Lurking behind the dusty visible portions of the universe lie objects with strange characteristics. Light cannot escape from these objects of mystery, rendering them virtually invisible. The power that they wield is so colossal that nothing can evade their intense gravitational grasp. Such objects seem implausible, yet do exist. Black holes, however comically and incorrectly named, have a place in this vast universe. Collapsed giant stars, black holes have been lurking in the shadows of the cosmos waiting to be found for millions of years. Fascinating since first discovered, black holes are tantalisingly close to, yet invisible from, Earth and contain untapped resources for the potential benefit of humanity.
In 1783, John Michell, a British natural philosopher, was the first to contemplate the likelihood of such objects as black holes. He was attempting to imagine a very compact star's effect on the light particles it emits (then called 'corpuscles'). He supposed that, to escape a star's gravitational pull, a particle must be projected at a sufficiently high velocity. The greater the gravitational pull, the more it would slow down these particles, so the greater the velocity required for them to escape. Michell calculated the minimum speed for a particle to escape using Newton's laws of gravity, and called this the 'escape velocity'. This escape velocity was determined by a particle's square root of its mass divided by its circumference. This means that the smaller the circumference (distance around the centre of the star), the stronger the pull of gravity at its surface. Thus, the escape velocity would increase.
Michell also stated that, once a star's circumference reached a certain size, the escape velocity would become the speed of light. Calling this the 'critical circumference', he predicted that the particles of light would just scarcely be able to escape. If a star existed with a greater circumference than this critical circumference, no light would be able to escape. Being unable to think of any laws to prevent this from happening, he decided that there could exist a great deal of these throughout the universe. Later on, however, light came to be thought of not as a particle, but as a wave. This made these 'dark stars' unable to fit into Newton's Laws of gravity (light would later become known as both under quantum theory).
Unfortunately, Michell's ideas were based on the Newtonian laws, where time was regarded as absolute. When the general theory of relativity was proposed, Michell's ideas did not fit into the new sense of time and space. His ideas, however, weren't forgotten. In 1916, Karl Schwarzschild found a way to fit the dark stars into general relativity. Not many people accepted this, and even Einstein didn't believe dark stars to exist. The reasons behind this lay in people's tendency not to believe in an object of huge mass, yet zero size. Not many people would accept this without a large amount of evidence in favour of the theory, little of which existed at the time! The importance of dark stars wouldn't be realised until John Wheeler studied Einstein's theory of general relativity, despite its seeming unimportance at the time.
Subrahmanyan Chandrasekhar, an Indian graduate student, went to England in 1928 to study under the astronomer Sir Arthur Eddington, an expert on general relativity. Chandrasekhar determined how a large star could exist and be able to endure its own gravity, even after it ran out of fuel. Once the star began to shrink, the particles of matter would be forced closer to each other, and repel one another. Thus the star would remain in equilibrium, its gravity balanced by these repulsive forces. He also calculated a limit to how large the star could become for this to occur. If the star was too massive, the force of repulsion would be overcome by the vehement gravity. This limit is reached by a star about 1.5 times more massive than our sun, and is known as the 'Chandrasekhar limit'. If such a star was more massive than this, it would collapse into itself at a single point. Eddington, who didn't believe this strange theory, inevitably caused Chandrasekhar to abandon this research.
In 1939, this problem was solved when Robert Oppenheimer came up with results that made Chandrasekhar's ideas comprehensible. His work on the atom bomb project in World War II delayed him, however. Oppenheimer determined that gravity affects light, therefore, at a certain critical radius, light itself wouldn't be able to escape a star. More people became interested in the 1960s when technology became more modern. This new technology allowed them to observe immense gravitational effects where there had previously appeared to be nothing, and the radiation let off by black holes at their poles.
In the constellation Cygnus, X-rays were found to be discharged at one thousand times per second from an invisible source. It was discovered that whatever was creating this was only 300 kilometres in diameter, and was right next to a giant blue star. The giant star's path was being pulled by something with an immense but unseen gravitational force. This was the first time that an object believed to be a black hole was detected in the sky. The X-rays were thought to be coming from the friction inside streams of dust and gas being pulled into the black hole from the star next to it. Many other stars have since been found that show the same phenomenon.
Black holes weren't named 'black holes' until later still. The first name proposed was by Michell, who called them 'dark stars,' because they were collapsed stars that were unable to emit light. Since Schwarzschild started working with the dark stars, and calculated a way for them to exist, they were then known as 'Schwarzschild singularities'. In 1969, John Wheeler upset many French people by naming the objects 'black holes'. The literal translation to French was an inappropriate phrase, so the French suggested the name be changed to 'astre occlu', literally translated as 'hidden star'. However, this wasn't nearly as popular a name as a 'black hole', so that was the name that stuck.
Unfortunately, this name doesn't accurately describe these mysterious objects. To understand a black hole, one must first understand how one is formed. First, a very large star, no less than thirty times the size of the sun, must run out of fuel. It will then cool down, and begin to collapse. Being too large for the aforementioned repulsion of its particles to keep itself stable, the star will collapse further still. The smaller it becomes, the stronger the gravitational pull at the surface. After it reaches the critical circumference, even light itself, having the highest attainable velocity, will no longer be able to escape from the surface. The star will then commence to collapse to a tiny point called a 'singularity', which is a point of zero size yet infinite density. Therefore, a black hole isn't a 'hole' in space, but an infinitely dense object pulling in and devouring everything in its path.
Once not even believed to exist, black holes are now known to be found throughout the universe. Very large black holes are found at the centres of galaxies, whereas smaller ones are scattered just as unevenly as the stars all around. The giant black hole at the centre of the Milky Way, Sagittarius A, was recently observed for two weeks. It was found to have high-energy eruptions, lobes of very hot gas at twenty million degrees Celsius, and faint X-ray streams up to one light year1 in length. All of these amazing findings are thought to be related to activity near the event horizon of the black hole.
The event horizon is equivalent to a point of no return. Once anything passes it, it can't escape, no matter what it does, as it cannot ever reach the escape velocity required, as this is greater than the speed of light. It is thought that if a person fell into a black hole, he or she would see the history of the universe right there in front of him or her. However, the difference in the gravitational pull from the person's head to his or her feet would be so great that he or she would be stretched out and most likely either resemble a thin piece of yarn, be very quickly dead, or, even more likely still, both. The gravitational pull of a black hole is so great that both time and space are greatly distorted in its vicinity, and, at the point of the singularity, anything pulled inside is torn apart. Along with the event horizon and the point of singularity, most black holes have large discs of hot gas encircling them, which is attracted by their extreme gravity.
Black holes that are very massive, even compared to other black holes, are known as 'supermassive black holes', and at least one is believed to be found at the centre of every galaxy. They are now thought to actually form before the surrounding galaxy. When viewing early galaxies, a study showed that there was already a fully formed supermassive black hole at each of their cores, despite the galaxy surrounding them not yet itself being fully formed. These black holes are thought to formed by gathering materials from the dense cores of other, older galaxies. Ohio State University researcher Marianne Vetergaard has said in light of this:
Looking at this evidence, I have to think that black holes start forming before galaxies do, or form at a much faster rate, or both.
As for the other end of a black hole's life, nothing will be able to be viewed for many, many years. However, Stephen Hawking predicted what would happen when a black hole stopped spinning. Known, and named after him, as 'Hawking Radiation', even black holes do still give off radiation, and thus lose energy. He also found that the temperature of the black hole is related to its surface gravity. Along with that idea was that, as the black hole loses energy and shrinks, its temperature and surface gravity will increase, causing it to evaporate. This will happen over a very long period of time, which is why no one will be able to witness one for many, many years, and possibly never will, but, after the black hole shrinks beyond the size of the nucleus of an atom, it will become so intensely hot that it will brutally explode in a tiny fraction of a second.
Some more interesting things about black holes are their potential uses. One use could be as an energy source. Roger Penrose of Oxford University calculated how this might be accomplished. First, in order to harness this energy, a structure would have to be built to orbit the black hole, which could supply a fuel to deposit into it. Each particle of the fuel would be broken into two pieces at the event horizon, with one falling inside it. The other particle would give off a boost of energy as its comrade was demolished. These bits of energy could be collected and used to sustain a ship or civilization.
However, this source of energy couldn't last forever. Cemetrios Christdoulou, a researcher for Princeton University, revealed that black holes would eventually stop spinning after all of this, and would not be able to release any more of their energy. Despite the prospect of great new energy supplies, the nearest black hole is well over 15 light years away, which is about eighty-eight trillion miles away. This is a short distance relative to the size of the universe, but seeing as travelling such distances is well outside the scope of our current technology, this energy cannot yet be harnessed.
Black holes may also even be used for time travel. The closer to the speed of light that an object travels, the slower time passes for it. Einstein abandoned the idea of absolute time, and one of his ideas was that:
Each person travelling in his or her own way must experience a different time flow than others, travelling differently.
Einstein himself couldn't perform an experiment to show this since the effects on time aren't easily observable until one reaches speeds far closer to the speed of light than anyone ever has. Later, experiments using very accurate atomic clocks and extremely fast jets were used to help prove this idea.
This relates to black holes, in that orbiting around a black hole would bring one to very high speeds, close to the speed of light, causing the time in that person's view to seem normal, but to everyone else very slow. The person moving that fast would feel as if he were travelling ordinarily, but would to himself be moving forward in time faster than everyone else, thus creating time travel. This can only go one way, though. Time travel backwards would be very difficult, if not impossible. Once again the nearest black hole would be too far away to even try this. The same experiments involving atomic clocks on high-speed jets have, again, proven this to be true.
Of course, going forward in time faster than usual would be difficult due to the immense amounts of energy that would be required. The amount of energy that would most likely be needed to move significantly more rapidly forward in time would be similar to that which a medium-sized star would use.
This is just a small sampling of the vast information on black holes. These enticing objects have made heads turn in the world of physics as well as the world in general. Their theorisation was first looked on as a joke, but today as a great idea of long ago. Black holes have intrigued humans since first discovered, and that has helped some to discover all of the fascinating facts about them, including their potential uses. Black holes may seem so very distant and invisible, but in actuality are closer than ever. Many scientists and philosophers have been calculating and thinking about these faceless wonders, and, thanks to their dedication and brilliance, humans today can have the knowledge of something virtually invisible in the abyss of the universe.