21,673 research outputs found
Flow Motifs Reveal Limitations of the Static Framework to Represent Human interactions
Networks are commonly used to define underlying interaction structures where
infections, information, or other quantities may spread. Although the standard
approach has been to aggregate all links into a static structure, some studies
suggest that the time order in which the links are established may alter the
dynamics of spreading. In this paper, we study the impact of the time ordering
in the limits of flow on various empirical temporal networks. By using a random
walk dynamics, we estimate the flow on links and convert the original
undirected network (temporal and static) into a directed flow network. We then
introduce the concept of flow motifs and quantify the divergence in the
representativity of motifs when using the temporal and static frameworks. We
find that the regularity of contacts and persistence of vertices (common in
email communication and face-to-face interactions) result on little differences
in the limits of flow for both frameworks. On the other hand, in the case of
communication within a dating site (and of a sexual network), the flow between
vertices changes significantly in the temporal framework such that the static
approximation poorly represents the structure of contacts. We have also
observed that cliques with 3 and 4 vertices con- taining only low-flow links
are more represented than the same cliques with all high-flow links. The
representativity of these low-flow cliques is higher in the temporal framework.
Our results suggest that the flow between vertices connected in cliques depend
on the topological context in which they are placed and in the time sequence in
which the links are established. The structure of the clique alone does not
completely characterize the potential of flow between the vertices
On Clifford Subalgebras, Spacetime Splittings and Applications
Z2-gradings of Clifford algebras are reviewed and we shall be concerned with
an alpha-grading based on the structure of inner automorphisms, which is
closely related to the spacetime splitting, if we consider the standard
conjugation map automorphism by an arbitrary, but fixed, splitting vector.
After briefly sketching the orthogonal and parallel components of products of
differential forms, where we introduce the parallel [orthogonal] part as the
space [time] component, we provide a detailed exposition of the Dirac operator
splitting and we show how the differential operator parallel and orthogonal
components are related to the Lie derivative along the splitting vector and the
angular momentum splitting bivector. We also introduce multivectorial-induced
alpha-gradings and present the Dirac equation in terms of the spacetime
splitting, where the Dirac spinor field is shown to be a direct sum of two
quaternions. We point out some possible physical applications of the formalism
developed.Comment: 22 pages, accepted for publication in International Journal of
Geometric Methods in Modern Physics 3 (8) (2006
Quantum critical superconductors in string theory and M-theory
We construct zero-temperature solutions of supergravity theories in five and
four dimensions which interpolate between two copies of anti-de Sitter space,
one of which preserves an abelian gauge symmetry while the other breaks it.
These domain wall solutions can be lifted to solutions of type IIB string
theory and eleven-dimensional supergravity. They describe quantum critical
behavior and emergent relativistic conformal symmetry in a superfluid or
superconducting state of a strongly coupled dual gauge theory. We include
computations of frequency-dependent conductivities which exhibit power law
scaling in the infrared, with exponents determined by irrelevant perturbations
to the symmetry-breaking anti-de Sitter background.Comment: 5 pages, 3 figures. v2: References slightly improved, mentioned F^F
constrain
Study of models of the sine-Gordon type in flat and curved spacetime
We study a new family of models of the sine-Gordon type, starting from the
sine-Gordon model, including the double sine-Gordon, the triple one, and so on.
The models appears as deformations of the starting model, with the deformation
controlled by two parameters, one very small, used to control a linear
expansion on it, and the other, which specifies the particular model in the
family of models. We investigate the presence of topological defects, showing
how the solutions can be constructed explicitly from the topological defects of
the sine-Gordon model itself. In particular, we delve into the double
sine-Gordon model in a braneworld scenario with a single extra dimension of
infinite extent, showing that a stable gravity scenario is admissible. Also, we
briefly show that the deformation procedure can be used iteratively, leading to
a diversity of possibilities to construct families of models of the sine-Gordon
type.Comment: 8 pages, 7 figures; Title changed, author and new results included;
version to appear in EPJ
Bulk viscosity of strongly coupled plasmas with holographic duals
We explain a method for computing the bulk viscosity of strongly coupled
thermal plasmas dual to supergravity backgrounds supported by one scalar field.
Whereas earlier investigations required the computation of the leading
dissipative term in the dispersion relation for sound waves, our method
requires only the leading frequency dependence of an appropriate Green's
function in the low-frequency limit. With a scalar potential chosen to mimic
the equation of state of QCD, we observe a slight violation of the lower bound
on the ratio of the bulk and shear viscosities conjectured in arXiv:0708.3459.Comment: 33 pages, 3 figure
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