28,260 research outputs found
Gravitons and Lightcone Fluctuations II: Correlation Functions
A model of a fluctuating lightcone due to a bath of gravitons is further
investigated. The flight times of photons between a source and a detector may
be either longer or shorter than the light propagation time in the background
classical spacetime, and will form a Gaussian distribution centered around the
classical flight time. However, a pair of photons emitted in rapid succession
will tend to have correlated flight times. We derive and discuss a correlation
function which describes this effect. This enables us to understand more fully
the operational significance of a fluctuating lightcone. Our results may be
combined with observational data on pulsar timing to place some constraints on
the quantum state of cosmological gravitons.Comment: 16 pages and two figures, uses eps
Nonlinear transformat
A technique for designing automatic flight controllers for aircraft which utilizes the transformation theory of nonlinear systems to linear systems is presently being developed at NASA Ames Research Center. A method is considered in which a given nonlinear is transformed to a controllable linear system in Brunovsky canonical form. A linear approximation is introduced to the nonlinear system called the modified tangent model. This model is easily computed. Constructing the transformation for this model enables the designer to find an approximate transformation for the nonlinear system
The Effects of Stress Tensor Fluctuations upon Focusing
We treat the gravitational effects of quantum stress tensor fluctuations. An
operational approach is adopted in which these fluctuations produce
fluctuations in the focusing of a bundle of geodesics. This can be calculated
explicitly using the Raychaudhuri equation as a Langevin equation. The physical
manifestation of these fluctuations are angular blurring and luminosity
fluctuations of the images of distant sources. We give explicit results for the
case of a scalar field on a flat background in a thermal state.Comment: 26 pages, 1 figure, new material added in Sect. III and in Appendices
B and
Disentanglement and Decoherence without dissipation at non-zero temperatures
Decoherence is well understood, in contrast to disentanglement. According to
common lore, irreversible coupling to a dissipative environment is the
mechanism for loss of entanglement. Here, we show that, on the contrary,
disentanglement can in fact occur at large enough temperatures even for
vanishingly small dissipation (as we have shown previously for decoherence).
However, whereas the effect of on decoherence increases exponentially with
time, the effect of on disentanglement is constant for all times,
reflecting a fundamental difference between the two phenomena. Also, the
possibility of disentanglement at a particular increases with decreasing
initial entanglement.Comment: 3 page
Restrictions on Negative Energy Density in Flat Spacetime
In a previous paper, a bound on the negative energy density seen by an
arbitrary inertial observer was derived for the free massless, quantized scalar
field in four-dimensional Minkowski spacetime. This constraint has the form of
an uncertainty principle-type limitation on the magnitude and duration of the
negative energy density. That result was obtained after a somewhat complicated
analysis. The goal of the current paper is to present a much simpler method for
obtaining such constraints. Similar ``quantum inequality'' bounds on negative
energy density are derived for the electromagnetic field, and for the massive
scalar field in both two and four-dimensional Minkowski spacetime.Comment: 17 pages, including two figures, uses epsf, minor revisions in the
Introduction, conclusions unchange
A simple algorithm for computing canonical forms
It is well known that all linear time-invariant controllable systems can be transformed to Brunovsky canonical form by a transformation consisting only of coordinate changes and linear feedback. However, the actual procedures for doing this have tended to be overly complex. The technique introduced here is envisioned as an on-line procedure and is inspired by George Meyer's tangent model for nonlinear systems. The process utilizes Meyer's block triangular form as an intermedicate step in going to Brunovsky form. The method also involves orthogonal matrices, thus eliminating the need for the computation of matrix inverses. In addition, the Kronecker indices can be computed as a by-product of this transformation so it is necessary to know them in advance
Averaged Energy Conditions in 4D Evaporating Black Hole Backgrounds
Using Visser's semi-analytical model for the stress-energy tensor
corresponding to the conformally coupled massless scalar field in the Unruh
vacuum, we examine, by explicitly evaluating the relevant integrals over
half-complete geodesics, the averaged weak (AWEC) and averaged null (ANEC)
energy conditions along with Ford-Roman quantum inequality-type restrictions on
negative energy in the context of four dimensional evaporating black hole
backgrounds. We find that in all cases where the averaged energy conditions
fail, there exist quantum inequality bounds on the magnitude and duration of
negative energy densities.Comment: Revtex, 13 pages, to appear in Phy. Rev.
Cosmological and Black Hole Horizon Fluctuations
The quantum fluctuations of horizons in Robertson-Walker universes and in the
Schwarzschild spacetime are discussed. The source of the metric fluctuations is
taken to be quantum linear perturbations of the gravitational field. Lightcone
fluctuations arise when the retarded Green's function for a massless field is
averaged over these metric fluctuations. This averaging replaces the
delta-function on the classical lightcone with a Gaussian function, the width
of which is a measure of the scale of the lightcone fluctuations. Horizon
fluctuations are taken to be measured in the frame of a geodesic observer
falling through the horizon. In the case of an expanding universe, this is a
comoving observer either entering or leaving the horizon of another observer.
In the black hole case, we take this observer to be one who falls freely from
rest at infinity. We find that cosmological horizon fluctuations are typically
characterized by the Planck length. However, black hole horizon fluctuations in
this model are much smaller than Planck dimensions for black holes whose mass
exceeds the Planck mass. Furthermore, we find black hole horizon fluctuations
which are sufficiently small as not to invalidate the semiclassical derivation
of the Hawking process.Comment: 22 pages, Latex, 4 figures, uses eps
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How accessible and acceptable are current GP referral mechanisms for IAPT for low-income patients? Lay and primary care perspectives
Background: Improving Access to Psychological Therapies (IAPT) constitutes a key element of England’s national mental health strategy. Accessing IAPT usually requires patients to self-refer on the advice of their GP. Little is known about how GPs perceive and communicate IAPT services with patients from low-income communities, nor how the notion of self-referral is understood and responded to by such patients.
Aims: This paper examines how IAPT referrals are made by GPs and how these referrals are perceived and acted on by patients from low-income backgrounds
Method: Findings are drawn from in-depth interviews with low-income patients experiencing mental distress (n = 80); interviews with GPs (n = 10); secondary analysis of video-recorded GP-patient consultations for mental health (n = 26).
Results: GPs generally supported self-referral, perceiving it an important initial step towards patient recovery. Most patients however, perceived self-referral as an obstacle to accessing IAPT, and felt their mental health needs were being undermined. The way that IAPT was discussed and the pathway for referral appears to affect uptake of these services.
Conclusions: A number of factors deter low-income patients from self-referring for IAPT. Understanding these issues is necessary in enabling the development of more effective referral and support mechanisms within primary care
Quantum Inequalities and Singular Energy Densities
There has been much recent work on quantum inequalities to constrain negative
energy. These are uncertainty principle-type restrictions on the magnitude and
duration of negative energy densities or fluxes. We consider several examples
of apparent failures of the quantum inequalities, which involve passage of an
observer through regions where the negative energy density becomes singular. We
argue that this type of situation requires one to formulate quantum
inequalities using sampling functions with compact support. We discuss such
inequalities, and argue that they remain valid even in the presence of singular
energy densities.Comment: 18 pages, LaTex, 2 figures, uses eps
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