TODAY, we have come to accept the idea of black holes, from the insides of which nothing, not even light, can escape. A black hole, which is a curious mathematical consequence of Albert Einstein’s general theory of relativity (GTR), is perhaps the strangest object in the universe. This mysterious object, once thought to be only the stuff of science fiction, is now accepted as real. Just about a year ago, astronomers were actually able to image a supermassive black hole’s immediate surroundings in a faraway galaxy. In 2015, exactly a century after Einstein came up with his theory, gravitational wave detectors picked up signals from the collision of two black holes (Figure 1).
This year’s Nobel Prize in Physics has been awarded to scientists who provided the theoretical and experimental basis for why black holes should and do exist. Roger Penrose, the 89-year-old British theorist from Oxford University, gets one half of the prize for his theoretical description of how gravitational collapse could result in a black hole. The Royal Swedish Academy of Sciences citation said that Penrose was chosen for the award “for the discovery that black hole formation is a robust prediction of the general theory of relativity”.
Sixty-eight-year-old Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics, Garching, Germany, and 55-year-old Andrea Ghez of the University of California at Los Angeles share the other half of the prize for their experiments that provided the observational evidence of a supermassive black hole at the centre of the earth’s galaxy, the Milky Way. The award citation for them said, “for the discovery of a supermassive compact object at the centre of our galaxy” .
Event horizon
Einstein himself did not believe that such objects could actually exist. In the very year after he formulated the theory, the German astrophysicist Karl Schwarzschild found a solution to Einstein’s equations of GTR which implied that, as a spherically symmetric object collapses under its own gravitational pull, there exists an “event horizon”, a characteristic radius associated with every quantity of mass (now called the Schwarzschild radius) at which the matter falling inwards under collapse will be totally cut off from observers outside that radius. The black hole remains hidden forever inside the event horizon. Everything that is inside it is gravitationally trapped.
In 1939, Robert Oppenheimer, the father of the A-bomb, and his graduate student Hartland Snyder wrote a paper in which they described how a star might collapse into an object so dense that not even light could escape from it. When giant stars run out of the fuel that keeps them shining and the radiation pressure counteracts the inward gravitational pull of the star’s own mass, they first explode as supernovae and then the remnants collapse into a dense object whose gravity pulls everything inside, including light. “The star thus tends to close itself off from any communication with a distant observer; only its gravitational field persists,” the duo wrote. However, scientists were not comfortable with such an inexorable collapse leading to weird stellar objects. The famous Russian physicist Lev Landau, in fact, wanted to modify quantum mechanics if required to ensure that such situations did not arise.
More problematic than the event horizon was that Einstein’s equations also predicted that a point of infinite density—a mathematical singularity—would result in the object’s interior. Physics does not have tools yet to handle such a catastrophic situation. One could think of this as the reverse of the Big Bang. To many physicists, this is a failure of the GTR, and they argue that when quantum theory and gravitation are merged into a single quantum theory of gravity, an area of intense current research, this unphysical situation of a star collapsing into an object of infinite density, where time and space would end, would be prevented. This is an idea that goes back to Landau, who had wanted quantum theory to prevent the formation of a black hole (though the name black hole itself only came in 1967).
Until the 1960s, physicists argued that such scenarios were theoretical oddities due to the assumptions made in the analyses that stars are perfectly round and absolutely symmetrical, which in reality is not the case. Indeed, Landau’s associates, E. Lifschitz and I.M. Khalatnikov, wrote a paper showing that these situations were the result of the idealised assumptions made. Penrose was not convinced by the Lifschitz-Khalatnikov arguments and set about finding a generalised solution to the question of all inward falling matter irrespective of their original geometry.
Referring to the Oppenheimer-Snyder work, Penrose said in a post-announcement interview with Nobel Media: “Well, it was a paper… with a theoretical model… of a dust cloud, and it was more or less the kind of situation we would now refer to as the collapse of a black hole. But the thing is they had first of all dust, and dust by definition is something with no pressure, so there’s nothing to stop it. And secondly it was completely symmetrical, so everything fell in towards the centre and so since there was nothing to stop it you got this singular point in the middle and actually a model which looks like a black hole. But not many people believed it, most particularly because of the symmetry… Lifschitz and Khalatnikov …had written a paper that more or less said that in the general case you would not get singularities…. I looked at the paper and I sort of thought that the way they were doing it wasn’t terribly convincing…. I didn’t know whether to trust it, and so I started thinking about it on my own and thinking about this problem in a more geometrical way, not really solving equations… and not making simple assumptions about symmetry because that’s the point, you mustn’t have that, so I produced arguments.”
Discovery of quasars
The trigger for scientists, including Penrose, to revisit the question of the existence of black holes was the discovery in 1963 of quasars, the brightest objects in the universe. For nearly a decade before that, astronomers were puzzled by the emission of radio waves from mysterious sources such as 3C273 in the Virgo constellation (“Star among astronomers”, Frontline , October 9, 2020). Visible light from that object finally revealed the distance at which it was located: 2.5 billion light years from the earth. Scientists reasoned that light from such a faraway source meant that the intensity of light emitted must be equivalent to several hundred galaxies. Emission of such an incredible amount of energy from within a small volume of a quasar is possible only if there was matter accreting into a massive black hole—a million-solar-mass black hole—at enormous speeds.
In 1964, Penrose considered the question of whether black holes could arise in real-world situations of stars that were not perfect spheres. Investigating the unsymmetrical collapse of matter from a purely topological perspective, he showed that the formation of a black hole was an inevitable consequence of the GTR; any sufficiently dense object must form a black hole. So black holes form when stellar matter gets packed with very high density, the greater the mass the larger the black hole, and its event horizon. To get an idea of the density required, consider that the mass equivalent of the sun has an event horizon with a diameter of about 3 km, whereas a mass such as that of the earth matter has to be packed into a diameter of just 9 mm.
Penrose needed to expand the methods used to study the GTR with new topological ideas. He argued that regardless of shape any object with an event horizon would contain what he called a “trapped surface”. All rays from a trapped surface point towards a centre irrespective of whether the surface is curved outwards or inwards. He described a trapped surface as follows. It has the property such that if you had light bulbs sitting on the surface, all the emitted light rays would converge irrespective of the surface curvature because of the strong gravitational pull. Penrose recalled later that this idea came to him at a crossroads in London near Birkbeck college, where he was a professor. The idea apparently just flashed into his mind when he stopped talking for a few minutes to watch out for traffic while waiting to cross the road.
Once a trapped surface is formed following the collapse of matter, nothing can stop the collapse from continuing. It is a one-way street beyond the event horizon that all matter can cross only in one direction. The implication of this property is that anything inside a black hole has only one future: falling towards the centre. Time replaces space inside the event horizon and all paths point towards an inescapable end, the singularity. Penrose proved that a black hole always hides a singularity. The topological concept of a trapped surface was central to his proof of the singularity theorem. His ground-breaking paper was published in Physical Review Letters in January 1965 and is regarded as one of the most important contributions to Einstein’s GTR.
But it is not possible for any external observer to peer beyond the event horizon because no signal can escape it, and so the singularity remains inaccessible. Penrose later formalised this idea into a physical principle called “cosmic censorship”. Alongside, Penrose also wanted astronomers to search the skies for black hole-related phenomena. In 1969, he wrote: “I only wish to make a plea for ‘black holes’ to be taken seriously and their consequences to be explored in full detail.”
Even though a black hole cannot be seen, its massive gravitational influence on the orbital motion of surrounding stars can be observed and measured and its properties inferred. This was the essence of Penrose’s plea. Reinhard Genzel and Andrea Ghez, who worked independently and led separate research groups, have done just that for over three decades. They meticulously measured the motion of stars in the centre of the Milky Way, a galaxy that is 100,000 light years across and consists of interstellar gas and dust and a few hundred billion stars, including the sun.
The intervening gas and dust prevented much of the visible light from the centre of the galaxy reaching an optical telescope on the earth. So, early on, observations of the galactic centre largely relied on infrared and radio astronomy. On the basis of the orbits of stars, Genzel and Andrea Ghez produced the most convincing evidence so far of an invisible supermassive object lurking at the centre of the Milky Way, as had been suspected soon after the discovery of quasars.
Adoptive optics
Karl Jansky, the father of radio astronomy, detected the first radio signal from the Milky Way in the direction of the constellation Sagittarius in 1931. Later observations confirmed his discovery, and a strong source of radio waves was found there; the region was called Sagittarius A*. By the end of the 1960s, it became clear that Sagittarius A* occupied the centre of the galaxy around which all the stars of the galaxy orbited (Figure 2). But more systematic studies of that object had to wait until Genzel and Andrea Ghez entered the scene in the1990s. They began their observations with bigger telescopes, better equipment and the advanced technique of “adaptive optics” (“For a true picture”, Frontline , November 4, 2011).
Genzel and his group started with the 3.58 m New Technology Telescope of the European Southern Observatory (ESO) on La Silla mountain in Chile. Later, they began to use the other big instrument in Chile, the Very Large Telescope (VLT) facility, also the ESO’s, on the Paranal mountain. With four big telescopes, each of 8.2 m diameter, the VLT has the world’s largest monolithic mirrors. Andrea Ghez’s team, on the other hand, used the Keck Observatory located at Mauna Kea in Hawaii, United States. Its mirrors are almost 10 m in diameter and are currently among the largest in the world. The mirrors are of a honeycomb design consisting of 36 hexagonal segments that can be moved separately to focus the starlight better.
Notwithstanding the fact that the bigger the telescope the better its resolution, the 100-km-deep atmosphere below which an observer on the earth is located distorts the incoming light, posing a limiting factor to that resolution and causing an inevitable blur. Adaptive optics was a crucial development in astronomy and enabled astronomers to correct for the image-distorting atmospheric turbulence and led to a more than thousand-fold improvement in resolution. This helped both Genzel and Andrea Ghez determine star positions and orbits much more precisely.
Following the motion of the 30-odd brightest stars at the centre of the galaxy, researchers found that stars moved extremely fast if they were within a radius of one light month from the centre not unlike a swarm of bees. But those outside this region followed more ordered elliptical orbits. One of the studied stars, called S2, completes its orbit around the galactic centre in less than 16 years, which is short compared with the 200 million years the sun takes to complete its orbit.
There has been extraordinary agreement between the measurements the two teams made, and these measurements led to the conclusion that the supermassive object at the centre of the galaxy must have a mass of about four million solar masses packed into a region about the size of the solar system.
Talking about his team’s work with great excitement, Genzel said in his post-announcement interview to the Nobel Media: “That [adaptive optics] was the first phase of innovation in the late ‘90s. But that’s not sufficient to come very close to the galactic centre. So, our most recent innovation has been to combine four of these telescopes, two of them cause an interferometer, so there’s four 8 m telescopes. And with that, we can then sense the motions of stars which are orbiting the black hole with exquisite precision. We also see actually gas… very close in the accretion zone around the black hole. That’s about as close as you can get because any closer all material has to disappear in the black hole. So in that sense we are seeing not it, but we are seeing, so to speak, we are sensing its gravity, and we are seeing, you know, gas and stars moving around it, and just by how the gas moves, how the stars move we can then infer with high precision what it is and that there must be a black hole. And also in fact that general relativity holds even in this super strong regime of curvature.”
But is that object indeed a black hole? That, as Genzel said, could be the only possible explanation. As the theorist Vitor Cardoso of the Technical University of Lisbon told the online news letter Physics of the American Institute of Physics: “The evidence is so overwhelming that a black hole is the least exotic explanation for the object sitting there.” In fact, since quasars were discovered, physicists have argued that most large galaxies would be harbouring supermassive black holes.
What remains is to actually picture Sagittarius A*. This is clearly a possibility. As mentioned in the beginning of the article, in April 2019, the Event Horizon Telescope network succeeded in imaging the immediate vicinity of a supermassive black hole located deep inside the galaxy known as Messier 87 (M87), 55 million light years away from the earth, which, as the Royal Swedish Academy of Sciences press note on the Nobel award said, “is a blacker than black eye surrounded by a ring of fire”. The gigantic core of M87 is more than a thousand times heavier than Sagittarius A*.
The first gravitational waves were detected in 2015 using the Laser Interferometer Gravitational-Wave Observatory (LIGO) detector in Louisiana, U.S., for which the research team was awarded the 2017 Nobel Prize. The waves were the result of the collision of two black holes that occurred 1.3 billion years ago. Of course, those black holes were much lighter, weighing only about 36 and 29 solar masses. But like black holes, gravitational waves were detected exactly 100 years after Einstein wrote the equations that predicted their existence.
Story is far from over
But the story is far from over. Penrose showed that in certain scenarios a singularity arises naturally as a consequence of the GTR, but at the singularity, GTR itself ceases to apply because infinite density does not have any physical meaning. Nature perhaps has a way out, and may be a quantum theory of gravity will reveal nature’s working.
