22,420 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
Quantum Inequalities on the Energy Density in Static Robertson-Walker Spacetimes
Quantum inequality restrictions on the stress-energy tensor for negative
energy are developed for three and four-dimensional static spacetimes. We
derive a general inequality in terms of a sum of mode functions which
constrains the magnitude and duration of negative energy seen by an observer at
rest in a static spacetime. This inequality is evaluated explicitly for a
minimally coupled scalar field in three and four-dimensional static
Robertson-Walker universes. In the limit of vanishing curvature, the flat
spacetime inequalities are recovered. More generally, these inequalities
contain the effects of spacetime curvature. In the limit of short sampling
times, they take the flat space form plus subdominant curvature-dependent
corrections.Comment: 18 pages, plain LATEX, with 3 figures, uses eps
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
Focusing Vacuum Fluctuations II
The quantization of the scalar and electromagnetic fields in the presence of
a parabolic mirror is further developed in the context of a geometric optics
approximation. We extend results in a previous paper to more general
geometries, and also correct an error in one section of that paper. We
calculate the mean squared scalar and electric fields near the focal line of a
parabolic cylindrical mirror. These quantities are found to grow as inverse
powers of the distance from the focus. We give a combination of analytic and
numerical results for the mean squared fields. In particular, we find that the
mean squared electric field can be either negative or positive, depending upon
the choice of parameters. The case of a negative mean squared electric field
corresponds to a repulsive Van der Waals force on an atom near the focus, and
to a region of negative energy density. Similarly, a positive value corresponds
to an attractive force and a possibility of atom trapping in the vicinity of
the focus.Comment: 26 pages, 15 figures; additional discussion added in Sects. IV and I
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
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
A quantum violation of the second law?
An apparent violation of the second law of thermodynamics occurs when an atom
coupled to a zero-temperature bath, being necessarily in an excited state, is
used to extract work from the bath. Here the fallacy is that it takes work to
couple the atom to the bath and this work must exceed that obtained from the
atom. For the example of an oscillator coupled to a bath described by the
single relaxation time model, the mean oscillator energy and the minimum work
required to couple the oscillator to the bath are both calculated explicitly
and in closed form. It is shown that the minimum work always exceeds the mean
oscillator energy, so there is no violation of the second law
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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
Electromagnetic field quantization in an anisotropic magnetodielectric medium with spatial-temporal dispersion
By modeling a linear, anisotropic and inhomogeneous magnetodielectric medium
with two independent set of harmonic oscillators, electromagnetic field is
quantized in such a medium. The electric and magnetic polarizations of the
medium are expressed as linear combinations of the ladder operators describing
the magnetodielectric medium. The Maxwell and the constitutive equations of the
medium are obtained as the Heisenberg equations of the total system. The
electric and magnetic susceptibilities of the medium are obtained in terms of
the tensors coupling the medium with the electromagnetic field. The explicit
forms of the electromagnetic field operators are obtained in terms of the
ladder operators of the medium.Comment: 18 pages, no figure
Zero-point energy of massless scalar fields in the presence of soft and semihard boundaries in D dimensions
The renormalized energy density of a massless scalar field defined in a
D-dimensional flat spacetime is computed in the presence of "soft" and
"semihard" boundaries, modeled by some smoothly increasing potential functions.
The sign of the renormalized energy densities for these different confining
situations is investigated. The dependence of this energy on for the cases
of "hard" and "soft/semihard" boundaries are compared.Comment: 36 pages, LaTeX, 4 figure
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