21,929 research outputs found
Comments on Noncommutative Field Theories
We discuss some aspects of noncommutative quantum field theories obtained
from the Seiberg-Witten limit of string theories in the presence of an external
B-field. General properties of these theories are studied as well as the
phenomenological potential of noncommutative QED.Comment: 15 pages. Based on talks at the 9th Adriatic Meeting (Dubrovnik,
Croatia) and at String Phenomenology 2003 (Durham, UK). v2 references adde
Topics in String Theory and Quantum Gravity
These are the lecture notes for the Les Houches Summer School on Quantum
Gravity held in July 1992. The notes present some general critical assessment
of other (non-string) approaches to quantum gravity, and a selected set of
topics concerning what we have learned so far about the subject from string
theory. Since these lectures are long (133 A4 pages), we include in this
abstract the table of contents, which should help the user of the bulletin
board in deciding whether to latex and print the full file.
1-FIELD THEORETICAL APPROACH TO QUANTUM GRAVITY: Linearized gravity;
Supergravity; Kaluza-Klein theories; Quantum field theory and classical
gravity; Euclidean approach to Quantum Gravity; Canonical quantization of
gravity; Gravitational Instantons.
2-CONSISTENCY CONDITIONS: ANOMALIES: Generalities about anomalies; Spinors in
2n dimensions; When can we expect to find anomalies?; The Atiyah-Singer Index
Theorem and the computation of anomalies; Examples: Green-Schwarz cancellation
mechanism and Witten's SU(2) global anomaly.
3-STRING THEORY I. BOSONIC STRING: Bosonic string; Conformal Field Theory;
Quantization of the bosonic string; Interaction in string theory and the
characterization of the moduli space; Bosonic strings with background fields.
Stringy corrections to Einstein equations; Toroidal compactifications.
-duality; Operator formalism
4-STRING THEORY II. FERMIONIC STRINGS: Fermionic String; Heterotic String;
Strings at finite temperature; Is string theory finite?
5-OTHER DEVELOPMENTS AND CONCLUSIONS: String ``Phenomenology''; Black Holes
and Related SubjectsComment: 133 pages, 22 figures (not included, available upon request), LaTe
The influence of persuasion in opinion formation and polarization
We present a model that explores the influence of persuasion in a population
of agents with positive and negative opinion orientations. The opinion of each
agent is represented by an integer number that expresses its level of
agreement on a given issue, from totally against to totally in favor
. Same-orientation agents persuade each other with probability ,
becoming more extreme, while opposite-orientation agents become more moderate
as they reach a compromise with probability . The population initially
evolves to (a) a polarized state for , where opinions' distribution is
peaked at the extreme values , or (b) a centralized state for ,
with most opinions around . When , polarization lasts for a
time that diverges as , where is the population's size. Finally,
an extremist consensus ( or ) is reached in a time that scales as
for
Powering AGNs with super-critical black holes
We propose a novel mechanism for powering the central engines of Active
Galactic Nuclei through super-critical (type II) black hole collapse. In this
picture, ~ of material collapsing at relativistic speeds can
trigger a gravitational shock, which can eject a large percentage of the
collapsing matter at relativistic speeds, leaving behind a "light" black hole.
In the presence of a poloidal magnetic field, the plasma collimates along two
jets, and the associated electron synchrotron radiation can easily account for
the observed radio luminosities, sizes and durations of AGN jets. For Lorentz
factors of order 100 and magnetic fields of a few hundred , synchrotron
electrons can shine for yrs, producing jets of sizes of order 100 kpc.
This mechanism may also be relevant for Gamma Ray Bursts and, in the absence of
magnetic field, supernova explosions.Comment: 4 pages, 1 figur
Interacting social processes on interconnected networks
We propose and study a model for the interplay between two different
dynamical processes --one for opinion formation and the other for decision
making-- on two interconnected networks and . The opinion dynamics on
network corresponds to that of the M-model, where the state of each agent
can take one of four possible values (), describing its level of
agreement on a given issue. The likelihood to become an extremist ()
or a moderate () is controlled by a reinforcement parameter .
The decision making dynamics on network is akin to that of the
Abrams-Strogatz model, where agents can be either in favor () or against
() the issue. The probability that an agent changes its state is
proportional to the fraction of neighbors that hold the opposite state raised
to a power . Starting from a polarized case scenario in which all agents
of network hold positive orientations while all agents of network have
a negative orientation, we explore the conditions under which one of the
dynamics prevails over the other, imposing its initial orientation. We find
that, for a given value of , the two-network system reaches a consensus
in the positive state (initial state of network ) when the reinforcement
overcomes a crossover value , while a negative consensus happens
for . In the phase space, the system displays a
transition at a critical threshold , from a coexistence of both
orientations for to a dominance of one orientation for
. We develop an analytical mean-field approach that gives an
insight into these regimes and shows that both dynamics are equivalent along
the crossover line .Comment: 25 pages, 6 figure
Gluon Saturation and Black Hole Criticality
We discuss the recent proposal in hep-th/0611312 where it was shown that the
critical anomalous dimension associated to the onset of non-linear effects in
the high energy limit of QCD coincides with the critical exponent governing the
radius of the black hole formed in the spherically symmetric collapse of a
massless scalar field. We argue that a new essential ingredient in this mapping
between gauge theory and gravity is continuous self-similarity, not present in
the scalar field case but in the spherical collapse of a perfect fluid with
barotropic equation of state. We identify this property with geometric scaling,
present in DIS data at small values of Bjorken x. We also show that the
Choptuik exponent in dimension five tends to the QCD critical value in the
traceless limit of the energy momentum tensor.Comment: Talk given at 12th International Conference on Elastic and
Diffractive Scattering: Forward Physics and QCD, Hamburg, DESY, Germany,
21-25 May 200
Agent Based Models of Language Competition: Macroscopic descriptions and Order-Disorder transitions
We investigate the dynamics of two agent based models of language
competition. In the first model, each individual can be in one of two possible
states, either using language or language , while the second model
incorporates a third state XY, representing individuals that use both languages
(bilinguals). We analyze the models on complex networks and two-dimensional
square lattices by analytical and numerical methods, and show that they exhibit
a transition from one-language dominance to language coexistence. We find that
the coexistence of languages is more difficult to maintain in the Bilinguals
model, where the presence of bilinguals in use facilitates the ultimate
dominance of one of the two languages. A stability analysis reveals that the
coexistence is more unlikely to happen in poorly-connected than in fully
connected networks, and that the dominance of only one language is enhanced as
the connectivity decreases. This dominance effect is even stronger in a
two-dimensional space, where domain coarsening tends to drive the system
towards language consensus.Comment: 30 pages, 11 figure
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