Black holes are still one of the most puzzling phenomena in the cosmos. But thanks to this year’s three Nobel Prize winners in physics, we know a little more about them. One half of the award goes to the British physicist Roger Penrose, who demonstrated how general relativity can lead to the formation of black holes. The second is shared by the German astronomer Reinhard Genzel and his US colleague Andrea Ghez, whose observations have proven the existence of a supermassive black hole in the center of the Milky Way.
Today black holes are almost part of everyday astronomy – but at the same time they are still one of the great puzzles in the cosmos. Because they are invisible, but have such a great force of gravity that even light cannot escape them. These giants of gravity give themselves away indirectly through a ring of radiant plasma, raging particle jets or bursts of rays. But astrophysicists can only guess what is going on within the event horizon. It wasn’t until 2019 that astronomers succeeded for the first time in photographing a supermassive black hole – a dark shadow was seen in the middle of a bright ring of light.
Roger Penrose and the “trapped area”
Even Albert Einstein did not want to believe that such exotic structures existed at all – although his theory of relativity did not rule this out. Because it says that massive objects bend space-time – and a black hole contains so much concentrated mass that it ultimately causes space-time to collapse. How this is to be described physically and what happens in the process remained unclear and speculative for a long time. But in 1944, while taking a walk, the British physicist Roger Penrose came up with the decisive idea: He used mathematics and topology to develop the concept of a “trapped surface”. Such a surface is a two-dimensional, closed structure – similar to a ring, which forces all radiation inwards towards the center.
Inside this ring, Penrose recognized, there is always a singularity, a point at which space and time can no longer be defined. This concept explains why light and matter can only pass the event horizon of a black hole in one direction – inward. Because, as Penrose noted, space and time exchange roles within this trapped surface. This turns the inward movement into the forward movement in time. This is exactly what makes a return from the black hole impossible – because it would be a journey back in time.
Sagittarius A * on the trail
In contrast, the achievements of Reinhard Genzel and Andrea Ghez are based on concrete observations. Because the two astronomers and their teams have independently proven that there is a supermassive black hole in the heart of our home galaxy – Sagittarius A *. It was already clear at the end of the 1960s that there was a source of strong radio radiation in the center of the Milky Way. But only Genzel and Ghez were able to narrow down the nature of this object. They achieved this by observing the movements of stars in the vicinity of this object for decades. Genzel and his team used the Very Large Telescope of the European Southern Observatory in Chile for their observations, Ghez and her colleagues aimed the optics of the Keck Observatory on the galactic center.
With the help of adaptive optics and observations in the infrared range, they were able to track the speed and orbits of stars in the galactic center amidst strong interference radiation and dust. From this they concluded that there is a gravity giant in the center of our galaxy that concentrates around four million solar masses in a relatively small space. “The pioneering achievements of Reinhard Genzel and Andrea Ghez paved the way for new generations of precise tests of general relativity and its most bizarre predictions,” said the Nobel Prize Committee. In fact, it was only recently that astronomers used the star S2 to verify and confirm both the precession predicted by Einstein and the gravitational redshift at the black hole Sagittarius A *.
Source: Nobelprize.org