Historical remarks about black holes

Black hole is a region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull. As far as 1784, an English geologist John Michell realized that it would be theoretically possible for gravity to be so overwhelming strong that nothing, even light could escape. In order to generate such gravity, an object must have to be a very massive and unimaginably dense. At that time, such an object seemed physically impossible.

The idea of black holes resurfaced in 1916, when Karl Schwarzschild succeeded to solve Einstein field equation. However, Schwarzschild limited the complexity of the field equation by assuming the star is perfectly spherical and that it did not rotate. His calculations yielded a solution aptly called the Schwarzschild solution. The name black hole was invented in 1968 by an American physicist, John Archibald Wheeler.

An artist depiction of two black holes merging.

In 1939 Robert Oppenheimer and H. Snyder suggested in a paper that, “When all thermonuclear sources of energy are exhausted, a sufficiently heavy star will collapse. Unless (something can somehow) reduce that star’s mass to order of that of the sun, this contraction will continue indefinitely…. The contraction will go past the phases of white dwarfs and neutron stars, to an object which is cut off from the communication of with the rest of the universe. The suggestion thus redirected attention to the Schwarzschild model of the exterior of a star. To this, the name black hole was invented later in 1968 by an American physicist, John Archibald Wheeler.

The collapse of a spherical star leading to formation of trapped surface, event horizon and spacetime singularity.

Another development took place in 1954 when a young Russian physicist, A.Z Petrov classified spacetime with the families of radially ingoing and outgoing light rays in the Schwarzschild model as Petrov type D classification. Petrov’s classification was slow in making into mainstream, but was crucial to the next major development.

In 1963, using Petrov type D classification and by examining an algebraically simple class of type D metric tensors, Roy Kerr succeeded in finding the long-sought metric of spinning black hole spacetime. The work was further advanced in 1967 by R.H Boyer and R.W Lindquist by introducing their elegant coordinate system. In the same paper, they found the maximally extended Kerr spacetime and investigated their geodesics. However, the full analysis of Kerr geodesics becomes possible only with the discovery of a fourth geodesic first integral by Brandon Carter in 1968.

At this moment, we are still unable to observe black hole directly. However in the recent years searches have been made to find the invisible partner of twin stars. In 1971, the telescopes aboard UHURU satellite detected rapid variations of the X-ray source, Cygnus XI and this provides the first evidence of the likely existence of the black holes.

On the theoretical level, there have been objections on the solutions being too special in the spherically symmetric property. It is not surprising, if all matter is moving radially towards the center, then it will ultimately results in a singularity there. However, perturbation of the Schwarzschild solutions have been considered and they appear to suggest that all asymmetries are eventually radiated away and that asymptotically in time, the system will settle down to Schwarzschild black hole.

Another important address to the abovementioned objection is given by Hawking and Penrose who managed to prove some remarkable theorems, the singularity theorems, which suggest that many of the qualitative features of this collapse picture remain in more general situations. Both Hawking and Penrose’s work is independent of the particular field equations of the general relativity.

During the 1970s, papers on black holes were so extensively written with many new ideas, mathematical models and new approaches. One of the radical approaches is to include quantum effects on black holes. The considerations of the quantum effect led to black holes being no longer considered as permanent structures as suggested by the classical theory. On this regard, Hawking suggested that the black holes seem to emit particles at a steady rate, which is known as Hawking radiation. The idea of radiation from black holes was the first example of a prediction that depended in an essential way on both the great theories of the last century, general relativity and the quantum mechanics. The works of Carter, Hawking and Bardeen together with Bekenstein were one of the most significant, by associating black hole with the thermodynamics. Later the work was extended to layout the thermodynamic behaviour of the black hole.