Frauendiener talks about wave solutions of the gravitational field: gravitational waves. He starts with an overview into the first developments in the research on gravitational waves, between 1962 and 1964. A key property of the metric is that it is conformal invariant: it is allowed to be multiplied by a number. Gravitational waves are these fluctuations in the metric. However, to study them one needs to define what the “stationary background“ is, on top of which the oscillations occur. This can be determined by studying isolated systems.