The three winners have been leaders of the decades-long effort to understand these dark cosmic mysteries


BLACK HOLES are some of the universe’s most exotic objects. They are so dense that nothing, not even light, can escape their immense gravitational pull. At a black hole’s centre is thought to be a point of infinite density, called a singularity, where the known laws of physics break down.

Understanding these dark cosmic objects has been a decades-long effort. On October 6th the Nobel-prize committee of Sweden’s Royal Academy of Science awarded the most famous honour in physics to three of those who have led that effort.

Half the prize of SKr10m ($1.1m) went to Sir Roger Penrose of Oxford University, who built on Albert Einstein’s general theory of relativity to predict how black holes could form in the universe. Andrea Ghez and Reinhard Genzel, of the University of California’s Los Angeles and Berkeley campuses respectively, shared the other half for their decades of work mapping the movements of stars around the centre of the Milky Way. Their studies demonstrated that this is the site of an enormous cosmic object, which is assumed to be a supermassive black hole.

Though black holes were predicted around a century ago, as a consequence of general relativity (which is actually a theory of how gravity shapes the structure and contents of the universe), early work suggested that they could form only from the collapse of perfectly symmetrical stars or gas clouds. That is hardly realistic, and Einstein himself doubted that they actually existed.

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They therefore remained a theoretical curiosity until 1965 when Dr Penrose, as he then was, worked out the specifics of how real matter could collapse into a black hole. He showed, using a mathematical concept which he called a trapped surface, that even asymmetric, clumpy stars and dust clouds could form black holes. This work provided the tools observational astronomers needed to go out hunting for them.

By definition, it is impossible to see black holes directly. Instead, physicists glean insights by studying how their gravity affects the motions of their stellar neighbours. Dr Ghez and Dr Genzel used this idea to gather evidence that Sagittarius A*—a bright source of radio waves at the centre of the Milky Way, Earth’s home galaxy—is actually a supermassive black hole around which all the stars in the galaxy, the Sun included, orbit.

Dr Ghez and her team employed the Keck Observatory telescope, in Hawaii, with its ten-metre-wide primary mirror, to make their observations. Dr Genzel’s group used a series of eight-metre-wide telescopes high in the mountains of the Atacama desert, in Chile. These instruments were sensitive enough to peer through the clouds of dust that otherwise obscure the heart of the Milky Way to observers from Earth.

Over three decades of observations, both sets of researchers, working independently, tracked around 30 of the brightest stars at the galactic centre (see chart). They found that the closer a star was to Sagittarius A*, the faster it moved. Whereas the Sun, which is about 26,000 light years from the centre, takes around 200m years to complete an orbit of Sagittarius A*, a star called S2, which comes as close as 17 light hours to it, completes a single orbit in a dizzying 16 years. These measurements have permitted astronomers to piece together a picture of Sagittarius A* as a black hole of around 4m solar masses, packed into a region of space that is the size of the solar system.

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April 2019 saw the release of the first-ever image of a black hole (Sagittarius A*’s local equivalent at the centre of a galaxy called M87, 53m light years from Earth). This was taken using the Event Horizon Telescope, a collaboration linking eight existing radio telescopes all around Earth that permits far higher resolution than any single instrument could manage. As technology improves, the Event Horizon Telescope could also one day provide a more detailed image of the region around Sagittarius A*.

Sir Roger, who had to step out of his shower to take the call from the academy telling him of his win, just before it was announced to the world, said the award was “an extreme honour and great pleasure”. Speaking afterwards from his home in Oxford, he recalled that astronomers had taken a long time to take his ideas on black holes seriously. “In the early days there was a lot of scepticism,” he said. Eventually, however, observational studies began gathering evidence for their existence. “It was remarkable and gratifying to see that [my] work has been shown to be something real in the universe.”

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Via The Economist