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What is a gravitational wave?

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Gravity, governed by Einstein, and everything else (strong, weak and electromagnetic interactions), governed by quantum physics, are the two independent rules known to govern everything in our Universe. Image credit: SLAC National Accelerator Laboratory.

After being awarded the CNRS gold medal in 2017, the work that allowed the direct detection of gravitational waves was rewarded with a Nobel Prize in physics in 2017. But what is a gravitational wave?

Imagine a calm water lake. This lake is our universe. But suddenly, two black holes that had gradually approached each other end up initiating a deadly dance before merging brutally in a formidable impact. Such a formidable collision makes space and time tremble. As if one had suddenly thrown a huge stone into the lake, and caused a wave of ripples slipping on the surface of the water.

The hypothesis of the existence of this space shiver goes back to the first decades of the 20th century. In 1915 precisely, when the general relativity theorized by Einstein shook our understanding of the world. Henceforth, one had to consider space, endowed with elasticity and shaped by matter, exactly as if the content (planets, stars and galaxies) curved the container, that is, the entire space.

Thus, Einstein tells us, the form of space depends on the matter that is housed in it. Therefore, any acceleration of mass should change it and this is manifested by a wave that runs through the entire Cosmos. The famous pebble launched into a lake! As it passes, this wave would expand and contract space. This is exactly what happens when two black holes collide. A part of the energy is then dissipated in the form of waves which make space and time vibrate: the any acceleration of mass should change it and this is manifested by a wave that runs through the entire Cosmos.

Thus, in theory, every object that is on the path of a gravitational wave sees its length vary: everything happens as if the space between the atoms of its molecules is distended and then tightened. This surprising observation indicating that in the distance two massive stars are approaching each other to collide or that a star explodes, ejecting its envelope.

At the first direct detection in 2016, the wave had been emitted by two black holes with respective masses of 29 and 36 times the mass of the Sun that approached and eventually merged 1.3 billion light years. The source is, therefore, more than a billion light-years away. The fusion gave rise to a gigantic black hole with a final mass of 62 solar masses. Now, 29 + 36 are supposed to make 65. This means that the equivalent of 3 solar masses has been expelled as gravitational waves.

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And it is this event of great intensity that caused this vibration of space-time,  titillating as its passage on Earth the detectors LIGO and Virgo. Better still, the source of these gravitational waves would be in the southern hemisphere, a hypothesis permitted by comparing the arrival times of gravitational waves in the two detectors located across the United States ( 7 milliseconds deviation) and the study of the signal characteristics measured by LIGO and Virgo. This discovery opens up a vast field of research,” says Benoît Mours, an astrophysicist at the Annecy-le-Vieux laboratory. It is even a  new astronomy which has just emerged as the Academy of Sciences has pointed out. Indeed, without such an instrument, the researchers would never have managed to detect the fusion of this duo of black holes. By proving their efficiency, these gravitational wave detectors provide a new tool to scrutinize and understand our Universe.

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