Traversable wormholes: minimum violation of null energy condition revisited

It was argued in literature that traversable wormholes can exist with arbitrarily small violation of null energy conditions. We show that if the amount of exotic material near the wormhole throat tends to zero, either this leads to a horn instead of a wormhole or the throat approaches the horizon in such a way that infnitely large stresses develop on the throat.Comment: 12 pages. To appear in PR

The Alert and Creative Entrepreneur: A Clarification

Israel M. Kirzner is the 2006 winner of The International Award for Entrepreneurship and Small Business Research (the FSF-Nutek Award). In this Prize Lecture he argues that a number of those who have commented on his work have misunderstood certain aspects of his theoretical system, and as a result the common distinction in the literature between “Schumpeterian” and “Kirznerian” entrepreneurs is flawed. He also argues that his understanding of the market process (set in motion by entrepreneurial decisions) provides a theoretical underpinning for public policy vis-à-vis entrepreneurship. Professor Kirzner’s main contributions to the economics of entrepreneurship were also presented and evaluated by Douhan, Eliasson and Henrekson (2007).Austrian economics; Economic development; Entrepreneurship; Small business economics

Albedo electrons between 12 and 1000 MeV

Balloon measurements of albedo electron fluxe

Incompressible fluid inside an astrophysical black hole?

It is argued that under natural hypothesis the Fermions inside a black hole formed after the collapse of a neutron star could form a non compressible fluid (well before reaching the Planck scale) leading to some features of integer Quantum Hall Effect. The relations with black hole entropy are analyzed. Insights coming from Quantum Hall Effect are used to analyze the coupling with Einstein equations. Connections with some cosmological scenarios and with higher dimensional Quantum Hall Effect are shortly pointed out.Comment: 30 pages, 2 figures. Accepted for publication on Physical Review D: references added, typos corrected, test polishe

String Supported Wormhole Spacetimes and Causality Violations

We construct a static axisymmetric wormhole from the gravitational field of two Schwarzschild particles which are kept in equilibrium by strings (ropes) extending to infinity. The wormhole is obtained by matching two three-dimensional timelike surfaces surrounding each of the particles and thus spacetime becomes non-simply connected. Although the matching will not be exact in general it is possible to make the error arbitrarily small by assuming that the distance between the particles is much larger than the radius of the wormhole mouths. Whenever the masses of the two wormhole mouths are different, causality violating effects will occur.Comment: 12 pages, LaTeX, 1 figur

Higher-Dimensional Bulk Wormholes and their Manifestations in Brane Worlds

There is nothing to prevent a higher-dimensional anti-de Sitter bulk spacetime from containing various other branes in addition to hosting our universe, presumed to be a positive-tension 3-brane. In particular, it could contain closed, microscopic branes that form the boundary surfaces of void bubbles and thus violate the null energy condition in the bulk. The possible existence of such micro branes can be investigated by considering the properties of the ground state of a pseudo-Wheeler-DeWitt equation describing brane quantum dynamics in minisuperspace. If they exist, a concentration of these micro branes could act as a fluid of exotic matter able to support macroscopic wormholes connecting otherwise distant regions of the bulk. Were the brane constituting our universe to expand into a region of the bulk containing such higher-dimensional macroscopic wormholes, they would likely manifest themselves in our brane as wormholes of normal dimensionality, whose spontaneous appearance and general dynamics would seem inexplicably peculiar. This encounter could also result in the formation of baby universes of a particular type.Comment: 21 pages, 1 figur