244 research outputs found
Linear Einstein equations and Kerr-Schild maps
We prove that given a solution of the Einstein equations for the
matter field , an autoparallel null vector field and a solution
of the linearized Einstein equation on the
given background, the Kerr-Schild metric ( arbitrary constant) is an exact solution of the Einstein equation for the
energy-momentum tensor . The mixed form of the Einstein equation for
Kerr-Schild metrics with autoparallel null congruence is also linear. Some more
technical conditions hold when the null congruence is not autoparallel. These
results generalize previous theorems for vacuum due to Xanthopoulos and for
flat seed space-time due to G\"{u}rses and G\"{u}rsey.Comment: 9 pages, accepted by Class. Quant. Gra
Is Quantum Mechanics Compatible with an Entirely Deterministic Universe?
A b s t r a c t It will be argued that 1) the Bell inequalities are not
equivalent with those inequalities derived by Pitowsky and others that indicate
the Kolmogorovity of a probability model, 2) the original Bell inequalities are
irrelevant to both the question of whether or not quantum mechanics is a
Kolmogorovian theory as well as the problem of determinism, whereas 3) the
Pitowsky type inequalities are not violated by quantum mechanics, hence 4)
quantum mechanics is a Kolmogorovian probability theory, therefore, 5) it is
compatible with an entirely deterministic universe.Comment: 15 pages, (compressed and uuencoded) Postscript (188 kb), preprint
94/0
Testing Newtonian Gravity with AAOmega: Mass-to-Light Profiles of Four Globular Clusters
Testing Newtonian gravity in the weak-acceleration regime is vital to our
understanding of the nature of the gravitational interaction. It has recently
been claimed that the velocity dispersion profiles of several globular clusters
flatten out at large radii, reminiscent of galaxy rotation curves, even though
globular clusters are thought to contain little or no dark matter. We
investigate this claim, using AAOmega observations of four globular clusters,
namely M22, M30, M53 and M68. M30, one such cluster that has had this claim
made for its velocity dispersion, was included for comparison with previous
studies. We find no statistically significant flattening of the velocity
dispersion at large radii for any of our target clusters and therefore we infer
the observed dynamics do not require that globular clusters are dark matter
dominated, or a modification of gravity. Furthermore, by applying a simple
dynamical model we determine the radial mass-to-light profiles for each
cluster. The isothermal rotations of each cluster are also measured, with M22
exhibiting clear rotation, M68 possible rotation and M30 and M53 lacking any
rotation, within the uncertainties.Comment: 7 pages, 4 figures and two tables. Accepted by MNRA
On the roots of the Poincare structure of asymptotically flat spacetimes
The analysis of vacuum general relativity by R. Beig and N. O Murchadha (Ann.
Phys. vol 174, 463 (1987)) is extended in numerous ways. The weakest possible
power-type fall-off conditions for the energy-momentum tensor, the metric, the
extrinsic curvature, the lapse and the shift are determined, which, together
with the parity conditions, are preserved by the energy-momentum conservation
and the evolution equations. The algebra of the asymptotic Killing vectors,
defined with respect to a foliation of the spacetime, is shown to be the
Lorentz Lie algebra for slow fall-off of the metric, but it is the Poincare
algebra for 1/r or faster fall-off. It is shown that the applicability of the
symplectic formalism already requires the 1/r (or faster) fall-off of the
metric. The connection between the Poisson algebra of the Beig-O Murchadha
Hamiltonians and the asymptotic Killing vectors is clarified. The value H[K^a]
of their Hamiltonian is shown to be conserved in time if K^a is an asymptotic
Killing vector defined with respect to the constant time slices. The angular
momentum and centre-of-mass, defined by the value of H[K^a] for asymptotic
rotation-boost Killing vectors K^a, are shown to be finite only for 1/r or
faster fall-off of the metric. Our center-of-mass expression is the difference
of that of Beig and O Murchadha and the spatial momentum times the coordinate
time. The spatial angular momentum and this centre-of-mass form a Lorentz
tensor, which transforms in the correct way under Poincare transformations.Comment: 34 pages, plain TEX, misleading notations changed, discussion
improved and corrected, appearing in Class. Quantum Gra
Barriers to psychological help-seeking in young men who have attempted suicide : an interpretative phenomenological analysis
The current paper reviews literature on help-seeking in relation to suicide and attempted suicide. An overview of the extensive research into risk factors associated with suicide is given highlighting the link between mental health problems and suicide. A minority of people with mental health problems seek professional help and even fewer people will go on to receive help from specialist mental health services. The same pattern is seen in those at risk of suicide. The majority will not be receiving specialist support at the time of their death although about half will have had recent contact with their GP. Reasons for not seeking help in times of emotional distress are discussed. Finally, clinical implications arising from the literature are addressed and suggestions are made for future research
- …