2,365 research outputs found
Quantum Gravity and Causal Structures: Second Quantization of Conformal Dirac Algebras
It is postulated that quantum gravity is a sum over causal structures coupled
to matter via scale evolution. Quantized causal structures can be described by
studying simple matrix models where matrices are replaced by an algebra of
quantum mechanical observables. In particular, previous studies constructed
quantum gravity models by quantizing the moduli of Laplace, weight and
defining-function operators on Fefferman-Graham ambient spaces. The algebra of
these operators underlies conformal geometries. We extend those results to
include fermions by taking an osp(1|2) "Dirac square root" of these algebras.
The theory is a simple, Grassmann, two-matrix model. Its quantum action is a
Chern-Simons theory whose differential is a first-quantized, quantum mechanical
BRST operator. The theory is a basic ingredient for building fundamental
theories of physical observables.Comment: 4 pages, LaTe
4D gravity on a brane from bulk higher-curvature terms
We study a gravity model where a tensionful codimension-one three-brane is
embedded on a bulk with infinite transverse length. We find that 4D gravity is
induced on the brane already at the classical level if we include
higher-curvature (Gauss-Bonnet) terms in the bulk. Consistency conditions
appear to require a negative brane tension as well as a negative coupling for
the higher-curvature terms.Comment: 10 pages, no figures; a minor change in wording (to appear in MPLA
Induced gravity on intersecting brane-worlds Part I: Maximally symmetric solutions
We explore models of intersecting brane-worlds with induced gravity terms on
codimension one branes and on their intersection. Maximally symmetric solutions
for the branes and the intersection are found. We find new self-accelerating
solutions. In a 6d spacetime, the solutions realize the see-saw modification of
gravity where the UV scale of the modification to 4d gravity is determined by
6d Planck scale given by eV and the IR scale of the
modification is determined by GeV where
is present-day Hubble scale. We find that it is increasingly difficult to
construct phenomenologically viable models in higher-dimensional spacetime due
to the necessity to have the lower value for the fundamental Planck scale to
realize the late time acceleration. It is found that the system also admits
self-tuning solutions where the tension at the intersection does not change the
geometry of the intersection. The induced gravity terms can avoid the necessity
to compactify the extra dimensions. Finally, we discuss the possibility to have
ordinary matter at the intersection, without introducing any regularisation,
using the induced gravity terms.Comment: 16 pages, some mistakes in the identification of the higher
codimensional singular structure corrected. Main results unchange
Fragile to strong crossover coupled to liquid-liquid transition in hydrophobic solutions
Using discrete molecular dynamics simulations we study the relation between
the thermodynamic and diffusive behaviors of a primitive model of aqueous
solutions of hydrophobic solutes consisting of hard spheres in the Jagla
particles solvent, close to the liquid-liquid critical point of the solvent. We
find that the fragile-to-strong dynamic transition in the diffusive behavior is
always coupled to the low-density/high-density liquid transition. Above the
liquid-liquid critical pressure, the diffusivity crossover occurs at the Widom
line, the line along which the thermodynamic response functions show maxima.
Below the liquid-liquid critical pressure, the diffusivity crossover occurs
when the limit of mechanical stability lines are crossed, as indicated by the
hysteresis observed when going from high to low temperature and vice versa.
These findings show that the strong connection between dynamics and
thermodynamics found in bulk water persists in hydrophobic solutions for
concentrations from low to moderate, indicating that experiments measuring the
relaxation time in aqueous solutions represent a viable route for solving the
open questions in the field of supercooled water.Comment: 6 pages, 4 figures. Accepted for publication on Physical Review
Quantum Darboux theorem
The problem of computing quantum mechanical propagators can be recast as a computation of a Wilson line operator for parallel transport by a flat connection acting on a vector bundle of wave functions. In this picture, the base manifold is an odd-dimensional symplectic geometry, or quite generically a contact manifold that can be viewed as a "phase-spacetime,"while the fibers are Hilbert spaces. This approach enjoys a "quantum Darboux theorem"that parallels the Darboux theorem on contact manifolds which turns local classical dynamics into straight lines. We detail how the quantum Darboux theorem works for anharmonic quantum potentials. In particular, we develop a novel diagrammatic approach for computing the asymptotics of a gauge transformation that locally makes complicated quantum dynamics trivial
Building a MultiAgent System from a User Workflow Specification
This paper provides a methodology to build
a MultiAgent System (MAS) described in terms of interactive
components from a domain-specic User Workow
Specication (UWS). We use a Petri nets-based notation
to describe workow specications. This, besides using a
familiar and well-studied notation, guarantees an highlevel
of description and independence with more concrete
vendor-specic process denition languages. In order to
bridge the gap between workow specications and MASs,
we exploit other intermediate Petri nets-based notations.
Transformation rules are given to translate a notation to
another. The generated agent-based application implements
the original workow specication. Run-time support is
provided by a middleware suitable for the execution of the
generated code
Urban Scale Monitoring Approach for the Assessment of Rising Damp Effects in Venice
In coastal areas, the rising damp of salty water is a well-known degradation factor of historical masonries, leading to visible features such as crusts, masonry erosion, and plaster loss. Venetian masonries are strongly affected by decay caused by rising damp exacerbated by direct contact with salty water. Recurrent flooding due to high tides and an increase in the frequency of flooding events, also related to climate change, raises concern about the impacts. Although several studies have been carried out on probable future scenarios, a valuation of the decay risk due to rising damp at the urban level still needs to be implemented. This paper proposes a non-invasive and economically sustainable approach for evaluating rising damp effects at an urban scale. The approach includes a collection of archive images of masonries affected by rising damp dating back to the 1990s; a visual survey of the actual conservation state of masonries; a classification based on significant descriptors; and a discussion on exposure conditions and conservation states. The descriptors chosen are rising damp levels, biological growth, plaster loss, efflorescence, and brick erosion. The evaluation was implemented in a georeferenced system suitable for future comparisons, thus providing a management tool for the city's preservation
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