Dielectica traverses through the literature on this topic – and summarizes as they appear.
Correspondence prepared by: Sayan Bayan1 and Suman Chakraborty2, 1S. N. Bose National Centre for Basic Sciences, Kolkata, India, email: sayan.bayan@gmail.com, 2Physical Research Laboratory, Gujrat, India, email: suman.chakrabarty37@gmail.com (16:10:2020 and 14:00)
Key Words: Black holes, Event horizon, General Theory of Relativity, Singularity
India: The Nobel Prize in Physics for the year 2020has been awarded in one half to Roger Penrose“for the discovery that black hole formation is a robust prediction of the general theory of relativity” and the other half jointly to Reinhard Genzel and Andrea Ghez “for the discovery of a supermassive compact object at the centre of our galaxy”. Henceforth, the three scientists shared this prize for their outstanding discoveries about the ‘black hole’ which is considered as one of the most mysterious themes in Physics. To give a thought on their discovery, one needs to understand what are black holes and why are they so mysterious?
A black hole can be defined as a region in space where gravity is so strong that even light cannot escape through it. The black hole is enclosed by a boundary called ‘event horizon’ which defines the region where the velocity required to escape exceeds the speed of light [1]. Such strong gravity originates from the fact that matter gets squeezed into a very tiny space. Such a situation can be realized during the death of a star having mass more than three times the mass of the Sun. Since light cannot get out of black holes, it is conventionally invisible in naked eyes. However, a black hole can be realized by the virtue of its gravitational pull on other stars around it. The strong gravitational force of a black hole will pull matter from other stars. During this process, the compression and heat generation (in millions of degrees) will lead to the production of X-rays that radiate in space. Thus the outward region of the black hole can be realized by the emission of X-rays. In many occasions, the spreading of radio waves has also been witnessed [2].
Now back to the past, theoretically, the existence of black holes has been traced from Einstein’s general theory of relativity, although Einstein himself denied the concept of black holes. According to this theory, the force of gravity arises from the warping of spacetime (fusion of three dimensional space and one dimensional time) around a body. The core of this theory is some nonlinear equations known as Einstein field equations. Soon after the appearance of this theory, Karl Schwarzschild found the non-trivial solution of the Einstein field equations which led to the concept of gravitational collapse and the condition of singularity [3]. A singularity is a location where the gravitational field becomes infinite. However, the concept of singularity was debated a lot as many scientists attempted to prove that singularities don’t appear in generic solutions. In 1955, A. K. Raychadhuri’s work remarkably transformed the field of general theory of relativity as the equations laid by him formed the basis of all singularity theorems [3]. The defining moment came with the singularity theorem in 1965 where Penrose sparked the idea of incompleteness to describe singular spacetime and for the first time he introduced the concept of a closed trapped surface. The concept of a closed trapped surface reveals the inner region of an event horizon and can be assumed as the surface from which light is not moving away. Thus Penrose established that singularities appear generically and concluded with his Nobel winning discovery. Penrose’s singularity theorem was ample and straightaway and led to the development of modern singularity theorems [3].
Penrose established the existence of black holes theoretically, while teams led by Ghez and Genzel’s demonstrated experimental evidence of such celestial giants at the heart of our Galaxy. Since a long time physicists suspected that most galaxies include a supermassive black hole at the centre. The bright Sagittarius A* (Sgr A*) – a radio source situated at the centre of Milky Way, was suspected as a black hole. In the 1980s, Genzel and Charles Townes (another Nobel laureate) exploited InfraRed (IR) spectroscopy to track gas clouds orbiting at the centre of the Milky Way. Although they could infer the presence of a massive, compact source of gravitation, the evidence was not ultimate.
The prime challenge was to detect the emission from the stars amidst the gas and dust obstacles. However in 1990, teams led by Ghez and Genzel came out to address this issue with the world’s biggest telescopes which work in the near-IR region suitable for sensing the light that can escape the dusty region of the galactic centre. Ghez and her team used the Keck Observatory, Hawaii, while Genzel and his co-workers used the Very Large Telescope on Cerro Paranal, Chile. With a series of developments in imaging techniques, they could improve the resolution and sensitivity to the faint light from the celestial world.
The two teams tracked and plotted the tracks of several stars near the centre of the galaxy for a decade, particularly the star called S0-2 by Ghez’s group or S2 by Genzel’s team. This led to the understanding that the motion and pattern of the stars are influenced by the presence of an invisible source of four million solar masses and which is certainly a supermassive black hole [4]. This was not only the strongest evidence of the presence of the giant black hole at the galaxy’s core, but also stimulates research on the immense gravitational effects on its stellar neighbours.
Sources:
[1] https://www.nasa.gov/audience/forstudents/k-4/stories/nasa-knows/what-is-a-black-hole-k4.html
[2] https://science.nasa.gov/astrophysics/focus-areas/black-holes
[3] J. M. M. Senovilla, D. Garfinkle, Class. Quantum Grav., 32, 124008 (2015).
https://iopscience.iop.org/article/10.1088/0264-9381/32/12/124008
[4] www.timesonline.co.uk/tol/news/uk/science/article5316001.ece