965 research outputs found
Newtonian Potential in Quantum Regge Gravity
We show how the Newtonian potential between two heavy masses can be computed
in simplicial quantum gravity. On the lattice we compute correlations between
Wilson lines associated with the heavy particles and which are closed by the
lattice periodicity. We check that the continuum analog of this quantity
reproduces the Newtonian potential in the weak field expansion. In the smooth
anti-de Sitter-like phase, which is the only phase where a sensible lattice
continuum limit can be constructed in this model, we attempt to determine the
shape and mass dependence of the attractive potential close to the critical
point in . It is found that non-linear graviton interactions give rise to a
potential which is Yukawa-like, with a mass parameter that decreases towards
the critical point where the average curvature vanishes. In the vicinity of the
critical point we give an estimate for the effective Newton constant.Comment: (47 pages), CERN-TH.7314/9
Wheeler-DeWitt Equation in 3 + 1 Dimensions
Physical properties of the quantum gravitational vacuum state are explored by
solving a lattice version of the Wheeler-DeWitt equation. The constraint of
diffeomorphism invariance is strong enough to uniquely determine the structure
of the vacuum wave functional in the limit of infinitely fine triangulations of
the three-sphere. In the large fluctuation regime the nature of the wave
function solution is such that a physically acceptable ground state emerges,
with a finite non-perturbative correlation length naturally cutting off any
infrared divergences. The location of the critical point in Newton's constant
, separating the weak from the strong coupling phase, is obtained, and it
is inferred from the structure of the wave functional that fluctuations in the
curvatures become unbounded at this point. Investigations of the vacuum wave
functional further suggest that for weak enough coupling, , a
pathological ground state with no continuum limit appears, where configurations
with small curvature have vanishingly small probability. One is then lead to
the conclusion that the weak coupling, perturbative ground state of quantum
gravity is non-perturbatively unstable, and that gravitational screening cannot
be physically realized in the lattice theory. The results we find are in
general agreement with the Euclidean lattice gravity results, and lend further
support to the claim that the Lorentzian and Euclidean lattice formulations for
gravity describe the same underlying non-perturbative physics.Comment: 44 pages, 5 figures. arXiv admin note: text overlap with
arXiv:1207.375
Therapy for word-finding in aphasia: working at the syntactic level
Therapy for word finding problems in aphasia has targeted semantic or phonological processing. However, an intermediate syntactic level may exist, and spoken word production in aphasia may be impaired here. We investigated semantic, syntactic, and phonological processing in noun production with two people with aphasia. The data indicate that syntactic processing was impaired for both speakers. We carried out two forms of therapy: phonological and syntactic. For one person both therapies were effective leading to gains in word-finding, but no change in syntactic processing; for the other person word-finding improved, and the syntactic therapy led to improved syntactic knowledge
Transurethral resection and surveillance of bladder cancer supported by 5-aminolevulinic acid-induced fluorescence endoscopy
Purpose: We determined whether neoplastic disease, which was missed under white light can be found during transurethral resection of bladder cancer by 5-aminolevulinic acid-induced porphyrin fluorescence. Materials and Methods: 5-Aminolevulinic acid-induced fluorescence endoscopy was carried out in 328 cases. A 3% 5-aminolevulinic acid solution was instilled intravesically in a mean time of 2.8 h before endoscopy, The fluorescence was excited by a special incoherent light source which provided blue light in addition to white light. Results: In 82 (25%) cases additional neoplastic lesions were found only because of their red porphyrin fluorescence which was induced by 5-aminolevulinic acid. 31% of these neoplastic foci which were found in normal and nonspecific inflamed mucosa had a poorly differentiated histology. Conclusions: 5-Aminolevulinjc acid facilitates detection of neoplastic disease during transurethral resection of bladder cancer and increases the accuracy of diagnosis
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