How the green light was given for gravitational wave search
  The recent detection of gravitational waves by the LIGO/VIRGO team is an
incredibly impressive achievement of experimental physics. It is also a
tremendous success of the theory of General Relativity. It confirms the
existence of black holes; shows that binary black holes exist; that they may
collide and that during the merging process gravitational waves are produced.
These are all predictions of General Relativity theory in its fully nonlinear
regime. The existence of gravitational waves was predicted by Albert Einstein
in 1916 within the framework of linearized Einstein theory. Contrary to common
belief, even the very \emph{definition} of a gravitational wave in the fully
nonlinear Einstein theory was provided only after Einstein's death. Actually,
Einstein had arguments against the existence of nonlinear gravitational waves
(they were erroneous but he did not accept this), which virtually stopped
development of the subject until the mid 1950s. This is what we refer to as the
\emph{Red Light} for gravitational waves research. In the following years, the
theme was picked up again and studied vigorously by various experts, mainly
Herman Bondi, Felix Pirani, Ivor Robinson and Andrzej Trautman, where the
theoretical obstacles concerning gravitational wave existence were successfully
overcome, thus giving the `Green Light' for experimentalists to start designing
detectors, culminating in the recent LIGO/VIRGO discovery. In this note we tell
the story of this theoretical breakthrough. Particular attention is given to
the fundamental 1958 papers of Trautman, which seem to be lesser known outside
the circle of General Relativity experts. A more detailed technical description
of these 2 papers is given in the Appendix.
