The principle of relativity requires that no interaction can propagate instantly. Gravitational waves (GW) must exist, propagating with the same speed as light. The specific characters of GW are predicted by Einstein’s general relativity (GR). After decades of efforts to develop detectors, on 11 February 2016, the LIGO and Virgo Collaboration published the discovery of a GW.

The elements of GR relevant for GW production, propagation and detection. How the GR field, which is the dimensionless metric tensor, differs from the other fundamental fields, which have physical dimensions. The instruments and the discovery. After the first observation, dozens of gravitational signals have been detected, the vast majority from merging black holes and one, on 17 August 2017, from the merger of neutron stars. In this case, electromagnetic signals are expected, and have been detected, providing unique information to astrophysics and to fundamental physics as well. The measurement of the speed of the GW and the establishment of a bound on the mass of the graviton.

We continue to carry out the programme of covariant quantisation, this time with emphasis on the massless closed string states: the graviton, the Kalb–Ramond or B-field, and the dilaton. The graviton is related to the linearised Einstein equations, the Kalb–Ramond field is used to discuss the properties and the dualisation of higher rank gauge fields, the dilaton serves as a first example of a ‘moduli field’ and its relation to the string coupling is explained.