This is what scientists think it looks like a black hole | Science

Display of the 1979 black hole by Jean-Pierre Luminet. Using computer data, he drew several thousand black dots on a white sheet by hand and took a photographic negative to get the final image. The gas strokes around the black hole towards us are brighter thanks to a Doppler reinforcement. The part of the gas disk behind the black hole is visible above it, because its light has been bent by the gravity of the black hole.

J.-P. Luminet

By Daniel Clery

More than half a dozen scientific press conferences are scheduled for April 10, raising the hope that astronomers have photographed for the first time a black hole, objects with gravitational fields so strong that even light can not escape. Although its existence is now almost universally accepted, mainly because of the effect of its gravity on nearby objects, nobody has actually seen one.

The black holes are completely dark and without distinctive features. The giants in the centers of galaxies are also surprisingly small, despite containing millions or billions of times the mbad of our sun. To make observing them even more difficult, those giants are wrapped in clouds of dust and gas. But the superheated gas currents revolve around the holes, emanating radio waves of about a millimeter of wavelength that can penetrate those clouds.

Two years ago, an international collaboration known as the Horizon Event Telescope (EHT) accumulated time in eight different radio telescopes around the world to try to visualize the supermbadive black hole at the center of our galaxy, known as Sagittarius A *, and another in the center of the nearby galaxy M87. They used a technique known as interferometry to combine the output of globally dispersed instruments to produce images as if they were from a single dish as wide as Earth. You need such a large dish to see the details of something that would fit easily within Mercury's orbit and is 26,000 light-years away.

His 5 nights of observation produced 4 petabytes of data. If that amount of data were music stored as MP3 files, it would take 8000 years to reproduce. The team has spent the last 2 years correlating, calibrating and interpreting the data and now they are preparing to show us the results.

If the EHT has an image, it can reveal the shadow of the event horizon of the black hole, the point of no return for anything that falls into the black hole, against a backdrop of the bright swirl of gas in orbit around it. The size of that shadow and the shape of the rotating gas, compensated by the gravity of the hole, will help confirm many theories about these enigmatic objects.

While we wait for this week's announcement, Science He spoke with someone who has spent much of his career imagining what black holes would be like. In February, anticipating the results of EHT, Jean-Pierre Luminet, of the Paris Observatory in Meudon, France, published an illustrated history of black hole images that records decades of progress from pen drawings and simulations of supercomputers and Hollywood films . This interview has been edited for clarity and brevity.

Q: What drove you to start working on black hole visualizations in 1978?

A: The challenge was to show something of an object that, by definition, is invisible, plus my natural interest in the optical illusions and distortions of space-time, and also, the fact that nobody had the idea of ​​calculating something realistic !

Improvements in the power of computers and software brought new, vivid images. This gallery shows a non-rotating black hole and its accretion disk from various angles.

J. A. Marck / J.-P. Luminet

Q: Were you surprised by the curious ways you discovered?

A: No, not at all, because before writing a computer program with equations, I always try to get a preliminary idea from geometric considerations. In this case, the simple geometric reasoning suggested that, due to the gravitational lens, no part of the disk could be hidden, even its back! And simple considerations about the relativistic rotation of the disc implied that a strong Doppler shift would cause a strong asymmetry of the apparent flow.

P: The first image he produced, using a pen and ink, was impressive. What was the reaction of astronomers and the public?

A: Since I was very young, it was my old doctorate. Advisor Brandon Carter, who began to publicize my work by showing the image at a meeting of the Royal Society in London. After receiving requests for reprint from around the world, my image was reproduced in popular scientific journals as American scientist, Sky and telescope, and so on, and in monographs of other astronomers.

Frame from a video produced in 1991 by Jean-Pierre Luminet and his colleagues for a French television documentary. Includes additional Doppler distortions and asymmetries.

J. A. Marck / J.-P. Luminet

Q: Soon after, astronomers realized that some, if not all, galaxies have a supermbadive black hole at their centers. What challenges did this pose?

A: The difficulty to visualize the environments of the mbadive black holes in the galactic centers is that it is not known if the accretion structure is a thin disk (as in most simulations), a thick one (like a 3D bull) or a cloud of gas, or if you have jets and so on. For example, in very active galactic nuclei and quasars, the accretion flow is very important and the disk is probably thick. Luckily, Sagittarius A * and M87 * are not active galaxies, so the hypothesis of a thin disk is reasonable. It also depends (but not so much) on whether the black hole is spinning or not.

Q: With Jean-Alain Marck in the 1990s, you swept motion animations around a black hole. Did this help understanding, or was it more for public participation?

A: It was essentially to provide more attractive images (colored, animated) to the public. Interestingly, the scientific community considered these simulations as a game and did not realize their future importance.

The black hole of Gargantua from the movie. Interstellar, produced by the visual effects company Double Negative in London.

AF / Alamy Archive Stock Photo

Q: What do you think about the visualizations produced for the movie? Interstellar?

A: I wrote many things about this in my blog. In short, it is geometrically good but physically incorrect because they neglected the physical properties of the accretion disk and the effects of Doppler shift.

Q: Has the EHT added impetus to the visualization field as people try to discover what they would actually see?

A: Of course. I stopped my history of black hole images in 2002 precisely because as soon as the image of a black hole with the EHT became a possibility, there was an explosion of so many simulations that I could not fit. Unfortunately, most of these simulations did. We will not cite our pioneers.

Q: What kind of image do you think the EHT team will reveal this week?

A: It will depend on many factors: for example, the angle of inclination of the observer with respect to the accretion disk. If it is almost in front, the asymmetry of brightness due to the Doppler shift will not be strong. Also, in Sagittarius A * and M87 * environments, in the thickness of the disk (if any) and so on. I suspect for several reasons that M87 * should provide a cleaner image than Sagittarius A *. In any case, if there is a thin accretion disk (as I am sure it is the case of M87 *), the image should not be far from one of the views calculated by Jean-Alain Marck in 1989.

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