Among all the forces of Nature, the gravitational force is the one that has been known to man for the longest time. One of its fundamental properties -that all bodies fall with the same acceleration- was recognized by Galileo at the beginning of the seventeenth century. Towards the end of the same century, Newton established the universal gravitation law connecting the force responsible of the fall of bodies to the gravitational force between planets. Finally, Einstein, with the theory of general relativity, connected the gravitational field with the structure of space-time. Nevertheless, we dispose of very few elements on properties of gravitational force in particular in extreme conditions such as during the primordial explosion or the collision of black holes. Contrarily to what one could believe the gravitational force is, among the fundamental forces, the less known.

Einstein’s theory predicts the existence of gravitational waves which are perturbations of the gravitational field spreading out in space at the speed of light, like ripples on the surface of a pond. However, while electromagnetic radiations (for example visible light) can be absorbed completely by matter, gravitational waves can travel through space without being absorbed neither by stars, nor by interstellar matter.

This very low interaction, together with the weakness of the gravitational force makes the detection of gravitational waves extraordinarly difficult. Actually, after 30 years of active research, we only have an indirect proof of their existence. It has not yet been possible to detect gravitational waves directly, this remains one of the major challenges of experimental physics.
The gravitational force
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