3,157 research outputs found
Canonical formalism for simplicial gravity
We summarise a recently introduced general canonical formulation of discrete
systems which is fully equivalent to the covariant formalism. This framework
can handle varying phase space dimensions and is applied to simplicial gravity
in particular.Comment: 4 pages, 5 figures, based on a talk given at Loops '11 in Madrid, to
appear in Journal of Physics: Conference Series (JPCS
Hot bang states of massless fermions
According to the characterization of local thermal equilibrium states in Local Quantum Physics proposed by Buchholz et al. microscopic and corresponding macroscopic observables are computed for the model of massless, free fermions on Minkowski space. An example for a local equilibrium state describing a hot bang is given, the main step being the proof of its positivity
Breaking and restoring of diffeomorphism symmetry in discrete gravity
We discuss the fate of diffeomorphism symmetry in discrete gravity.
Diffeomorphism symmetry is typically broken by the discretization. This has
repercussions for the observable content and the canonical formulation of the
theory. It might however be possible to construct discrete actions, so--called
perfect actions, with exact symmetries and we will review first steps towards
this end.Comment: to appear in the Proceedings of the XXV Max Born Symposium "The
Planck Scale", Wroclaw, 29 June - 3 July, 200
Socioeconomic impact of photovoltaic power at Schuchuli, Arizona
The social and economic impact of photovoltaic power on a small, remote native American village is studied. Village history, group life, energy use in general, and the use of photovoltaic-powered appliances are discussed. No significant impacts due to the photovoltaic power system were observed
Operator Spin Foams: holonomy formulation and coarse graining
A dual holonomy version of operator spin foam models is presented, which is
particularly adapted to the notion of coarse graining. We discuss how this
leads to a natural way of comparing models on different discretization scales,
and a notion of renormalization group flow on the partially ordered set of
2-complexes.Comment: 5 pages, 3 figures, to appear in Journal of Physics: Conference
Series. (JPCS
From the discrete to the continuous - towards a cylindrically consistent dynamics
Discrete models usually represent approximations to continuum physics.
Cylindrical consistency provides a framework in which discretizations mirror
exactly the continuum limit. Being a standard tool for the kinematics of loop
quantum gravity we propose a coarse graining procedure that aims at
constructing a cylindrically consistent dynamics in the form of transition
amplitudes and Hamilton's principal functions. The coarse graining procedure,
which is motivated by tensor network renormalization methods, provides a
systematic approximation scheme towards this end. A crucial role in this coarse
graining scheme is played by embedding maps that allow the interpretation of
discrete boundary data as continuum configurations. These embedding maps should
be selected according to the dynamics of the system, as a choice of embedding
maps will determine a truncation of the renormalization flow.Comment: 22 page
From covariant to canonical formulations of discrete gravity
Starting from an action for discretized gravity we derive a canonical
formalism that exactly reproduces the dynamics and (broken) symmetries of the
covariant formalism. For linearized Regge calculus on a flat background --
which exhibits exact gauge symmetries -- we derive local and first class
constraints for arbitrary triangulated Cauchy surfaces. These constraints have
a clear geometric interpretation and are a first step towards obtaining
anomaly--free constraint algebras for canonical lattice gravity. Taking higher
order dynamics into account the symmetries of the action are broken. This
results in consistency conditions on the background gauge parameters arising
from the lowest non--linear equations of motion. In the canonical framework the
constraints to quadratic order turn out to depend on the background gauge
parameters and are therefore pseudo constraints. These considerations are
important for connecting path integral and canonical quantizations of gravity,
in particular if one attempts a perturbative expansion.Comment: 37 pages, 5 figures (minor modifications, matches published version +
updated references
Approximating the physical inner product of Loop Quantum Cosmology
In this article, we investigate the possibility of approximating the physical inner product of constrained quantum theories. In particular, we calculate the physical inner product of a simple cosmological model in two ways: Firstly, we compute it analytically via a trick, secondly, we use the complexifier coherent states to approximate the physical inner product defined by the master constraint of the system. We will find that the approximation is able to recover the analytic solution of the problem, which solidifies hopes that coherent states will help to approximate solutions of more complicated theories, like loop quantum gravity
Brain computer interfaces: psychology and pragmatic perspectives for the future.
Whilst technologies, such as psychophysiological
measurements in general and electroencephalograms (EEG) in
particular, have been around and continually improving for many years, future technologies promise to revolutionise the emerging Information Society through the development of brain-computer interfaces and augmented cognition solutions. This paper explores critical psychological and pragmatic issues that must be understood before these technologies can deliver their potential well. Within the context of HCI, we examined a sample (n =105) BCI papers and found that the majority of research aimed to provide communication and control resources to people with
disabilities or with extreme task demands. However, the concepts of usability and accessibility, and respective findings from their substantial research literatures were rarely applied explicitly but referenced implicitly. While this suggests an increased awareness of these concepts and the related large research literatures, the task remains to sharpen these concepts and to articulate their obvious relevance to BCI work
Gauge-invariant coherent states for loop quantum gravity: I. Abelian gauge groups
In this paper, we investigate the properties of gauge-invariant coherent states for loop quantum gravity, for the gauge group U(1). This is done by projecting the corresponding complexifier coherent states defined by Thiemann and Winkler to the gauge-invariant Hilbert space. This being the first step toward constructing physical coherent states, we arrive at a set of gauge-invariant states that approximate well the gauge-invariant degrees of freedom of Abelian loop quantum gravity (LQG). Furthermore, these states turn out to encode explicit information about the graph topology, and show the same pleasant peakedness properties known from the gauge-variant complexifier coherent states. In a companion paper, we will turn to the more sophisticated case of SU(2)
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