17,916 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
Anomalous diffusion in quantum Brownian motion with colored noise
Anomalous diffusion is discussed in the context of quantum Brownian motion
with colored noise. It is shown that earlier results follow simply and directly
from the fluctuation-dissipation theorem. The limits on the long-time
dependence of anomalous diffusion are shown to be a consequence of the second
law of thermodynamics. The special case of an electron interacting with the
radiation field is discussed in detail. We apply our results to wave-packet
spreading
Quantum Field Theory Constrains Traversable Wormhole Geometries
Recently a bound on negative energy densities in four-dimensional Minkowski
spacetime was derived for a minimally coupled, quantized, massless, scalar
field in an arbitrary quantum state. The bound has the form of an uncertainty
principle-type constraint on the magnitude and duration of the negative energy
density seen by a timelike geodesic observer. When spacetime is curved and/or
has boundaries, we argue that the bound should hold in regions small compared
to the minimum local characteristic radius of curvature or the distance to any
boundaries, since spacetime can be considered approximately Minkowski on these
scales. We apply the bound to the stress-energy of static traversable wormhole
spacetimes. Our analysis implies that either the wormhole must be only a little
larger than Planck size or that there is a large discrepancy in the length
scales which characterize the wormhole. In the latter case, the negative energy
must typically be concentrated in a thin band many orders of magnitude smaller
than the throat size. These results would seem to make the existence of
macroscopic traversable wormholes very improbable.Comment: 26 pages, plain LaTe
Two-loop effective potential for a general renormalizable theory and softly broken supersymmetry
I compute the two-loop effective potential in the Landau gauge for a general
renormalizable field theory in four dimensions. Results are presented for the
\bar{MS} renormalization scheme based on dimensional regularization, and for
the \bar{DR} and \bar{DR}' schemes based on regularization by dimensional
reduction. The last of these is appropriate for models with softly broken
supersymmetry, such as the Minimal Supersymmetric Standard Model. I find the
parameter redefinition which relates the \bar{DR} and \bar{DR}' schemes at
two-loop order. I also discuss the renormalization group invariance of the
two-loop effective potential, and compute the anomalous dimensions for scalars
and the beta function for the vacuum energy at two-loop order in softly broken
supersymmetry. Several illustrative examples and consistency checks are
included.Comment: 38 pages. Typos in equations (3.5), (3.11), and (6.3) are fixed.
Explicit claim of renormalization group invariance in the general case of
softly-broken supersymmetry is added. Additional discussion of cases of
multiple simple or U(1) groups. Equations in Appendix B rewritten in a more
useful for
Nevirapine- and efavirenz-associated hepatotoxicity under programmatic conditions in Kenya and Mozambique.
To describe the frequency, risk factors, and clinical signs and symptoms associated with hepatotoxicity (HT) in patients on nevirapine- or efavirenz-based antiretroviral therapy (ART), we conducted a retrospective cohort analysis of patients attending the ART clinic in Kibera, Kenya, from April 2003 to December 2006 and in Mavalane, Mozambique, from December 2002 to March 2007. Data were collected on 5832 HIV-positive individuals who had initiated nevirapine- or efavirenz-based ART. Median baseline CD4+ count was 125 cells/ÎĽL (interquartile range [IQR] 55-196). Over a median follow-up time of 426 (IQR 147-693) days, 124 (2.4%) patients developed HT. Forty-one (54.7%) of 75 patients with grade 3 HT compared with 21 (80.8%) of 26 with grade 4 had associated clinical signs or symptoms (P = 0.018). Four (5.7%) of 124 patients with HT died in the first six months compared with 271 (5.3%) of 5159 patients who did not develop HT (P = 0.315). The proportion of patients developing HT was low and HT was not associated with increased mortality. Clinical signs and symptoms identified 50% of grade 3 HT and most cases of grade 4 HT. This suggests that in settings where alanine aminotransferase measurement is not feasible, nevirapine- and efavirenz-based ART may be given safely without laboratory monitoring
Quantum measurement and decoherence
Distribution functions defined in accord with the quantum theory of
measurement are combined with results obtained from the quantum Langevin
equation to discuss decoherence in quantum Brownian motion. Closed form
expressions for wave packet spreading and the attenuation of coherence of a
pair of wave packets are obtained. The results are exact within the context of
linear passive dissipation. It is shown that, contrary to widely accepted
current belief, decoherence can occur at high temperature in the absence of
dissipation. Expressions for the decoherence time with and without dissipation
are obtained that differ from those appearing in earlier discussions
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
Exponential Divergence and Long Time Relaxation in Chaotic Quantum Dynamics
Phase space representations of the dynamics of the quantal and classical cat
map are used to explore quantum--classical correspondence in a K-system: as
, the classical chaotic behavior is shown to emerge smoothly and
exactly. The quantum dynamics near the classical limit displays both
exponential separation of adjacent distributions and long time relaxation, two
characteristic features of classical chaotic motion.Comment: 10 pages, ReVTeX, to appear in Phys. Rev. Lett. 13 figures NOT
included. Available either as LARGE (uuencoded gzipped) postscript files or
hard-copies from [email protected]
Averaged Energy Conditions and Quantum Inequalities
Connections are uncovered between the averaged weak (AWEC) and averaged null
(ANEC) energy conditions, and quantum inequality restrictions on negative
energy for free massless scalar fields. In a two-dimensional compactified
Minkowski universe, we derive a covariant quantum inequality-type bound on the
difference of the expectation values of the energy density in an arbitrary
quantum state and in the Casimir vacuum state. From this bound, it is shown
that the difference of expectation values also obeys AWEC and ANEC-type
integral conditions. In contrast, it is well-known that the stress tensor in
the Casimir vacuum state alone satisfies neither quantum inequalities nor
averaged energy conditions. Such difference inequalities represent limits on
the degree of energy condition violation that is allowed over and above any
violation due to negative energy densities in a background vacuum state. In our
simple two-dimensional model, they provide physically interesting examples of
new constraints on negative energy which hold even when the usual AWEC, ANEC,
and quantum inequality restrictions fail. In the limit when the size of the
space is allowed to go to infinity, we derive quantum inequalities for timelike
and null geodesics which, in appropriate limits, reduce to AWEC and ANEC in
ordinary two-dimensional Minkowski spacetime. We also derive a quantum
inequality bound on the energy density seen by an inertial observer in
four-dimensional Minkowski spacetime. The bound implies that any inertial
observer in flat spacetime cannot see an arbitrarily large negative energy
density which lasts for an arbitrarily long period of time.Comment: 20pp, plain LATEX, TUTP-94-1
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