31 research outputs found
Multi-hazard risks in New York City
Megacities are predominantly
concentrated along coastlines, making them exposed to a diverse mix of
natural hazards. The assessment of climatic hazard risk to cities rarely has
captured the multiple interactions that occur in complex urban systems. We
present an improved method for urban multi-hazard risk assessment. We then
analyze the risk of New York City as a case study to apply enhanced methods
for multi-hazard risk assessment given the history of exposure to multiple
types of natural hazards which overlap spatially and, in some cases,
temporally in this coastal megacity. Our aim is to identify hotspots of
multi-hazard risk to support the prioritization of adaptation strategies that
can address multiple sources of risk to urban residents. We used
socioeconomic indicators to assess vulnerabilities and risks to three
climate-related hazards (i.e., heat waves, inland flooding and coastal flooding) at high spatial resolution.
The analysis incorporates local experts' opinions to identify sources of
multi-hazard risk and to weight indicators used in the multi-hazard risk
assessment. Results demonstrate the application of multi-hazard risk
assessment to a coastal megacity and show that spatial hotspots of
multi-hazard risk affect similar local residential communities along the
coastlines. Analyses suggest that New York City should prioritize adaptation
in coastal zones and consider possible synergies and/or trade-offs to
maximize impacts of adaptation and resilience interventions in the spatially
overlapping areas at risk of impacts from multiple hazards.</p
Quantization of static space-times
A 4-dimensional Lorentzian static space-time is equivalent to 3-dimensional
Euclidean gravity coupled to a massless Klein-field. By canonically quantizing
the coupling model in the framework of loop quantum gravity, we obtain a
quantum theory which actually describes quantized static space-times. The
Kinematical Hilbert space is the product of the Hilbert space of gravity with
that of imaginary scalar fields. It turns out that the Hamiltonian constraint
of the 2+1 model corresponds to a densely defined operator in the underlying
Hilbert space, and hence it is finite without renormalization. As a new point
of view, this quantized model might shed some light on a few physical problems
concerning quantum gravity.Comment: 14 pages, made a few modifications, added Journal-re
operator for canonical quantum gravity
We study the properties of operator on the kinematical
Hilbert space for canonical quantum gravity. Its complete spectrum
with respect to the spin network basis is obtained. It turns out that
is diagonalized in this basis, and it is a well defined
self-adjoint operator on . The same conclusions are also tenable on
the SU(2) gauge invariant Hilbert space with the gauge invariant spin network
basis.Comment: 10 pages, minor modefication, reference update
``Sum over Surfaces'' form of Loop Quantum Gravity
We derive a spacetime formulation of quantum general relativity from
(hamiltonian) loop quantum gravity. In particular, we study the quantum
propagator that evolves the 3-geometry in proper time. We show that the
perturbation expansion of this operator is finite and computable order by
order. By giving a graphical representation a' la Feynman of this expansion, we
find that the theory can be expressed as a sum over topologically inequivalent
(branched, colored) 2d surfaces in 4d. The contribution of one surface to the
sum is given by the product of one factor per branching point of the surface.
Therefore branching points play the role of elementary vertices of the theory.
Their value is determined by the matrix elements of the hamiltonian constraint,
which are known. The formulation we obtain can be viewed as a continuum version
of Reisenberger's simplicial quantum gravity. Also, it has the same structure
as the Ooguri-Crane-Yetter 4d topological field theory, with a few key
differences that illuminate the relation between quantum gravity and TQFT.
Finally, we suggests that certain new terms should be added to the hamiltonian
constraint in order to implement a ``crossing'' symmetry related to 4d
diffeomorphism invariance.Comment: Seriously revised version. LaTeX, with revtex and epsfi
A candidate for a background independent formulation of M theory
A class of background independent membrane field theories are studied, and
several properties are discovered which suggest that they may play a role in a
background independent form of M theory. The bulk kinematics of these theories
are described in terms of the conformal blocks of an algebra G on all oriented,
finite genus, two-surfaces. The bulk dynamics is described in terms of causal
histories in which time evolution is specified by giving amplitudes to certain
local changes of the states. Holographic observables are defined which live in
finite dimensional states spaces associated with boundaries in spacetime. We
show here that the natural observables in these boundary state spaces are, when
G is chosen to be Spin(D) or a supersymmetric extension of it, generalizations
of matrix model coordinates in D dimensions. In certain cases the bulk dynamics
can be chosen so the matrix model dynamics is recoverd for the boundary
observables. The bosonic and supersymmetric cases in D=3 and D=9 are studied,
and it is shown that the latter is, in a certain limit, related to the matrix
model formulation of M theory. This correspondence gives rise to a conjecture
concerning a background independent form of M theory in terms of which
excitations of the background independent membrane field theory that correspond
to strings and D0 branes are identified.Comment: Latex 46 pages, 21 figures, new results included which lead to a
modification of the statement of the basic conjecture. Presentation improve