Nathalie Deruelle, director of research at CNRS, is a member of the Astroparticle and Cosmology Laboratory at Université Paris 7-Paris Diderot and affiliated professor at the Yukawa Institute in Kyoto.

Jean-Pierre Lasota is director of research emeritus at the Paris Institute of Astrophysics, Sorbonne Université, and professor at the Nicolas Copernicus Center of Astronomy in Warsaw. He is also the author of “La science des trous noirs” (2010, Editions Odile Jacob).

In 1916 Albert Einstein discovered them in his general relativity equations, but he later came to doubt their existence. In the 1960s, attempts on the part of a visionary physicist to demonstrate their existence ended in a notorious fiasco. It was not until the autumn of 2015 that two detectors in the USA at last began to vibrate, one after the other, as they registered the passage of a gravitational wave emitted by two merging black holes, 1.5 billion years ago. Two years later, after observations of several other black hole mergers, the LIGO and Virgo interferometers were able to detect gravitational waves emitted by the merger of two neutron stars, an event followed by an electromagnetic firework. The Nobel Prize awarded for these discoveries in 2017 testify to their importance; they are to revolutionize astronomy.

Einstein’s General Theory of Relativity explains gravitation — from falling objects to the expansion of the Universe — by replacing Newton's universal attractive force with the geometry of space-time. When two celestial bodies circle each other, space-time is deformed, as it always is around a massive object, but it then folds in on itself, in oscillations which propagate at the speed of light. These waves, coming from the edges of the Universe, initially very intense, become tiny by the time they reach Earth, so their detection requires instruments of extreme precision.

The gravitational waves detected by the these detectors have opened a new window in astronomy, just as four centuries ago Galileo’s telescope opened a new window on the visible world. Who knows what strange objects, in years to come, will be revealed to us by observing the sky in gravitational waves.