4,090 research outputs found
Ultraviolet behavior in background independent quantum field theory
We describe a background independent quantization of the scalar field that
provides an explicit realization of Fock-like states and associated operators
in a polymer Hilbert space. The vacuum expectation values of the commutator and
anti-commutator of the creation and annihilation operators become energy
dependent, and exhibit a surprising transition to fermionic behavior at high
energy. Furthermore the approach yields a modified dispersion relation with a
leading correction proportional to the momentum cubed. These results suggests a
fundamental change in the ultraviolet properties of quantum fields.Comment: 8 pages, 5 figure
Dynamic attentional modulation of vision across space and time after right hemisphere stroke and in ageing
This article is available open access and is shared under a Creative Commons licence (http://creativecommons.org/licenses/by/3.0/). Copyright @ 2012 Elsevier Ltd.Introduction - Attention modulates the availability of sensory information to conscious perception. In particular, there is evidence of pathological, spatial constriction of the effective field of vision in patients with right hemisphere damage when a central task exhausts available attentional capacity. In the current study we first examined whether this constriction might be modulated across both space and time in right hemisphere stroke patients without neglect. Then we tested healthy elderly people to determine whether non-pathological ageing also leads to spatiotemporal impairments of vision under conditions of high attention load.
Methods - Right hemisphere stroke patients completed a task at fixation while attempting to discriminate letters appearing in the periphery. Attentional load of the central task was modulated by increasing task difficulty. Peripheral letters appeared simultaneously with the central task or at different times (stimulus onset asynchronies, SOAs) after it. In a second study healthy elderly volunteers were tested with a modified version of this paradigm.
Results - Under conditions of high attention load right hemisphere stroke patients have a reduced effective visual field, over a significantly extended ‘attentional blink’, worse for items presented to their left. In the second study, older participants were unable to discriminate otherwise salient items across the visual field (left or right) when their attention capacity was loaded on the central task. This deficit extended temporally, with peripheral discrimination ability not returning to normal for up to 450 msec.
Conclusions - Dynamically tying up attention resources on a task at fixation can have profound effects in patient populations and in normal ageing. These results demonstrate that items can escape conscious detection across space and time, and can thereby impact significantly on visual perception in these groups.The European Commission, Brunel University and the Wellcome Trust
Dimension-Dependence of the Critical Exponent in Spherically Symmetric Gravitational Collapse
We study the critical behaviour of spherically symmetric scalar field
collapse to black holes in spacetime dimensions other than four. We obtain
reliable values for the scaling exponent in the supercritical region for
dimensions in the range . The critical exponent increases
monotonically to an asymptotic value at large of . The
data is well fit by a simple exponential of the form: .Comment: 5 pages, including 7 figures New version contains more data points,
one extra graph and more accurate error bars. No changes to result
General covariance, and supersymmetry without supersymmetry
An unusual four-dimensional generally covariant and supersymmetric SU(2)
gauge theory is described. The theory has propagating degrees of freedom, and
is invariant under a local (left-handed) chiral supersymmetry, which is half
the supersymmetry of supergravity. The Hamiltonian 3+1 decomposition of the
theory reveals the remarkable feature that the local supersymmetry is a
consequence of Yang-Mills symmetry, in a manner reminiscent of how general
coordinate invariance in Chern-Simons theory is a consequence of Yang-Mills
symmetry. It is possible to write down an infinite number of conserved
currents, which strongly suggests that the theory is classically integrable. A
possible scheme for non-perturbative quantization is outlined. This utilizes
ideas that have been developed and applied recently to the problem of
quantizing gravity.Comment: 17 pages, RevTeX, two minor errors correcte
Quantum resolution of black hole singularities
We study the classical and quantum theory of spherically symmetric spacetimes
with scalar field coupling in general relativity. We utilise the canonical
formalism of geometrodynamics adapted to the Painleve-Gullstrand coordinates,
and present a new quantisation of the resulting field theory. We give an
explicit construction of operators that capture curvature properties of the
spacetime and use these to show that the black hole curvature singularity is
avoided in the quantum theory.Comment: 5 pages, version to appear in CQ
2+1 Gravity without dynamics
A three dimensional generally covariant theory is described that has a 2+1
canonical decomposition in which the Hamiltonian constraint, which generates
the dynamics, is absent. Physical observables for the theory are described and
the classical and quantum theories are compared with ordinary 2+1 gravity.Comment: 9 page
Group study of an 'undercover' test for visuospatial neglect: Invisible cancellation can reveal more neglect than standard cancellation
Visual neglect is a relatively common deficit after brain damage, particularly strokes. Cancellation tests provide standard clinical measures of neglect severity and deficits in daily life. A recent single-case study introduced a new variation on standard cancellation. Instead of making a visible mark on each target found, the patient made invisible marks (recorded with carbon paper underneath, for later scoring). Such invisible cancellation was found to reveal more neglect than cancellation with visible marks. Here we test the generality of this. Twenty three successive cases with suspected neglect each performed cancellation with visible or invisible marks. Neglect of contralesional targets was more pronounced with invisible marks. Indeed, about half of the patients only showed neglect in this version. For cases showing more neglect with invisible marks, stronger neglect of contralesional targets correlated with more revisits to ipsilesional targets for making additional invisible marks upon them. These results indicate that cancellation with invisible marks can reveal more neglect than standard cancellation with visible marks, while still providing a practical bedside test. Our observations may be consistent with recent proposals that demands on spatial working memory (required to keep track of previously found items only when marked invisibly) can exacerbate spatial neglect
Two dimensional general covariance from three dimensions
A 3d generally covariant field theory having some unusual properties is
described. The theory has a degenerate 3-metric which effectively makes it a 2d
field theory in disguise. For 2-manifolds without boundary, it has an infinite
number of conserved charges that are associated with graphs in two dimensions
and the Poisson algebra of the charges is closed. For 2-manifolds with boundary
there are additional observables that have a Kac-Moody Poisson algebra. It is
further shown that the theory is classically integrable and the general
solution of the equations of motion is given. The quantum theory is described
using Dirac quantization, and it is shown that there are quantum states
associated with graphs in two dimensions.Comment: 10 pages (Latex), Alberta-Thy-19-9
An evaluation of subjective experiences, effects and overall satisfaction with clozapine treatment in a UK forensic service
Objectives: Patients prescribed clozapine were surveyed to assess (a) the effects, both positive and adverse, and overall satisfaction with clozapine in comparison to previously prescribed antipsychotics and (b) the relative significance of effects experienced, both positive and adverse, in terms of impact on subjective well-being.
Methods: A total of 56 male patients prescribed clozapine at a forensic psychiatric hospital were surveyed using a 27-item questionnaire. All patients had been prescribed clozapine for a minimum of 3 months. Respondents were asked to rate effects and satisfaction with clozapine treatment in comparison with previously prescribed antipsychotic medication on a five-point scale. Respondents were also asked to rate effects experienced with clozapine treatment in terms of impact on subjective well-being on a five-point scale.
Results: A total of 89% of respondents reported greater satisfaction with clozapine than with previously prescribed antipsychotic medication. A majority of patients reported positive effects in terms of an improvement in their quality of life (68%) and social abilities (52%) with clozapine in comparison with previously prescribed antipsychotics. Nocturnal hypersalivation (84%) and weight gain (57%) were the most common adverse effects. Hedonic responses were assessed for each effect in order to determine the associated subjective experiences. The most positive hedonic responses were for quality of life, mood and alertness. In terms of adverse impact on subjective well-being, nocturnal hypersalivation ranked highest.
Conclusions: Patients in a UK forensic sample are largely satisfied with clozapine treatment. The subjective effects of clozapine treatment should be taken into account by clinicians when assessing response. This may provide an opportunity to highlight the positive changes and prioritize management of the most undesirable adverse effects, which is likely to promote compliance and improve longer term treatment outcomes
On Singularity Resolution in Quantum Gravity
We examine the singularity resolution issue in quantum gravity by studying a
new quantization of standard Friedmann-Robertson-Walker geometrodynamics. The
quantization procedure is inspired by the loop quantum gravity programme, and
is based on an alternative to the Schr\"odinger representation normally used in
metric variable quantum cosmology. We show that in this representation for
quantum geometrodynamics there exists a densely defined inverse scale factor
operator, and that the Hamiltonian constraint acts as a difference operator on
the basis states. We find that the cosmological singularity is avoided in the
quantum dynamics. We discuss these results with a view to identifying the
criteria that constitute "singularity resolution" in quantum gravity.Comment: 12 page
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