3,950 research outputs found
Sonic analog of gravitational black holes in Bose-Einstein condensates
It is shown that, in dilute-gas Bose-Einstein condensates, there exist both
dynamically stable and unstable configurations which, in the hydrodynamic
limit, exhibit a behavior resembling that of gravitational black holes. The
dynamical instabilities involve creation of quasiparticle pairs in positive and
negative energy states, as in the well-known suggested mechanism for black hole
evaporation. We propose a scheme to generate a stable sonic black hole in a
ring trap.Comment: RevTeX 3.1, 1 figure, 4 page
Detection of SiO emission from a massive dense cold core
We report the detection of the SiO (J = 2 - 1) transition from the massive
cold dense core G333.125-0.562. The core remains undetected at wavelengths
shorter than 70 micron and has compact 1.2 mm dust continuum. The SiO emission
is localised to the core. The observations are part of a continuing
multi-molecular line survey of the giant molecular cloud G333. Other detected
molecules in the core include 13CO, C18O, CS, HCO+, HCN, HNC, CH3OH, N2H+, SO,
HC3N, NH3, and some of their isotopes. In addition, from NH3 (1,1) and (2,2)
inversion lines, we obtain a temperature of 13 K. From fitting to the spectral
energy distribution we obtain a colour temperature of 18 K and a gas mass of 2
x 10^3 solar mass. We have also detected a 22 GHz water maser in the core,
together with methanol maser emission, suggesting the core will host massive
star formation. We hypothesise that the SiO emission arises from shocks
associated with an outflow in the cold core.Comment: 6 pages, 4 figures, 1 table, to be published in MNRA
A non-resonant dark-side solution to the solar neutrino problem
We re-analyse spin-flavour precession solutions to the solar neutrino problem
in the light of the recent SNO CC result as well as the 1258--day
Super-Kamiokande data and the upper limit on solar anti-neutrinos. In a
self-consistent magneto-hydrodynamics approach the resulting scheme has only 3
effective parameters: , and the neutrino mixing angle
. We show how a rates-only analysis for fixed slightly
favours spin-flavour precession (SFP) solutions over oscillations (OSC). In
addition to the resonant solution (RSFP for short), there is a new non-resonant
solution (NRSFP) in the ``dark-side''. Both RSFP and NRSFP lead to flat recoil
energy spectra in excellent agreement with the latest SuperKamiokande data. We
also show that in the presence of a neutrino transition magnetic moment of
Bohr magneton, a magnetic field of 80 KGauss eliminates all large
mixing solutions other than the so-called LMA solution.Comment: 12 pages, 3 postscript figures, using elsart.cls. Published versio
An Improved Approximate Consensus Algorithm in the Presence of Mobile Faults
This paper explores the problem of reaching approximate consensus in
synchronous point-to-point networks, where each pair of nodes is able to
communicate with each other directly and reliably. We consider the mobile
Byzantine fault model proposed by Garay '94 -- in the model, an omniscient
adversary can corrupt up to nodes in each round, and at the beginning of
each round, faults may "move" in the system (i.e., different sets of nodes may
become faulty in different rounds). Recent work by Bonomi et al. '16 proposed a
simple iterative approximate consensus algorithm which requires at least
nodes. This paper proposes a novel technique of using "confession" (a mechanism
to allow others to ignore past behavior) and a variant of reliable broadcast to
improve the fault-tolerance level. In particular, we present an approximate
consensus algorithm that requires only nodes, an
improvement over the state-of-the-art algorithms.
Moreover, we also show that the proposed algorithm is optimal within a family
of round-based algorithms
The Minimal Length and Large Extra Dimensions
Planck scale physics represents a future challenge, located between particle
physics and general relativity. The Planck scale marks a threshold beyond which
the old description of spacetime breaks down and conceptually new phenomena
must appear. Little is known about the fundamental theory valid at Planckian
energies, except that it necessarily seems to imply the occurrence of a minimal
length scale, providing a natural ultraviolet cutoff and a limit to the
possible resolution of spacetime.
Motivated by String Theory, the models of large extra dimensions lower the
Planck scale to values soon accessible. These models predict a vast number of
quantum gravity effects at the lowered Planck scale, among them the production
of TeV-mass black holes and gravitons. Within the extra dimensional scenario,
also the minimal length comes into the reach of experiment and sets a
fundamental limit to short distance physics.
We review the status of Planck scale physics in these effective models.Comment: 18 pages, 5 figures, brief review to appear in Mod. Phys. Let.
Efficient UC Commitment Extension with Homomorphism for Free (and Applications)
Homomorphic universally composable (UC) commitments allow for the sender to reveal the result of additions and multiplications of values contained in commitments without revealing the values themselves while assuring the receiver of the correctness of such computation on committed values.
In this work, we construct essentially optimal additively homomorphic UC commitments from any (not necessarily UC or homomorphic) extractable commitment. We obtain amortized linear computational complexity in the length of the input messages and rate 1.
Next, we show how to extend our scheme to also obtain multiplicative homomorphism at the cost of asymptotic optimality but retaining low concrete complexity for practical parameters.
While the previously best constructions use UC oblivious transfer as the main building block, our constructions only require extractable commitments and PRGs, achieving better concrete efficiency and offering new insights into the sufficient conditions for obtaining homomorphic UC commitments.
Moreover, our techniques yield public coin protocols, which are compatible with the Fiat-Shamir heuristic.
These results come at the cost of realizing a restricted version of the homomorphic commitment functionality where the sender is allowed to perform any number of commitments and operations on committed messages but is only allowed to perform a single batch opening of a number of commitments.
Although this functionality seems restrictive, we show that it can be used as a building block for more efficient instantiations of recent protocols for secure multiparty computation and zero knowledge non-interactive arguments of knowledge
Dynamics for a 2-vertex Quantum Gravity Model
We use the recently introduced U(N) framework for loop quantum gravity to
study the dynamics of spin network states on the simplest class of graphs: two
vertices linked with an arbitrary number N of edges. Such graphs represent two
regions, in and out, separated by a boundary surface. We study the algebraic
structure of the Hilbert space of spin networks from the U(N) perspective. In
particular, we describe the algebra of operators acting on that space and
discuss their relation to the standard holonomy operator of loop quantum
gravity. Furthermore, we show that it is possible to make the restriction to
the isotropic/homogeneous sector of the model by imposing the invariance under
a global U(N) symmetry. We then propose a U(N) invariant Hamiltonian operator
and study the induced dynamics. Finally, we explore the analogies between this
model and loop quantum cosmology and sketch some possible generalizations of
it.Comment: 28 pages, v2: typos correcte
Quantum time uncertainty in Schwarzschild-anti-de Sitter black holes
The combined action of gravity and quantum mechanics gives rise to a minimum
time uncertainty in the lowest order approximation of a perturbative scheme, in
which quantum effects are regarded as corrections to the classical spacetime
geometry. From the nonperturbative point of view, both gravity and quantum
mechanics are treated on equal footing in a description that already contains
all possible backreaction effects as those above in a nonlinear manner. In this
paper, the existence or not of such minimum time uncertainty is analyzed in the
context of Schwarzschild-anti-de Sitter black holes using the isolated horizon
formalism. We show that from a perturbative point of view, a nonzero time
uncertainty is generically present owing to the energy scale introduced by the
cosmological constant, while in a quantization scheme that includes
nonperturbatively the effects of that scale, an arbitrarily high time
resolution can be reached.Comment: 10 pages, version published in Physical Review
Fairness and Efficiency in DAG-based Cryptocurrencies
Bitcoin is a decentralised digital currency that serves as an alternative to
existing transaction systems based on an external central authority for
security. Although Bitcoin has many desirable properties, one of its
fundamental shortcomings is its inability to process transactions at high
rates. To address this challenge, many subsequent protocols either modify the
rules of block acceptance (longest chain rule) and reward, or alter the
graphical structure of the public ledger from a tree to a directed acyclic
graph (DAG). Motivated by these approaches, we introduce a new general
framework that captures ledger growth for a large class of DAG-based
implementations. With this in hand, and by assuming honest miner behaviour, we
(experimentally) explore how different DAG-based protocols perform in terms of
fairness, i.e., if the block reward of a miner is proportional to their hash
power, as well as efficiency, i.e. what proportion of user transactions a
ledger deems valid after a certain length of time. Our results demonstrate
fundamental structural limits on how well DAG-based ledger protocols cope with
a high transaction load. More specifically, we show that even in a scenario
where every miner on the system is honest in terms of when they publish blocks,
what they point to, and what transactions each block contains, fairness and
efficiency of the ledger can break down at specific hash rates if miners have
differing levels of connectivity to the P2P network sustaining the protocol
Dynamics of a Massive Binary at Birth
Almost all massive stars have bound stellar companions, existing in binaries
or higher-order multiples. While binarity is theorized to be an essential
feature of how massive stars form, essentially all information about such
properties is derived from observations of already formed stars, whose orbital
properties may have evolved since birth. Little is known about binarity during
formation stages. Here we report high angular resolution observations of 1.3 mm
continuum and H30alpha recombination line emission, which reveal a massive
protobinary with apparent separation of 180 au at the center of the massive
star-forming region IRAS07299-1651. From the line-of-sight velocity difference
of 9.5 km/s of the two protostars, the binary is estimated to have a minimum
total mass of 18 solar masses, consistent with several other metrics, and
maximum period of 570 years, assuming a circular orbit. The H30alpha line from
the primary protostar shows kinematics consistent with rotation along a ring of
radius of 12 au. The observations indicate that disk fragmentation at several
hundred au may have formed the binary, and much smaller disks are feeding the
individual protostars.Comment: Published in Nature Astronomy. This is author's version. Full article
is available here (https://rdcu.be/brENk). 47 pages, 10 figures, including
methods and supplementary informatio
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