Ever since Einstein formulated general relativity in 1915 a little more than a century ago, a tantalizing consequence of this theory has intrigued physicists: there can be objects in the astronomical universe within which the gravitational field is so strong that nothing – even light – can escape from them. For the better part of the century, however, the generally held view within the community of physicists was that this is merely a strange theoretical possibility. Such objects, christened ‘black holes‘ by John Wheeler, were suspected not to exist. This year’s Nobel Prize is an official recognition that black holes are no longer a scientific fantasy, but very much an integral part of the physicist’s current view of our universe. Half of the Prize has been given to two astrophysicists who demonstrated the existence of a monster black hole at the centre of our galaxy. Although light takes about 25,000 years to reach us from this galactic centre, present-day astrophysicists think of it merely as our backyard when contrasted with the distances of the most faraway galaxies revealed by our telescopes.
At least one Nobel Prize in the past was for a work which had a direct bearing on black holes. Subramanyan Chandrasekhar, recipient of the 1983 Nobel Prize, showed for the first time in the 1930s how a black hole might arise. In a normal star, energy is produced by nuclear reactions in the central core. The tremendous pressure caused by the heat of these nuclear reactions balances the gravity of the star. All stars eventually run out of their nuclear fuel. When that happens, Chandrasekhar found that stars more massive than about 1.4 solar masses would embark on an unstoppable process of contraction – suggesting a mechanism by which black holes can form.
If the contracting star was rotating, then the centrifugal force due to rotation would surely make the contraction in the equatorial region slower compared to the contraction in the polar region. Can this kind of uneven contraction somehow avoid the formation of a black hole? Such doubts could be put to rest only in the 1960s when Roger Penrose (recipient of half the Nobel Prize) and Stephen Hawking proved the very powerful singularity theorems of general relativity. They showed that an inevitable and inescapable conclusion follows from the basic equations of the subject. The gravitational contraction of a massive astronomical object must lead to what is called a singularity in spacetime – a point beyond which we cannot proceed any more with our known laws of physics. In other words, if general relativity is correct, then a gravitational contraction has to result in a black hole. The singularity theorems also established another result of great astrophysical significance. We know that our universe is expanding with time. A natural conclusion from this would be that the universe began its expansion from a singular state in the past – what astrophysicists call a big bang. An important question again was whether the universe could have evolved in such a manner that there was no initial big bang. The singularity theorems showed that the universe must have begun from a singularity if general relativity holds. The big bang must have taken place.
The work of Penrose and Hawking is unanimously regarded as the most important theoretical breakthrough in general relativity since Einstein’s original formulation of the theory. We Indians can take pride in the fact that there was an Indian prelude to this breakthrough. Amal Raychadhuri working in Calcutta in 1955 arrived at an important equation from the basic principles of general relativity. The Raychaudhuri equation was the starting point for Penrose and Hawking to prove the singularity theorems.
Physics is an experimental science. Physicists justifiably have a healthy scepticism towards any novel theoretical idea until it is confirmed by experiments. If light cannot escape from black holes, then we certainly cannot ‘see’ black holes. What kinds of observations can then astrophysicists do for establishing the existence of black holes? A black hole would exert a strong gravitational pull on matter in the surrounding regions. The presence of the black hole has to be ascertained by studying and analyzing what is happening in its vicinity. If matter in the surrounding region falls into the black hole, then the gravitational potential energy lost in this process can get converted into intense radiation emitted before the matter disappears into the black hole. Astrophysicists in Moscow and Cambridge formulated the important theoretical ideas of how this happens.
X-rays from astronomical objects cannot reach us at the surface of the earth, since they are absorbed by the atmosphere. X-ray detecting instruments flown in man-made satellites in the 1960s discovered X-rays coming from some binary stars (two stars orbiting around each other). It was concluded that one member of some of these binary star systems is a black hole, on which gas from the other star is falling. X-rays come from this very hot infalling gas. This was the first astronomical evidence for the existence of black holes. The black holes discovered in this manner are typically somewhat more massive than the sun – consistent with the limit found by Chandrasekhar.
Astronomical evidence started mounting that much more massive black holes may form in the centres of some galaxies where matter from the surrounding regions may keep accumulating due to the strong gravitational attraction there. A normal galaxy is a collection of a very large number of stars. Astrophysicists discovered that some galaxies appeared to be something more than a mere collection of stars. Intense radiation comes from very small central cores of some galaxies. They are called active galaxies. Quasars discovered in 1960s are the most extreme examples of active galaxies. It was inferred that supermassive black holes from a million to a a billion times heavier than the sun exist at the centres of these galaxies and the radiation is coming from matter falling into these black holes. Since our galaxy appears to be a ‘normal’ galaxy and not an active galaxy, one interesting question is whether there is a black hole at the centre of our galaxy, which is in the direction of the constellation Sagittarius. Visible light coming from this galactic centre is obscured by gas and dust in the intervening space. The groups of Reinhard Genzel and Andrea Ghez studied stars in this region in the near infra-red light which can reach us by penetrating this cosmic veil of gas and dust. After several years of painstaking and meticulous work, they reached the conclusion that the observed very rapid motions of some stars in this region could be explained only by the existence of a black hole four million times more massive than the sun at the galactic centre. A science Nobel Prize is not given to more than three recipients. Had Hawking been alive, the Nobel committee surely would have difficulties in deciding the list of recipients. While the work of Genzel and Ghez is first-rate science and undoubtedly deserved the Nobel Prize, we should not forget the tremendously important contributions of many others in providing a definitive proof of the existence of black holes.
Professor Arnab Rai Choudhuri teaches at the Department of Physics, Indian Institute of Science, Bangalore.