  We discuss on different issues pertaining the theory of gravity, which pose
some unresolved fundamental questions. First we tackle the problem of observers
in general relativity, with particular emphasis in tilted observers. We explain
why these observers may detect dissipative processes in systems which appear
isentropic to comoving observers. Next we analyze the strange relationship
between vorticity and radiation, and underline the potential observational
consequences of such a link. Finally we summarize all the results that have
been obtained so far on the physical properties of the sources of gravitational
radiation. We conclude with a list of open questions which we believe deserve
further attention.
