3,550 research outputs found
Intelligent environments: a manifesto
We explain basic features of an emerging area called Intelligent Environments. We give a short overview on how
it has developed, what is the current state of the art and what are the challenges laying ahead. The aim of the
article is to make aware the Computer Science community of this new development, the differences with
previous dominant paradigms and the opportunities that this area offers to the scientific community and society
On supersymmetric interfaces for string theory
We construct the world-sheet interface which preserves space-time
supersymmetry in type II superstring theories in the Green-Schwarz formalism.
This is an analog of the conformal interface in two-dimensional conformal field
theory. We show that a class of the supersymmetric interfaces generates
T-dualities of type II theories, and that these interfaces have a geometrical
interpretation in the doubled target space. We compute the partition function
with a pair of the supersymmetric interfaces inserted, from which we read off
the spectrum of the modes coupled to the interfaces and the Casimir energy
between them. We also derive the transformation rules under which a set of
D-branes is transformed to another by the interface.Comment: 1+23 pages, 1 figure; (v2) added comments, made changes in
presentatio
Study of the interacting system NGC 6845
We present optical spectroscopy, B, V, R and I CCD photometry and VLA neutral
hydrogen observations of the interacting quartet NGC 6845, also know as Klemola
30. NGC 6845 A, the dominant component, sports a broad and bright tidal bridge
and a faint tidal tail, which bifurcate. The tidal bridge has a (B-I) color
bluer than that of NGC 6845 A inner disk. Five strong condensations, identified
as HII regions brighter than the brightest in our Galaxy, are found along the
tidal bridge, with the two most luminous located at the bridge tip. Two giant
HII regions, comparable to 30 Dor, are located where the tidal bridge and the
tidal tail joint the disk of NGC 6845 A. Since the age of the HII regions are
3-8 Myr, star formation has been occurring along the tidal bridge and the tidal
arm well after they had begun to be torn apart (>= 100 Myr). Satoh model
fitting to the rotation curve of the A component reveals a kinematical mass of
4.4(+-1.2)E11 M_sun, inside the central 12 kpc (H_0=75 km/s/Mpc). The HI
emission shows two components, a more massive one that belongs to NGC 6845 A,
and a second one associated to NGC 6845 B. We do not detect gas associated to
galaxies C and D. The total amount of HI is 1.4E10 M_sun, five time the HI
content of the Milky Way. The HI kinematics indicates an amount of dark matter
associated to the A component two times higher than the mass inside its central
12 kpc. The group kinematics indicates an M/L ~ 43(+-2) or M/L ~ 66(+-2) (solar
units), according to two different prescriptions for the internal absorption
correction. In spite of this difference, both values furnish similar group mass
(~1E13 M_sun). Although preliminary results on N-Body simulations indicate that
either B or C might well create a tidal feature like the bridge of the A
component, the collision with B appears to be more favourable.Comment: 24 pages, 11 JPEG figures, uses aastex.cls and natbib.sty (included).
To appear in the June/1999 issue of the Astronomical Journa
Penrose limits and Green-Schwarz strings
We discuss the Green-Schwarz action for type IIB strings in general
plane-wave backgrounds obtained as Penrose limits from any IIB supergravity
solutions with vanishing background fermions. Using the normal-coordinate
expansion in superspace, we prove that the light-cone action is necessarily
quadratic in the fermionic coordinates. This proof is valid for more general
pp-wave backgrounds under certain conditions. We also write down the complete
quadratic action for general bosonic on-shell backgrounds in a form in which
its geometrical meaning is manifest both in the Einstein and string frames.
When the dilaton and 1-form field strength are vanishing, and the other field
strengths are constant, our string-frame action reduces, up to conventions, to
the one which has been written down using the supercovariant derivative.Comment: 18 pages, latex, no figures; (v2) relation to ref.14 clarified; (v3)
typos corrected, minor change
Strings in AdS_3 and the SL(2,R) WZW Model. Part 2: Euclidean Black Hole
We consider the one-loop partition function for Euclidean BTZ black hole
backgrounds or equivalently thermal AdS_3 backgrounds which are quotients of
H_3 (Euclidean AdS_3). The one-loop partition function is modular invariant and
we can read off the spectrum which is consistent to that found in
hep-th/0001053. We see long strings and discrete states in agreement with the
expectations.Comment: 23 pages, 3 figure
Quantum Coherent String States in AdS_3 and SL(2,R) WZWN Model
In this paper we make the connection between semi-classical string
quantization and exact conformal field theory quantization of strings in 2+1
Anti de Sitter spacetime. More precisely, considering the WZWN model
corresponding to SL(2,R) and its covering group, we construct quantum {\it
coherent} string states, which generalize the ordinary coherent states of
quantum mechanics, and show that in the classical limit they correspond to
oscillating circular strings. After quantization, the spectrum is found to
consist of two parts: A continuous spectrum of low mass states (partly
tachyonic) fulfilling the standard spin-level condition necessary for unitarity
|j|< k/2, and a discrete spectrum of high mass states with asymptotic behaviour
m^2\alpha'\propto N^2 (N positive integer). The quantization condition for the
high mass states arises from the condition of finite positive norm of the
coherent string states, and the result agrees with our previous results
obtained using semi-classical quantization. In the k\to\infty limit, all the
usual properties of coherent or {\it quasi-classical} states are recovered. It
should be stressed that we consider the circular strings only for simplicity
and clarity, and that our construction can easily be used for other string
configurations too. We also compare our results with those obtained in the
recent preprint hep-th/0001053 by Maldacena and Ooguri.Comment: Misprints corrected. Final version to appear in Phys. Rev.
Local Inhomogeneity Effects on Nucleation Process in a High External Bias
Quantum nucleation processes in the presence of local moderate
inhomogeneities are studied theoretically at high biases. The quantum
nucleation rate Gamma is calculated for one-dimensional systems in a form Gamma
= A e^(-B/hbar) by using the `bounce' method. The bias-dependence of the
exponent B is shown to be changed by inhomogeneities. This change is explained
by the reduction of the effective spatial dimension of the system. By studying
the system-size dependence of the prefactor A, the condition for the appearance
of inhomogeneity effects is evaluated. Nucleation rates in thermal activation
regimes are also calculated, and compared with quantum tunneling regimes. For
higher-dimensional systems, it is shown that the local approximation of
inhomogeneity does not hold, and that spatial profiles of inhomogeneity become
important.Comment: 10 pages, 6 figure
Backflow and dissipation during the quantum decay of a metastable Fermi liquid
The particle current in a metastable Fermi liquid against a first-order phase
transition is calculated at zero temperature. During fluctuations of a droplet
of the stable phase, in accordance with the conservation law, not only does an
unperturbed current arise from the continuity at the boundary, but a backflow
is induced by the density response. Quasiparticles carrying these currents are
scattered by the boundary, yielding a dissipative backflow around the droplet.
An energy of the hydrodynamic mass flow of the liquid and a friction force
exerted on the droplet by the quasiparticles have been obtained in terms of a
potential of their interaction with the droplet.Comment: 5 pages (REVTeX), to be published in Phys. Rev.
Stability of vortex lines in liquid 3He-4He mixtures at zero temperature
At low temperatures and 3He concentrations below 6.6 %, there is experimental
evidence about the existence in liquid helium mixtures, of stable vortices with
3He-rich cores. When the system is either supersaturated or submitted to a
tensile strength, vortices lose stability becoming metastable and eventually
completely unstable, so that their cores freely expand. Within a density
functional approach, we have determined the pressure-3He concentration curve
along which this instability appears at zero temperature.Comment: Typeset using Revtex, 9 pages and 5 Postscript figure
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