General relativity predicts the occurrence of gravitational waves, whose properties should resemble in some respects those of electromagnetic waves: they should travel at the same speed, c, and they should be polarized. Joseph Weber, an American physicist, announced in 1969 that he had detected events that might be caused by incoming gravitational waves--namely, vibrations occurring simultaneously in pairs of large aluminum cylinders, about 1,000 kilometres apart and each weighing several tons. Although these detectors had been insulated with great care from all other potential sources of such vibrations, the separation of gravitational signals from ordinary thermal noise (Brownian motion) presents delicate problems of instrumentation and interpretation, which proved difficult to resolve to the satisfaction of other experimenters attempting to repeat Weber's observations.

Weber's approach has been refined by the choice of different materials for the vibrating masses, by cryogenic techniques reducing the level of thermal noise, and by other improvements. A fundamentally different technique, replacing Weber's stationary cylinders by independently moving masses whose distances from each other would be measured by interferometric means, also has been investigated. While these efforts at direct detection of gravitational waves were under way, observations of the binary pulsar PSR 1913+16 indicated that this double star system is losing energy at precisely the rate that corresponds to the emission of gravitational radiation according to the theory of general relativity.

The discovery of gravitational waves would represent an important confirmation of the validity of the theory. Also, such waves might become the basis of an entirely new technology of astronomical observation, as they are believed to be the most penetrating kind of radiation imaginable.

The properties of certain astronomical objects, such as quasars (see below Relativistic cosmology), pulsars (extremely dense stars that emit electromagnetic pulses with great regularity), very bright galaxies at the cores of which extraordinary amounts of energy are being emitted, and jets of matter moving at relativistic speeds, imply that there are processes involving gravitational fields so strong that general relativity is needed to interpret the observations, which in turn will provide new tests of that theory.