18,023 research outputs found
Quantum Uncertainty Considerations for Gravitational Lens Interferometry
The measurement of the gravitational lens delay time between light paths has
relied, to date, on the source having sufficient variability to allow
photometric variations from each path to be compared. However, the delay times
of many gravitational lenses cannot be measured because the intrinsic source
amplitude variations are too small to be detectable. At the fundamental quantum
mechanical level, such photometric time stamps allow which-path knowledge,
removing the ability to obtain an interference pattern. However, if the two
paths can be made equal (zero time delay) then interference can occur. We
describe an interferometric approach to measuring gravitational lens delay
times using a quantum-eraser/restorer approach, whereby the time travel along
the two paths may be rendered measurably equal. Energy and time being
non-commuting observables, constraints on the photon energy in the energy-time
uncertainty principle, via adjustments of the width of the radio bandpass,
dictate the uncertainty of the time delay and therefore whether the path taken
along one or the other gravitational lens geodesic is knowable. If one starts
with interference, for example, which-path information returns when the
bandpass is broadened (constraints on the energy are relaxed) to the point
where the uncertainty principle allows a knowledge of the arrival time to
better than the gravitational lens delay time itself, at which point the
interference will disappear. We discuss the near-term feasibility of such
measurements in light of current narrow-band radio detectors and known short
time-delay gravitational lenses.Comment: 22 page
Discrete analogue computing with rotor-routers
Rotor-routing is a procedure for routing tokens through a network that can
implement certain kinds of computation. These computations are inherently
asynchronous (the order in which tokens are routed makes no difference) and
distributed (information is spread throughout the system). It is also possible
to efficiently check that a computation has been carried out correctly in less
time than the computation itself required, provided one has a certificate that
can itself be computed by the rotor-router network. Rotor-router networks can
be viewed as both discrete analogues of continuous linear systems and
deterministic analogues of stochastic processes.Comment: To appear in Chaos Special Focus Issue on Intrinsic and Designed
Computatio
Coherent-feedback quantum control with a dynamic compensator
I present an experimental realization of a coherent-feedback control system
that was recently proposed for testing basic principles of linear quantum
stochastic control theory [M. R. James, H. I. Nurdin and I. R. Petersen, to
appear in IEEE Transactions on Automatic Control (2008),
arXiv:quant-ph/0703150v2]. For a dynamical plant consisting of an optical
ring-resonator, I demonstrate ~ 7 dB broadband disturbance rejection of
injected laser signals via all-optical feedback with a tailored dynamic
compensator. Comparison of the results with a transfer function model pinpoints
critical parameters that determine the coherent-feedback control system's
performance.Comment: 4 pages, 4 EPS figure
Planetological implications of mass loss from the early Sun
The element lithium is observed to be underabundant in the Sun by a factor of approx. equal to 100. To account for this depletion, Boothroyd et al. (Ap. J., in press 1991) proposed a model in which the Sun's zero-age-main-sequence mass was approx. 1.1 solar magnitude. If this is the explanation for the lithium depletion, then astronomical observations of F/G dwarfs in clusters suggest that the timescale for mass loss is approx. equal to 0.6 Gyr. Assuming this approximate timescale, the authors investigated several planetological implications of the astrophysical model
VCE early acoustic test results of General Electric's high-radius ratio coannular plug nozzle
Results of variable cycle engine (VCE) early acoustic engine and model scale tests are presented. A summary of an extensive series of far field acoustic, advanced acoustic, and exhaust plume velocity measurements with a laser velocimeter of inverted velocity and temperature profile, high radius ratio coannular plug nozzles on a YJ101 VCE static engine test vehicle are reviewed. Select model scale simulated flight acoustic measurements for an unsuppressed and a mechanical suppressed coannular plug nozzle are also discussed. The engine acoustic nozzle tests verify previous model scale noise reduction measurements. The engine measurements show 4 to 6 PNdB aft quadrant jet noise reduction and up to 7 PNdB forward quadrant shock noise reduction relative to a fully mixed conical nozzle at the same specific thrust and mixed pressure ratio. The influences of outer nozzle radius ratio, inner stream velocity ratio, and area ratio are discussed. Also, laser velocimeter measurements of mean velocity and turbulent velocity of the YJ101 engine are illustrated. Select model scale static and simulated flight acoustic measurements are shown which corroborate that coannular suppression is maintained in forward speed
Plenary Panel Discussion: Challenges and opportunities for the future of control
This panel reflects the scope and diversity of the unprecedented challenges and opportunities for the systems and controls community that has been created by several research themes from the basic sciences to advanced technologies. Connecting physical processes at multiple time and space scales in quantum, statistical, fluid, and solid mechanics, remains not only a central scientific challenge but also one with increasing technological implications. This is particular so in highly organized and nonequilibrium systems as in biology and nanotechnology, where interconnection, feedback, and dynamics are playing an increasingly central role
Rayleigh scattering in the transmission spectrum of HAT-P-18b
We have performed ground-based transmission spectroscopy of the hot Jupiter
HAT-P-18b using the ACAM instrument on the William Herschel Telescope (WHT).
Differential spectroscopy over an entire night was carried out at a resolution
of using a nearby comparison star. We detect a bluewards slope
extending across our optical transmission spectrum which runs from 4750 to
9250\AA. The slope is consistent with Rayleigh scattering at the equilibrium
temperature of the planet (852K). We do not detect enhanced sodium absorption,
which indicates that a high-altitude haze is masking the feature and giving
rise to the Rayleigh slope. This is only the second discovery of a Rayleigh
scattering slope in a hot Jupiter atmosphere from the ground, and our study
illustrates how ground-based observations can provide transmission spectra with
precision comparable to the Hubble Space Telescope.Comment: 11 pages, 9 figures, accepted for publication in MNRA
Calculating effective resistances on underlying networks of association schemes
Recently, in Refs. \cite{jsj} and \cite{res2}, calculation of effective
resistances on distance-regular networks was investigated, where in the first
paper, the calculation was based on stratification and Stieltjes function
associated with the network, whereas in the latter one a recursive formula for
effective resistances was given based on the Christoffel-Darboux identity. In
this paper, evaluation of effective resistances on more general networks which
are underlying networks of association schemes is considered, where by using
the algebraic combinatoric structures of association schemes such as
stratification and Bose-Mesner algebras, an explicit formula for effective
resistances on these networks is given in terms of the parameters of
corresponding association schemes. Moreover, we show that for particular
underlying networks of association schemes with diameter such that the
adjacency matrix possesses distinct eigenvalues, all of the other
adjacency matrices , can be written as polynomials of ,
i.e., , where is not necessarily of degree . Then, we use
this property for these particular networks and assume that all of the
conductances except for one of them, say , are zero to give a
procedure for evaluating effective resistances on these networks. The
preference of this procedure is that one can evaluate effective resistances by
using the structure of their Bose-Mesner algebra without any need to know the
spectrum of the adjacency matrices.Comment: 41 page
A Dynamical Analysis of the Proposed Circumbinary HW Virginis Planetary System
In 2009, the discovery of two planets orbiting the evolved binary star system
HW Virginis was announced, based on systematic variations in the timing of
eclipses between the two stars. The planets invoked in that work were
significantly more massive than Jupiter, and moved on orbits that were mutually
crossing - an architecture which suggests that mutual encounters and strong
gravitational interactions are almost guaranteed. In this work, we perform a
highly detailed analysis of the proposed HW Vir planetary system. First, we
consider the dynamical stability of the system as proposed in the discovery
work. Through a mapping process involving 91,125 individual simulations, we
find that the system is so unstable that the planets proposed simply cannot
exist, due to mean lifetimes of less than a thousand years across the whole
parameter space. We then present a detailed re-analysis of the observational
data on HW Vir, deriving a new orbital solution that provides a very good fit
to the observational data. Our new analysis yields a system with planets more
widely spaced, and of lower mass, than that proposed in the discovery work, and
yields a significantly greater (and more realistic) estimate of the uncertainty
in the orbit of the outermost body. Despite this, a detailed dynamical analysis
of this new solution similarly reveals that it also requires the planets to
move on orbits that are simply not dynamically feasible. Our results imply that
some mechanism other than the influence of planetary companions must be the
principal cause of the observed eclipse timing variations for HW Vir. If the
sys- tem does host exoplanets, they must move on orbits differing greatly from
those previously proposed. Our results illustrate the critical importance of
performing dynamical analyses as a part of the discovery process for
multiple-planet exoplanetary systems.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Societ
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