1,287 research outputs found

### Optimal generalization of power filters for gravitational wave bursts, from single to multiple detectors

Searches for gravitational wave signals which do not have a precise model
describing the shape of their waveforms are often performed using power
detectors based on a quadratic form of the data. A new, optimal method of
generalizing these power detectors so that they operate coherently over a
network of interferometers is presented. Such a mode of operation is useful in
obtaining better detection efficiencies, and better estimates of the position
of the source of the gravitational wave signal. Numerical simulations based on
a realistic, computationally efficient hierarchical implementation of the
method are used to characterize its efficiency, for detection and for position
estimation. The method is shown to be more efficient at detecting signals than
an incoherent approach based on coincidences between lists of events. It is
also shown to be capable of locating the position of the source.Comment: 16 pages, 5 figure

### Time-frequency detection algorithm for gravitational wave bursts

An efficient algorithm is presented for the identification of short bursts of
gravitational radiation in the data from broad-band interferometric detectors.
The algorithm consists of three steps: pixels of the time-frequency
representation of the data that have power above a fixed threshold are first
identified. Clusters of such pixels that conform to a set of rules on their
size and their proximity to other clusters are formed, and a final threshold is
applied on the power integrated over all pixels in such clusters. Formal
arguments are given to support the conjecture that this algorithm is very
efficient for a wide class of signals. A precise model for the false alarm rate
of this algorithm is presented, and it is shown using a number of
representative numerical simulations to be accurate at the 1% level for most
values of the parameters, with maximal error around 10%.Comment: 26 pages, 15 figures, to appear in PR

### Topological representations of matroid maps

The Topological Representation Theorem for (oriented) matroids states that
every (oriented) matroid can be realized as the intersection lattice of an
arrangement of codimension one homotopy spheres on a homotopy sphere. In this
paper, we use a construction of Engstr\"om to show that structure-preserving
maps between matroids induce topological mappings between their
representations; a result previously known only in the oriented case.
Specifically, we show that weak maps induce continuous maps and that the
process is a functor from the category of matroids with weak maps to the
homotopy category of topological spaces. We also give a new and conceptual
proof of a result regarding the Whitney numbers of the first kind of a matroid.Comment: Final version, 21 pages, 8 figures; Journal of Algebraic
Combinatorics, 201

### Optimal detection of burst events in gravitational wave interferometric observatories

We consider the problem of detecting a burst signal of unknown shape. We
introduce a statistic which generalizes the excess power statistic proposed by
Flanagan and Hughes and extended by Anderson et al. The statistic we propose is
shown to be optimal for arbitrary noise spectral characteristic, under the two
hypotheses that the noise is Gaussian, and that the prior for the signal is
uniform. The statistic derivation is based on the assumption that a signal
affects only affects N samples in the data stream, but that no other
information is a priori available, and that the value of the signal at each
sample can be arbitrary. We show that the proposed statistic can be implemented
combining standard time-series analysis tools which can be efficiently
implemented, and the resulting computational cost is still compatible with an
on-line analysis of interferometric data. We generalize this version of an
excess power statistic to the multiple detector case, also including the effect
of correlated noise. We give full details about the implementation of the
algorithm, both for the single and the multiple detector case, and we discuss
exact and approximate forms, depending on the specific characteristics of the
noise and on the assumed length of the burst event. As a example, we show what
would be the sensitivity of the network of interferometers to a delta-function
burst.Comment: 21 pages, 5 figures in 3 groups. Submitted for publication to
Phys.Rev.D. A Mathematica notebook is available at
http://www.ligo.caltech.edu/~avicere/nda/burst/Burst.nb which allows to
reproduce the numerical results of the pape

### Perfect mirrors and the self-accelerating box paradox

We consider the question raised by Unruh and Wald of whether mirrored boxes
can self-accelerate in flat spacetime (the ``self-accelerating box paradox'').
From the point of view of the box, which perceives the acceleration as an
impressed gravitational field, this is equivalent to asking whether the box can
be supported by the buoyant force arising from its immersion in a perceived
bath of thermal (Unruh) radiation. The perfect mirrors we study are of the type
that rely on light internal degrees of freedom which adjust to and reflect
impinging radiation. We suggest that a minimum of one internal mirror degree of
freedom is required for each bulk field degree of freedom reflected. A short
calculation then shows that such mirrors necessarily absorb enough heat from
the thermal bath that their increased mass prevents them from floating on the
thermal radiation. For this type of mirror the paradox is therefore resolved.
We also observe that this failure of boxes to ``float'' invalidates one of the
assumptions going into the Unruh-Wald analysis of entropy balances involving
boxes lowered adiabatically toward black holes. Nevertheless, their broad
argument can be maintained until the box reaches a new regime in which
box-antibox pairs dominate over massless fields as contributions to thermal
radiation.Comment: 11 pages, Revtex4, changes made in response to referee and to enhance
clarity, discussion of massive fields correcte

### Motion of Inertial Observers Through Negative Energy

Recent research has indicated that negative energy fluxes due to quantum
coherence effects obey uncertainty principle-type inequalities of the form
|\Delta E|\,{\Delta \tau} \lprox 1\,. Here $|\Delta E|$ is the magnitude of
the negative energy which is transmitted on a timescale $\Delta \tau$. Our main
focus in this paper is on negative energy fluxes which are produced by the
motion of observers through static negative energy regions. We find that
although a quantum inequality appears to be satisfied for radially moving
geodesic observers in two and four-dimensional black hole spacetimes, an
observer orbiting close to a black hole will see a constant negative energy
flux. In addition, we show that inertial observers moving slowly through the
Casimir vacuum can achieve arbitrarily large violations of the inequality. It
seems likely that, in general, these types of negative energy fluxes are not
constrained by inequalities on the magnitude and duration of the flux. We
construct a model of a non-gravitational stress-energy detector, which is
rapidly switched on and off, and discuss the strengths and weaknesses of such a
detector.Comment: 18pp + 1 figure(not included, available on request), in LATEX,
TUPT-93-

### Quantum Gravity effects near the null black hole singularity

The structure of the Cauchy Horizon singularity of a black hole formed in a
generic collapse is studied by means of a renormalization group equation for
quantum gravity. It is shown that during the early evolution of the Cauchy
Horizon the increase of the mass function is damped when quantum fluctuations
of the metric are taken into account.Comment: 15 Pages, one figure. Minor changes in the presentation, to appear on
Phys.Rev.

### Multi-parameter scaling of the Kondo effect in quantum dots with an even number of electrons

We address a recent theoretical discrepancy concerning the Kondo effect in
quantum dots with an even number of electrons where spin-singlet and -triplet
states are nearly degenerate. We show that the discrepancy arises from the fact
that the Kondo scaling involves many parameters, which makes the results depend
on concrete microscopic models. We illustrate this by the scaling calculations
of the Kondo temperature, $T_K$, as a function of the energy difference between
the singlet and triplet states $\Delta$. $T_K(\Delta)$ decreases with
increasing $\Delta$, showing a crossover from a power law with a universal
exponent to that with a nonuniversal exponent. The crossover depends on the
initial parameters of the model.Comment: 8 pages, 3 figure

### Thermodynamics of charged and rotating black strings

We study thermodynamics of cylindrically symmetric black holes. Uncharged as
well as charged and rotating objects have been discussed. We derive surface
gravity and hence the Hawking temperature and entropy for all these cases. We
correct some results in the literature and present new ones. It is seen that
thermodynamically these black configurations behave differently from
spherically symmetric objects

### Appearance and Stability of Anomalously Fluctuating States in Shor's Factoring Algorithm

We analyze quantum computers which perform Shor's factoring algorithm, paying
attention to asymptotic properties as the number L of qubits is increased.
Using numerical simulations and a general theory of the stabilities of
many-body quantum states, we show the following: Anomalously fluctuating states
(AFSs), which have anomalously large fluctuations of additive operators, appear
in various stages of the computation. For large L, they decohere at anomalously
great rates by weak noises that simulate noises in real systems. Decoherence of
some of the AFSs is fatal to the results of the computation, whereas
decoherence of some of the other AFSs does not have strong influence on the
results of the computation. When such a crucial AFS decoheres, the probability
of getting the correct computational result is reduced approximately
proportional to L^2. The reduction thus becomes anomalously large with
increasing L, even when the coupling constant to the noise is rather small.
Therefore, quantum computations should be improved in such a way that all AFSs
appearing in the algorithms do not decohere at such great rates in the existing
noises.Comment: 11 figures. A few discussions were added in verion 2. Version 3 is
the SAME as version 2; only errors during the Web-upload were fixed. Version
4 is the publised version, in which several typos are fixed and the reference
list is update

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