispersion energies of superconductors
We present an efficient expression for the analytic continuation to arbitrary
complex frequencies of the complex optical and AC conductivity of a homogeneous
superconductor with arbitrary mean free path. Knowledge of this quantity is
fundamental in the calculation of thermodynamic potentials and dispersion
energies involving type-I superconducting bodies. When considered for imaginary
frequencies, our formula evaluates faster than previous schemes involving
Kramers--Kronig transforms. A number of applications illustrates its
efficiency: a simplified low-frequency expansion of the conductivity, the
electromagnetic bulk self-energy due to longitudinal plasma oscillations, and
the Casimir free energy of a superconducting cavity.Comment: 20 pages, 7 figures, calculation of Casimir energy adde
Casimir Energy of a Spherical Shell
The Casimir energy for a conducting spherical shell of radius is computed
using a direct mode summation approach. An essential ingredient is the
implementation of a recently proposed method based on Cauchy's theorem for an
evaluation of the eigenfrequencies of the system. It is shown, however, that
this earlier calculation uses an improper set of modes to describe the waves
exterior to the sphere. Upon making the necessary corrections and taking care
to ensure that no mathematically ill-defined expressions occur, the technique
is shown to leave numerical results unaltered while avoiding a longstanding
criticism raised against earlier calculations of the Casimir energy.Comment: LaTeX, 14 pages, 1 figur
Observability of the Bulk Casimir Effect: Can the Dynamical Casimir Effect be Relevant to Sonoluminescence?
The experimental observation of intense light emission by acoustically
driven, periodically collapsing bubbles of air in water (sonoluminescence) has
yet to receive an adequate explanation. One of the most intriguing ideas is
that the conversion of acoustic energy into photons occurs quantum
mechanically, through a dynamical version of the Casimir effect. We have argued
elsewhere that in the adiabatic approximation, which should be reliable here,
Casimir or zero-point energies cannot possibly be large enough to be relevant.
(About 10 MeV of energy is released per collapse.) However, there are
sufficient subtleties involved that others have come to opposite conclusions.
In particular, it has been suggested that bulk energy, that is, simply the
naive sum of , which is proportional to the volume, could
be relevant. We show that this cannot be the case, based on general principles
as well as specific calculations. In the process we further illuminate some of
the divergence difficulties that plague Casimir calculations, with an example
relevant to the bag model of hadrons.Comment: 13 pages, REVTe
Effects of phase transitions in devices actuated by the electromagnetic vacuum force
We study the influence of the electromagnetic vacuum force on the behaviour
of a model device based on materials, like germanium tellurides, that undergo
fast and reversible metal-insulator transitions on passing from the crystalline
to the amorphous phase. The calculations are performed at finite temperature
and fully accounting for the behaviour of the material dielectric functions.
The results show that the transition can be exploited to extend the distance
and energy ranges under which the device can be operated without undergoing
stiction phenomena. We discuss the approximation involved in adopting the
Casimir expression in simulating nano- and micro- devices at finite
temperature
Calculation of the Casimir Force between Similar and Dissimilar Metal Plates at Finite Temperature
The Casimir pressure is calculated between parallel metal plates, containing
the materials Au, Cu, or Al. Our motivation for making this calculation is the
need of comparing theoretical predictions, based on the Lifshitz formula, with
experiments that are becoming gradually more accurate. In particular, the
finite temperature correction is considered, in view of the recent discussion
in the literature on this point. A special attention is given to the case where
the difference between the Casimir pressures at two different temperatures,
T=300 K and T=350 K, is involved. This seems to be a case that will be
experimentally attainable in the near future, and it will be a critical test of
the temperature correction.Comment: 23 latex pages, 12 figures. Introductory section expanded, 4 new
references. To appear in J. Phys. A: Math. Ge
- …