What you need to know about black holes that won the 2020 Nobel Prize in Physics

The honor goes to Roger Penrose, Reinhard Genzel and Andrea Ghez for their work on black holes and the confirmation of the theory of relativity

(Image: Getty Images) Black holes. One of the most fascinating, mysterious and frightening entities in our Universe. Of which until a century ago we did not even suspect the existence, and which only last year we were finally able to photograph. It is to them that this year's Nobel Prize in Physics is dedicated, awarded to Roger Penrose, Reihnard Genzel and Andrea Ghez, three scientists who with their work have shed light - in the words of the Royal Swedish Academy of Sciences - on " darkest secrets of the Universe ". In particular, Penrose was awarded for discovering that the formation of black holes is consistent with Albert Einstein's theory of general relativity, while Genzel and Getz were awarded the distinction for having discovered a supermassive object at the center of the Milky Way. Supermassive object that is very likely a huge black hole.

To understand the scientific significance of these discoveries we must go back over a century ago, and precisely to November 1915, when a young Albert Einstein presented his theory of general relativity, completely unhinging the concepts of time, space and gravity as they had been until then. General relativity, in particular, describes how the geometry of spacetime (the fabric with which the Universe is sewn) is modified by gravity: a force that, in addition to keeping us steady on Earth, regulates the orbits of the planets around the Sun and of the Sun around the center of the Milky Way, to cause stars to be born from gas dust and collapse under their own weight, is also able to influence the shape of space and the passage of time. As bizarre as it may seem to us, it seems that this is exactly how the world works: all the tests to which the theory of general relativity has been subjected so far have in fact successfully passed the test of experimentation. Einstein's equations predict, for example, that an extremely heavy mass is able to bend space and slow down the passage of time.

Browse gallery But you can take a further step forward: what happens in presence of an even larger mass? One of the first to ask - apart from Einstein, of course - was the German astrophysicist Karl Schwartzschild, who fiddling with the complicated field equations of general relativity showed how a very large mass could even swallow a portion of spacetime, creating what would then be it was called, in fact, a black hole. Several subsequent studies even discussed some of its characteristics, and in particular the so-called event horizon, a sort of circular veil that wraps and hides the black hole and everything inside it. To give an idea of ​​how dense and massive these objects are, think that, if the mass of the Sun were sufficient to make it a black hole, its event horizon would have a diameter of just three kilometers. If Earth were, the diameter of its event horizon would be nine millimeters long.

Until the 1960s, however, black holes were considered to be nothing more than one of the possible mathematical solutions of Einstein's field equations . A theoretical speculation of whose concrete existence few were convinced. Things began to change in 1963 following the discovery of quasars, the brightest objects in the Universe. It all began about ten years earlier, with the observation of radio waves from mysterious sources: among these there was for example an object called 3C273, which was finally located in the constellation of Virgo. When the astrophysicists managed to capture the visible light emitted by 3C273, they realized that, despite the source being over a billion light years away, it possessed an intensity similar to that of the light of several hundreds of galaxies. The only possible explanation that justified the emission of such a large amount of energy was the fall of matter into a black hole.

And so, then, black holes come into play and Roger Penrose enters the scene. (and his colleague Stephen Hawking). The British scientist says that in 1964, while walking with a colleague in London, he was enlightened by the idea of ​​the so-called trapped surfaces, an indispensable mathematical concept for the description of a black hole. It is, greatly simplifying the concept, of a surface that forces everything that hits it to point towards its center, regardless of the curvature of the surface itself. With this mathematical architecture Penrose was able to show that a black hole always hides a singularity, that is, a point with infinite density of matter. A point where time and space are dilated and deformed to infinity, and for the description of which there are no tools or theories yet. Basically, after matter begins to collapse on itself to form a black hole and create a trapped surface, says Penrose, there is no longer any way to escape the event horizon and return to the other side. And this is the reason why what is inside a black hole is totally inaccessible from the outside. The laws of physics that we know up to now prescribe its impenetrability ab aeterno.

Not being able to look inside, or behind, however, does not necessarily mean not understanding. The existence, characteristics and behavior of black holes can in fact also be inferred indirectly, by observing their effects on other celestial bodies and on the light emitted by the stars. At this point in the story, Genzel and Ghez enter the scene, who with their research groups have long probed the center of our galaxy, the Milky Way: studying the motion of different stars with telescopes on the Earth and in orbit, the two scientists they came to the conclusion that a supermassive object must necessarily exist in the center of the Milky Way. And that this supermassive object, consistent with other conjectures and observations, must necessarily be a black hole. The measurements of Genzel and Getz, among other things, made it possible to test the theory of relativity and its predictions even more precisely and laid the foundation for other even more resolute observations. Because, as they know from Stockholm, "the Universe still has many secrets and surprises to discover".