Comment on "What does the Letelier-Gal'tsov metric describe?"

We show that the Letelier-Gal'tsov (LG) metric describing multiple crossed strings in relative motion does solve the Einstein equations, in spite of the discontinuity uncovered recently by Krasnikov [gr-qc/0502090] provided the strings are straight and moving with constant velocities.Comment: 3 page

The Letelier-Gal'tsov spacetime revisited

Contrary to a recent claim by Anderson ["The Mathematical Theory of Cosmic Strings", I.O.P. Publishing, Bristol 2003], we show that the Letelier-Gal'tsov metric does represent a system of crossed straight infinite cosmic strings moving with arbitrary constant velocities.Comment: 3 page

Rotating non-asymptotically flat black rings in charged dilaton gravity

We derive new rotating, non-asymptotically flat black ring solutions in five-dimensional Einstein-Maxwell-dilaton gravity with dilaton coupling constant $\alpha=\sqrt{8/3}$ which arises from a six-dimensional Kaluza-Klein theory. As a limiting case we also find new rotating, non-asymptotically flat five-dimensional black holes. The solutions are analyzed and the mass, angular momentum and charge are computed. A Smarr-like relation is found. It is shown that the first law of black hole thermodynamics is satisfied.Comment: 21 pages, LaTeX; v2 a reference added, typos correcte

Soft singularity and the fundamental length

It is shown that some regular solutions in 5D Kaluza-Klein gravity may have interesting properties if one from the parameters is in the Planck region. In this case the Kretschman metric invariant runs up to a maximal reachable value in nature, i.e. practically the metric becomes singular. This observation allows us to suppose that in this situation the problems with such soft singularity will be much easier resolved in the future quantum gravity then by the situation with the ordinary hard singularity (Reissner-Nordstr\"om singularity, for example). It is supposed that the analogous consideration can be applied for the avoiding the hard singularities connected with the gauge charges.Comment: 5 page

Evaluation of a wave-vector-frequency-domain method for nonlinear wave propagation

A wave-vector-frequency-domain method is presented to describe one-directional forward or backward acoustic wave propagation in a nonlinear homogeneous medium. Starting from a frequency-domain representation of the second-order nonlinear acoustic wave equation, an implicit solution for the nonlinear term is proposed by employing the Green’s function. Its approximation, which is more suitable for numerical implementation, is used. An error study is carried out to test the efficiency of the model by comparing the results with the Fubini solution. It is shown that the error grows as the propagation distance and step-size increase. However, for the specific case tested, even at a step size as large as one wavelength, sufficient accuracy for plane-wave propagation is observed. A two-dimensional steered transducer problem is explored to verify the nonlinear acoustic field directional independence of the model. A three-dimensional single-element transducer problem is solved to verify the forward model by comparing it with an existing nonlinear wave propagation code. Finally, backward-projection behavior is examined. The sound field over a plane in an absorptive medium is backward projected to the source and compared with the initial field, where good agreement is observed

Bounds on the force between black holes

We treat the problem of N interacting, axisymmetric black holes and obtain two relations among physical parameters of the system including the force between the black holes. The first relation involves the total mass, the angular momenta, the distances and the forces between the black holes. The second one relates the angular momentum and area of each black hole with the forces acting on it.Comment: 13 pages, no figure

G2G_2 generating technique for minimal D=5 supergravity and black rings

A solution generating technique is developed for D=5 minimal supergravity with two commuting Killing vectors based on the $G_2$ U-duality arising in the reduction of the theory to three dimensions. The target space of the corresponding 3-dimensional sigma-model is the coset $G_{2(2)}/(SL(2,R)\times SL(2,R))$. Its isometries constitute the set of solution generating symmetries. These include two electric and two magnetic Harrison transformations with the corresponding two pairs of gauge transformations, three $SL(2,R) S$-duality transformations, and the three gravitational scale, gauge and Ehlers transformations (altogether 14). We construct a representation of the coset in terms of $7\times 7$ matrices realizing the automorphisms of split octonions. Generating a new solution amounts to transforming the coset matrices by one-parametric subgroups of $G_{2(2)}$ and subsequently solving the dualization equations. Using this formalism we derive a new charged black ring solution with two independent parameters of rotation.Comment: 21 pages revtex-4, 1 figure, typo corrected in Eq. (103

Relaxation of atomic polarization in paraffin-coated cesium vapor cells

The relaxation of atomic polarization in buffer-gas-free, paraffin-coated cesium vapor cells is studied using a variation on Franzen's technique of ``relaxation in the dark'' [Franzen, Phys. Rev. {\bf 115}, 850 (1959)]. In the present experiment, narrow-band, circularly polarized pump light, resonant with the Cs D2 transition, orients atoms along a longitudinal magnetic field, and time-dependent optical rotation of linearly polarized probe light is measured to determine the relaxation rates of the atomic orientation of a particular hyperfine level. The change in relaxation rates during light-induced atomic desorption (LIAD) is studied. No significant change in the spin relaxation rate during LIAD is found beyond that expected from the faster rate of spin-exchange collisions due to the increase in Cs density.Comment: 14 pages, 14 figure

A detailed record of the Lower Jaramillo polarity transition from a southern hemisphere deep sea sediment core

A detailed record of the lower Jaramillo (reversed to normal) polarity transition was obtained from a southern hemisphere, deep-sea sediment core (latitude = 35.91°E, longitude = 59.97°E) The record consists of over 850 samples taken across 140 cm of section. The transition itself is recorded across approximately 70 cm and is represented by more than 475 specimens from about 160 levels giving intermediate directions. The transition is identified by a nearly 180° shift from directions in good agreement with a reversed, axial dipole field to those closely aligned with a normal, axial dipole field for the core site latitude. The inclinations shallow gradually early in the reversal and pass through very steep negative values (-80°) late in the transition. The declinations show little appreciable variation until the inclinations have moved through the near vertical, and then slowly approach values in agreement with a normal polarity field. An intensity low accompanies the directional change during which the intensity drops to less than 15% of the maximum values observed in this sample interval. The intensity fluctuation spans a wider interval than the directional change, decreasing prior to any systematic change in the directions and then increasing to pre-transition levels by the same depth at which the directions have stabilized. The VGP path constructed for this reversal is longitudinally constrained to a certain extent, between 140° and 230° for intermediate VGP latitudes and is roughly centered 120° from the site longitude. This path is therefore a far-sided VGP path in Hoffman's [1977] terminology. Assuming a constant sedimentation rate (67m/Ma) through the Jaramillo Subchron, the duration of the transition is estimated to be 11,200 years to 4,500 years (depending on the criteria) for the directional change, whereas the associated intensity variation occurred over 15,000 to 20,000 years. Considered together with records of the most recent reversal (Matuyama/Brunhes) in light of current transitional field models, this record strongly suggests that the lower Jaramillo transitional field was dominated by different harmonics than the Matuyam/Brunhes transitional field