Wheeler’s Legacy

In 1905, Einstein presented the special theory of relativity, wherein he established that the laws of physics appear the same in all inertial frames, and he confirmed the constancy of the speed of light in a vacuum. Following the discovery of the special theory of relativity, Einstein dedicated another decade to incorporating acceleration into his theory. In 1915, he formulated the general theory of relativity, which addressed the behavior of an accelerating observer [1]. The general theory of relativity (GR) introduced a groundbreaking concept in physics by unifying space and time into what is now known as spacetime, a departure from the Newtonian understanding of gravity [2]. Among the most captivating and remarkable predictions of GR is the existence of black holes within our universe. Black holes represent regions in spacetime where the gravitational force is so intense that not even light can escape from them. Their exceptional characteristics make black holes some of the most enigmatic objects in astrophysics, and their properties can be elucidated by solving Einstein's field equations.

The notion of escape velocity exceeding the speed of light from the surface of a compact object was initially broached by John Mitchell in 1783 [3]. Subsequently, in 1796, Pierre Simon de Laplace further explored this concept [4]. In the 20th century, shortly after the discovery of GR, German mathematician Schwarzschild made a groundbreaking contribution by precisely solving Einstein's field equations for a vacuum, resulting in the formulation of the first black hole metric, known as the Schwarzschild metric [5]. The Schwarzschild metric encompasses two distinct types of singularity: a central singularity located at its center (r = 0) and a removable coordinate singularity occurring at its Schwarzschild radius (rs = 2GM). It was later revealed that there exists a maximum mass limit for a white dwarf, often referred to as Chandrasekhar's limit [6]. When a white dwarf star surpasses this limit (1.4M⊙), it undergoes further gravitational collapse, transforming into either a neutron star or a black hole. Subsequently, Fritz Zwicky and Walter Baade thoroughly investigated various properties of white dwarfs, excluding their maximum mass, prompting extensive research into this specific aspect, with notable contributions by Landau and others [7, 8, 9, 10, 11].

Oppenheimer and Volkoff demonstrated the upper bound limit of mass for a massive star during the formation of a neutron star [12]. They, along with Synder, subsequently conducted an in-depth study on the gravitational collapse of massive stars, leading to the formation of black holes [13]. The process of a sufficiently massive star collapsing into a black hole is such that, for a distant observer, it takes an infinite amount of time. Consequently, such collapsing stars were termed "frozen stars" during that era [14]. Nearly four decades after Schwarzschild's discovery of the black hole solution, Kerr derived the rotating counterpart of the Schwarzschild metric [15]. Additionally, Wheeler provided a physical model for collapsing stars [16] and coined the term "black holes" in his seminal paper.

Black holes exhibit an extremely high degree of matter compression, resulting in the extraordinary strength of their gravitational fields. Any external matter that enters the gravitational field of a black hole either crosses its event horizon and plunges into the black hole or accumulates in the form of an accretion disc around it. The event horizon, serving as the boundary of the black hole, marks the point beyond which any matter is irretrievably lost [17].