Shielding of Space Vehicles by Magnetic Fields

Spacecraft shielding by magnetic field

Gravitating Fluxbranes

We consider the effect that gravity has when one tries to set up a constant background form field. We find that in analogy with the Melvin solution, where magnetic field lines self-gravitate to form a flux-tube, the self-gravity of the form field creates fluxbranes. Several exact solutions are found corresponding to different transverse spaces and world-volumes, a dilaton coupling is also considered.Comment: 14 pages, 5 figure

Nucleating Black Holes via Non-Orientable Instantons

We extend the analysis of black hole pair creation to include non- orientable instantons. We classify these instantons in terms of their fundamental symmetries and orientations. Many of these instantons admit the pin structure which corresponds to the fermions actually observed in nature, and so the natural objection that these manifolds do not admit spin structure may not be relevant. Furthermore, we analyse the thermodynamical properties of non-orientable black holes and find that in the non-extreme case, there are interesting modifications of the usual formulae for temperature and entropy.Comment: 27 pages LaTeX, minor typos are correcte

Two loop and all loop finite 4-metrics

In pure Einstein theory, Ricci flat Lorentzian 4-metrics of Petrov types III or N have vanishing counter terms up to and including two loops. Moreover for pp-waves and type-N spacetimes of Kundt's class which admit a non-twisting, non expanding, null congruence all possible invariants formed from the Weyl tensor and its covariant derivatives vanish. Thus these Lorentzian metrics suffer no quantum corrections to all loop orders. By contrast for complete non-singular Riemannian metrics the two loop counter term vanishes only if the metric is flat.Comment: 4 pages Latex file, no figure

Charged Dilaton Black Holes with a Cosmological Constant

The properties of static spherically symmetric black holes, which are either electrically or magnetically charged, and which are coupled to the dilaton in the presence of a cosmological constant, are considered. It is shown that such solutions do not exist if the cosmological constant is positive (in arbitrary spacetime dimension >= 4). However, asymptotically anti-de Sitter black hole solutions with a single horizon do exist if the cosmological constant is negative. These solutions are studied numerically in four dimensions and the thermodynamic properties of the solutions are derived. The extreme solutions are found to have zero entropy and infinite temperature for all non-zero values of the dilaton coupling constant.Comment: 12 pages, epsf, phyzzx, 4 in-text figures incl. (minor typos fixed, 1 reference added

Multi-black holes and instantons in effective string theory

The effective action for string theory which takes into account non-minimal coupling of moduli admits multi-black hole solutions. The euclidean continuation of these solutions can be interpreted as an instanton mediating the splitting and recombination of the throat of extremal magnetically charged black holes.Comment: 10 pages, plain Te

Generalized Killing equations and Taub-NUT spinning space

The generalized Killing equations for the configuration space of spinning particles (spinning space) are analysed. Simple solutions of the homogeneous part of these equations are expressed in terms of Killing-Yano tensors. The general results are applied to the case of the four-dimensional euclidean Taub-NUT manifold.Comment: 10 pages, late

Non-asymptotically flat, non-AdS dilaton black holes

We show that previously known non-asymptotically flat static black hole solutions of Einstein-Maxwell-dilaton theory may be obtained as near-horizon limits of asymptotically flat black holes. Specializing to the case of the dilaton coupling constant $\alpha^2 = 3$, we generate from the non-asymptotically flat magnetostatic or electrostatic black holes two classes of rotating dyonic black hole solutions. The rotating dyonic black holes of the ``magnetic'' class are dimensional reductions of the five-dimensional Myers-Perry black holes relative to one of the azimuthal angles, while those of the ``electric'' class are twisted dimensional reductions of rotating dyonic Rasheed black strings. We compute the quasi-local mass and angular momentum of our rotating dyonic black holes, and show that they satisfy the first law of black hole thermodynamics, as well as a generalized Smarr formula. We also discuss the construction of non-asymptotically flat multi-extreme black hole configurations.Comment: Minor corrections. 2 references added. To appear in Physical Review

The Decay of Magnetic Fields in Kaluza-Klein Theory

Magnetic fields in five-dimensional Kaluza-Klein theory compactified on a circle correspond to ``twisted'' identifications of five dimensional Minkowski space. We show that a five dimensional generalisation of the Kerr solution can be analytically continued to construct an instanton that gives rise to two possible decay modes of a magnetic field. One decay mode is the generalisation of the ``bubble decay" of the Kaluza-Klein vacuum described by Witten. The other decay mode, rarer for weak fields, corresponds in four dimensions to the creation of monopole-anti-monopole pairs. An instanton for the latter process is already known and is given by the analytic continuation of the \KK\ Ernst metric, which we show is identical to the five dimensional Kerr solution. We use this fact to illuminate further properties of the decay process. It appears that fundamental fermions can eliminate the bubble decay of the magnetic field, while allowing the pair production of Kaluza-Klein monopoles.Comment: 25 pages, one figure. The discussion of fermions has been revised: We show how fundamental fermions can eliminate the bubble-type instability but still allow pair creation of monopole

Particle Production and Positive Energy Theorems for Charged Black Holes in deSitter

We study quantum mechanical and classical stability properties of Reissner-Nordstrom deSitter spacetimes, which describe black holes with mass $M$ and charge $Q$ in a background with cosmological constant $\Lambda \ge 0$. There are two sources of particle production in these spacetimes; the black hole horizon and the cosmological horizon. A scattering calculation is done to compute the Hawking radiation in these spacetimes. We find that the flux from the black hole horizon equals the flux from the cosmological horizon, if and only if $|Q|=M$, indicating that this is a state of thermodynamic equilibrium. The spectrum, however, is not thermal. We also show that spacetimes containing a number of charge equal to mass black holes with $\Lambda \ge 0$, have supercovariantly constant spinors, suggesting that they may be minimum energy states in a positive energy construction. As a first step in this direction, we present a positive energy construction for asymptotically deSitter spacetimes with vanishing charge. Because the construction depends only on a spatial slice, our result also holds for spacetimes which are asymptotically Robertson-Walker.Comment: 11 pages (1 figure not included), UMHEP-39