On 6th of October, the Royal Swedish Academy announced that one half of the Physics Nobel Prize has been awarded to Sir Roger Penrose “for the discovery that black hole formation is a robust prediction of the general theory of relativity”, while the other half was awarded jointly to Reinhard Genzel and Andrea Ghez “for the discovery of a supermassive compact object at the centre of our galaxy”. Thus, one half was a tribute to the brilliant minds that worked out the theoretical predictions of the reality defining Theory of Relativity while the other half went to the experimental findings that shed further lights on our own galaxy; nevertheless, all these works helped us in forming a clearer understanding of our universe. In the relay race of advancing human civilization, the baton of knowledge is passed down from one generation to another. The brilliant work which culminated in this year’s Nobel Prize for Physics was borne out of tireless striving by a few of the most brilliant minds of the world including Albert Einstein, Stephen Hawking, and surprisingly an Indian, of whom not many people have heard about: Amal Kumar Raychaudhuri. How the work of Nobel Prize awardee Roger Penrose connects back to all these names and especially Amal Kumar Raychaudhuri, is a story that must be told.
In 1915, when Einstein delivered his universe defining General Theory of Relativity (GTR), it aroused many fundamental questions about our existence, universe, and singularity. What is a singularity? It’s a point where all physical laws are indistinguishable from one another, where space and time are no longer interrelated realities, but merge indistinguishably and cease to have any independent meaning. As a result of Einstein’s GTR, singularities were predicted resulting in the theoretical existence of black holes. According to GTR, any star reaching a certain point in its mass (aka. the Schwarzchild Radius) would produce an intense gravitational force to collapse the star. At this point, nothing would be capable of escaping its surface, including light and thus the name Black Hole. For earth, the Schwarzchild Radius is 1 cm i.e. packing the earth into the tip of your thumb would produce a black hole while for the Sun, it’s 3 km. But physicists around the world were scratching their heads about how singularities are produced from gravitation. Then came two young and outstanding scientists who worked out a set of their own theorems that dared to answer the question of singularities being produced out of the womb of gravitation. Roger Penrose, in 1965 proposed the Penrose Singularity Theorem, in which he proposed that whenever matter reaches a certain energy-condition inside a black hole, the space-time curvature in it becomes infinite, virtually turning it into a trapped surface where time literally stands still, producing a time-like singularity! Stephen Hawking added to this theorem by formulating his Hawking Singularity Theorem, stating that a space-like singularity can occur when matter is forcibly compressed to a point, causing the rules that govern matter to break down. Both these theorems were merged together into Penrose-Hawking Singularity Theorem, to tackle the mind baffling question of singularity generation from gravitation.
The GTR was proposed in 1915 and for the next 50 years, all the scientific bigshots in the world were splitting their hair about the singularity problem. Finally, the problem was somewhat theoretically settled down by the young Penrose and Hawking in 1965. But the question generally arises, how did they do it? That’s where the story gets interesting, with the arrival of Indian scientist Amal Kumar Raychaudhuri aka AKR. Einstein attempted to describe the universe via mathematical models, considering it as static by introducing solution to his field equations. In his belief of a static universe, Einstein introduced cosmological constant to his field equations. But that was later proved incorrect by ground-breaking works by two scientists – Alexander Friedmann and Georges Lemaitre – who showed that the universe was indeed expanding. All the debates regarding the static universe were put to rest in 1929, when Edwin Hubble (after whom the Hubble’s space telescope is named) experimentally showed that the universe was expanding. Now to any logical mind it will strike, if the universe is expanding, then it must have begun somewhere, that beginning of the universe is the case of singularity or colloquially the Big Bang, the point from where our universe began. Thus, singularity was established, but the question that how it arises still remained unsolved. None of the scientists were being able to account for the appearance of singularity, when suddenly AKR burst into the scene. In 1955, when AKR was still working at Indian Association for the Cultivation of Science, Kolkata, he came up with his eponymous ‘Raychaudhuri Equation’. It was the first successful attempt, which described the dynamical factors like gravity, spin and shear that govern the expansion and contraction of the universe. Within a few months of publication of this paper in the Physical Review journal, it started getting noticed by the leading scientists of the day. When it reached the hands of the young Roger Penrose and Stephen Hawking, they used it and reached their conclusions that Singularity is unavoidable, except under strange energy conditions. The Raychaudhuri Equation served as a fundamental lemma or fundamental theorem, on which the conclusions of Penrose-Hawking Singularity Theorems were based, and the world was never the same again.
Since then Black-Holes were discovered, Einstein’s legend grew even more, Stephen Hawking grasped the universe from the confines of his wheel chair and Roger Penrose, the only person still alive among them, won one half of the 2020 Nobel Prize. And what about AKR? Well, he must be dwelling somewhere out there, beyond all ideas of physical laws, beyond space-time, he must have become one with the eternal, he must have found his own Singularity.
-Aishik Bhattacharya (Freelancer at IndianFolk.com and Senior Research Fellow at IACS Kolkata)
-Cosmology for the Curious, A Vilenkin and Delia Perlov
-Biog. Mem. Fell. INSA, N. Delhi 30 169-180 (2006), J V Narlikar
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