On April 10, the world first saw a black hole in a photograph. This supermassive object is located 53 million light-years from Earth and looks like a dark circle with an orange halo. Despite the fact that many people know what a black hole looks like, before that, all its images were reconstructions based on solutions of the Einstein equations. Now scientists are sure: black holes really look like they were represented. Lenta.ru tells how the picture was taken and what it means for science.
The black hole, named by the Hawaiian name Poehi (Powehi) – “a bottomless creation decorated with a dark source” – is so far from the Earth that it is impossible to discern it in detail using one radio telescope. Like other black holes, it is an object of enormous density and has such powerful gravity that it collapses around itself a space-time continuum. The curvature is so great that it forms an area from which none of the possible trajectories lead to the outside. The boundary of this area is called the event horizon, and everything that penetrates it (including visible light and other electromagnetic waves) cannot return.
In recent decades, scientists have not doubted the existence of black holes, although the very nature of these objects impedes their direct observation. Researchers used indirect methods, including observing objects that revolve around empty areas of space, or measuring the mass and size of objects that are sources of intense radiation. But to see the blackness of the horizon of events against the bright background of stars and gas, so far no one has been able to.
Bit by bit
Event Horizon Telescope is a unified network of eight observatories around the world, whose radio telescopes are synchronized with ultra-precise atomic clocks. Despite the fact that they work as one huge telescope with a diameter of 10 thousand kilometers, such a system is still significantly inferior to an imaginary radio telescope with a plate of a similar size in terms of the amount of information received. This limitation can be overcome a little because of the rotation of the Earth around its axis, so that you can collect some more radio waves. The main problem is that the final image will still be very noisy. Kathy Bowman's algorithm allows you to remove noise and build an acceptable picture.
The information obtained by radio telescopes can be interpreted in different ways and thus a whole “zoo” of images can be generated. However, one should not think that the researchers simply pulled the result to their ideas about how a black hole should look. There are strict limitations dictated by what astronomers know about space. Scientists know what astronomical objects should look like and what they don't look like. This allows you to weed out a huge number of options that depict what can not be in the center of galaxies.
Suppose we run a simulation in which a black hole is generated in accordance with the predictions of Einstein's theory of relativity, after which an exotic object is placed in the center of the Milky Way. As a result, the data is simulated, which in this case should receive the Event Horizon Telescope. If the black hole actually looked different (or it did not exist at all), the data from the telescopes would be completely different and the Bowman algorithm could get completely different images.
The algorithm, in turn, is similar to the puzzle builder. He analyzes the scanty data obtained by telescopes, and builds on their basis the overall picture, using fragments of thousands of images of cosmic and even earthly objects entered into it. If the image of a black hole (which we simulated) is obtained from different sets of images, then scientists can be sure that the algorithm is working correctly.
That is, to some extent, a reconstructed photograph of a black hole is a collage of fragments of various shots, even everyday ones. If the algorithm were bad, the result would strongly depend on the set of entered images, and instead of a black hole, researchers would get, for example, a photo from the wedding ceremony.
Everything came together
The resulting image of a supermassive black hole in the galaxy M87 corresponds to the predictions of Einstein's theory of relativity, which allows determining the mass and diameter of this exotic object. Its size exceeds the solar system and reaches 40 billion kilometers. In addition, it contains a mass of 6.5 billion suns. However, the most remarkable thing in the photograph, for the sake of which it was taken, is the dark circle in the center of the halo painted in conditional colors. This is the shadow of a black hole that corresponds to the horizon of events.
The black hole itself cannot be seen, but its shadow is clearly visible against the background of the absorbed substance. Poeha's Pole looks at Earth, so astronomers see hot gas rotating around a black hole “from above”. However, even if the black hole was visible from the side, calculations show that the substance moves along such paths that the shadow would still be visible. Interestingly, the shape of the shadow can determine the various properties of a black hole (for example, whether it is rotating) and distinguish it from the wormhole (wormhole).
In order to learn new details about the space monster in M87, scientists will have to study the photo in detail. In addition, now researchers are busy processing data obtained from observing the center of the Milky Way, where the black hole Sgr A * is located. It is possible that a more impressive snapshot of a supermassive black hole, located much closer to Poeha, “only” 25 thousand light-years from Earth will be published soon. Since the Milky Way is much calmer than the elliptical and active M87, astronomers can learn more about the behavior of black holes in different environments.
In the future, astronomers will get even more tools that will be included in the Event Horizon Telescope network. Thus, the Kitt Peak National Observatory in Arizona (USA) and the NOEMA millimeter grid in the French Alps to join the project in 2020. This will allow a better look at the processes occurring in close proximity to a black hole. These include the relativistic jet, which is ejected from the core of M87 and extends over five thousand light years. And the use of electromagnetic radiation of a slightly higher frequency should somewhat increase the clarity of new photos.