It is a major discovery in the history of astronomy: the detection of gravitational waves, a theory of Einstein which until now had never been able to be directly identified. The team behind the investigation published June 28, 2023 in Astronomy and Astrophysics And The Astrophysical Journal the conclusions of this scientific breakthrough, which requires quickly resuming the work of Newton and Einstein to understand how scientists were able to “hear” these waves.
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A complex Einstein theory to observe
We must take up the work of Newton and Einstein. In the 17th century, Newton theorized universal gravitation: masses attract each other with variations depending on their weight and distance. The movement of objects is aligned with this gravity: the Moon, moving in one direction, is however constantly subject to gravity which attracts it towards the Earth. It therefore emits a rotation around the Earth since it is both attracted towards the Earth and moving in a different direction, placing it in a circular trajectory.
Einstein will propose to the twentieth century a different vision of things: the mass of objects will deform space-time, and thus cause the attraction of objects. If you place a bowling ball in the center of a stretched sheet, the sheet will sink in the middle. If we roll a ball on this sheet, it will therefore also sink in the direction of the center and the bowling ball. But it is not the bowling ball which attracts the ball: it is the deformation of the space created by the mass of the bowling ball around it which will cause this attraction. In other words, it is not the mass of the object that creates the attraction, but the deformation of space-time following its mass. But what does this have to do with these gravitational waves?
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Well-hidden gravitational waves
The greater the mass of the object, the greater the curvature of spacetime, as well as gravity. A displacement of a very massive object in space-time will displace this high curvature, to the point that in certain cases a disturbance of space-time may appear which will diffuse: this is the gravitational wave, theorized by Einstein.
But we have never succeeded in detecting them, and for good reason: they require objects of staggering mass and are of a very low frequency. However, they have an essential role for the study of space, since they can provide information on high-mass objects and the functioning of the universe. So, how to detect waves that are still theoretical and truly elusive?
The pulsar as a relay of gravitational waves
This is where pulsars, dead stars from supernova explosions, come into play. These celestial objects send out incredibly regular radio signals, some several hundred times per second, hence their name. pulsating star, vibrating star in the language of Molière). By using a network of more than 100 pulsars, the idea was therefore to identify minute indices of frequency changes in the signals emitted by the pulsars: we are talking here about variations of the order of nanoseconds.
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Two gravitational wave detectors had already been able to detect a quiver in 2015; the scientists of the International Puslar Timing Array (IPTA) consortium this time succeeded in “hearing” a more stretched signal, a sign of a phenomenon on a larger scale. This detected signal is equivalent to “changes of less than a millionth of a second over more than 20 years“, according to Antoine Petiteau, of the Atomic Energy Commission, questioned by our colleagues from AFP. This disturbance is common to each of the hundred pulsars, proof of a gravitational wave. The cause? Black holes of “several million to several billion times the mass of the Sun“and greater than the size of our Solar System, according to information given to our colleagues atAFP by Gilles Theureau, astronomer at the Paris-PSL Observatory.
Einstein’s theory put forward more than a century ago is therefore clearly observable, and suggests advances in the field of astrophysics: in the words of Gilles Theureau, “we open a new window on the Universe“.
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