1,627 research outputs found
Analog black holes in flowing dielectrics
We show that a flowing dielectric medium with a linear response to an
external electric field can be used to generate an analog geometry that has
many of the formal properties of a Schwarzschild black hole for light rays, in
spite of birefringence. We also discuss the possibility of generating these
analog black holes in the laboratory.Comment: Revtex4 file, 7 pages, 4 eps figures, a few changes in presentation,
some references added, conclusions unchange
Self-Calibration Methods for Uncontrolled Environments in Sensor Networks: A Reference Survey
Growing progress in sensor technology has constantly expanded the number and
range of low-cost, small, and portable sensors on the market, increasing the
number and type of physical phenomena that can be measured with wirelessly
connected sensors. Large-scale deployments of wireless sensor networks (WSN)
involving hundreds or thousands of devices and limited budgets often constrain
the choice of sensing hardware, which generally has reduced accuracy,
precision, and reliability. Therefore, it is challenging to achieve good data
quality and maintain error-free measurements during the whole system lifetime.
Self-calibration or recalibration in ad hoc sensor networks to preserve data
quality is essential, yet challenging, for several reasons, such as the
existence of random noise and the absence of suitable general models.
Calibration performed in the field, without accurate and controlled
instrumentation, is said to be in an uncontrolled environment. This paper
provides current and fundamental self-calibration approaches and models for
wireless sensor networks in uncontrolled environments
And what if gravity is intrinsically quantic ?
Since the early days of search for a quantum theory of gravity the attempts
have been mostly concentrated on the quantization of an otherwise classical
system. The two most contentious candidate theories of gravity, sting theory
and quantum loop gravity are based on a quantum field theory - the latter is a
quantum field theory of connections on a SU(2) group manifold and former a
quantum field theory in two dimensional spaces. Here we argue that there is a
very close relation between quantum mechanics and gravity. Without gravity
quantum mechanics becomes ambiguous. We consider this observation as the
evidence for an intrinsic relation between these fundamental laws of nature. We
suggest a quantum role and definition for gravity in the context of a quantum
universe, and present a preliminary formulation for gravity in a system with a
finite number of particles.Comment: 8 pages, 1 figure. To appear in the proceedings of the DICE2008
conference, Castiglioncello, Tuscany, Italy, 22-26 Sep. 2008. V2: some typos
remove
Acoustic horizons for axially and spherically symmetric fluid flow
We investigate the formation of acoustic horizons for an inviscid fluid
moving in a pipe in the case of stationary and axi-symmetric flow. We show
that, differently from what is generally believed, the acoustic horizon forms
in correspondence of either a local minimum or maximum of the flux tube
cross-section. Similarly, the external potential is required to have either a
maximum or a minimum at the horizon, so that the external force has to vanish
there. Choosing a power-law equation of state for the fluid, , we solve the equations of the fluid dynamics and show that the two
possibilities are realized respectively for and . These results
are extended also to the case of spherically symmetric flow.Comment: 6 pages, 3 figure
Analog model for an expanding universe
Over the last few years numerous papers concerning analog models for gravity
have been published. It was shown that the dynamical equation of several
systems (e.g. Bose-Einstein condensates with a sink or a vortex) have the same
wave equation as light in a curved-space (e.g. black holes). In the last few
months several papers were released which deal with simulations of the
universe.
In this article the de-Sitter universe will be compared with a freely
expanding three-dimensional spherical Bose-Einstein condensate. Initially the
condensate is in a harmonic trap, which suddenly will be switched off. At the
same time a small perturbation will be injected in the center of the condensate
cloud.
The motion of the perturbation in the expanding condensate will be discussed,
and after some transformations the similarity to an expanding universe will be
shown.Comment: Presented at the 4th Australasian conference on General Relativity
and Cosmology, Monash U, Melbourne, 7-9 January 200
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