The 2020 Nobel prize in physics has been awarded with one half to the theoretical physicist, Roger Penrose and the other half jointly to the two observational astrophysicists, Reinhard Genzel and Andrea Ghez. This year’s Nobel prize has been awarded in recognition of the pioneering work carried out by the three physicists on black holes in the cosmos.
A black hole is one of the outstanding predictions of the general theory of relativity (GTR), developed by Albert Einstein in 1915. GTR is probably one of the greatest triumphs of the human ingenuity and is based on the simple equivalence principle that states that Newton’s inertial mass is same as the gravitational mass. This theory elucidated that gravity is caused by the curvature of space-time and also solved many mysteries, for instance, the anomalous orbital motion of the planet mercury that Newton’s gravitational law failed to explain.
A few months after Einstein announced GTR, Karl Schwarzschild, an astronomer, pointed out that Einstein’s equations contained a catastrophic solution with space-time collapsing into a point having infinite density, what is referred to as a singularity. This discovery of singular solution of the GTR equations, which basically predicts the existence of black holes, was not further explored by the scientists at that time as Einstein considered the solution to be a mathematical construct and argued that nature would find a way to avoid such catastrophic situations.
There were several important developments before research in GTR and black holes became the mainstream research disciplines in physics in 1960. The most important advancement occurred in 1931 when Subrahmanyan Chandrasekhar showed that non-rotating body of electron degenerate matter above a certain limiting mass of 1.2 times the solar mass (the currently accepted value is 1.4), what is now called as Chandrasekhar limit, has no stable solution. [It is interesting to note that the basic idea for this important finding was conceptualized when Chandrasekhar was travelling to England from India to pursue higher studies.] The most significant consequence of the Chandrasekhar limit is that a star having core mass of 1.4 times the solar mass can collapse, which under certain conditions may result in the formation of a black hole.
The important mechanism in the formation and the collapse of stars in the cosmos is the gravitational contraction or collapse. The astronomical objects contract under the influence of their own gravity that draws matter towards the centre of gravity of the objects. The gradual contraction of the interstellar matter under the gravitational inward pull leads to the formation of the stars. This gravitational collapse continues till the contraction raises the temperature of the core matter sufficient enough to set-off the thermonuclear fusion reaction. The outward thermal pressure exerted by the thermonuclear reactions occurring in the core counterbalances the inward pressure of gravity. When the two pressures are equal and opposite, further gravitational collapse is halted and the star reaches a state of dynamic equilibrium. The sun in our solar system is right now in this phase of equilibrium and is shinning due to the thermonuclear reactions occurring in the core of it. As soon as the material inside the star is exhausted to sustain the fusion process, the star begins to collapse again under the gravitational pressure until it reaches a new equilibrium state.
The new equilibrium state depends on the mass of the collapsing star. If its mass is less than 10 times the solar mass, it will decay into what is called as a white dwarf, which is a stellar core remnant mostly composed of electron-degenerate matter. The nearest known white dwarf is Sirius B, which is about 8.6 light years away and is the smaller component of the Sirius binary star. If the collapsing star has mass between 10 to 25 times the solar mass, the core becomes one of the densest stellar objects, what is called as the neutron star. The gravitational collapse of still heavier stars is considered to be dominant mechanism through which stellar mass black holes can be formed.
During the period from 1960 to 1975, what is called as the golden era of relativity, studies in black holes became a frontier subject of research in astrophysics. The observation of newer objects in the cosmos like pulsars, which were shown to be rapidly rotating neutron stars, was one of the reasons that research in black hole physics witnessed resurgence. Further, Roger Penrose, the Nobel Laureate, invented a new approach in 1965 to depict space-time, so called Penrose diagrams, that circumvented the mathematical intricacies of GTR. Using this technique, Penrose and Hawking demonstrated that singularity in GTR equations appear quite generically. Earlier, it was thought that singularity may be an artifact of the symmetry restrictions imposed on the solutions. They also showed that GTR predicts that the universe must have had a beginning. Although, most of the astrophysicists greeted the news of Nobel prize of 2020 being awarded to the work done on black holes, but some were dismayed that Stephen Hawking who was arguably the world’s leading figure on black holes, could not be part of the Nobel prize winning team. Hawking died in 2018, making him ineligible for the Nobel prize. The prize cannot be granted posthumously as per the statute added in 1974.
The most important question that needs to be addressed is what is the observational evidence that back holes exist in the cosmos. The problem is that black holes are opaque bodies and don’t emit any electromagnetic radiation to prove their existence directly. The indirect methodology that astrophysicists have adopted to detect the presence of a black hole in space is through its gravitational influence on the surrounding bodies. It is believed that at the centre of each galaxy in the universe is a supermassive black hole. The movement of stars near the centre of our own Milky Way galaxy provides a strong observational evidence that these stars are orbiting around a massive black hole. The invisible object is also the radio source called as Sagittarius A*.
The groups led by other two Nobel Laureates of 2020, Reinhard Genzel and Andrea Ghez, have been painstakingly observing about 90 stars around Sagittarius A* for decades. Recently, after one of the stars called S2, completed a full orbital motion, the scientists were able to infer that an object of mass of about 4.3 million times the solar mass must be contained in a volume with radius of 0.002 light years to cause the observable motion of the orbiting stars. There is also other observational evidence that this object might have an event horizon, a unique feature of a black hole. The event horizon is an envelope around a black hole at which the escape velocity of the particle must exceed the speed of light, which is impossible. This implies that any object falling on the event horizon will disappear, because light emitted by or reflecting off the object can no longer escape.
This is the first and certainly not the last Nobel prize to be awarded for unraveling the mysteries of the black holes. These are very exclusive objects in the cosmos where GTR and quantum mechanics intersect. General theory of relativity, on the one hand, explains the large scale structures and has completely revolutionized our perception of the universe. Quantum mechanics, on the other hand, is the underlying principle behind the workings of the microscopic world of atoms and molecules. Most of the modern appliances, for instance, mobile phone, computer, transistor, laser, electron microscope and the global positioning system are based on the quantum mechanical laws. To understand the physics of black holes, both quantum mechanics and GTR must be applied as these objects are small and at the same time are supermassive. The problem is that GTR and quantum mechanics are mutually exclusive theories as they are based on different axioms and cannot be applied simultaneously. Presently, one of the greatest challenges in physics is to unify the two theories. The unified approach shall provide insight into the inner workings of a black hole and the people who will develop the new theory shall certainly deserve a Nobel prize.
Prof. Sheikh Javid Ahmed is presently Professor of Physics at University of Kashmir.