14,342 research outputs found
Non-Existence of Time-Periodic Solutions of the Dirac Equation in a Reissner-Nordstrom Black Hole Background
It is shown analytically that the Dirac equation has no normalizable,
time-periodic solutions in a Reissner-Nordstrom black hole background; in
particular, there are no static solutions of the Dirac equation in such a
background field. The physical interpretation is that Dirac particles can
either disappear into the black hole or escape to infinity, but they cannot
stay on a periodic orbit around the black hole.Comment: 24 pages, 2 figures (published version
Virtual noiseless amplification and Gaussian post-selection in continuous-variable quantum key distribution
The noiseless amplification or attenuation are two heralded filtering
operations that enable respectively to increase or decrease the mean field of
any quantum state of light with no added noise, at the cost of a small success
probability. We show that inserting such noiseless operations in a transmission
line improves the performance of continuous-variable quantum key distribution
over this line. Remarkably, these noiseless operations do not need to be
physically implemented but can simply be simulated in the data post-processing
stage. Hence, virtual noiseless amplification or attenuation amounts to perform
a Gaussian post-selection, which enhances the secure range or tolerable excess
noise while keeping the benefits of Gaussian security proofs.Comment: 8 pages, 5 figure
Towards Intelligent Databases
This article is a presentation of the objectives and techniques
of deductive databases. The deductive approach to databases aims at extending
with intensional definitions other database paradigms that describe
applications extensionaUy. We first show how constructive specifications can
be expressed with deduction rules, and how normative conditions can be defined
using integrity constraints. We outline the principles of bottom-up and
top-down query answering procedures and present the techniques used for
integrity checking. We then argue that it is often desirable to manage with
a database system not only database applications, but also specifications of
system components. We present such meta-level specifications and discuss
their advantages over conventional approaches
The Neural Representation Benchmark and its Evaluation on Brain and Machine
A key requirement for the development of effective learning representations
is their evaluation and comparison to representations we know to be effective.
In natural sensory domains, the community has viewed the brain as a source of
inspiration and as an implicit benchmark for success. However, it has not been
possible to directly test representational learning algorithms directly against
the representations contained in neural systems. Here, we propose a new
benchmark for visual representations on which we have directly tested the
neural representation in multiple visual cortical areas in macaque (utilizing
data from [Majaj et al., 2012]), and on which any computer vision algorithm
that produces a feature space can be tested. The benchmark measures the
effectiveness of the neural or machine representation by computing the
classification loss on the ordered eigendecomposition of a kernel matrix
[Montavon et al., 2011]. In our analysis we find that the neural representation
in visual area IT is superior to visual area V4. In our analysis of
representational learning algorithms, we find that three-layer models approach
the representational performance of V4 and the algorithm in [Le et al., 2012]
surpasses the performance of V4. Impressively, we find that a recent supervised
algorithm [Krizhevsky et al., 2012] achieves performance comparable to that of
IT for an intermediate level of image variation difficulty, and surpasses IT at
a higher difficulty level. We believe this result represents a major milestone:
it is the first learning algorithm we have found that exceeds our current
estimate of IT representation performance. We hope that this benchmark will
assist the community in matching the representational performance of visual
cortex and will serve as an initial rallying point for further correspondence
between representations derived in brains and machines.Comment: The v1 version contained incorrectly computed kernel analysis curves
and KA-AUC values for V4, IT, and the HT-L3 models. They have been corrected
in this versio
The Canis Major Dwarf Galaxy
Recent observational evidence suggests that the Sagittarius dwarf galaxy
represents the only major ongoing accretion event in the Galactic halo,
accounting for the majority of stellar debris identified there. This paper
summarizes the recent discovery of another potential Milky Way accretion event,
the Canis Major dwarf galaxy. This dwarf satellite galaxy is found to lie just
below the Galactic plane and appears to be on an equatorial orbit. Unlike
Sagittarius, which is contributing to the Galactic halo, the location and
eventual demise of Canis Major suggests that it represents a building block of
the thick disk.Comment: Refereed contribution to "Structure & Dynamics in the Local Universe,
a workshop to honour Brent Tully's 60th birthday", Nov 2003. 4 pages + 2
figures (quality reduced due to size restrictions). To appear in PAS
On the origin of variable structures in the winds of hot luminous stars
Examination of the temporal variability properties of several strong optical
recombination lines in a large sample of Galactic Wolf-Rayet (WR) stars reveals
possible trends, especially in the more homogeneous WC than the diverse WN
subtypes, of increasing wind variability with cooler subtypes. This could imply
that a serious contender for the driver of the variations is stochastic,
magnetic subsurface convection associated with the 170 kK partial-ionization
zone of iron, which should occupy a deeper and larger zone of greater mass in
cooler WR subtypes. This empirical evidence suggests that the heretofore
proposed ubiquitous driver of wind variability, radiative instabilities, may
not be the only mechanism playing a role in the stochastic multiple
small-scaled structures seen in the winds of hot luminous stars. In addition to
small-scale stochastic behaviour, subsurface convection guided by a global
magnetic field with localized emerging loops may also be at the origin of the
large-scale corotating interaction regions as seen frequently in O stars and
occasionally in the winds of their descendant WR stars.Comment: 8 pages, 2 figures and 2 tables. Monthly Notices of the Royal
Astronomical Society 201
Security of continuous-variable quantum key distribution against general attacks
We prove the security of Gaussian continuous-variable quantum key
distribution against arbitrary attacks in the finite-size regime. The novelty
of our proof is to consider symmetries of quantum key distribution in phase
space in order to show that, to good approximation, the Hilbert space of
interest can be considered to be finite-dimensional, thereby allowing for the
use of the postselection technique introduced by Christandl, Koenig and Renner
(Phys. Rev. Lett. 102, 020504 (2009)). Our result greatly improves on previous
work based on the de Finetti theorem which could not provide security for
realistic, finite-size, implementations.Comment: 5 pages, plus 11 page appendi
Superkicks in ultrarelativistic encounters of spinning black holes
We study ultrarelativistic encounters of two spinning, equal-mass black holes
through simulations in full numerical relativity. Two initial data sequences
are studied in detail: one that leads to scattering and one that leads to a
grazing collision and merger. In all cases, the initial black hole spins lie in
the orbital plane, a configuration that leads to the so-called "superkicks". In
astrophysical, quasicircular inspirals, such kicks can be as large as ~3,000
km/s; here, we find configurations that exceed ~15,000 km/s. We find that the
maximum recoil is to a good approximation proportional to the total amount of
energy radiated in gravitational waves, but largely independent of whether a
merger occurs or not. This shows that the mechanism predominantly responsible
for the superkick is not related to merger dynamics. Rather, a consistent
explanation is that the "bobbing" motion of the orbit causes an asymmetric
beaming of the radiation produced by the in-plane orbital motion of the binary,
and the net asymmetry is balanced by a recoil. We use our results to formulate
some conjectures on the ultimate kick achievable in any black hole encounter.Comment: 10 pages, 6 figures, 2 table
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