911 research outputs found
Casimir force on a piston
We consider a massless scalar field obeying Dirichlet boundary conditions on
the walls of a two-dimensional L x b rectangular box, divided by a movable
partition (piston) into two compartments of dimensions a x b and (L-a) x b. We
compute the Casimir force on the piston in the limit L -> infinity. Regardless
of the value of a/b, the piston is attracted to the nearest end of the box.
Asymptotic expressions for the Casimir force on the piston are derived for a <<
b and a >> b.Comment: 10 pages, 1 figure. Final version, accepted for publication in Phys.
Rev.
Site assessment of Douglas Shoal ship grounding in the Great Barrier Reef
The bulk carrier Shen Neng 1 ran aground on Douglas Shoal in the Great Barrier Reef Marine Park in April 2010. At over 40 hectares, this is the largest ship grounding scar known in the Great Barrier Reef, and possibly the largest reef-related grounding in the world. Challenges for assessment of the site included its large scale and the remote nature of Douglas Shoal coupled with its high exposure to wind, wave conditions and fauna that may pose safety hazards. Marine surveys used multiple and novel methods including sediment sampling combined with visual and acoustic survey techniques
Stability of the self-phase-locked pump-enhanced singly resonant parametric oscillator
Steady-state and dynamics of the self-phase-locked (3\omega ==> 2\omega,
\omega) subharmonic optical parametric oscillator are analyzed in the
pump-and-signal resonant configuration, using an approximate analytical model
and a full propagation model. The upper branch solutions are found always
stable, regardless of the degree of pump enhancement. The domain of existence
of stationary states is found to critically depend on the phase-mismatch of the
competing second-harmonic process.Comment: LateX2e/RevteX4, 4 pages, 5 figures. Submitted to Phys. Rev. A
(accepted on Jan. 17, 2003
Filamentous green algae from the Early Devonian Rhynie chert
A relatively neglected element of the biota of the Lower Devonian Rhynie chert Lagerstätte are filamentous green algae exceptionally preserved by silicification. Palynological processing of sediments associated with the cherts has yielded palynomorphs that we also interpret as the remains of filamentous green algae and one such taxon is described herein. Cells occur individually, in masses or joined end-to-end as an unbranched filament. The cells are characterised by end walls that form a ‘collar’ structure and inner bodies interpreted as reproductive structures. Because of a lack of preserved characters taxonomic precision is limited, although we suggest the fossils are most likely either zygnematalean or oedogonialean algae that inhabited ponds or lakes and were either attached to substrates and/or free-floating
Quantum properties of transverse pattern formation in second-harmonic generation
We investigate the spatial quantum noise properties of the one dimensional
transverse pattern formation instability in intra-cavity second-harmonic
generation. The Q representation of a quasi-probability distribution is
implemented in terms of nonlinear stochastic Langevin equations. We study these
equations through extensive numerical simulations and analytically in the
linearized limit. Our study, made below and above the threshold of pattern
formation, is guided by a microscopic scheme of photon interaction underlying
pattern formation in second-harmonic generation. Close to the threshold for
pattern formation, beams with opposite direction of the off-axis critical wave
numbers are shown to be highly correlated. This is observed for the fundamental
field, for the second harmonic field and also for the cross-correlation between
the two fields. Nonlinear correlations involving the homogeneous transverse
wave number, which are not identified in a linearized analysis, are also
described. The intensity differences between opposite points of the far fields
are shown to exhibit sub-Poissonian statistics, revealing the quantum nature of
the correlations. We observe twin beam correlations in both the fundamental and
second-harmonic fields, and also nonclassical correlations between them.Comment: 18 pages, 17 figures, submitted to Phys. Rev.
Approximate Deadline-Scheduling with Precedence Constraints
We consider the classic problem of scheduling a set of n jobs
non-preemptively on a single machine. Each job j has non-negative processing
time, weight, and deadline, and a feasible schedule needs to be consistent with
chain-like precedence constraints. The goal is to compute a feasible schedule
that minimizes the sum of penalties of late jobs. Lenstra and Rinnoy Kan
[Annals of Disc. Math., 1977] in their seminal work introduced this problem and
showed that it is strongly NP-hard, even when all processing times and weights
are 1. We study the approximability of the problem and our main result is an
O(log k)-approximation algorithm for instances with k distinct job deadlines
Soil quality: to regulate or to manage?
The concept of soil quality was conceived in the early 1990s as a parallel to those of air
and water quality in response to concerns about soil 'health,' sustainability and environmentally
`friendly' crop production. The concept has the potential to be used by researchers to link soil
research issues to broader environmental issues when applying for funds.
However, unlike air and water, soils have no defined 'pure' state against which measures can be taken
and comparisons made. The physical, chemical and biological composition of soils varies widely and
no single attribute or soil type can be established as a standard. The choice of appropriate soil
properties and their standards depends on the use to which the soil is put.
We suggest concentrating on quality management of the soil, rather than managing generically-chosen
soil properties, collectively called soil 'quality'. Quality management puts the onus on managers to
use the technical tools that are readily available to manage soils and landscapes, and on scientists to
develop new tools
What is "system": the information-theoretic arguments
The problem of "what is 'system'?" is in the very foundations of modern
quantum mechanics. Here, we point out the interest in this topic in the
information-theoretic context. E.g., we point out the possibility to manipulate
a pair of mutually non-interacting, non-entangled systems to employ
entanglement of the newly defined '(sub)systems' consisting the one and the
same composite system. Given the different divisions of a composite system into
"subsystems", the Hamiltonian of the system may perform in general
non-equivalent quantum computations. Redefinition of "subsystems" of a
composite system may be regarded as a method for avoiding decoherence in the
quantum hardware. In principle, all the notions refer to a composite system as
simple as the hydrogen atom.Comment: 13 pages, no figure
Stellar populations of classical and pseudo-bulges for a sample of isolated spiral galaxies
In this paper we present the stellar population synthesis results for a
sample of 75 bulges in isolated spiral Sb-Sc galaxies, using the spectroscopic
data from the Sloan Digital Sky Survey and the STARLIGHT code. We find that
both pseudo-bulges and classical bulges in our sample are predominantly
composed of old stellar populations, with mean mass-weighted stellar age around
10 Gyr. While the stellar population of pseudo-bulges is, in general, younger
than that of classical bulges, the difference is not significant, which
indicates that it is hard to distinguish pseudo-bulges from classical bulges,
at least for these isolated galaxies, only based on their stellar populations.
Pseudo-bulges have star formation activities with relatively longer timescale
than classical bulges, indicating that secular evolution is more important in
this kind of systems. Our results also show that pseudo-bulges have a lower
stellar velocity dispersion than their classical counterparts, which suggests
that classical bulges are more dispersion-supported than pseudo-bulges.Comment: 10 pages, 8 figures. Accepted for publication in Astrophysics & Space
Scienc
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