3,591 research outputs found
Non-negative curvature obstructions in cohomogeneity one and the Kervaire spheres
In contrast to the homogeneous case, we show that there are compact
cohomogeneity one manifolds, that do not support invariant metrics of
non-negative sectional curvature. In fact we exhibit infinite families of such
manifolds including the exotic Kervaire spheres. Such examples exist for any
codimension of the singular orbits except for the case where both are equal to
two, where existence of non-negatively curved metrics is known.Comment: 10 page
The ^(54)Mn Clock and Its Implications for Cosmic Ray Propagation and Fe Isotope Studies
Radioactive ^(54)Mn suggested as a 'clock' for measuring the lifetime of heavy cosmic rays, has a poorly known β-decay half-life estimated to be in the range from ~10^5 to 10 ^7 yr. Some years ago Koch et al. concluded from measurements of the Mn/Fe ratio that a significant fraction of low-energy (<1 GeV/nucleon) ^(54)Mn produced by Fe fragmentation had decayed. Using a propagation code that includes improved fragmentation cross-sections, and recent data from HEAO 3 and a number of other spacecraft, we have reexamined the evidence for ^(54)Mn decay in cosmic rays. We conclude that present cosmic-ray data cannot establish the degree of ^(54)Mn decay, but point out that this question has important implications for studies of the ^(54)Fe abundance in cosmic-ray source material, as well as for cosmic-ray propagation studies
Radiation from a uniformly accelerating harmonic oscillator
We consider a radiation from a uniformly accelerating harmonic oscillator
whose minimal coupling to the scalar field changes suddenly. The exact time
evolutions of the quantum operators are given in terms of a classical solution
of a forced harmonic oscillator. After the jumping of the coupling constant
there occurs a fast absorption of energy into the oscillator, and then a slow
emission follows. Here the absorbed energy is independent of the acceleration
and proportional to the log of a high momentum cutoff of the field. The emitted
energy depends on the acceleration and also proportional to the log of the
cutoff. Especially, if the coupling is comparable to the natural frequency of
the detector () enormous energies are radiated away
from the oscillator.Comment: 26 pages, 1 eps figure, RevTeX, minor correction in grammar, add a
discussio
Modern technologies of adaptation young specialists in the organization
In this article the main directions and technologies of adaptation of young specialists are considered. The author has revealed new technology of adaptation of young specialists which will allow new employees to feel more comfortably on a new workplace, to join collective, and also will lead to reduction of a dissatisfaction and turnover of staff at an initial stage of adaptation
The Rotating Quantum Vacuum
We derive conditions for rotating particle detectors to respond in a variety
of bounded spacetimes and compare the results with the folklore that particle
detectors do not respond in the vacuum state appropriate to their motion.
Applications involving possible violations of the second law of thermodynamics
are briefly addressed.Comment: Plain TeX, 10 pages (to appear in PRD
The Energy-Momentum Tensor in Fulling-Rindler Vacuum
The energy density in Fulling-Rindler vacuum, which is known to be negative
"everywhere" is shown to be positive and singular on the horizons in such a
fashion as to guarantee the positivity of the total energy. The mechanism of
compensation is displayed in detail.Comment: 9 pages, ULB-TH-15/9
The Singularity Problem for Space-Times with Torsion
The problem of a rigorous theory of singularities in space-times with torsion
is addressed. We define geodesics as curves whose tangent vector moves by
parallel transport. This is different from what other authors have done,
because their definition of geodesics only involves the Christoffel connection,
though studying theories with torsion. We propose a preliminary definition of
singularities which is based on timelike or null geodesic incompleteness, even
though for theories with torsion the paths of particles are not geodesics. The
study of the geodesic equation for cosmological models with torsion shows that
the definition has a physical relevance. It can also be motivated, as done in
the literature, remarking that the causal structure of a space-time with
torsion does not get changed with respect to general relativity. We then prove
how to extend Hawking's singularity theorem without causality assumptions to
the space-time of the ECSK theory. This is achieved studying the generalized
Raychaudhuri equation in the ECSK theory, the conditions for the existence of
conjugate points and properties of maximal timelike geodesics. Hawking's
theorem can be generalized, provided the torsion tensor obeys some conditions.
Thus our result can also be interpreted as a no-singularity theorem if these
additional conditions are not satisfied. In other words, it turns out that the
occurrence of singularities in closed cosmological models based on the ECSK
theory is less generic than in general relativity. Our work is to be compared
with previous papers in the literature. There are some relevant differences,
because we rely on a different definition of geodesics, we keep the field
equations of the ECSK theory in their original form rather than casting them in
a form similar to general relativity with a modified energy momentum tensor,Comment: 17 pages, plain-tex, published in Nuovo Cimento B, volume 105, pages
75-90, year 199
Prospectus, December 12, 1972
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Nonthermal nature of incipient extremal black holes
We examine particle production from spherical bodies collapsing into extremal
Reissner-Nordstr\"om black holes. Kruskal coordinates become ill-defined in the
extremal case, but we are able to find a simple generalization of them that is
good in this limit. The extension allows us to calculate the late-time
worldline of the center of the collapsing star, thus establishing a
correspondence with a uniformly accelerated mirror in Minkowski spacetime. The
spectrum of created particles associated with such uniform acceleration is
nonthermal, indicating that a temperature is not defined. Moreover, the
spectrum contains a constant that depends on the history of the collapsing
object. At first sight this points to a violation of the no-hair theorems;
however, the expectation value of the stress-energy-momentum tensor is zero and
its variance vanishes as a power law at late times. Hence, both the no-hair
theorems and the cosmic censorship conjecture are preserved. The power-law
decay of the variance is in distinction to the exponential fall-off of a
nonextremal black hole. Therefore, although the vanishing of the stress
tensor's expectation value is consistent with a thermal state at zero
temperature, the incipient black hole does not behave as a thermal object at
any time and cannot be regarded as the thermodynamic limit of a nonextremal
black hole, regardless of the fact that the final product of collapse is
quiescent.Comment: 13 pages, 2 epsf figures, RevTeX 3. Minor changes, version published
in PR
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