Gravitational waves distort space-time and produce forces in such a way that the distance between free masses will alternately decrease and increase during the passage of a gravitational wave. An important characteristic is that when there is elongation in one direction there is compression in the perpendicular one. As a result, a circle made of free masses will get successively elongated and contracted in two perpendicular directions.

The amplitude of gravitational waves, the dimensionless parameter “h”, is measured by the relative variation of distance between two free masses. The absolute variation is therefore proportional to the distance between the two masses. It would typically be “as large” as the size of an atom if one could monitor the distance from the earth to the sun, and it would be about one hundred millions times smaller for two points separated by a distance of a few kilometers.

Such a small variation of distance can however be detected through the phenomenon of interference.
A laser Michelson interferometer is very sensitive to differential length variations between its two arms and is ideally suited to the detection of gravitational waves. Because of the extremely high sensitivity required, the length of the arms must be hundreds of kilometers. Since this cannot be practically achieved on earth, one uses multiple reflections between two mirrors to artificially increase the measuring length. Fabry-Perot resonant cavities made of two mirrors are currently employed in gravitational waves interferometers.