One of the consequences of Einstein’s general theory of relativity is the prediction of gravitational waves, disturbances which affect the geometry of space-time and travel at the speed of light. When a sound is emitted, the passing of the sound wave modifies the air pressure whilst the emission of an electromagnetic wave changes the electrical characteristics of the environment in which it moves. In the same way, when a massive body is accelerated space-time must permanently readjust around it, which is expressed by miniscule disturbances which travel at the speed of light. Miniscule as the gravitational interaction is very weak. The gravitational waves thus interact very little with matter, which thus causes the difficulty of bringing them to light and, to this day, no gravitational wave has been directly detected (we only have indirect proof of their existence).
The perturbation which the spreading of these waves provokes within space-time is expressed by the fact that the shortest route between two points will lengthen and then contract; in other words a light ray will take longer and then shorter to travel from one point to the other.

Two major experiments in particular are under way in order to try and detect these waves: VIRGO, near Pisa, which is a joint Franco-Italian project, and LIGO, in the United States. Both rest on the same principle: a laser beam is split into two rays, which take distinct trajectories within the two perpendicular arms (of a length from 3 to 4km) of an interferometer. At the end of these arms mirrors reflect the rays and send them back to meet up again; their interference is expressed by more or less light depending on the difference in length between the journeys. Yet the arrival of a gravitational wave should modify these lengths, which would be expressed by modifications of the interference pattern. But for that it is required that the interferometer is very highly sensitive, for it must be capable of detecting a relative variation of length (ΔL/L) of the order of 10^-21, or even more (the equivalent of the size of an atom compared to the distance between the Earth and the Sun) in a fraction of a second! And in that is rooted the importance of research into the means of reaching ultimate precisions in measurements in physics (read the article Measuring beyond the standard quantum limit).