Cosmic strings, as topological spacetime defects, show striking resemblance to defects in solid continua: distortions, which can be classified into disclinations and dislocations, are linelike defects characterized by a delta function-valued curvature and torsion distribution giving rise to rotational and translational holonomy. We exploit this analogy and investigate how distortions can be adapted in a systematic manner from solid state systems to Einstein-Cartan gravity. As distortions are efficiently described within the framework of a SO(3) ⊃ × T (3) gauge theory of solid continua with line defects, we are led in a straightforward way to a Poincaré gauge approach to gravity which is a natural framework for introducing the notion of distorted spacetimes. Constructing all ten possible distorted spacetimes, we recover, inter alia, the wellknown exterior spacetime of a spin-polarized cosmic string as a special case of such a geometry. In a second step, we search for matter distributions which, in Einstein-Cartan gravity, act as sources of distorted spacetimes. The resulting solutions, appropriately matched to the distorted vacua, are cylindrically symmetric and are interpreted as spin-polarized cosmic strings and cosmic dislocations.

The form of Maxwell's theory is well known in the framework of general relativity, a fact that is related to the applicability of the principle of equivalence to electromagnetic phenomena. We pose the question whether this form changes if torsion and/or nonmetricity fields are allowed for in spacetime. Starting from the conservation laws of electric charge and magnetic flux, we recognize that the Maxwell equations themselves remain the same, but the constitutive law must depend on the metric and, additionally, may depend on quantities related to torsion and/or nonmetricity. We illustrate our results by putting an electric charge on top of a spherically symmetric exact solution of the metric-affine gauge theory of gravity (comprising torsion and nonmetricity). All this is compared to the recent results of Vandyck. file vandyck8.tex, 1997-02-11, Classical and Quantum Gravity, to be published (1997)

Recently Paw lowski and Raczka (P&R) proposed a unified model for the fundamental interactions which does not contain a physical Higgs field. The gravitational field equation of their model is rederived under heavy use of the computer algebra system Mathematica and its package MathTensor.

The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use.Typesetting: Camera-ready copy from the authors using a Springer TEX macro package SPIN 10515617 56/3144 -5 4 3 2 10 -Printed on acid-free paper VI PrefaceOur particular thanks go to the Silicon Graphics branch in Cologne and to U. Jager who provided us with the graphics hardware necessary for an effective presentation of the visualization demonstrations.Different people helped us at different stages of the project: Dr. J. Debrus (Physics Center), Jutta Adam (Heraeus Foundation), Dr. H.J. K6lsch (Springer), and F. Gronwald, R. Metzler, and J. Weidmann. We thank all of them for their support.

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