why is it so complicated to make a picture of a black hole?

Event Horizon Telescope (EHT) does it again. In 2019, scientists from this network of telescopes with great fanfare presented the very first image of a black hole, M87 *, located in the center of the galaxy of the same name. An already revolutionary announcement … But also a very, very small disappointment: the scientific community expected to discover the image of another black hole, Sagittarius A *. The latter is located at the center of our galaxy, the Milky Way.

First image of the shadow of a black hole: the supermassive black tower in the center of the galaxy M87, observed by the EHT network.

The EHT collaboration

Three years later, would scientists have managed to achieve this shot? LOn April 28, the European Southern Observatory (ESO) and the Event Horizon Telescope (EHT) announced that they will present on May 12 during a press conference, excellent results ». ESO’s press release mentions that their announcement will focus on the Milky Way – and EHT is used precisely to catch black holes. This image of Sagittarius A *, like the image of M87 *, would be a scientific and technological achievement. Because the barriers to trapping a black hole are significant.

invisible objects

First downside: they are by definition … black as no light escapes from them. Gravity actually presupposes that all objects attract each other according to their mass and the distance that separates them. This is the famous myth of Newton’s apple: If you lose the fruit, it falls, attracted to the Earth, due to gravity. The more massive an object is, the more it attracts the other: gravity is weaker on the Moon than on Earth. On the other hand, by gaining speed, the bodies manage to escape from gravity and to move away from the Earth (or the Moon): this is the principle of rockets. The more massive the celestial object (a planet, a star, etc.), the faster you have to go to move away from it and counteract gravity.

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Exactly, black holes are so massive – they concentrate the mass of much larger objects to a very small size. that you had to go faster than the light to escape. But Einstein’s theory of relativity says that nothing can exceed the speed of light. Which means nothing can come out of a black hole, not even a ray of light.

The simulation of a black hole released in 1979.

Jean-Pierre Luminet / CNRS Photo Library

It would therefore be of no use trying to see them directly. To succeed in making them visible, researchers are focusing on the surroundings of the black hole. We can actually see a luminous circle, called a growth disk. ” It is gas from stars that is swallowed by the black hole “, Explains Éric Lagadec, astronomer and president of the French Society of Astronomy and Astrophysics. The presence of this light disk had enabled Jean-Pierre Luminet, a French astrophysicist, to produce in 1979 the first theoretical modeling of a black hole thanks to a computer. About ten years later, his colleague Jean-Alain Marck produced an even more accurate video simulation. First challenge met!

But to go from representation to photography, you have to cross another barrier: given the distance from the Earth where they are located, black holes are small, very small. It therefore requires a very good zoom to capture the accretion disk that surrounds them. That is why EHT was born. ” It is not a telescope, but a series of telescopes that form a kind of super telescope », The astronomer continues. It brings together eight instruments located in different places on the planet: in Hawaii, Mexico, Arizona, the Atacama Desert in Chile and Antarctica. If they existed before EHT, the latter make it possible to synchronize their observations.

What interest? ” The larger a telescope is, the more light it receives and sees more detail.explains Eric Lagadec. But by combining the light from two instruments, its size becomes equivalent to that which separates the two telescopes. If 100 meters separate them, the virtual instrument resulting from the combination of the observations of two telescopes measures 100 meters. Earth. Which allows you to zoom in more than anyone else. ” Enough to read a newspaper in New York from a café in Paris “, Mentions for example ESO. Or an orange on the Moon from Earth. Even a million-euro euro coin. More than enough to offer humanity the right instrument to start chasing black holes.

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The only thing left for the photographers of the sky was to turn to the right candidate: a black hole that was large enough and close enough that this super-powered zoom might interest it. This is the case with the M87 *. If the latter is located at the center of a galaxy 50 million light-years away from Earth, it has a mass of 6.5 billion times that of the Sun. This makes it the densest giant black hole on our planet, and therefore the most accessible to catch. The image obtained in 2019 by EHT thus shows the silhouette of the M87 surrounded by its accretion disk. More than validating the simulation of Jean-Pierre Luminet and Albert Einstein’s forecasts, it makes it possible to determine the mass and size of the black hole. And is further proof of the existence of black holes.


Why, with all this knowledge in your pocket, did you not succeed in capturing Sagittarius A *? If the latter is much smaller than M87 *, with 4.3 million solar masses, it is also much closer as it is located in our galaxy. ” Seen from Earth, they are both about equal in size Says Eric Lagadec. Sagittarius A * is therefore available for the EHT’s zoom – that’s not the problem.

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But there are a lot of celestial bodies between us and the center of our galaxy that prevent us from visualizing it well. “, The astronomer argues, adding that Sagittarius A * is” variable “. In other words, when we try to catch it, it is” trembling », A bit like taking a picture of someone running. ” It’s hard to get a clear picture summarizes Eric Lagadec. This explains why we had to “settle” with the M87 three years ago. Have the EHT researchers succeeded in overcoming this last obstacle?

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