October 11, 2020

The 2020 Nobel in Physics and the Monster of a Black Hole in Our Galaxy

Prabir Purkayastha

THE Nobel Prize in physics this year has been shared by Roger Penrose, the mathematical physicist, for his work on the theoretical basis of black holes, and Reinhard Genzel, and Andrea Ghez, two astronomers, who led independent teams, for verifying the existence of such a black hole at the centre of our Milky Way galaxy.  Penrose showed that the consequence of Einstein’s general theory of relativity is the formation of black holes, not simply of collapsing stars, but also in certain dense regions of space.  Such black holes capture everything: nothing can come out, not even light. Genzel and Ghez and their teams independently showed by tracking the trajectory of a star that a super heavy object – around four million solar masses – exists at the centre of the Milky Way galaxy. Ghez is the fourth woman to win a Nobel prize in physics, the first one being Marie Curie in 1903.

There are two connections to India on black holes. The first is through physics. It was S Chandrasekhar, an Indian physicist, who had shown in 1930 that if a star was larger than 1.4 times the solar mass, it would not stop collapsing. Chandrasekhar was C V Raman’s nephew, India’s first Nobel laureate in physics. He moved to the US in 1936 and assumed American citizenship in 1953. Below this mass – known now as the Chandrasekhar limit – the star would become a white dwarf. If the mass of the star was higher, he did not know what would happen. We now know that it would blow-up in a supernova; or blow-up and then collapse, with their atoms squeezed into the nucleus-sized spaces forming a neutron star; or not stop collapsing creating a black hole. Chandrasekhar received the Nobel Prize for physics in 1983.

The second Indian connection, and an unhappy one, is from its name. It is now established Phillip Dicke, the Albert Einstein Professor in Science in Princeton, was the first to coin the term black hole for the gravitational collapse of a star creating a singularity. And his family remember his use of black hole whenever he could not find something in the house, asking whether it had disappeared into the Black Hole of Calcutta. Black Hole of Calcutta was, as we know, a grossly overblown myth about a number of English soldiers and East India Company European employees being shut in a small prison room with two small windows, killing a number of them due to suffocation. The numbers that were claimed then by the East India Company have been disputed by a number of historians, but provided the justification of wholesale killings, plunder and the seizure of lands that finally become the British Empire in India. It overshadowed –in English minds – the innumerable colonial massacres that the British carried out and the devastating famines that accompanied British rule.

Black hole by its very name conjures up a place where all matter is lost and from which nothing can emerge. So how do we show that it exists? One is to mathematically derive it from a theory that has been proven already. This is what Penrose did, deriving it from Einstein’s General Theory of Relativity. He also showed using mathematical topology, now known as Penrose transforms, that unlike other such derivations for black holes from stars with a certain mass, his approach did not need perfect symmetry of the collapsing matter. The only requirement was that if there was enough density of matter in a given space, it would lead – applying the General Theory of Relativity – to the formation of a black hole without the need for symmetry. This removed one of the major objections about black holes, that no collapse could be perfectly symmetrical. Even Einstein did not really believe that black holes could exist, even if his General Theory predicted this possibility.

Such a theoretical derivation is not enough for physicists; physics needs experimental evidence to confirm a theory. Or not enough for the Nobel Prize and the Swedish Academy that privileges experimental physics over theory. An observation that confirms the existence of a super heavy object that does not emit any energy, would provide a verification of Penrose prediction of a black hole. This is what Genzel and Ghez achieved, finding that the Milky Way galaxy, like most galaxies, hosts a massive black hole at its centre.

Einstein’s General Theory of Relativity that he formulated in 1915, had an unexpected fallout. Within a few months of its publication, a physicist, Karl Schwarzschild, who was then serving in the German Army, published a solution to Einstein’s field equations which showed that if matter and energy exceeded a certain bound, it would cause space-time to collapse on itself producing a singularity; or a black hole. The external world would feel its gravitational effect but no mass or even light could escape from such a black hole.

Einstein became world famous for having turned the familiar world of Newtonian physics upside down. But in spite of his worldwide fame, he had his enemies both in Germany and in academia, for his opposition to the First World War, his radical views including socialism, and being a Jew. The prevailing orthodoxy of physics including the Nobel Committee hated Einstein for all those reasons, and argued that Einstein’s theories were only theories, and lacked experimental proof.

In 1919, to end this argument Arthur Eddington, the English astronomer, proposed an experimental verification of Theory of Relativity. If a massive object curved space around itself due to its mass, it should be possible to observe this curvature by measuring starlight passing close to the sun during an eclipse. Eddington did this during a solar eclipse and was able to show that the results agreed with the predictions of Einstein’s General Theory of Relativity. London Times declared, "Revolution in Science, New Theory of the Universe"; a New York Times headline wrote, "Given the Speed, Time is Naught." Einstein became a rockstar in physics, a stature unmatched by any scientist.

But even that did not get him the Nobel Prize in 1920 and 1921. The science historian Robert Friedman wrote in his book, The Politics of Excellence, that the Committee could not stomach, "political and intellectual radical, who – it was said – did not conduct experiments, crowned as the pinnacle of physics." The 1920 prize went to an eminently forgettable discovery of an inert nickel-steel alloy, and in 1921, the prize was not awarded. By then, denying Einstein was possible for the Committee but not bestowing it on another. Finally, in 1922, Einstein was awarded the held-over Nobel of 1921, not for the Theory of Relativity for which he was most famous. It was for the discovery of the photoelectric effect – that light also behaves as a particle – that Einstein had made in 1905. It was also the same year that he had published his first of relativity papers, the special theory of relativity.

Penrose’s work had laid a firm mathematical basis for black holes and in the heart of such a hole, a space-time singularity. Hawking developed this further to propose that the entire universe started with a singularity in time; or with a Big Bang. Although Hawking achieved an iconic status, perhaps the most famous physicist after Einstein, he never received the Nobel Prize. Penrose’s Nobel Prize for the space-time singularity is perhaps a bow to Hawking for the Nobel which he never received.

Theories in physics open out possibilities to understand our universe. But without experimental verification, there is still a niggling doubt that some new phenomena could contradict the theory. So the search for experimental verification, the supposed gold standard of physics. And when it comes to astrophysics, a daunting task of performing experiments with stars light-years away! This is why Chandrasekhar’s Nobel Prize took more than 50 years, Penrose’s 55! And as Nobel is not awarded posthumously, some physicists never.

Dr Andrea Ghez is a professor in the University of California, Los Angeles and Dr Genzel, the Director of the Max Planck Institute in Garching, Germany. Ghez’s team used the Keck Observatory in Hawaii, while Genzel’s group used telescopes in Chile operated by the European Southern Observatory (ESO). Both the teams have been in “competition” for some time and have jointly received many honours. In this case, it was over tracking stars close to the Galactic centre of the Milky Way. Both teams tracked the same star, called So2 by Ghez’s team and S2 by Genzel, which had a very short orbiting period around the centre of Milky Way, of only about 16 years compared to the sun’s orbit of 200 million years. Both teams’ results, using different telescopes and data sets over decades, have shown that they are in close agreement that a super heavy object, with a mass of about four million suns, lies at the centre of our galaxy. In the staid language of the Nobel Committee, “A robust interpretation of these observations is that the compact object at the Galactic centre is compatible with being a supermassive black hole.”

We have come a long way from Einstein’s Theory of Relativity and Chandrasekhar’s stellar collapse. So let me end with Chandrasekhar’s Nobel speech, where he quotes Rabindranath Tagore:
Where the mind is without fear and the head is held high;
Where knowledge is free;
Where words come out from the depth of truth;
Where tireless striving stretches its arms towards perfection;
Where the clear stream of reason has not lost its way into the dreary desert sand of dead habit;
Into that haven of freedom, let me awake.

Oft quoted, perhaps overused, but nevertheless appropriate for our dark times.