1,646 research outputs found
No-go theorem for bimetric gravity with positive and negative mass
We argue that the most conservative geometric extension of Einstein gravity
describing both positive and negative mass sources and observers is bimetric
gravity and contains two copies of standard model matter which interact only
gravitationally. Matter fields related to one of the metrics then appear dark
from the point of view of an observer defined by the other metric, and so may
provide a potential explanation for the dark universe. In this framework we
consider the most general form of linearized field equations compatible with
physically and mathematically well-motivated assumptions. Using gauge-invariant
linear perturbation theory, we prove a no-go theorem ruling out all bimetric
gravity theories that, in the Newtonian limit, lead to precisely opposite
forces on positive and negative test masses.Comment: 19 pages, no figures, journal versio
Multimetric extension of the PPN formalism: experimental consistency of repulsive gravity
Recently we discussed a multimetric gravity theory containing several copies
of standard model matter each of which couples to its own metric tensor. This
construction contained dark matter sectors interacting repulsively with the
visible matter sector, and was shown to lead to cosmological late-time
acceleration. In order to test the theory with high-precision experiments
within the solar system we here construct a simple extension of the
parametrized post-Newtonian (PPN) formalism for multimetric gravitational
backgrounds. We show that a simplified version of this extended formalism
allows the computation of a subset of the PPN parameters from the linearized
field equations. Applying the simplified formalism we find that the PPN
parameters of our theory do not agree with the observed values, but we are able
to improve the theory so that it becomes consistent with experiments of
post-Newtonian gravity and still features its promising cosmological
properties.Comment: 19 pages, no figures, journal versio
Bistable Gradient Networks II: Storage Capacity and Behaviour Near Saturation
We examine numerically the storage capacity and the behaviour near saturation
of an attractor neural network consisting of bistable elements with an
adjustable coupling strength, the Bistable Gradient Network (BGN). For strong
coupling, we find evidence of a first-order "memory blackout" phase transition
as in the Hopfield network. For weak coupling, on the other hand, there is no
evidence of such a transition and memorized patterns can be stable even at high
levels of loading. The enhanced storage capacity comes, however, at the cost of
imperfect retrieval of the patterns from corrupted versions.Comment: 15 pages, 12 eps figures. Submitted to Phys. Rev. E. Sequel to
cond-mat/020356
MYND: Unsupervised Evaluation of Novel BCI Control Strategies on Consumer Hardware
Neurophysiological studies are typically conducted in laboratories with
limited ecological validity, scalability, and generalizability of findings.
This is a significant challenge for the development of brain-computer
interfaces (BCIs), which ultimately need to function in unsupervised settings
on consumer-grade hardware. We introduce MYND: A framework that couples
consumer-grade recording hardware with an easy-to-use application for the
unsupervised evaluation of BCI control strategies. Subjects are guided through
experiment selection, hardware fitting, recording, and data upload in order to
self-administer multi-day studies that include neurophysiological recordings
and questionnaires. As a use case, we evaluate two BCI control strategies
("Positive memories" and "Music imagery") in a realistic scenario by combining
MYND with a four-channel electroencephalogram (EEG). Thirty subjects recorded
70.4 hours of EEG data with the system at home. The median headset fitting time
was 25.9 seconds, and a median signal quality of 90.2% was retained during
recordings.Neural activity in both control strategies could be decoded with an
average offline accuracy of 68.5% and 64.0% across all days. The repeated
unsupervised execution of the same strategy affected performance, which could
be tackled by implementing feedback to let subjects switch between strategies
or devise new strategies with the platform.Comment: 9 pages, 5 figures. Submitted to PNAS. Minor revisio
Unilateral Ovarian Abscess Caused by Salmonella
Background: Patients with unilateral ovarian abscesses due to Salmonella are rare. Case report: A 48-year-old woman with a left ovarian abscess caused by Salmonella group O7 is reported. Conclusion: In our patient, the ovary may have been seeded hematogenously by salmonellae and may have evolved into a local infection
Enhancement of Stochastic Resonance in distributed systems due to a selective coupling
Recent massive numerical simulations have shown that the response of a
"stochastic resonator" is enhanced as a consequence of spatial coupling.
Similar results have been analytically obtained in a reaction-diffusion model,
using "nonequilibrium potential" techniques. We now consider a field-dependent
diffusivity and show that the "selectivity" of the coupling is more efficient
for achieving stochastic-resonance enhancement than its overall value in the
constant-diffusivity case.Comment: 10 pgs (RevTex), 4 figures, submitted to Phys.Rev.Let
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