Musa Fahad
This paper firstly provides a brief introduction to black holes. It is about the comparison of the study of black holes. It differentiates between the theories on black holes in the past compared with the present day knowledge on black holes which accepts or rejects those theories. It further talks about the concept of singularities, and how modern-day scientists have provided alternative explanations to the concept of singularities. It also provides information regarding the types of black holes. Black holes have always been a part of nature that has astonished scientists till this date.Objects billions of times larger than our sun, not even allowing light to escape, and altering the dimensions of spacetime, the black holes are nothing, but a monstrous force of the universe. The concept of black holes was first introduced by John Michell, a professor at Cambridge University, in 1783. He wrote a letter to Henry Cavendish on 27th November, 1783, stating, “On the means of discovering the distance, magnitude of the fixed stars, in consequence of the diminution of the velocity of their light, in case such a diminution should be found to take place in any of them, and such other data should be procured from observations, as would be farther necessary for that purpose.” (The Royal Society Publishing) Michell accepted Newton’s theory that lights consists of small material particles. He reasoned that such particles, emerging from the surface of a star, would have their speed reduced by the star’s gravitational pull. He then questioned what would happen if a star’s gravitational pull was so strong that its escape velocity exceeded the speed of light? Then, light would not escape from the star, and fall back to it. Michell then used Ole Roemer’s speed of light which he discovered in the 17th Century to hypothesise that the escape velocity would exceed the speed of light on a star 500 times bigger than the sun. Light would not be able to escape from such a star, making it invisible to the outside world. This was the first ever concept of a black hole given by John Michell.The concept of black holes came into light again after Einstein. Published his theory of General Relativity in 1915 which became the base of black hole studies. Einstein sent his theoretical solutions to his colleague Karl Schwarzschild, the director of the Astrophysical Observatory in Potsdam, an accomplished theorist and mathematician. At the time, Karl had joined World War I despite being in his 40s. Weeks before, Einstein had completed his work of a whole decade, successfully expanding his special theory of relativity to include gravitational forces along with electricity and magnetism. The theory of General Relativity is summarised by John Wheeler as, “Space-Time tells matter how to move; matters tells Space-Time how to curve.” Within a matter of days, Schwarschild wrote back to Einstein with the first known solution to Einstein’s field equations. Tragically, Karl passed away a year later on the frontlines. (Astronomy: A brief History of Black Holes, August 29,2019). However, he left behind a solution that completely describes how space-time is warped outside a spherical object like a planet or star. One of the features of this mathematical solution is that for very compact, high-density stars, it becomes much harder to escape the gravitational field of the star. Eventually, there comes a point where every particle, even light, becomes gravitationally trapped. This point of no escape is called the event horizon. As one approaches the event horizon, time slows to a complete standstill. Early physicists, having no explanation to this phenomenon gave it the name of frozen stars. It wasn’t until 1967, when John Wheeler proposed the name of black holes. Many physicists at first believed that such an object cannot exist. Arthur Eddington quoted,“There should be a law of nature to prevent a star from behaving in this absurd way.” It took many years until black holes were accepted beyond just a mathematical curiosity. Over the years, multiple types of black holes have been discovered since the 20th Century. The first type of black holes is Stellar Mass black holes born from the death of stars i.e when stars run out of nuclear fuel and collapse under their own gravity. Other stellar mass black holes form from the collision of two neutron stars. These stars were detected by LIGO and Virgo in 2017. These are probably the most common type of black holes. The second type of black holes are called supermassive black holes. They are present at the centre of nearly every galaxy. Their masses range from 100,000 times to billions of times the mass of a sun. The EHT (Event Horizon Telescope Collaboration) captured the first ever image of a black hole, the supermassive black hole at the centre of galaxy M87 in 2019. Intermediate mass black holes are the third type, being the least found. They weight 100 to 10,000 times the mass of our sun. After the discovery of black holes in the past, it has been hypothesized that at the centre of every black hole, exists a singularity. A singularity is a region of space where the density of matter, or the curvature of space-time becomes infinite and the concepts of space and time cease to have any meaning. At the point the whole fabric of space-time ruptures and the precepts of Einstein’s theory of General Relativity along with general physics rules break down and no longer apply.
However, modern scientists have other theories as to what happens at the centre of a black hole, denying the existence of singularities. According to latest research published in the journal Physics Review D, black holes can be completely different celestial bodies called gravastars. João Luís Rosa, study co-author and professor of Physics at the University of Gdańsk in Poland said, “Gravastars are hypothetical astronomical objects that were introduced (in 2001) as alternatives to black holes, they can be interpreted as stars made of vacuum energy or dark energy: the same type of energy that propels the accelerated expansion of the universe”.
Paul M. Sutter, astrophysicist at SUNY Stony brook and the Flatiron institute, host of Ask a Spaceman and Space radio, and author of How to Die in Space provides other possible theories as well. He states that another possible alternative explanation to reject singularities is the theory of Planck Stars. Inside the black hole, matter is not crushed to an
infinitely tiny point, rather there could be the smallest possible configuration of matter. As matter is crushed under the massive gravitational weight of a dying star, it is met with resistance. The discreetness of space-time prevents matter from reaching anything smaller than the Planck length (around 1.68 times 10^-35 metres). Allthe matter that has ever fallen into a black hole gets compressed into a ball not much bigger than this. The resistance to continued pressure eventually causes the matter to uncollapse (i.e explode) making black holes only temporary objects. But, because of time dilation effects around black holes, it takes billion to trillions of years for black holes to explode from our perspective. (Space.com: what happens at the centre
of a black hole, (February 9, 2022). The third theory to counter the existence of singularities is the singularity acting as a portal. The idea of a single point of infinite density comes from our conception of stationary, non-rotating black holes. However, when black holes spin, scientists hypothesise that the spin of a black hole stretches the singularity into a ring. This theory does not deny the existence of singularities, instead it implies thst the
role of a singularity in the black hole is not just being a point of infinity where the laws of physics break down. It suggests that according to
Einstein’s theory of General Relativity, one you pass through a ring singularity, you enter a wormhole (a passage between two points in space-time shortening the distance between the two actual points) and 7 come out through a white hole at the speed of light, landing in an
entirely new part of the universe. White holes are exactly the opposites of black holes; black holes attract everything inside while white holes
push everything outside of themselves. (Space.com: what happens at the centre of a black hole, (February 9, 2022) All of these are just theories to counter the idea of singularities. Scientists develop these theories to counter the idea of an infinite point of time, as according to physics, any point in space-time cannot be infinite, while any theory leading to such a conclusion is considered to have a fault.
More than 200 years after John Michell gave the idea for the possible existence of black holes, when his idea was so ahead of time that it did not receive any attention, scientists of the modern era now have the first
photograph of black holes. With countless science-fiction movies on black holes, and latest groundbreaking discoveries about their shape and structure, black holes are a significant branch of astrophysics today. Even now, there is hardly enough known about these giants of nature to
validate any of the theories given by scientists as to what is at the centre of these black holes, scientists continue to question these claims, finding
new evidences everyday. Over the next few decades, we may have much more knowledge about black holes than we do today, which may
completely convert our concepts into something much more complex and different than our understanding of these celestial supergiants today.