15,790 research outputs found
Dinosaur tracks from the Kilmaluag Formation (Bathonian, Middle Jurassic) of Score Bay, Isle of Skye, Scotland, UK
Tracks of a juvenile theropod dinosaur with footprint lengths of between 2 and 9 cm as well as adults of the same ichnospecies with footprints of about 15–25 cm in length were found in the Bathonian (Middle Jurassic) Kilmaluag Formation of Score Bay, northwestern Trotternish Peninsula, Isle of Skye, Scotland, UK. Two footprint sizes occur together on the same bedding plane in the central portion of Score Bay, both in situ and on loose blocks. Another horizon containing footprints above this was also identified. The footprints from the lowest horizon were produced in a desiccated silty mud that was covered with sand. A close association of both adults and juveniles with similar travel direction indicated by the footprints may suggest post-hatching care in theropod dinosaurs. Other footprints, produced on a rippled sandy substrate, have been found on the slightly higher bedding plane at this locality. Loose blocks found 130 m to the northeast in the central part of Score Bay have not been correlated with any in situ sediments, but were preserved in a similar manner to those from the higher bedding plane. These tracks represent the youngest dinosaur remains yet found in Scotland
Dynamical Evolution of the Scalar Condensate in Heavy Ion Collisions
We derive the effective coarse-grained field equation for the scalar
condensate of the linear sigma model in a simple and straightforward manner
using linear response theory. The dissipative coefficient is calculated at tree
level on the basis of the physical processes of sigma-meson decay and of
thermal sigma-mesons and pions knocking sigma-mesons out of the condensate. The
field equation is solved for hot matter undergoing either one or three
dimensional expansion and cooling in the aftermath of a high energy nuclear
collision. The results show that the time constant for returning the scalar
condensate to thermal equilibrium is of order 2 fm/c.Comment: 19 pages, 3 figures are embedded at the end. The effect of the time
dependence of the condensate v is included in this revised version. Numerical
work is redone accordingl
The Representation of Natural Numbers in Quantum Mechanics
This paper represents one approach to making explicit some of the assumptions
and conditions implied in the widespread representation of numbers by composite
quantum systems. Any nonempty set and associated operations is a set of natural
numbers or a model of arithmetic if the set and operations satisfy the axioms
of number theory or arithmetic. This work is limited to k-ary representations
of length L and to the axioms for arithmetic modulo k^{L}. A model of the
axioms is described based on states in and operators on an abstract L fold
tensor product Hilbert space H^{arith}. Unitary maps of this space onto a
physical parameter based product space H^{phy} are then described. Each of
these maps makes states in H^{phy}, and the induced operators, a model of the
axioms. Consequences of the existence of many of these maps are discussed along
with the dependence of Grover's and Shor's Algorithms on these maps. The
importance of the main physical requirement, that the basic arithmetic
operations are efficiently implementable, is discussed. This conditions states
that there exist physically realizable Hamiltonians that can implement the
basic arithmetic operations and that the space-time and thermodynamic resources
required are polynomial in L.Comment: Much rewrite, including response to comments. To Appear in Phys. Rev.
Topological Entanglement Entropy from the Holographic Partition Function
We study the entropy of chiral 2+1-dimensional topological phases, where
there are both gapped bulk excitations and gapless edge modes. We show how the
entanglement entropy of both types of excitations can be encoded in a single
partition function. This partition function is holographic because it can be
expressed entirely in terms of the conformal field theory describing the edge
modes. We give a general expression for the holographic partition function, and
discuss several examples in depth, including abelian and non-abelian fractional
quantum Hall states, and p+ip superconductors. We extend these results to
include a point contact allowing tunneling between two points on the edge,
which causes thermodynamic entropy associated with the point contact to be lost
with decreasing temperature. Such a perturbation effectively breaks the system
in two, and we can identify the thermodynamic entropy loss with the loss of the
edge entanglement entropy. From these results, we obtain a simple
interpretation of the non-integer `ground state degeneracy' which is obtained
in 1+1-dimensional quantum impurity problems: its logarithm is a
2+1-dimensional topological entanglement entropy.Comment: 16 pages, 2 figure
Molecular Tracers of the Central 12 pc of the Galactic Center
We have used the BIMA array to image the Galactic Center with a 19-pointing
mosaic in HCN(1-0), HCO+(1-0), and H 42-alpha emission with 5 km/s velocity
resolution and 13'' x 4'' angular resolution. The 5' field includes the
circumnuclear ring (CND) and parts of the 20 and 50 km/s clouds. HCN(1-0) and
HCO+ trace the CND and nearby giant molecular clouds while the H 42-alpha
emission traces the ionized gas in Sgr A West. We find that the CND has a
definite outer edge in HCN and HCO+ emission at ~45'' radius and appears to be
composed of two or three distinct streams of molecular gas rotating around the
nucleus. Outside the CND, HCN and HCO+ trace dense clumps of high-velocity gas
in addition to optically thick emission from the 20 and 50 km/s clouds. A
molecular ridge of compressed gas and dust, traced in NH3 emission and
self-absorbed HCN and HCO+, wraps around the eastern edge of Sgr A East. Just
inside this ridge are several arcs of gas which have been accelerated by the
impact of Sgr A East with the 50 km/s cloud. HCN and HCO+ emission trace the
extension of the northern arm of Sgr A West which appears to be an independent
stream of neutral and ionized gas and dust originating outside the CND. Broad
line widths and OH maser emission mark the intersection of the northern arm and
the CND. Comparison to previous NH3 and 1.2mm dust observations shows that HCN
and HCO+ preferentially trace the CND and are weaker tracers of the GMCs than
NH3 and dust. We discuss possible scenarios for the emission mechanisms and
environment at the Galactic center which could explain the differences in these
images.Comment: 24 pages, including 17 figures; to appear in The Astrophysical
Journa
Hormonal Signal Amplification Mediates Environmental Conditions during Development and Controls an Irreversible Commitment to Adulthood
Many animals can choose between different developmental fates to maximize fitness. Despite the complexity of environmental cues and life history, different developmental fates are executed in a robust fashion. The nematode Caenorhabditis elegans serves as a powerful model to examine this phenomenon because it can adopt one of two developmental fates (adulthood or diapause) depending on environmental conditions. The steroid hormone dafachronic acid (DA) directs development to adulthood by regulating the transcriptional activity of the nuclear hormone receptor DAF-12. The known role of DA suggests that it may be the molecular mediator of environmental condition effects on the developmental fate decision, although the mechanism is yet unknown. We used a combination of physiological and molecular biology techniques to demonstrate that commitment to reproductive adult development occurs when DA levels, produced in the neuroendocrine XXX cells, exceed a threshold. Furthermore, imaging and cell ablation experiments demonstrate that the XXX cells act as a source of DA, which, upon commitment to adult development, is amplified and propagated in the epidermis in a DAF-12 dependent manner. This positive feedback loop increases DA levels and drives adult programs in the gonad and epidermis, thus conferring the irreversibility of the decision. We show that the positive feedback loop canalizes development by ensuring that sufficient amounts of DA are dispersed throughout the body and serves as a robust fate-locking mechanism to enforce an organism-wide binary decision, despite noisy and complex environmental cues. These mechanisms are not only relevant to C. elegans but may be extended to other hormonal-based decision-making mechanisms in insects and mammals
Mixtures in non stable Levy processes
We analyze the Levy processes produced by means of two interconnected classes
of non stable, infinitely divisible distribution: the Variance Gamma and the
Student laws. While the Variance Gamma family is closed under convolution, the
Student one is not: this makes its time evolution more complicated. We prove
that -- at least for one particular type of Student processes suggested by
recent empirical results, and for integral times -- the distribution of the
process is a mixture of other types of Student distributions, randomized by
means of a new probability distribution. The mixture is such that along the
time the asymptotic behavior of the probability density functions always
coincide with that of the generating Student law. We put forward the conjecture
that this can be a general feature of the Student processes. We finally analyze
the Ornstein--Uhlenbeck process driven by our Levy noises and show a few
simulation of it.Comment: 28 pages, 3 figures, to be published in J. Phys. A: Math. Ge
Creating quanta with "annihilation" operator
An asymmetric nature of the boson `destruction' operator and its
`creation' partner is made apparent by applying them to a
quantum state different from the Fock state . We show that it is
possible to {\em increase} (by many times or by any quantity) the mean number
of quanta in the new `photon-subtracted' state . Moreover, for
certain `hyper-Poissonian' states the mean number of quanta in the
(normalized) state can be much greater than in the
`photon-added' state . The explanation of this
`paradox' is given and some examples elucidating the meaning of Mandel's
-parameter and the exponential phase operators are considered.Comment: 10 pages, LaTex, an extended version with several references added
and the text divided into sections; to appear in J. Phys.
The strong coupling, unification, and recent data
The prediction of the strong coupling assuming (supersymmetric) coupling
constant unification is reexamined. We find, using the new electroweak data,
. The implications of the large
value are discussed. The role played by the beauty width is
stressed. It is also emphasized that high-energy (but not low-energy)
corrections could significantly diminish the prediction. However, unless
higher-dimension operators are assumed to be suppressed, at present one cannot
place strong constraints on the super-heavy spectrum. Non-leading electroweak
threshold corrections are also discussed.Comment: 12 pages, LaTex + RevTex, uuencoded postscript file (including 13
figures) is attached. Also available at ftp://dept.physics.upenn.edu/pub/Ni
Designing electronic collaborative learning environments
Electronic collaborative learning environments for learning and working are in vogue. Designers design them according to their own constructivist interpretations of what collaborative learning is and what it should achieve. Educators employ them with different educational approaches and in diverse situations to achieve different ends. Students use them, sometimes very enthusiastically, but often in a perfunctory way. Finally, researchers study them and—as is usually the case when apples and oranges are compared—find no conclusive evidence as to whether or not they work, where they do or do not work, when they do or do not work and, most importantly, why, they do or do not work. This contribution presents an affordance framework for such collaborative learning environments; an interaction design procedure for designing, developing, and implementing them; and an educational affordance approach to the use of tasks in those environments. It also presents the results of three projects dealing with these three issues
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