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ORIGINAL FRENCH ARTICLE: Le trou noir sort enfin de l’ombre

by Laurent Mouloud

A Black Hole Emerges At Last from the Darkness

Translated Saturday 13 April 2019, by Henry Crapo

For the first time in the history of Astronomy, a team of scientists revealed, on 10 April, the veritable image of these mysterious celestial objects.

A dark sphere, surrounded by a ghostly orange halo ... It is 15h07 on this Wednesday 10 April, in the offices of the European Commission. On the giant screen of the press room, plunged into obscurity, there appears the photograph that the entire scientific community impatiently awaited in recent months: the first truly real image — hardly a bit blurred — of a black hole. Until now, these cosmic monsters had been theoretically predicted, modeled by numerical simulation, and even detected. But never observed. "We had all the indirect proofs of their existence, and we even know the physical process of their fabrication", emphasizes Guy Perrin, astronomer at the Paris Observatory - PSL. "We lacked but one thing, to see one, for real." This historic moment for astronomy has finally arrived. Bringing with it the final proof of what had been predicted by Albert Einstein, based on his equations of general relativity. That genial wise man can now sleep soundly.

Photo: European Southern Observatory/AFP

The event was prepared with great pomp and circumstance. No less than six simultaneous press conferences were held yesterday across the globe (Brussels, Washington, Tokyo, Taipei, Shanghai, Santiago). To unveil the achievement in all languages. But also to remind one that this landmark photo is the result of a vast international collaboration, called Event Horizon Telescope (EHT), which brings together eight radio telescopes spread around the globe, from Europe to the South Pole, including Chile and Hawaii. Why such a team?"  The EHT uses the principle of interferometry, explains astrophysicist Pierre Léna [1] to l’Humanité. This involves combining information from different telescopes and using comparisons between received signals to reconstruct a virtual telescope, as large as the distance between the different devices, knowing that the larger the distance, the more accurate the observation will be..." In this case, the EHT represents a giant instrument of some 10,000 km in diameter, close to the size of the earth!

On April 5, 2017, the eight grouped telescopes targeted two "supermassive" black holes: Sagittarius A*, the closest to us, in the heart of our Milky Way, and its counterpart in the centre of the elliptical galaxy Messier 87, alias M87, in the constellation of the Virgin, about 50 million light years from Earth. And then what? And then months of waiting and stress. "The method is technically very complex," says Guy Perrin. Researchers look for a common signal from these telescopes, which operate in a heterogeneous way. "Due to the southern winter, it took eight months for the results of the South Pole Telescope, which did not arrive until December 23, 2017, to join the 4 petabytes (4 million billion bytes) of data already collected ... Once the common signal was found, it took more than a year of work to transcribe it all into photos. "For greater safety, the work was done four times, by four different teams," says French researcher Frédéric Gueth, a CNRS astronomer and deputy director of the Institut de Radioastronomie Millimétrique (Iram), a research partner.

The most expensive photograph on earth

All lead to the same image. And, a surprise, it was finally the black hole in the M87 galaxy, 1,500 times more massive than Sagittarius A*, that turned out to be the most photogenic. A Grail for researchers. "I never thought I’d see a real one in my lifetime," enthuses the astrophysicist at CNRS Jean-Pierre Luminet, author of the first digital simulation in 1979. Well, "see", in a way of speaking. According to the law of general relativity published in 1915 by Einstein, the gravitational attraction exerted by these monsters of density (as if the Earth were to be compressed into a thimble) is such that nothing can escape. Neither matter nor light. The result: they are invisible to our eyes. Astronomers therefore observe, instead, the immediate environment of the black hole.

And what do we see? First of all, the "accretion disc". In other words, very hot gas - plasma - and pieces of dislocated stars that spiral around the abyss before finally diving into it, generating a brilliant burst of ultraviolet light. It is these millimeter waves that the EHT is able to detect just before the final dive. "What we also see in the image is the shadow of the "non-return" limit (called the event horizon) of the black hole on the brilliant accretion disk," explains Frédéric Gueth, interviewed by the AFP. Observations determined that the giant’s mass was 6.5 billion times that of the Sun. But also that it was turning clockwise. "We are observing exactly what was planned. That’s a pleasure," adds the researcher.

And now what? The EHT will be back in service. And try to determine the exact density of the matter around the black hole, to better understand the magnetic field or how the matter in the disc rotates. "This field of research is fascinating," says Pierre Léna. The astrophysicist is leaving for Chile next week. With the hope of being able to measure the rotation of a black hole. "Which would also be a first," smiles the researcher.

[1Author of "A Story of Blur. Mirrors, black holes and other worlds" (Une histoire de flou. Miroirs, trous noirs et autres mondes). Éditions le Pommier, April 2019.

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