The universe is an awe-inspiring place. It has been a matter of scrupulous perusal and research. One of the most intriguing parts of the universe is the concept of black holes. The study of black holes dates back to the 18th century. However, major advances in the study of black holes were only made in the early 20th century. Post the popularisation of the concept, black holes also featured in fiction.
Earth, a novel by David Brin talks about an artificial black hole that has fallen into the Earth’s core. The events and the plot described in the book addresses a lot of other issues at hand, like global warming, endangered species and eco-terrorism. The highly acclaimed fictional book also claims that the only way to protect Earth from the impending disaster is to let the human race die out and let the evolutionary clock start over. While there have been several works of fiction around the subject, there has been significant amount of research gone into it as well.
Black Holes: Voids Left Behind by Former Stars
NASA defines a black hole as a region in the space with a gravitational force so strong, it pulls in light too. The magnitude of such gravity can be credited to the fact that black holes are tiny spaces of tightly squeezed and packed matter. They are cold remnants of former stars in the form of highly compressed objects. The underlying concept behind a black hole was introduced in the 18th century by the English geologist, John Mitchell.
The forgotten genius had opened up a plethora of scientific facts and intriguing questions when he realized that if the Sun could be compressed down by several orders of magnitude, it would develop such a strong gravitational pull that only entities travelling at a speed faster than that of light could escape it. In his paper published at the Philosophical Transactions of the Royal Society of London in 1783, he largely spoke about these objects that he then called ‘dark stars’. A few years later, a similar suggestion was made independently by a French scientist Marquis de Laplace. Both these notions considered light as particulate matter that could be slowed down due to gravity.
Expounding the blur
A famous experiment carried out by Michelson and Morley in 1887, however, questioned this belief. The experiment proved that light traveled at a constant speed of 186,000 miles a second, questioning how gravity could pull it back. In 1915, Einstein’s revolutionary General Theory of Relativity united time and space as related entities curved and warped by energy and matter in it. According to this theory, the stars curve and distort the spacetime dimensions around them. The massive stars create a bottomless hole with an extremely high gravitational pull once it burns its nuclear fuel, cools and shrinks below a critical size.
To visualize the theory described above , think of spacetime as a rubber-sheet with weight on top of it resembling stars. The weight of the stars will create a depression on the sheet. Higher the weight of the star, deeper is the depression. The term ‘black hole’ was finally coined in 1967, by an American physicist, John Wheeler.
How are Black Holes formed?
Black holes were just abstract mathematical concepts before the astronomer Chandrasekhar calculated the formation of these exotic spatial objects. Stars are held in perfect balance and form by two opposing comparable forces: the inward gravitational pressure, counteracted by the outward pressure of its emitted radiation. Stars release a tremendous amount of nuclear energy, owing to the exothermic reaction occurring at its core. This reaction produces gamma radiation due to conversion of millions of tonnes of hydrogen to helium every second.
This conversion process is extended from helium to carbon, and eventually to oxygen as heavier elements continue to form. Most stars, like the Sun stop at oxygen as they are not massive enough to support fusion reaction beyonds that. They transform into a white dwarf and cool down. However, stars bigger in size and volume continue the fusion reaction to elements higher up in the periodic table like silicon, aluminum, potassium, and so on, all the way to iron. It is known that no energy can be produced by fusing iron atoms together.
As a result, the radiation from the star turns off immediately. The gravitational force exceeds the outward pressure and the star implodes. The cataclysmic detonation of these stars, called ‘Supernovae‘ often scatters a part of a star into the void of space, in the form of gas and dust particles. The remnants however form a massive ‘cold’ object that collapses down into a much smaller volume of space. The velocity required to escape from this collapsing star increases till a point where even the speed of light is not sufficient. This is how a black hole is formed.
Any object can become a Black Hole
The distant explosions of Supernovae scatter elements and radiations into the space. The material so produced may lead to the formation of planets, spatial objects and even living organisms. This explains the fundamental link between us and black holes. However, the crux of the concept is that any object can become a black hole.
Since, we are all made of atoms, we have a lot of empty space. The electron cloud around the nucleus of the atom can be compressed, thus accounting for a huge room for probable compression into a tight object. With a high enough concentration of mass at any point in space, it can have an escape velocity greater than that of speed of light. On attaining this state, any object would be a black hole. However, the size to which an object needs to be compressed in order to become a black hole is the question.
In 1915, a German physicist, Karl Schwarzschild used Einstein’s field equations of General relativity to come up with a quantitative justification to the theory. He furnished an equation that could calculate the radius of the sphere an object would have to be squished to the size of, for it to become a black hole. The equation (Rs = 2*GM/c2 ) needs you to plug in your mass (M) and Newton’s Gravitational Constant (G) along with the speed of light (c) to calculate the Schwarzschild-radius (Rs).
… But cannot suck in everything around it!
Having said that any object can transform into a black hole, it is of foremost importance to bust the notion that it can pull everything into it. This brings us to the point of introduction of another concept: the Event Horizon. Once a star turns into a black hole, it is exiled from the rest of the universe by a clear boundary. This boundary is called the Event Horizon, basically the region where gravity is just strong enough to drag light back, and prevent it escaping.
Anything crossing the line between the interior and the exterior of the black hole, will be lost forever. The boundary thus acts like a one-way membrane. The span of the event horizon depends on the mass of the star that formed the black hole. The event horizon formed by a star with a mass several times that of the Sun could be a few kilometers across. However, this could grow once the black hole starts engulfing dust, particles and other black holes.
It is important to note that any object being converted into a black hole will maintain a constant mass. Hence, it will have the same gravitational impact on the surroundings as it did before compression. When it is said that nothing escapes a black hole, it can be rephrased as ‘anything that crosses the event horizon of a black hole, cannot escape it’.
Black Holes continue to intrigue
A campaign launched in April 2017 attempted to capture the first-ever image of a black hole. Astronomers and researchers attmpted several experiments using a system of radio-telescopes to photograph the massive black hole at the center of the Milky Way, Sagittarius A*. This project, called Event Horizon Telescope (EHT) links observatories across the globe and attempted to demystify several smudges around the concept.
The project also aimed to shed some light on the information-paradox, a puzzle combining quantum mechanics and general relativity. It is believed that physical information along with other states can disappear permanently into the black holes. However, there has to be a limit to the amount of information that a black hole can contain. Since information is a function of energy (which in turn is dependent on mass, courtesy: E=mc2), there is only a certain amount of it that can be packed into a confined space.
While the entire concept is still a bit muddy, a lot has been written about it. Stephen Hawking has an insightful article on it that attempts to break the complex concept down. While more is being spoken about it, we on Earth can breathe for a while as the Sun will never turn into a black hole, owing to its mass. Space organisations are using the most advanced hardware and theories to assimilate this learning. One can only wait to watch, what eventually comes of it.