2,028 research outputs found
Exact Gravitational Wave Signatures from Colliding Extreme Black Holes
The low-energy dynamics of any system admitting a continuum of static
configurations is approximated by slow motion in moduli (configuration) space.
Here, following Ferrell and Eardley, this moduli space approximation is
utilized to study collisions of two maximally charged Reissner--Nordstr{\"o}m
black holes of arbitrary masses, and to compute analytically the gravitational
radiation generated by their scattering or coalescence. The motion remains slow
even though the fields are strong, and the leading radiation is quadrupolar. A
simple expression for the gravitational waveform is derived and compared at
early and late times to expectations.Comment: 6 page
Hairy Black Holes and Null Circular Geodesics
Einstein-matter theories in which hairy black-hole configurations have been
found are studied. We prove that the nontrivial behavior of the hair must
extend beyond the null circular orbit (the photonsphere) of the corresponding
spacetime. We further conjecture that the region above the photonsphere
contains at least 50% of the total hair's mass. We support this conjecture with
analytical and numerical results.Comment: 5 page
Universality in an integer Quantum Hall transition
An integer Quantum Hall effect transition is studied in a modulation doped
p-SiGe sample. In contrast to most examples of such transitions the
longitudinal and Hall conductivities at the critical point are close to 0.5 and
1.5 (e^2/h), the theoretically expected values. This allows the extraction of a
scattering parameter, describing both conductivity components, which depends
exponentially on filling factor. The strong similarity of this functional form
to those observed for transitions into the Hall insulating state and for the
B=0 metal- insulator transition implies a universal quantum critical behaviour
for the transitions. The observation of this behaviour in the integer Quantum
Hall effect, for this particular sample, is attributed to the short-ranged
character of the potential associated with the dominant scatterers
Privacy and Fairness in Recommender Systems via Adversarial Training of User Representations
Latent factor models for recommender systems represent users and items as low
dimensional vectors. Privacy risks of such systems have previously been studied
mostly in the context of recovery of personal information in the form of usage
records from the training data. However, the user representations themselves
may be used together with external data to recover private user information
such as gender and age. In this paper we show that user vectors calculated by a
common recommender system can be exploited in this way. We propose the
privacy-adversarial framework to eliminate such leakage of private information,
and study the trade-off between recommender performance and leakage both
theoretically and empirically using a benchmark dataset. An advantage of the
proposed method is that it also helps guarantee fairness of results, since all
implicit knowledge of a set of attributes is scrubbed from the representations
used by the model, and thus can't enter into the decision making. We discuss
further applications of this method towards the generation of deeper and more
insightful recommendations.Comment: International Conference on Pattern Recognition and Method
The Quantized Hall Insulator: A New Insulator in Two-Dimensions
Quite generally, an insulator is theoretically defined by a vanishing
conductivity tensor at the absolute zero of temperature. In classical
insulators, such as band insulators, vanishing conductivities lead to diverging
resistivities. In other insulators, in particular when a high magnetic field
(B) is added, it is possible that while the magneto-resistance diverges, the
Hall resistance remains finite, which is known as a Hall insulator. In this
letter we demonstrate experimentally the existence of another, more exotic,
insulator. This insulator, which terminates the quantum Hall effect series in a
two-dimensional electron system, is characterized by a Hall resistance which is
approximately quantized in the quantum unit of resistance h/e^2. This insulator
is termed a quantized Hall insulator. In addition we show that for the same
sample, the insulating state preceding the QHE series, at low-B, is of the HI
kind.Comment: 4 page
The fastest way to circle a black hole
Black-hole spacetimes with a "photonsphere", a hypersurface on which massless
particles can orbit the black hole on circular null geodesics, are studied. We
prove that among all possible trajectories (both geodesic and non-geodesic)
which circle the central black hole, the null circular geodesic is
characterized by the {\it shortest} possible orbital period as measured by
asymptotic observers. Thus, null circular geodesics provide the fastest way to
circle black holes. In addition, we conjecture the existence of a universal
lower bound for orbital periods around compact objects (as measured by
flat-space asymptotic observers): , where is the
mass of the central object. This bound is saturated by the null circular
geodesic of the maximally rotating Kerr black hole.Comment: 5 page
Evidence for a Finite Temperature Insulator
In superconductors the zero-resistance current-flow is protected from
dissipation at finite temperatures (T) by virtue of the short-circuit condition
maintained by the electrons that remain in the condensed state. The recently
suggested finite-T insulator and the "superinsulating" phase are different
because any residual mechanism of conduction will eventually become dominant as
the finite-T insulator sets-in. If the residual conduction is small it may be
possible to observe the transition to these intriguing states. We show that the
conductivity of the high magnetic-field insulator terminating superconductivity
in amorphous indium-oxide exhibits an abrupt drop, and seem to approach a zero
conductance at T<0.04 K. We discuss our results in the light of theories that
lead to a finite-T insulator
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