2,799 research outputs found
Galactic Disk Warps
This review addresses recent developments in the field of disk galaxy warps.
Both results from a new HI survey of edgeon disk galaxies, and of simulations
of the interaction between a disk+halo and an orbiting satelite, will be
discussed.Comment: paper presented at ``Galaxy Disks and Disk Galaxies'', Rome, June
200
No unique solution to the seismological problem of standing kink MHD waves
The aim of this paper is to point out that the classic seismological problem
using observations and theoretical expressions for the periods and damping
times of transverse standing magnetohydrodynamic (MHD) waves in coronal loops
is better referred to as a reduced seismological problem. Reduced emphasises
the fact that only a small number of characteristic quantities of the
equilibrium profiles can be determined. Reduced also implies that there is no
unique solution to the full seismological problem. Even the reduced
seismological problem does not allow a unique solution. Bayesian inference
results support our mathematical arguments and offer insight into the
relationship between the algebraic and the probabilistic inversions.Comment: 10 pages, accepted in A&
Least Upper Bounds of the Powers Extracted and Scattered by Bi-anisotropic Particles
The least upper bounds of the powers extracted and scattered by
bi-anisotropic particles are investigated analytically. A rigorous derivation
for particles having invertible polarizability tensors is presented, and the
particles with singular polarizability tensors that have been reported in the
literature are treated explicitly. The analysis concludes that previous upper
bounds presented for isotropic particles can be extrapolated to bi-anisotropic
particles. In particular, it is shown that neither nonreciprocal nor
magnetoelectric coupling phenomena can further increase those upper bounds on
the extracted and scattered powers. The outcomes are illustrated further with
approximate circuit model examples of two dipole antennas connected via a
generic lossless network.Comment: 9 pages, 1 figure
Learning through play: an educational computer game to introduce radar fundamentals
The information exchange has evolved from traditional books to computers and Internet in a few years' time. Our current university students were born in this age: they learn and have fun with different methods as previous generations did. These digital natives enjoy computer games. Thus, designing games for learning some selected topics could be a good teaching strategy for such collective and also for undergraduate university students. This paper describes the development and test of an educational computer game revolving around radar. The objective of the game RADAR Technology is to teach students about the fundamentals of radar, while having fun during the learning experience. Based on the principle that you learn better what you practice, the authors want to induce students to discover a difficult to understand topic by proposing them a different experience, in a format better adapted to their generation skills. The computer game has been tested with actual students and the obtained results seem to be very promising
Gaussian process hyper-parameter estimation using parallel asymptotically independent Markov sampling
Gaussian process emulators of computationally expensive computer codes
provide fast statistical approximations to model physical processes. The
training of these surrogates depends on the set of design points chosen to run
the simulator. Due to computational cost, such training set is bound to be
limited and quantifying the resulting uncertainty in the hyper-parameters of
the emulator by uni-modal distributions is likely to induce bias. In order to
quantify this uncertainty, this paper proposes a computationally efficient
sampler based on an extension of Asymptotically Independent Markov Sampling, a
recently developed algorithm for Bayesian inference. Structural uncertainty of
the emulator is obtained as a by-product of the Bayesian treatment of the
hyper-parameters. Additionally, the user can choose to perform stochastic
optimisation to sample from a neighbourhood of the Maximum a Posteriori
estimate, even in the presence of multimodality. Model uncertainty is also
acknowledged through numerical stabilisation measures by including a nugget
term in the formulation of the probability model. The efficiency of the
proposed sampler is illustrated in examples where multi-modal distributions are
encountered. For the purpose of reproducibility, further development, and use
in other applications the code used to generate the examples is freely
available for download at https://github.com/agarbuno/paims_codesComment: Computational Statistics \& Data Analysis, Volume 103, November 201
Spin squeezing and entanglement for arbitrary spin
A complete set of generalized spin-squeezing inequalities is derived for an
ensemble of particles with an arbitrary spin. Our conditions are formulated
with the first and second moments of the collective angular momentum
coordinates. A method for mapping the spin-squeezing inequalities for spin-1/2
particles to entanglement conditions for spin-j particles is also presented. We
apply our mapping to obtain a generalization of the original spin-squeezing
inequality to higher spins. We show that, for large particle numbers, a
spin-squeezing parameter for entanglement detection based on one of our
inequalities is strictly stronger than the original spin-squeezing parameter
defined in [A. Sorensen et al., Nature 409, 63 (2001)]. We present a coordinate
system independent form of our inequalities that contains, besides the
correlation and covariance tensors of the collective angular momentum
operators, the nematic tensor appearing in the theory of spin nematics.
Finally, we discuss how to measure the quantities appearing in our inequalities
in experiments.Comment: 18 pages including 3 figures, revtex4; v2: references added, typos
corrected; v3: typos corrected, published versio
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