Man has always needed vision to measure physical phenomena, but our vision is not efficient.
What we see with our eyes is the small range of vibrations found in the electromagnetic spectrum between 380 nanometers for the violet and 780 nanometers for the red while the image is built by our brain .
The wavelength of light is closely related to the concept of color but there are colors that we will never see, those located beyond the purple and below the red. They will never be interpreted by our brain.
The paradox with a black hole is that it has no color by definition because light, matter and energy are trapped in the black hole.
How did the physicists of the EHT (Horizon of Events) project transform information from an invisible supermassive black hole into visible light for the human eye?
Here again a black hole emits no information, it is a mysterious object whose surface is neither solid nor liquid nor gaseous, it is a simple immaterial border called "horizon of the events". We must invent another way to "see" or rather a way to "see" the nothing.
If the black hole is invisible, its environment, it is not, and it is thanks to him that we will be able to determine the horizon of events and thus see the shadow of the black hole.
According to the black hole theory, matter close to the event horizon is strongly heated by accretion before being absorbed by the monster and then disappearing forever in its gravitational well.
But before disappearing forever, the dust and the surrounding gas as well as the residues of stars that were once broken by the tidal forces, emit a characteristic radiation in the field of millimeter waves (a light not visible by the eye).
This radiation could reveal us around the black hole. The millimetric light turning around the black hole will therefore provide us with a lot of information about the black hole (its gravitational force, the way it distorts the space-time, the effects of gravitational lenses, etc.).
In summary, a black hole, although invisible, "lights" in its own way the material it attracts.
The image on the right represents a black hole lit by the material that it has itself heated.
It is by using the technique of the interferometry with very long base, that is to say by combining eight radio telescopes distributed on our planet (Europe, Chile, United States, Hawaii and Antarctic) that the astronomers created a telescope virtual giant whose diameter is as large as the distance that separates them (about 10,000 km).
The targets concerned were the two most visible "black holes" from Earth.
The first target was Sagittarius A * (Sgr A), located at the center of our Milky Way, 26,000 light-years away from us, whose mass is equal to 4.1 million times that of the Sun. The other target was the supermassive black hole (1,500 times Sagittarius A * or 6 billion solar masses), located 50 million light-years away from the giant M87 elliptical galaxy.
The EHT teams had only a window of about two weeks each year to make an attempt to group observations.
To build the high-resolution image, physicists had to combine the signals picked up by the different antennas of the network with their own atomic clocks, ie the arrival times of signals to the nearest tenth of a billionth of a second, then compare and triangulate them with their point of origin to reconstruct a gigantic global image. A single night of observation collected 2 petabytes of data (2 x 1015). It is possible that in this mountain of data is hidden other information that would allow us to better understand the very particular physics prevailing in the environment close to black holes, including the huge jets of particles and radiation that some of them project into space at speeds close to that of light.
The hard data drives stored in Antarctica then had to wait until the end of the long freezing winter to be transported to the MIT Haystack Observatory and the Max Planck Institute in Bonn.
This first image of a black hole that you see on your screen is composed of only 33 KB of data (33 x 103), a thousand thousand times simpler than the data collected.
This is "seeing" for our brain, it simplifies to the extreme the complexity of reality, a simple black silhouette surrounded by a bright spot blurred enough to our happiness.
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Image: in April 2017, the information from the first image of a black hole was collected on the hard disks of the various telescopes and transported by an airliner to the Haystack Observatory in Massachusetts. Then, they were processed, compared and analyzed by clusters of supercomputers and it was not until April 2019 that physicists were able to unveil this simple halo of orange-yellow, blurry and asymmetrical light that stands out against a black background .
The cosmic monster, located about 50 million light-years from Earth, was imaged in the center of the M87 galaxy, by an international collaboration called Event Horizon Telescope (EHT).
Photo credit: EHT Collaboration
Jean Pierre Luminet in 1978, taking into account the complex distortions that the strong gravitational field imparts to the space-time and the trajectories of the light rays that marry the frame, created the first virtual image of a black hole (image -against).
In its simulation, the event horizon looks like a slightly flattened disc. The gravitational field so strongly curves the paths of the light rays in the vicinity of the black hole that the rear part of the disc is "raised".
Thus a secondary image makes it possible to see the other face of the accretion disk, that located behind the black hole which took the form of a thin halo of light glued to the central black disk.
But the main feature of this simulation is the difference in brightness between the different regions of the disc.
The maximum brightness is greatest in the areas closest to the horizon, because this is where the gas is the warmest. For a distant observer, the received luminous flux is amplified on the side of the image where the gas approaches the observer. The observer is not stationary with respect to the black hole, it introduces a distortion of the images by Doppler effect.
The black hole can be rotated and generated asymmetry, the black hole is no longer spherical.
In addition, the gravitational lens effect caused by the black hole amplifies the apparent size of its event horizon.
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Image: the first simulation by Jean-Pierre Luminet (1978), a young researcher at the Observatoire de Paris-Meudon. Photo credit: EHT Collaboration