934 research outputs found
Models for Discrete Quantum Gravity
We first discuss a framework for discrete quantum processes (DQP). It is
shown that the set of q-probability operators is convex and its set of extreme
elements is found. The property of consistency for a DQP is studied and the
quadratic algebra of suitable sets is introduced. A classical sequential growth
process is "quantized" to obtain a model for discrete quantum gravity called a
quantum sequential growth process (QSGP). Two methods for constructing concrete
examples of QSGP are provided.Comment: 15 pages which include 2 figures which were created using LaTeX and
contained in the fil
A Lorentzian Gromov-Hausdoff notion of distance
This paper is the first of three in which I study the moduli space of
isometry classes of (compact) globally hyperbolic spacetimes (with boundary). I
introduce a notion of Gromov-Hausdorff distance which makes this moduli space
into a metric space. Further properties of this metric space are studied in the
next papers. The importance of the work can be situated in fields such as
cosmology, quantum gravity and - for the mathematicians - global Lorentzian
geometry.Comment: 20 pages, 0 figures, submitted to Classical and quantum gravity,
seriously improved presentatio
Symmetries, Horizons, and Black Hole Entropy
Black holes behave as thermodynamic systems, and a central task of any
quantum theory of gravity is to explain these thermal properties. A statistical
mechanical description of black hole entropy once seemed remote, but today we
suffer an embarrassment of riches: despite counting very different states, many
inequivalent approaches to quantum gravity obtain identical results. Such
``universality'' may reflect an underlying two-dimensional conformal symmetry
near the horizon, which can be powerful enough to control the thermal
characteristics independent of other details of the theory. This picture
suggests an elegant description of the relevant degrees of freedom as
Goldstone-boson-like excitations arising from symmetry breaking by the
conformal anomaly.Comment: 6 pages; first prize essay, 2007 Gravity Research Foundation essay
contes
Numerical determination of entanglement entropy for a sphere
We apply Srednicki's regularization to extract the logarithmic term in the
entanglement entropy produced by tracing out a real, massless, scalar field
inside a three dimensional sphere in 3+1 flat spacetime. We find numerically
that the coefficient of the logarithm is -1/90 to 0.2 percent accuracy, in
agreement with an existing analytical result
Where are the degrees of freedom responsible for black hole entropy?
Considering the entanglement between quantum field degrees of freedom inside
and outside the horizon as a plausible source of black hole entropy, we address
the question: {\it where are the degrees of freedom that give rise to this
entropy located?} When the field is in ground state, the black hole area law is
obeyed and the degrees of freedom near the horizon contribute most to the
entropy. However, for excited state, or a superposition of ground state and
excited state, power-law corrections to the area law are obtained, and more
significant contributions from the degrees of freedom far from the horizon are
shown.Comment: 6 pages, 4 figures, Invited talk at Theory Canada III, Edmonton,
Alberta, Canada, June 16, 200
Is entanglement entropy proportional to area?
It is known that the entanglement entropy of a scalar field, found by tracing
over its degrees of freedom inside a sphere of radius , is
proportional to the area of the sphere (and not its volume). This suggests that
the origin of black hole entropy, also proportional to its horizon area, may
lie in the entanglement between the degrees of freedom inside and outside the
horizon. We examine this proposal carefully by including excited states, to
check probable deviations from the area law.Comment: 6 pages. Based on talk by S. Das at Theory Canada 1, Vancouver, 3
June, 2005. To be published in a special edition of the Canadian Journal of
Physics. Minor changes to match published versio
Causal Fermion Systems: A Quantum Space-Time Emerging from an Action Principle
Causal fermion systems are introduced as a general mathematical framework for
formulating relativistic quantum theory. By specializing, we recover earlier
notions like fermion systems in discrete space-time, the fermionic projector
and causal variational principles. We review how an effect of spontaneous
structure formation gives rise to a topology and a causal structure in
space-time. Moreover, we outline how to construct a spin connection and
curvature, leading to a proposal for a "quantum geometry" in the Lorentzian
setting. We review recent numerical and analytical results on the support of
minimizers of causal variational principles which reveal a "quantization
effect" resulting in a discreteness of space-time. A brief survey is given on
the correspondence to quantum field theory and gauge theories.Comment: 23 pages, LaTeX, 2 figures, footnote added on page
Tinted Semi-Transparent Solar Panels Allow Concurrent Production of Crops and Electricity on the Same Cropland
Agrivoltaics describes concurrent agricultural production of crops and photovoltaic generation of electricity on the same cropland. By using tinted semi-transparent solar panels, this study introduces a novel element to transform the concept of agrivoltaics from just solar-sharing to selective utilisation of different light wavelengths. Agrivoltaic growth of basil and spinach was tested. When compared with classical agriculture, and based on the feed-in-tariff of the experimental location, agrivoltaic co-generation of biomass and electricity is calculated to result in an estimated financial gross gain up to +2.5% for basil and +35% for spinach. Marketable biomass yields did not change significantly for basil, while a statistically significant loss was observed for spinach. This was accompanied by a relative increase in the protein content for both plants grown under agrivoltaic conditions.
Agrivoltaics implemented with tinted solar panels improved the biomass production per unit amount of solar radiation up to 68%, with up to 63% increase in the ratio of leaf and stem biomass to root. Agrivoltaics can enrich the portfolio of farmers, mitigate risks associated with climate, and vastly enhance global photovoltaics capacity without compromising agricultural production.Leverhulme Trust RPG-2015-393
Italian Ministry of University and Research (to co-author A Schievano
FRW cosmologies between chaos and integrability
A recent paper by Castagnino, Giacomini and Lara concludes that there is no
chaos in a conformally coupled closed Friedmann-Robertson-Walker universe,
which is in apparent contradiction with previous works. We point out that
although nonchaotic the quoted system is nonintegrable.Comment: Revtex, 2 pages, no figure
- …