The course of understanding any object in science often starts with its visualization then studying it's properties, analysing its effects on surrounding and finally building mathematical model describing its nature. This is the reason we use the word "Dis-cover" for the process of unraveling unkown in science. Visual or sensory experience of object precedes the theoretical/mathematical inquiry of its existence. But what if we have constructed a whole theory, mathematical model around an object that we haven't been able to see, feel or hear? The existence of one such phenomenon was haunting the science since 18th century. Physicist have build detailed models of it's nature but often were suspitious of it's existance. Yes, you all guessed it right, Black Holes: "one of the most exotic object in the universe" as described by the Nobel Committee while announcing the winners of this year's Nobel Prize in Physics.
Roger Penroz “for the discovery that black hole formation is a robust prediction of the general theory of relativity” and Reinhard Genzel with Andrea Ghez “for the discovery of a supermassive compact object at the center of our galaxy” have awarded Nobel Prize in Physics 2020, half to Roger Penroz and other half to Reinhard Genzel with Andrea Ghez jointly.
Although an object of extreme mathematical complexity, black holes might be the only such concept that has been hugely popular amongst general public thanks to numerous science fictions, Hollywood movies and rumours of apocalypse caused by its possible encounter with us often spread by popular media.
Scientifically, Black hole is extremely dense region of space where gravity is so strong that no object, even light cant escape its gravity. When supermassive star that is much larger than our sun burns out its fuel, it eventually collapses under its own gravity to form such dense region. Such black holes are called stellar black holes and have mass in range of 5-60 times the mass of sun. However, there are many black holes which are with mass many millions and billions times the mass of sun called supermassive black holes. How these supermassive black holes are formed is quite a mystery yet. Gravity of the black hole is so tremendous that any object in its vicinity is stretched like spaghetti, shredded into pieces and swallowed by it which further increases black hole's mass and makes its gravity even stronger. The gigantic mass of the black hole which ranges from 5 solar masses to some billion solar masses is concentrated at a single point called singularity which stretches the space infinitely encapsulating it from the outside universe. Singularity is surrounded by Event Horizon, the boundary around singularity which separates it from the outside universe. Path to the event horizon is unidirectional. Once an object falls into the event horizon, it cannot escape black hole‟s gravitational pull. Inside the event horizon all the known laws of physics collapse. Time stand still hence no event can takes place.
Every object around us consists of mass. As per Newton's universal law of gravity, there always exists the gravitational attraction between two masses. This is the reason why every object around us falls back to earth when thrown upwards. However, if we throw an object with sufficient force it can escape earth's gravitational pull never to fall back on earth. This is the way scientists and engineers send satellites in space using extreme thrust generated in the opposite direction to that of gravity using rockets. Greater the mass of object, greater it's gravitational pull hence greater thrust required for escaping its gravity. So, it will require only 1/6 th of the force for an object to escape moon's gravity than the force required for it to escape earth's gravity (as moon is much smaller and less massive than earth) .
The idea that there could be stars so massive that even their own light could not be escaped from the clutches of their gravity was first proposed by John Michell in 1783. He called them "Dark Stars". He suggested that there might be many such dark stars in the universe. However, we will not be able to detect them by just sight as no visual information is being passed by them. Sadly the idea of dark star was so ahead of its time that it made no major impact on science of the time and forgotten with his death until his writings resurfaced in 1970s.The first major possibility of its existence was emerged from the revolutionary General theory of relativity published by Einstein in 1916. Equations of general theory of relativity eventually directs to the strange, dense, massive object which will curve the space time to the extent that it engulfs the light emitting from star making it invisible to outside world. The German physicist astronomer Karl Schwarzschild provided the first exact solution to the Einstein field equation to general relativity and formulated black hole mathematically. However until 1970s the black hole was considered mere a mathematical possibility. Even Einstein didn't think that it could exist. However ten years after Einstein's death in 1965 British physicist Roger Penerose (NobelLaureate for 2020 Physics Nobel) invented the mathematical methods to show that General Theory of Relativity eventually leads to formation of black hole and also described it's properties. Before Penerose, the solution to notoriously complex equations of general theory of relativity were solved using completely spherical and symmetric model of black holes and were regarded theoretical speculation as nothing in the universe is perfectly symmetrical. Through his quirky idea of trapped surfaces he devised the crucial mathematical tool to describe real world black holes.
While Penrose's work laid down the mathematical bedrock to describe the black hole, the advances in telescope technology began to unravel the mysteries of the universe. Invention of Radio Telescope in 1930s and Infrared Telescope in 1960s allowed us to venture into different regions of electromagnetic spectrum. Discovery of Quasars (star sized object in early universe which emit the energy equivalent to few galaxies) and Neutron Star (highly dense star entirely made up of Neutron Particle) in 1963 and 1967 respectively, increased speculations of real existence of Black Hole.
Even though we cannot see black hole, we can observe effect of its gravity on surrounding stars. If there are other stars orbiting black hole, their orbits can be used for calculating mass and location of the black hole. Astronomers have identified numerous such black hole candidates in multiple binary star systems (stars orbiting each other). In August 1931 Karl Jansky considered as father of Radio Astronomy, discovered bright radio signal coming from Sagittarius constellation. Source of this signal was hidden in the clouds and dust around galactic center. Because of this, astronomers were unable to observe it using optical telescopes. In February 1974, astronomers discovered bright but very compact radio source „Sagittarius A*‟ using infrared telescopes. Looking at very compact size of the radio source Sagittarius A*, physicistshave been suspecting the existence of the black hole at the center of Milky Way since then. In early 1990s Reinhard Genzel and Andrea Ghez (other Nobel Laureates for 2020 Physics Nobel, Andrea being only fourth woman to win physics nobel) leading separate research teams with refined methodologies, better equipment and bigger telescopes started to map the orbits of brightest stars circling the galactic center. Both of these teams have monitored the motion of the stars at galactic center tirelessly for three decades. Calculations of both the groups found existence of heavy invisible galactic center forcing nearby stars swirl around it with astounding speed. The invisible mass has a four million solar masses crumbled into the region no longer than our solar system pointing to the only one possible candidate, a supermassive Black hole.
On 14th September 2015, LIGO (LASER INTRFEROMETER GRAVITATIONAL WAVE OBSERVATORY) collaboration observed first ever signal of Gravitational Wave, which is another outcome of General Theory of Relativity that Einstein hardly believed to be ever detectable (Nobel Prize of Physics 2017 ). These are ripples in the space time, caused by some of the most energetic events in the universe. The strongest gravitational waves of the magnitude detected by LIGO can be generated by most violent events in the universe like collision of Black Holes or Neutron Stars. Subsequent discoveries of more such gravitational wave events by LIGO, VIRGO Interferometer have validated the existence of massive invisible objects like Black Hole in recent time. Also in 2019 the EVENT HORIZON Telescope astronomy network succeeded in imaging the closest surrounding of a supermassive black hole M87 (Black eye surrounded by ring of fire).
This year's Nobel Prize in physics celebrates humans endeavor for seeing the invisible. It honors the contribution of best minds in the history to the discovery of most exotic object in nature. Further, it encourages us to further investigate into the many secrets of the universe that are waiting to be discovered.
Nice, well written and very simple to understand.
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