Nonlinear electrodynamics is skilled with knots

The aims of this letter are three-fold: First is to show that nonlinear generalizations of electrodynamics support various types of knotted solutions in vacuum. The solutions are universal in the sense that they do not depend on the specific Lagrangian density, at least if the latter gives rise to a well-posed theory. Second is to describe the interaction between probe waves and knotted background configurations. We show that the qualitative behaviour of this interaction may be described in terms of Robinson congruences, which appear explicitly in the causal structure of the theory. Finally, we argue that optical arrangements endowed with intense background fields could be the natural place to look for the knots experimentally.Comment: 5 pages, 1 figur

Organization of the specialized office, with emphasis on the insurance agency office

Thesis (M.B.A.)--Boston Universit

Geometric scalar theory of gravity

We present a geometric scalar theory of gravity. Our proposal will be described using the "background field method" introduced by Gupta, Feynman and others as a field theory formulation of general relativity. We analyze previous criticisms against scalar gravity and show how the present proposal avoids these difficulties. This concerns not only the theoretical complaints but also those related to observations. In particular, we show that the widespread belief of the conjecture that the source of scalar gravity must be the trace of the energy-momentum tensor - which is one of the main difficulties to couple gravity with electromagnetic phenomenon in previous models - does not apply to our geometric scalar theory. Some consequences of the new scalar theory are explored.Comment: We did some modifications which do not change the content of the tex

Remarks on the algebraic structure of (2, 2) double forms

We study the algebraic features of covariant tensors of valence four containing two blocks of skew indices. After a rather general treatment, we specialize ourselves to four-dimensional spacetimes and discuss several complementary aspects of these objects. In particular, we focus our attention on the corresponding invariant subspaces and generalise previous relations such as the Ruse-Lanczos identity, the Bel-Matte decomposition and the Lovelock-like quadratic identities. We conclude pointing out some possible applications of the formalism