8,372 research outputs found
Causal evolution of spin networks
A new approach to quantum gravity is described which joins the loop
representation formulation of the canonical theory to the causal set
formulation of the path integral. The theory assigns quantum amplitudes to
special classes of causal sets, which consist of spin networks representing
quantum states of the gravitational field joined together by labeled null
edges. The theory exists in 3+1, 2+1 and 1+1 dimensional versions, and may also
be interepreted as a theory of labeled timelike surfaces. The dynamics is
specified by a choice of functions of the labelings of d+1 dimensional
simplices,which represent elementary future light cones of events in these
discrete spacetimes. The quantum dynamics thus respects the discrete causal
structure of the causal sets. In the 1+1 dimensional case the theory is closely
related to directed percolation models. In this case, at least, the theory may
have critical behavior associated with percolation, leading to the existence of
a classical limit.Comment: latex, 32 pages, 17 figure
Statistical Mechanics and Information-Theoretic Perspectives on Complexity in the Earth System
Peer reviewedPublisher PD
Stability analysis of coupled map lattices at locally unstable fixed points
Numerical simulations of coupled map lattices (CMLs) and other complex model
systems show an enormous phenomenological variety that is difficult to classify
and understand. It is therefore desirable to establish analytical tools for
exploring fundamental features of CMLs, such as their stability properties.
Since CMLs can be considered as graphs, we apply methods of spectral graph
theory to analyze their stability at locally unstable fixed points for
different updating rules, different coupling scenarios, and different types of
neighborhoods. Numerical studies are found to be in excellent agreement with
our theoretical results.Comment: 22 pages, 6 figures, accepted for publication in European Physical
Journal
Quantum Gravity: Has Spacetime Quantum Properties?
The incompatibility between GR and QM is generally seen as a sufficient
motivation for the development of a theory of Quantum Gravity. If - so a
typical argumentation - QM gives a universally valid basis for the description
of all natural systems, then the gravitational field should have quantum
properties. Together with the arguments against semi-classical theories of
gravity, this leads to a strategy which takes a quantization of GR as the
natural avenue to Quantum Gravity. And a quantization of the gravitational
field would in some sense correspond to a quantization of geometry. Spacetime
would have quantum properties. But, this strategy will only be successful, if
gravity is a fundamental interaction. - What, if gravity is instead an
intrinsically classical phenomenon? Then, if QM is nevertheless fundamentally
valid, gravity can not be a fundamental interaction. An intrinsically classical
gravity in a quantum world would have to be an emergent, induced or residual,
macroscopic effect, caused by other interactions. The gravitational field (as
well as spacetime) would not have any quantum properties. A quantization of GR
would lead to artifacts without any relation to nature. The serious problems of
all approaches to Quantum Gravity that start from a direct quantization of GR
or try to capture the quantum properties of gravity in form of a 'graviton'
dynamics - together with the, meanwhile, rich spectrum of approaches to an
emergent gravity and/or spacetime - make this latter option more and more
interesting for the development of a theory of Quantum Gravity. The most
advanced emergent gravity (and spacetime) scenarios are of an
information-theoretical, quantum-computational type.Comment: 31 page
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