407 research outputs found
Quantizing speeds with the cosmological constant
Considering the Barrett-Crane spin foam model for quantum gravity with
(positive) cosmological constant, we show that speeds must be quantized and we
investigate the physical implications of this effect such as the emergence of
an effective deformed Poincare symmetry.Comment: 4 pages, revtex4, 3 figure
Coupling of spacetime atoms and spin foam renormalisation from group field theory
We study the issue of coupling among 4-simplices in the context of spin foam
models obtained from a group field theory formalism. We construct a
generalisation of the Barrett-Crane model in which an additional coupling
between the normals to tetrahedra, as defined in different 4-simplices that
share them, is present. This is realised through an extension of the usual
field over the group manifold to a five argument one. We define a specific
model in which this coupling is parametrised by an additional real parameter
that allows to tune the degree of locality of the resulting model,
interpolating between the usual Barrett-Crane model and a flat BF-type one.
Moreover, we define a further extension of the group field theory formalism in
which the coupling parameter enters as a new variable of the field, and the
action presents derivative terms that lead to modified classical equations of
motion. Finally, we discuss the issue of renormalisation of spin foam models,
and how the new coupled model can be of help regarding this.Comment: RevTeX, 18 pages, no figure
A simple background-independent hamiltonian quantum model
We study formulation and probabilistic interpretation of a simple
general-relativistic hamiltonian quantum system. The system has no unitary
evolution in background time. The quantum theory yields transition
probabilities between measurable quantities (partial observables). These
converge to the classical predictions in the limit. Our main tool
is the kernel of the projector on the solutions of Wheeler-deWitt equation,
which we analyze in detail. It is a real quantity, which can be seen as a
propagator that propagates "forward" as well as "backward" in a local parameter
time. Individual quantum states, on the other hand, may contain only "forward
propagating" components. The analysis sheds some light on the interpretation of
background independent transition amplitudes in quantum gravity
Axially and radially expandable modular helical soft actuator for robotic implantables
Soft robotics has advanced the field of biomedical engineering by creating safer technologies for interfacing with the human body. One of the challenges in this field is the realization of modular soft basic constituents and accessible assembly methods to increase the versatility of soft robots. We present a soft pneumatic actuator composed of two elastomeric strands that provide interdependent axial and radial expansion due to the modularity of the components and their helical arrangement. The actuator reaches 35% of elongation with respect to its initial height and both chambers achieve forces of 1N at about 19kPa. We describe the design, fabrication, modeling and benchtop testing of the soft actuator towards realizing 3D functional structures with potential medical applications. An example of application for soft medical robots is tissue regenerative for the long-gap esophageal atresia condition
Colored Group Field Theory
Group field theories are higher dimensional generalizations of matrix models.
Their Feynman graphs are fat and in addition to vertices, edges and faces, they
also contain higher dimensional cells, called bubbles. In this paper, we
propose a new, fermionic Group Field Theory, posessing a color symmetry, and
take the first steps in a systematic study of the topological properties of its
graphs. Unlike its bosonic counterpart, the bubbles of the Feynman graphs of
this theory are well defined and readily identified. We prove that this graphs
are combinatorial cellular complexes. We define and study the cellular homology
of this graphs. Furthermore we define a homotopy transformation appropriate to
this graphs. Finally, the amplitude of the Feynman graphs is shown to be
related to the fundamental group of the cellular complex
1/f Noise in Electron Glasses
We show that 1/f noise is produced in a 3D electron glass by charge
fluctuations due to electrons hopping between isolated sites and a percolating
network at low temperatures. The low frequency noise spectrum goes as
\omega^{-\alpha} with \alpha slightly larger than 1. This result together with
the temperature dependence of \alpha and the noise amplitude are in good
agreement with the recent experiments. These results hold true both with a
flat, noninteracting density of states and with a density of states that
includes Coulomb interactions. In the latter case, the density of states has a
Coulomb gap that fills in with increasing temperature. For a large Coulomb gap
width, this density of states gives a dc conductivity with a hopping exponent
of approximately 0.75 which has been observed in recent experiments. For a
small Coulomb gap width, the hopping exponent approximately 0.5.Comment: 8 pages, Latex, 6 encapsulated postscript figures, to be published in
Phys. Rev.
Thermal noise properties of two aging materials
In this lecture we review several aspects of the thermal noise properties in
two aging materials: a polymer and a colloidal glass.
The measurements have been performed after a quench for the polymer and
during the transition from a fluid-like to a solid-like state for the gel. Two
kind of noise has been measured: the electrical noise and the mechanical noise.
For both materials we have observed that the electric noise is characterized
by a strong intermittency, which induces a large violation of the Fluctuation
Dissipation Theorem (FDT) during the aging time, and may persist for several
hours at low frequency. The statistics of these intermittent signals and their
dependance on the quench speed for the polymer or on sample concentration for
the gel are studied. The results are in a qualitative agreement with recent
models of aging, that predict an intermittent dynamics. For the mechanical
noise the results are unclear. In the polymer the mechanical thermal noise is
still intermittent whereas for the gel the violation of FDT, if it exists, is
extremely small.Comment: to be published in the Proceedings of the XIX Sitges Conference on
''Jammming, Yielding and Irreversible Deformation in Condensed Matter'',
M.-C.Miguel and M. Rubi eds.,Springer Verlag, Berli
On the influence of a Coulomb-like potential induced by the Lorentz symmetry breaking effects on the Harmonic Oscillator
In this work, we obtain bound states for a nonrelativistic spin-half neutral
particle under the influence of a Coulomb-like potential induced by the Lorentz
symmetry breaking effects. We present a new possible scenario of studying the
Lorentz symmetry breaking effects on a nonrelativistic quantum system defined
by a fixed space-like vector field parallel to the radial direction interacting
with a uniform magnetic field along the z-axis. Furthermore, we also discuss
the influence of a Coulomb-like potential induced by Lorentz symmetry violation
effects on the two-dimensional harmonic oscillator.Comment: 14 pages, no figure, this work has been accepted for publication in
The European Physical Journal Plu
Fermions in three-dimensional spinfoam quantum gravity
We study the coupling of massive fermions to the quantum mechanical dynamics
of spacetime emerging from the spinfoam approach in three dimensions. We first
recall the classical theory before constructing a spinfoam model of quantum
gravity coupled to spinors. The technique used is based on a finite expansion
in inverse fermion masses leading to the computation of the vacuum to vacuum
transition amplitude of the theory. The path integral is derived as a sum over
closed fermionic loops wrapping around the spinfoam. The effects of quantum
torsion are realised as a modification of the intertwining operators assigned
to the edges of the two-complex, in accordance with loop quantum gravity. The
creation of non-trivial curvature is modelled by a modification of the pure
gravity vertex amplitudes. The appendix contains a review of the geometrical
and algebraic structures underlying the classical coupling of fermions to three
dimensional gravity.Comment: 40 pages, 3 figures, version accepted for publication in GER
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