This year's Nobel Prize for Physics is awarded to research related to black holes. Half the award went to British Professor Sir Roger Penrose for his theoretical research, which confirmed that the black hole is a robust prediction of Einstein's General Theory of Relativity. Penrose shared the prestigious award with Prof Andrea Ghez and Prof Reinhard Genzel for their experimental work, discovering the presence of a supermassive compact object at the heart of our Milky Way galaxy.
Prof Penrose was awarded the prize for research he carried out some 55 years ago. His research used Einstein's General Theory of Relativity to prove that a singularity and the formulation for a black hole is, mathematically, possible. The Penrose-Hawking singularity theorems, as this set of results are called, also answers the question of when does gravitation produce singularities. These theorems made use of the Raychoudhuri equation " a key ingredient to explain the space-time singularities and gravitational focusing properties in cosmology.
Though the above research is a collaborative effort between Sir Roger and Stephen Hawking, the latter shot into the limelight. Penrose, on the other hand, didn't attract media attention, and confined himself to academic pursuits. Penrose is a versatile intellectual, and a talented mathematician with wide interests. One can't help but wonder what took the Nobel Committee this long to honour him with the Prize.
Significance of Penrose's discovery
We know, from observing the stars in our galaxy and others, that stars form and die. Stars with a low mass, like our Sun, live for a long time and die as 'white dwarfs'. Bigger stars that are over eight times our Sun's mass, die in a spectacular explosion that we call a supernova. For stars that are much more massive " say, over 20 times the mass of the Sun " the central collapsing core is thought to be much, much larger. The core becomes immensely dense and compact, forming a black hole.
This extraordinary event, when a black hole forms, and matter gets crushed to limits beyond the nuclear scale, is a complex one to describe theoretically. It asks for a combination of gravity and quantum physics and mathematics. The closest we can get without this theoretical work is a vague estimate of a black hole's density, from studying other compact objects in the universe.
Only recently was the first direct image of a black hole from a nearby galaxy (M87) captured by astronomers in the globe, in a remarkable feat. However, there's still a huge gap in the modern understanding of black holes. We're yet to understand the singularity that was present in the beginning, before the big bang which supposedly gave rise to everything in the universe. We also don't know what happens to an observer that falls into a black-hole singularity.
Before the Penrose-Hawking singularity theorems came along in the 70s, singularities were thought to only form in certain situations. For instance, in a supernova explosion where a black hole is formed from the collapse of a star's core, it might be possible for a spinning star to partly counteract its own gravity. It was thought that a singularity wouldn't form under these circumstances " till the Penrose-Hawking singularity theorems disproved this belief. It showed that a singularity is formed in every instance there is an event horizon " the notional boundary around a black hole beyond which no light can escape.
Much-needed recognition for theoretical work
If there were two ideas that have tugged at the imagination of many-a-theoretical astrophysicist, they are black holes and Einstein's General Theory of Relativity. One of the most fascinating questions under astrophysics remains, "what is a black hole?". It is a curiosity that holds the interest of school children and astrophysicists alike. Of more real-world consequence, many nations have poured enormous funds into building experimental detectors to detect a black hole's presence. One of these experimental results, led by Ghaz and Genzel, was awarded half the Nobel Prize for Physics in 2020. Still, one wonders, isn't funding for international megaprojects like LIGO and VIRGO an acknowledgement of veritable theoretical work in need of recognition?
Experiments to detect black holes have been put forward, funded and are now fully-functional research hubs. The 2017 Nobel in Physics was awarded to >Rainer Weiss, Barry C. Barish and >Kip S. Thorne for decisive contributions to the LIGO detector, and the observation of gravitational waves. The signal that was detected using the LIGO observatory came from the merger of two black holes. It was, again, surprising to find that the prize was not awarded to those who made the actual predictions. In the meanwhile, the world lost one of the most brilliant minds in astrophysics, Professor Stephen Hawking, in 2018. Nobel prizes aren't awarded posthumously. And so, Hawking, for all his contributions, won't ever be awarded the Nobel Prize.
Penrose's Nobel decades overdue?
On a brief visit to the Indian Institute of Astrophysics in 2-9 January 1994, Sir Penrose delivered lectures at an international conference on (non-accelerator) particle physics. We were expecting him to receive a Nobel Prize in the next few years. I was a PhD student in the Institute at the time, and we were elated to hear him speak, and to interact with him. During the conference, Prof Penrose also wrote an article 'On the role of gravity in quantum state reduction' for the Proceedings of the International Conference on Non-Accelerator Particle Physics, edited by IIA's then-director, Prof R Cowsik. Prof Penrose was knighted the same year, for his "services to science". It would take another 26 years for him to win the Nobel Prize for his contributions to the theoretical understanding of black holes. It is heartening to see, despite the delay, that Sir Penrose was recognized by the Nobel Committee.
Subramanyan Chandrasekhar, an Indian-origin astrophysicist, received the Nobel Prize for Physics in 1983, for the work he did in the early 1930s. It took about 50 years for the Nobel Committee to recognize his work. Lest we forget that Albert Einstein never won the Nobel Prize for his timeless work on the General Theory of Relativity. Many great theoretical works and discovered are awarded late or missed altogether from consideration for the Nobel Prize.
Perhaps this is because Alfred Nobel intended for the prize to be given to those inventions that most benefit mankind?
The author is the director of the Indian Institute of Astrophysics, Bangalore, and works in the study of star clusters, stellar evolution and population in galaxies and Magellanic clouds. She tweets at @fiddlingstars