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The 2020 Nobel Prize in Physics has been awarded to Roger Penrose for his theoretical work showing that the formation of black holes is a direct consequence of Albert Einstein’s theory of general relativity, and to Reinhard Genzel and Andrea Ghez for each discovery of the hole. supermassive black at the center of our own galaxy, the Milky Way.
“The discoveries of this year’s awardees have broken new ground in the study of compact and supermassive objects,” David Haviland, chairman of the Nobel Committee for Physics, said in the organization’s press release. He continued: “But these exotic objects still raise many questions that beg for answers and motivate future research. Not just questions about its internal structure, but also questions about how to test our theory of gravity under extreme conditions in the vicinity of a black hole. “
Roger Penrose began dabbling in general relativity early in his career. Penrose, born in Colchester, Essex in 1931, developed new methods for studying the geometric properties and spatial relationships of various shapes and figures, a field known as topology, in his early 20s. Penrose specialized in making objects that fold in on themselves and were cyclical in nature, and he was so proficient that he inspired many of the Dutch artist MC Escher’s most famous geometric illusions.
Over the next decade, Penrose applied these talents to studying the inherently curved nature of spacetime described by general relativity to answer a very basic question: can black holes exist?
The concept of a black hole, an object so dense and with such a great gravitational pull that not even light can escape, is not new to general relativity. The idea was first proposed by the English astronomer John Mitchell in 1783 and by the French mathematician Pierre-Simon Laplace in 1796-1799. Using the framework provided by Newton during the previous century, they realized, with minimal assumptions, that it is theoretically possible to make an object so massive that no light can escape its gravitational pull.
This work was expanded upon by Einstein and many others in the months and years after Einstein completed his work on a theory of universal gravitation in November 2015. The first solution to Einstein’s equations, derived by Karl Schwarzschild in January 1916 while deployed in Germany. army during World War I on the Russian front, suggested an object that has so much gravity that at some point, regardless, not even light, can escape from it. Furthermore, the result coincided with the Newtonian value proposed by Mitchell and Laplace. Schwarzschild died while still deployed as an artillery officer four months later and was unable to make further contributions.
However, although the mathematics was resolved, for decades there was disagreement over whether such an object could be formed or not. American physicist Robert Oppenheimer suggested that a massive spherical ball of matter, like a star, could collapse into such an incredibly dense object, known as a singularity. Einstein himself disagreed and the debate continued until the 1960s.
Enter Penrose in 1964 and 1965, who applied his understanding of topology to the concept of black holes. He discovered that he was able to connect the point of no return, the event horizon, to the theoretical singularity hidden within a concept that is now known as trapped surfaces. Penrose showed that once beyond the event horizon, it is not that matter cannot escape, it is that its movement is always directed towards the singularity.
This has various implications. One of the most counterintuitive is that trying to escape a black hole only makes the problem worse. To escape Earth’s gravity, for example, a rocket can be used to propel itself above the planet’s escape velocity and visit other parts of the Solar System. In contrast, any attempt to escape an event horizon actually accelerates the descent into the singularity, a kind of cosmic quicksand.
These results have been central to our understanding of black holes ever since and provided a framework for the experimental discoveries that made up the other half of this year’s Nobel.
While Penrose and many others were laying down the theoretical foundations for black holes, a variety of experiments at the time strongly suggested that such supermassive objects exist and that they can be indirectly observed by their gravitational interactions with other pieces of matter. Observations in the 1950s and 1960s uncovered astronomical bodies that were roughly the size of our Solar System but with an energy output a thousand times that of our entire galaxy.
As more of these objects, so-called quasars, were discovered, the only plausible explanation was that vast amounts of matter were turning into black holes, in turn emitting enough light to shine through hundreds of millions or even billions. light years. It was then postulated that these quasars were actually the initial stages of galactic formation, and that most, if not all, galaxies have a black hole at their center, forming the core of the most common visible structures in the Universe.
The most immediate difficulty in studying black holes arose from the fact that the largest, and therefore the most extreme, are very far away. The only suspected supermassive black hole near Earth, in cosmic terms, was the object at the center of our own galaxy, known as Sagittarius A *. The other option was that there could be a collection of large stars that are energetic enough to mimic the energy output of a black hole.
This led to two observing teams, one led by Genzel at the Max Planck Institute for Extraterrestrial Physics in Germany and the other by Ghez at the University of California, Los Angeles, to attempt to follow the orbits of stars near the galactic center from The 1990s. . If the motion of the stars is generally random, that is evidence that there is no large central object. However, if stars accelerate the closer they get to the galactic nucleus, like comets orbiting the Sun, that’s evidence for a black hole.
The task was daunting. Genzel and Ghez had to track individual stars in the most populated region of the galaxy, and one that visible light doesn’t easily pass through. As such, they had to use the latest and most advanced optical techniques to pierce through the clouds of dust and gas from such a dense area. Their efforts were supported and largely made possible by the hundreds of other astronomers, engineers, and technicians who helped them make and operate some of the most advanced contemporary Earth-imaging equipment.
The results paid off after more than ten years of observations. One particular star, labeled S2, orbited a small region with high gravity once every 16 years. This immediately pointed to the existence of a supermassive black hole. Other studies published in the late 2000s ruled out other options, while at the same time establishing that Sagittarius A * is an object about 4 million times the mass of the Sun with a density far beyond what is theoretically needed to form a black hole. It is no longer postulated, but now accepted, that supermassive black holes are at the center of virtually all galaxies.
This experimental breakthrough is one of many black holes that have been investigated in recent years. The observations made by LIGO, which won the Nobel Prize in 2017, and the Event Horizon Telescope images of the accretion disk surrounding the supermassive black hole in the galaxy M87 by the Event Horizon Telescope, are further confirmations of both the existence of black holes and the role they play. in cosmic evolution. They also continue to confirm Einstein’s original theory, while at the same time opening up new avenues to study the cosmos.
