Tilted shear-free axially symmetric fluids
  We carry on a systematic study of the physical properties of axially
symmetric fluid distributions, which appear to be geodesic, shear--free,
irrotational, non--dissipative and purely electric, for the comoving congruence
of observers, from the point of view of the tilted congruence. The vanishing of
the magnetic part of the Weyl tensor for the comoving congruence of observers,
suggests that no gravitational radiation is produced during the evolution of
the system. Instead, the magnetic part of the Weyl tensor as measured by tilted
observers is non vanishing (as well as the shear, the four--acceleration, the
vorticity and the dissipation), giving rise to a flux of gravitational
radiation that can be characterized through the super--Poynting vector. This
result strengthens further the relevance of the role of observers in the
description of a physical system. An explanation of this dual interpretation in
terms of the information theory, is provided.
