00:00Alors je me souviens très bien, le 14 septembre, c'était un lundi.
00:04Nous avons mesuré pour la première fois sur Terre une déformation de l'espace.
00:08On a observé qu'un bras de l'interféromètre sera consisté, sera allongé périodiquement.
00:14Et on a eu l'info par e-mail qu'il y avait un signal qui était peut-être intéressant.
00:19Et il y a eu d'abord une phase un peu de « Ah, tiens, c'est nouveau, une injection,
00:24on regarde, on teste l'instrument ».
00:26Et très vite on s'est rendu compte que c'était beaucoup plus sérieux et l'enthousiasme est très vite
00:32monté.
00:35This historic signal was detected independently by both instruments of the LIGO observatory in the US,
00:42marking the first direct observation of gravitational waves.
00:46Predicted a century ago by Einstein, these tiny waves distort the space-time they pass through.
00:52The strongest waves are created by the most extrêmes events in the universe,
00:56like the collision of two black holes.
01:04Donc il faut imaginer ces deux mastodontes qui pèsent chacun plus de 30 fois notre Soleil,
01:10qui tournent l'un autour de l'autre 75 fois par seconde,
01:15alors qu'ils sont séparés de quelques centaines de kilomètres,
01:17et qu'ils ont une vitesse qui commence à approcher celle de la lumière.
01:20Donc l'espace-temps autour est tout chamboulé,
01:23et ce sont ces perturbations qui ont voyagé pendant plus d'un milliard d'années
01:28jusqu'à traverser la Terre le 14 septembre vers 10 heures.
01:33LIGO est part of a worldwide network of gravitational wave detectors.
01:37Virgo, which you can see on these images,
01:39is the European detector based near Pisa in Italy.
01:43It is currently being upgraded, and once associated with LIGO,
01:47Virgo will enable scientists to triangulate the gravitational signals arriving from the cosmos.
01:56These gravitational signals are very weak signals,
01:59they cross the Earth without any problem.
02:01And the instruments that we put on the surface of the Earth,
02:04in the United States or here, will observe the same signals.
02:08And by working together, and that's why we put together our data,
02:11and we exploit together these instruments LIGO and Virgo,
02:14we can go back to the direction of the source,
02:16and take a lot more on the source.
02:19These gigantic detectors built in an L shape have arms several kilometers long
02:25and are sensitive to the contraction and expansion of space itself.
02:29Les distances qu'on cherche à mesurer avec Virgo ou avec LIGO,
02:34c'est des variations de distance d'un milliardième de la taille d'un atome.
02:40In order to achieve such extreme sensitivities,
02:43the LIGO and Virgo detectors use a special technique called interferometry.
02:47A laser is divided into two beams by a special mirror.
02:51The beams will travel down both arms before being reflected
02:54and then combined into a single beam.
02:57The phases of the light waves are inverted and cancel each other out.
03:01When a gravitational wave travels through the detector,
03:04the arm's length varies, which in turn modifies the interference of the laser.
03:08The signal is then detectable.
03:10And if the goal is to measure variations that are as small as a billionth the size of an atom,
03:15the instruments must be perfectly stabilized and shielded from all vibrations.
03:19The laser beams travel in ultra-high vacuum tubes,
03:23and the mirrors must be suspended from a series of seismic attenuators
03:27that are also in a vacuum.
03:29At such sensitivities, even waves crashing onshore several dozens of kilometers away
03:35could create vibrations that would contaminate the measure.
03:39Optics is at the heart of the interferometer.
03:42The laser is guided by highly reflective mirrors.
03:45Today, engineers are setting up a brand new optical table
03:48that will control the laser beam at the end of one of the arms.
04:08For 20 years, hundreds of researchers, engineers and technicians
04:12have been working on these giant interferometers.
04:14And finally, after 20 years of silence,
04:18the scientific community has heard the slight whisper
04:21of a tremendous astrophysical event.
04:23Many more are to come.
04:25A new window has opened up a new form of astronomy,
04:32a new window on the universe,
04:36a new way to observe the universe in these extreme phenomena
04:39and that after a few years or a dozen years,
04:44the gravitational zones are part of astronomy
04:46as well as the optical telescopes or the particles or neutrinos.
04:53A new window has opened onto the cosmos.
04:56This new astronomy will help us understand gravity,
04:59the weakest fundamental force,
05:01which nonetheless governs the largest superstructures in the universe.
05:05Yet this universe remains mysterious in many ways.
05:0995% of it, black matter and black energy,
05:12are still invisible to us,
05:14although they do have a gravitational effect.
05:44That's what we're seeing in the universe.
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