[en] The concept of a 'field' is analysed, in connection with different physical situations, specifically, concerning the creation of radiation. The role of a field in relativistic physics shows that it is an independent physical entity that should be considered on the same grounds as matter particles. (author)

[en] The properties of principal null directions of a perturbed black hole are investigated. It is shown that principal null directions are directly observable quantities characterizing the spacetime. A definition of a perturbed spacetime, generalizing that given by Stewart and Walker, is proposed. This more general framework allows one to include descriptions of a given spacetime other than by a pair (M, g) where M is a four-dimensional differential manifold and g a Lorentz metric. Examples of alternative characterizations are the curvature representation of Karlhede and others, the Newman-Penrose representation or observable quantities involving principal null directions. The conditions are studied under which the various alternative choices of observables provide equivalent descriptions of the spacetime

No abstract available

No abstract available

[en] An elementary (mass-normalized) model of observers and emitters of light in free-fall within a black hole's radius is investigated in terms of the redshift spectrum induced. All observers and emitters follow the same kinds of trajectories, radially inward and starting from rest at spatial infinity. The major results are concerned with demonstrating the types of redshifts possible in all directions on a typical observer's celestial sphere. These are simulated by considering all equatorial light paths inside and generalizing to three dimensions by symmetry. Under certain assumptions a direction for maximum redshift and one for minimum redshift are obtained; these lie on antipodal points on the observer's celestial sphere. No multiple imaging or focusing is possible from isotropic sources inside r = 2m, however. At this stage no luminosity distances or intensity results are developed; these more complicated relationships would be required to simulate the actual picture getting through to an observer. Some of the redshift results are applied to a black hole whose scale is cosmological. This extreme example is included mainly as a curiosity to illustrate the impact of a simple change of scale and to reemphasize the importance of the microwave isotropy to theoretical models. A careful analytical formulation of general relativistic redshifts as seen in local Lorentz frames provides the tools for this investigation. (author)

[en] When considering relativistic motion within the special theory of relativity the question of the gravity free fall result for relativistic radial velocities is an interesting and surprisingly difficult one. Can Newton's force law for gravity be generalized and a Lorentz covariant equation of gravitational free fall be obtained. Einstein found that gravitation does not easily fit within the structure of Lorentz covaraince and was thus led to generalize the theory. Gravity, he suggested, causes a fundamental change in relativistic physics. Space-time itself becomes distorted (curved). It is considered by the author that the nice discussion of the general orbital motion in curved space given by Markley (Am. J. Phys.; 41:45 (1972)) is generally beyond the scope of introductory physics classes and a simplified answer is here given for the relativistic correction for which it is only necessary to impose the restriction of small distance free fall at relativistic speeds. (U.K.)

[en] In this paper, we will analyze the Fantappie group and its properties in connection with Cantorian space-time. Our attention will be focused on the possibility of extending special relativity. The cosmological consequences of such extension appear relevant, since thanks to the Fantappie group, the model of the Big Bang and that of stationary state become compatible. In particular, if we abandon the idea of the existence of only one time gauge, since we do not see the whole Universe but only a projection, the two models become compatible. In the end we will see the effects of the projective fractal geometry also on the galactic and extra-galactic dynamics

[en] Because of its exceptional significance in the history of great ideas in science, Einstein's first paper on relativity, especially its first part, deserves a more careful translation into English than presently exists. A new and annotated translation of this first part is presented here, together with a brief discussion of certain aspects of Einstein's paper

No abstract available

[en] An empirical test of Schlegel's ''interaction interpretation'' of the theory of special relativity is presented. Analysis of the UTC time scales maintained at various observatory sites over the world indicates that neither Schlegel's ''interaction interpretation'' of the theory of relativity nor the conventional ''space-time coordinate transformation interpretation'' of relativity can significantly improve agreement between the UTC time scales. Instead evidence for the effects of accelerations on clock rates is suggested