The Nobel Prize in Physics in 2020 was split between three scientists. The famous theoretical physicist Roger Penrose will receive half of the prize, and the rest will be divided equally between astronomers Reinhard Genzel and Andrei Gez. In the formulation of the Nobel Committee, Penrose was awarded for “the discovery that the formation of black holes is an accurate prediction of general relativity”, and Hantzel and Geuze – for “the discovery of a supermassive compact object in the center of our galaxy.”
For the second year in a row, the award was presented for achievements in the field of astrophysics – last year celebrated the discovery of planets orbiting distant stars. It can be recalled that moreover, only three years ago astrophysicists became Nobeliates, who for the first time registered gravitational waves from the merger of black holes.
Another characteristic feature of the awarded works is their age. Penrose’s seminal paper on black holes dates from 1965! The scientist himself celebrated his 89th birthday just two months ago. The other two laureates are younger, but their observations leading to the presentation of the award were carried out back in the 1990s.
Nevertheless, the theoretical prediction of the existence of black holes and the direct observation of cosmic objects with all the properties characteristic of these initially hypothetical objects is undoubtedly one of the most important achievements in astrophysics and has earned a high award. What contribution did each of the laureates make to this achievement?
How the idea of black holes was born
Roger Penrose, by the mid-1960s, had a series of strong mathematical papers and rose to the status of professor of applied mathematics at Birkbeck College, University of London. In 1964, he learns from the famous cosmologist John Wheeler about the problems of the mathematical theory of black holes, and proceeds to solve them.
In general, the idea of the existence of objects so massive that even light cannot overcome the force of their attraction, is old by the standards of modern physics. Back in the 18th century, it was independently expressed by the Englishman John Michell and the Frenchman Pierre-Simon Laplace. Their reasoning, however, was based on Newton’s laws, which, as it turned out later, are not applicable to light and supermassive bodies.
Albert Einstein created the correct theory of attraction for bodies of any mass and is also valid for light in 1915. This theory is called the general theory of relativity. It is she who is now considered the main theory of gravity and is widely used both to describe individual cosmic bodies and to describe the entire Universe. Already in 1916, for the equations of this theory, Karl Schwarshild obtained a mathematical solution describing the simplest black hole.
In subsequent years, the problem of the existence of black holes did not attract much attention, although some interesting results were obtained. The problem was that they were all obtained in the simplest idealized cases, and many scientists doubted that the formation of a black hole in realistic conditions is possible.
How Penrose made black holes real
Meanwhile, in the early 1960s, amazing space objects were discovered, called quasars. According to estimates, their brightness exceeds the brightness of thousands of galaxies, and they are poorly visible only because they are at a huge distance from us, somewhere closer to the edge of the visible Universe. Such a bright object could not be a simple star, since it is known that the brighter the star, the shorter its life, since it quickly burns up the entire supply of hydrogen necessary for thermonuclear combustion. Therefore, it was hypothesized that quasars are in fact super-large mass black holes, which, absorbing the surrounding matter, emit part of the energy of this matter in the form of bright fluxes of radiation.
It was in connection with this hypothesis that scientists returned to the problem of a more rigorous mathematical substantiation of the possibility of the formation of black holes. And it was Roger Penrose who managed to solve it. Using an ingenious mathematical apparatus, he showed that Einstein’s equations of general relativity have solutions that describe the formation of black holes even in far from ideal conditions. This evidence, coupled with the observation of quasars, convinced most astrophysicists of the reality of the existence of such exotic objects. Actually, the term “black hole” itself firmly entered into use only after these works.
How to find a black hole?
Soon after, it was hypothesized that supermassive black holes exist not only in distant quasars, but also in the center of almost any galaxy. Including the galaxy in which we live – the Milky Way. Their mass is not as large as the mass of quasars, so they are not as bright, but how then can they be detected?
Until very recently, the resolving power of telescopes was in no way sufficient to see directly the vicinity of even the largest black holes. Only with the launch of the Event Horizon Telescope network last year was it possible to finally get an image of the “black hole shadow”. The groups led by Reinhard Gentzel and Andrei Gez acted in a different way.
They decided to measure the speed of the stars orbiting the center of our galaxy – astronomers for historical reasons call it the object Sagittarius A * (“Arrow A with an asterisk”) – at a short distance from it. If there is a relatively small massive object in the center, then the speed of the stars revolving around it should be the lower, the further the star is located – this follows from the well-known Kepler’s laws and similarly to how the speed of the planets in the solar system is the lower, the further the planet is from The sun. If there is no compact object in the center, and all the mass is distributed over a large volume, then the speed of the stars would change with distance in some other way.
The first problem with such an observation is that there are a lot of stars in the center of the galaxy and even more interstellar gas and dust. That is, the center of the galaxy is simply poorly visible. This problem was solved by observing not in visible light, but in the infrared range – such waves are much weaker absorbed.
Another problem was that stars move slowly across the sky, and it takes a long time to measure their speeds. This makes observations with space telescopes, such as the famous Hubble Telescope, impractical – other observations also take time. For this reason, we had to use ground-based telescopes.
But there is another problem with ground-based telescopes: they get in the way of the earth’s atmosphere. When astronomers try to see fine details in the sky, even small fluctuations in the air caused by winds or uneven heating result in severe distortion. It was the solution to this technical problem that led the groups of Hänsel and Guez to the discovery.
How did you see the black hole in the center of our galaxy?
used a short exposure time, that is, they opened the camera lens for a split second. During this time, a small amount of light fell on the camera film, so a special supersensitive detector was developed.
Secondly, due to the movement of air, the images of stars in different images were displaced relative to each other, so astronomers used a specially developed method of aligning them, called speckle visualization.
Finally, thirdly, both groups carried out their measurements independently for several years. Hansel is in Chile at the La Silla Observatory, and Guez is in Hawaii at the Keck Observatory. Then they compared their observations with each other and found excellent agreement between the results. The velocities of the stars ideally coincided with Kepler’s laws, which was proof of the presence of a compact object of extremely large mass in the center of the galaxy.
Further observations using more sophisticated methods only confirmed the initial conclusions, and in addition, made it possible to obtain stronger arguments in favor of the fact that this massive object is indeed a black hole. In particular, back in 1992, the star S2 was discovered, which makes a complete revolution around the center in just 16 years. That is, during the observation period, it has already managed more than one revolution. It turned out that its motion cannot be described by Newton’s laws, but it is in excellent agreement with Einstein’s general theory of relativity. Well, as mentioned above, literally last year, with the help of a whole network of telescopes, it was possible to see the immediate vicinity of the center of the galaxy, and show that their image coincides with what was expected to be seen from a supermassive black hole.
Black holes are one of the most exotic objects in the universe. But they are interesting not only because not a single object, even light, can do them, but a large number of sometimes difficult to solve paradoxes are associated with their theory. Black holes are objects in which, according to scientists, there is an area where modern physical theories cease to work, and for their full description it is required to create an absolutely new theory – quantum gravity, which would combine the laws of the microworld, quantum physics, and the laws of gravitational attraction bodies that rule the world of stars.
This is the author’s version of an article originally published in Meduza