Here's what scientists like a black hole look like | Science

More than half a dozen scientific press conferences are scheduled for April 10, leaving hope that astronomers have for the first time imaged a black hole, objects with such powerful gravitational fields that even light can s & # 39; escape. Although their existence is now universally accepted, mainly because of the effect of their severity on nearby objects, no one has actually seen one.

The black holes themselves are entirely dark and featureless. Giants in the center of galaxies are also surprisingly small, even though they contain millions or billions of times the mass of our sun. To make their observation even more difficult, these giants are enveloped in clouds of dust and gas. But streams of superhot gas swirl around the holes, emitting radio waves of a wavelength of about one millimeter that can penetrate these clouds.

Two years ago, an international collaboration known as Event Horizon Telescope (EHT) brought together eight different radio telescopes around the world to try to image the supermassive black hole in the center of our galaxy, Sagittarius A *, and another at the center of the nearby M87 galaxy. They used a technique known as interferometry to combine the output of instruments scattered around the world to produce images from a single antenna as wide as the Earth. Such a big dish is needed to see the details of something that would easily fit in the orbit of Mercury and is 26,000 light-years away.

Their 5 nights of observation yielded 4 petabytes of data. If this amount of data was music stored in MP3 format, it would take 8,000 years to read it. The team has spent the last two years correlating, calibrating and interpreting the data and is now preparing to show us the results.

If the EHT has an image, it can reveal the shadow of the horizon of the events of the black hole, the point of no return of anything that falls in the black hole, on bottom of gas vortex in orbit. The size of this shadow and the shape of the swirling gas, focused by the gravity of the hole, will help to confirm many theories about these enigmatic objects.

Waiting for this week's announcement, Science He's been talking to someone who has spent much of his career imagining what black holes might look like. In February, in anticipation of the results of the EHT, Jean-Pierre Luminet of the Paris Observatory in Meudon, France, published an illustrated story of black hole imagery which records decades of progress, from fountain pen drawings to supercomputer simulations and Hollywood films. This interview has been modified for clarity and brevity.

Q: What prompted you to start working on visualizing black holes in 1978?

A: The challenge was to show something invisible by definition, as well as my natural interest in optical illusions and space-time distortions, not to mention the fact that no one had the idea of ​​calculating anything realistic!

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

A: 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, a simple geometric reasoning suggested that because of the gravitational lens, no part of the disc could be hidden, even its back side! And simple considerations on the relativistic rotation of the disc implied that a strong Doppler shift would cause a strong asymmetry of the apparent flux.

Q: The first image you produced, with a pen and ink, was impressive. What was the reaction of astronomers and the public?

A: As I was very young, it was my former doctorate advisor Brandon Carter who began to publicize my work by showing the photo at a meeting of the Royal Society in London. After receiving reprint requests from all over the world, my photo has been reproduced in popular science magazines such as American scientist, Sky and telescopeand so on, as well as in the monographs of other astronomers.

Q: Shortly after, astronomers realized that some, if not all, galaxies had a supermassive black hole in their center. What challenges did it pose?

A: The difficulty in visualizing the massive black hole environments at the galactic centers is that you do not know if the accretion structure is a thin disk (as in most simulations), thick (like a 3D torus) or a cloud of gas. , or if you have throws and so on. For example, in very active galactic nuclei and quasars, the accretion flux is very important and the disk is probably thick. Fortunately, Sagittarius A * and M87 * are not active galaxies, so the hypothesis of a thin disc is reasonable. It also depends (but not so much) on whether the black hole is rotating or not.

Q: With Jean-Alain Marck in the 1990s, you moved to motion animation of movement around a black hole. Did that help you understand, or was it more about public engagement?

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

Q: What do you think of the visualizations produced for the film? Interstellar?

A: I have written a lot about this in my blog. In short, geometrically good but physically wrong, as they neglected the physical properties of the accretion disk and Doppler shift effects.

Q: Has EHT added a boost to the visualization field as people were trying to figure out what they would actually see?

A: Sure. I stopped my black hole imagery story in 2002 precisely because, as soon as imaging a black hole with the EHT became a possibility, there was a burst of many simulations that I could not integrate them. Unfortunately, most of these simulations did not mention our pioneers.

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

A: This depends on many factors: for example, the angle of the observer's inclination to the accretion disk. If it is almost face up, the asymmetry of brightness due to Doppler shift will not be strong. In addition, on Sagittarius A * and M87 * environments, on the thickness of the disc (if any), etc. I suspect for various reasons that M87 * should provide a sharper picture than Sagittarius A *. In any case, if there is a thin accretion disk (as I'm sure it is for M87 *), the image should not be far from one of the views calculated by Jean-Alain Marck in 1989