Kramers-Kronig relations beyond the optical approximation

We extend Kramers-Kronig relations beyond the optical approximation, that is to dielectric functions $\varepsilon(q,\omega)$ that depend not only on the frequency but on the wave number as well. This implies extending the notion of causality commonly used in the theory of Kramers-Kronig relations to include the fact that signals cannot propagate faster than light in vacuo. The results we derive do not apply exclusively to electrodynamics but also to other theories of isotropic linear response in which the response function depends on both wave number and frequency.Comment: 1 figur

On the Bell's spaceships paradox and proper length for accelerated bodies

We study the Dewan-Beran-Bell thought experiment of two spaceships connected by a thread that start accelerated motion and discuss the proper length of the thread by means of Born's definition of proper length for arbitrary motion.Comment: 3 figure

An Extension of Poincaré group based on generalized Fermi-Walker coordinates

The class of accelerated and rotating reference frames has been studied on the basis of generalized Fermi-Walker coordinates. We obtain the infinitesimal transformations connecting any two of these frames and also their commutation relations. We thus have an infinite dimensional extension of the Poincaré algebra and, although it turns out to be Abelian extension, and hence trivial, it is noteworthy that, contrarily to Lorentz boosts, acceleration and rotational boost generators commute with each other and with the generators of Poincaré group as wel

Energy-momentum tensor for the electromagnetic field in a dispersive medium

On the basis of a non-local Lagrangian for Maxwell equations in a dispersive medium, the energy-momentum tensor of the field is derived. We obtain the Field equations through variational methods and an extension of Noether theorem for a non-local Lagrangian is obtained as well. The electromagnetic energy-momentum tensor obtained in the general context is then specialized to the case of a field with slowly varying amplitude on a rapidly oscillating carrier