22,658 research outputs found
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Supporting Computer-supported collaborative work (CSCW) in conceptual design
In order to gain a better understanding of online conceptual collaborative design processes this paper investigates how student designers make use of a shared virtual synchronous environment when engaged in conceptual design. The software enables users to talk to each other and share sketches when they are remotely located. The paper describes a novel methodology for observing and analysing collaborative design processes by adapting the concepts of grounded theory. Rather than concentrating on narrow aspects of the final artefacts, emerging “themes” are generated that provide a broader picture of collaborative design process and context descriptions. Findings on the themes of “grounding – mutual understanding” and “support creativity” complement findings from other research, while important themes associated with “near-synchrony” have not been emphasised in other research. From the study, a series of design recommendations are made for the development of tools to support online computer-supported collaborative work in design using a shared virtual environment
New Types of Thermodynamics from -Dimensional Black Holes
For normal thermodynamic systems superadditivity , homogeneity \H and
concavity \C of the entropy hold, whereas for -dimensional black holes
the latter two properties are violated. We show that -dimensional black
holes exhibit qualitatively new types of thermodynamic behaviour, discussed
here for the first time, in which \C always holds, \H is always violated
and may or may not be violated, depending of the magnitude of the black
hole mass. Hence it is now seen that neither superadditivity nor concavity
encapsulate the meaning of the second law in all situations.Comment: WATPHYS-TH93/05, Latex, 10 pgs. 1 figure (available on request), to
appear in Class. Quant. Gra
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Dietary manipulation of broiler breeder growth through the feeding of conjugated linoleic acid
Reactor antineutrino spectra and their application to antineutrino-induced reactions. II
The antineutrino and electron spectra associated with various nuclear fuels are calculated. While there are substantial differences between the spectra of different uranium and plutonium isotopes, the dependence on the energy and flux of the fission-inducing neutrons is very weak. The resulting spectra can be used for the calculation of the antineutrino and electron spectra of an arbitrary nuclear reactor at various stages of its refueling cycle. The sources of uncertainties in the spectrum are identified and analyzed in detail. The exposure time dependence of the spectrum is also discussed. The averaged cross sections of the inverse neutron β decay, weak charged and neutral-current-induced deuteron disintegration, and the antineutrino-electron scattering are then evaluated using the resulting ν̅_e spectra.
[RADIOACTIVITY, FISSION 235U, 238U, (^239)Pu, (^240)Pu, (^241)Pu, antineutrino and electron spectra calculated. σ for ν̅ induced reactions analyzed.
Symmetry Breaking Using Value Precedence
We present a comprehensive study of the use of value precedence constraints
to break value symmetry. We first give a simple encoding of value precedence
into ternary constraints that is both efficient and effective at breaking
symmetry. We then extend value precedence to deal with a number of
generalizations like wreath value and partial interchangeability. We also show
that value precedence is closely related to lexicographical ordering. Finally,
we consider the interaction between value precedence and symmetry breaking
constraints for variable symmetries.Comment: 17th European Conference on Artificial Intelligenc
Self-completeness and spontaneous dimensional reduction
A viable quantum theory of gravity is one of the biggest challenges facing
physicists. We discuss the confluence of two highly expected features which
might be instrumental in the quest of a finite and renormalizable quantum
gravity -- spontaneous dimensional reduction and self-completeness. The former
suggests the spacetime background at the Planck scale may be effectively
two-dimensional, while the latter implies a condition of maximal compression of
matter by the formation of an event horizon for Planckian scattering. We
generalize such a result to an arbitrary number of dimensions, and show that
gravity in higher than four dimensions remains self-complete, but in lower
dimensions it is not. In such a way we established an "exclusive disjunction"
or "exclusive or" (XOR) between the occurrence of self-completeness and
dimensional reduction, with the goal of actually reducing the unknowns for the
scenario of the physics at the Planck scale. Potential phenomenological
implications of this result are considered by studying the case of a
two-dimensional dilaton gravity model resulting from dimensional reduction of
Einstein gravity.Comment: 12 pages, 3 figures; v3: final version in press on Eur. Phys. J. Plu
Covariance and Time Regained in Canonical General Relativity
Canonical vacuum gravity is expressed in generally-covariant form in order
that spacetime diffeomorphisms be represented within its equal-time phase
space. In accordance with the principle of general covariance, the time mapping
{\T}: {\yman} \to {\rman} and the space mapping {\X}: {\yman} \to {\xman}
that define the Dirac-ADM foliation are incorporated into the framework of the
Hilbert variational principle. The resulting canonical action encompasses all
individual Dirac-ADM actions, corresponding to different choices of foliating
vacuum spacetimes by spacelike hypersurfaces. In this framework, spacetime
observables, namely, dynamical variables that are invariant under spacetime
diffeomorphisms, are not necessarily invariant under the deformations of the
mappings \T and \X, nor are they constants of the motion. Dirac observables
form only a subset of spacetime observables that are invariant under the
transformations of \T and \X and do not evolve in time. The conventional
interpretation of the canonical theory, due to Bergmann and Dirac, can be
recovered only by postulating that the transformations of the reference system
({\T},{\X}) have no measurable consequences. If this postulate is not deemed
necessary, covariant canonical gravity admits no classical problem of time.Comment: 41 pages, no figure
Escape path complexity and its context dependency in Pacific blue-eyes (Pseudomugil signifer)
The escape trajectories animals take following a predatory attack appear to
show high degrees of apparent 'randomness' - a property that has been described
as 'protean behaviour'. Here we present a method of quantifying the escape
trajectories of individual animals using a path complexity approach. When fish
(Pseudomugil signifer) were attacked either on their own or in groups, we find
that an individual's path rapidly increases in entropy (our measure of
complexity) following the attack. For individuals on their own, this entropy
remains elevated (indicating a more random path) for a sustained period (10
seconds) after the attack, whilst it falls more quickly for individuals in
groups. The entropy of the path is context dependent. When attacks towards
single fish come from greater distances, a fish's path shows less complexity
compared to attacks that come from short range. This context dependency effect
did not exist, however, when individuals were in groups. Nor did the path
complexity of individuals in groups depend on a fish's local density of
neighbours. We separate out the components of speed and direction changes to
determine which of these components contributes to the overall increase in path
complexity following an attack. We found that both speed and direction measures
contribute similarly to an individual's path's complexity in absolute terms.
Our work highlights the adaptive behavioural tactics that animals use to avoid
predators and also provides a novel method for quantifying the escape
trajectories of animals.Comment: 9 page
Warming of the Antarctic ice-sheet surface since the 1957 International Geophysical Year
Assessments of Antarctic temperature change have emphasized the contrast between strong warming of the Antarctic Peninsula and slight cooling of the Antarctic continental interior in recent decades. This pattern of temperature change has been attributed to the increased strength of the circumpolar westerlies, largely in response to changes in stratospheric ozone. This picture, however, is substantially incomplete owing to the sparseness and short duration of the observations. Here we show that significant warming extends well beyond the Antarctic Peninsula to cover most of West Antarctica, an area of warming much larger than previously reported. West Antarctic warming exceeds 0.1 °C per decade over the past 50 years, and is strongest in winter and spring. Although this is partly offset by autumn cooling in East Antarctica, the continent-wide average near-surface temperature trend is positive. Simulations using a general circulation model reproduce the essential features of the spatial pattern and the long-term trend, and we suggest that neither can be attributed directly to increases in the strength of the westerlies. Instead, regional changes in atmospheric circulation and associated changes in sea surface temperature and sea ice are required to explain the enhanced warming in West Antarctica
Composition of Jupiter irregular satellites sheds light on their origin
Irregular satellites of Jupiter with their highly eccentric, inclined and
distant orbits suggest that their capture took place just before the giant
planet migration. We aim to improve our understanding of the surface
composition of irregular satellites of Jupiter to gain insight into a narrow
time window when our Solar System was forming. We observed three Jovian
irregular satellites, Himalia, Elara, and Carme, using a medium-resolution
0.8-5.5 micro m spectrograph on the National Aeronautics and Space
Administration (NASA) Infrared Telescope Facility (IRTF). Using a linear
spectral unmixing model we have constrained the major mineral phases on the
surface of these three bodies. Our results confirm that the surface of Himalia,
Elara, and Carme are dominated by opaque materials such as those seen in
carbonaceous chondrite meteorites. Our spectral modeling of NIR spectra of
Himalia and Elara confirm that their surface composition is the same and
magnetite is the dominant mineral. A comparison of the spectral shape of
Himalia with the two large main C-type asteroids, Themis (D 176 km) and Europa
(D 352 km), suggests surface composition similar to Europa. The NIR spectrum of
Carme exhibits blue slope up to 1.5 microm and is spectrally distinct from
those of Himalia and Elara. Our model suggests that it is compositionally
similar to amorphous carbon. Himalia and Elara are compositionally similar but
differ significantly from Carme. These results support the hypotheses that the
Jupiter irregular satellites are captured bodies that were subject to further
breakup events and clustered as families based on their similar physical and
surface compositions
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