The story so far: The Nobel Prize for Physics, announced on October 6 , was shared by three laureates. One half went to Roger Penrose, now at the University of Oxford, for ‘the discovery that black hole formation is a robust prediction of the general theory of relativity’. The other half is shared by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics, Garching, Germany, and Andrea M. Ghez of the University of California, Los Angeles (UCLA), for ‘the discovery of a supermassive compact object at the centre of our galaxy’. A statement from the Royal Swedish Academy of Sciences, which selects the awardees for the Prize, said the three laureates were awarded “for their discoveries about one of the most exotic phenomena in the universe, the black hole”.
What is the early history of the black hole?
Over a hundred years before Albert Einstein published his theory of relativity, John Michell and Pierre-Simon Laplace had speculated that extremely dense stars could have such high gravity that not even light could escape them. These would become invisible (dark stars). This idea came back to life in 1916, when just a few weeks after Einstein had published his theory of relativity, German astrophysicist Karl Schwarzschild found a solution to these equations, having a feature that was later named ‘event horizon’ — the point of no return, beyond which even light, the fastest object in the world, cannot escape. However, these concepts and their implications were so bizarre then that even Einstein refused to believe that it was possible.
An Indian researcher, B. Datt, and independently, Robert Oppenheimer and Hartland Snyder, made the first calculations of the gravitational collapse of a star in the 1930s. However, they had made the simplifying assumption of spherical symmetry.
By the early 1960s, observations of ‘quasars’, which are now called active galactic nuclei, were preparing the ground for the eventual observational evidence for supermassive blackholes.
What were the key questions when Dr. Penrose came into the picture?
The scenario of a spherically symmetric massive star collapsing under its own gravity until it falls below the Schwarzschild radius (event horizon) to form an infinitely dense singularity had been described independently by Datt and Oppenheimer and Snyder.
“This was, however, conditional on the high degree of symmetry [spherical symmetry], which almost certainly does not hold for realistic astrophysical collapse,” says Prof. Ghanashyam Date, Chennai Mathematical Institute. The question was: would a complete gravitational collapse happen without spherical symmetry?
What was Dr. Penrose’s achievement?
Prof. Date sums it up thus: while more solutions of the Einstein equation were found, suggesting blackholes hiding singularities, they all had special symmetries and their realisation under generic astrophysical conditions was in doubt.
Dr. Penrose, through his singularity theorems, conceptualised the formation of ‘trapped surfaces’ as the condition for formation of black holes in a generic manner. He made it possible for the world of physics to accept that black holes can and will form in the universe, as described by Einstein’s field equations.
How did observations of the supermassive black hole at the centre of the Milky Way begin?
American astronomer Harlow Shapley, about a hundred years ago, was the first person to identify the centre of the galaxy. By the 1960s, we learnt that there was a source of radio waves sitting there. This was named ‘Sagittarius A*’, and it is approximately 26,000 light years away, i.e., light from this point would take 26,000 years to reach us. In comparison, light from the Sun takes approximately eight minutes to reach the Earth.
In the 1990s, larger telescope facilities became available and Dr. Ghez started a research programme from the Keck Observatory atop Mauna Kea in Hawaii. Dr. Genzel’s group first used the New Technology Telescope in La Silla, Chile, and later moved to the Very Large Telescope on the Cerro Paranal mountain in Chile.
How did they determine that there was a black hole?
All the stars in the Milky Way orbit the centre. For example, the Sun orbits Sagittarius A* in more than 200 million years. For nearly three decades, the groups observed some thirty stars, particularly one that was named S2 by one group and S02 by the other. They found that the stars move in perfect elliptical orbits, just as if the object about which they were orbiting (Sagittarius A*) is a concentrated mass [compact object] and not diffused or scattered. Given its calculated mass of about four million solar masses, and its invisibility, this could only be a supermassive black hole, they deduced.
Dr. Ghez is the fourth woman to receive the Physics Nobel. Who are the other three?
Since 1901, a total of 114 physics Nobel prizes have been awarded. Before Dr. Ghez, only three had gone to women: Marie Curie (1903, for radiation phenomena), Maria Goeppert-Mayer (1963, nuclear shell structure), Donna Strickland (2018, ultrashort, high-intensity laser pulses).