Black holes are postulated to be one of the most destructive forces known in the universe, with nothing being able to escape from its gaping maw, not even light. Even, supposedly, all known laws of physics fall apart inside the depths of the black hole. So how does the universe allow these objects to exist? Or rather do they exist or are they just a product of humanity's misunderstanding? Fundamentally black holes are a product of gravity, the weakest of the four known forces in the universe. It was with Isaac Newton in which the first seeds of knowledge concerning black holes began to grow.
With his laws of universal gravitation, Newton proposed that this gravitational force acts on everything, even light. As Novikov (1990) says "It is with the understanding of the fact that light is also subject to gravitational force that the early history of black holes began." The first tentative step into theorising the existence of black holes was taken by the British natural philosopher John Michell in 1783. In his explanation to show what very compact stars should look like, Michell tried to combine Newton's law of gravitation with the corpuscular description of light. Michell came up with the notion of 'escape velocity', which explains the velocity with which a particle needs to escape a body's gravity. Anything under the escape velocity will, by the force of gravity, be pulled back down. Developing this idea further Michell came up with the theory of a critical circumference, for which the speed of the light is its escape velocity for this body's mass.
Any thing under this critical circumference then light would even be pulled back down by gravity's overwhelming force. Michell speculated that there was a number of stars in the universe, which were under the critical circumference, these he called dark stars. These 'dark stars' were the eighteenth century version of black holes. Laplace thirteen years later also predicted these dark stars to exist, but in the third edition of his famous work 'Le System du Monde' there was no mention of these dark stars at all. It was in the following two centuries that amazing discoveries in science were forged, which were to help predict the existence of black holes. James Clark Maxwell's (1865) electromagnetic theory united electricity and magnetism, which at the end of the nineteenth century were two of the known three forces in the universe, the other being gravity.
Maxwell came up with four equations, which are known as the theory of the electromagnetic field, and it was with this that the first visible cracks in the Newtonian view of the universe began to show. The second crack appeared when Albert Michelson began timing the propagation of light. In the Newtonian view of the universe space and time are absolute and the speed of light is relative. Using what is now called the 'Michelson inferometer', a highly accurate experimental technique, Michelson found that there was no variation in the speed of light.
It was the same in all directions irrespective of motion. It was with the advent of one physicist, Albert Einstein, which changed the fundamental nature of astrophysics. It was while working on the problem of trying to fit Maxwell's electromagnetic laws with the Newtonian view of the universe, that Einstein in a flash of inspiration turned the known laws of physics upon its head. In one swift stroke Einstein rejected Newton's view of absolute space and time, demanding that space and time are relative. With this he devised two new fundamental principles. The Principle of the absoluteness of the speed of light-Whatever might be their nature, space and time must be so constituted as to make the speed of light absolutely the same in all directions, and absolutely independent of the motion of the person who measures it.
The principle of Relativity- Whatever might be their nature, the laws of physics must treat all states of motion as relative. These two principles started Einstein on his quest to properly explain the phenomena of the universe, which would inevitably lead to a renewed enquiry into the existence of black holes. Einstein's first principles led him onto some amazing discoveries: mass could be converted into energy (the famous E = MC 2) and the principle that nothing could go faster than light. With this first set of relativity theory, named special relativity, Einstein turned to the problem of fitting gravity into this relativistic framework. It wasn't Einstein, though, that first unified space and time. Minkowski (1908), who built on Einstein's work, discovered that the universe is made up of a four-dimensional 'spacetime' fabric that is absolute and not relative, and exists independently in all reference frames.
But following this discovery, which Einstein first rejected, Einstein began to develop his theory of gravity. The conclusions, which he came up with, were devastating for the Newtonian view of gravity. He proposed that space and time are warped, which made him realise that what was previously thought of as tidal gravity, was actually the manifestation of spacetime curvature (or warpage). In 1915 published his results into marrying his relativistic laws with gravity, which he called General Relativity, and with this the renewed interest into the dark stars of Michell and Laplace began. Karl Schwarzschild first took the tentative first step into black holes, after the publication of the general relativity theory. From Einstein's field equation he calculated the spacetime curvature outside any non-spinning spherical star, which led to the formation of Schwarzschild geometry, which was to expand knowledge of gravity and the universe enormously.
The Schwarzschild equations found that if a mass is compressed into a sufficiently small radius space-time becomes so severely distorted that even light can't escape from the force of gravity. This is the same critical circumference that Michell found more than a century earlier. The theory says that a star two or three times the mass of the sun light will not to be escape the pull of gravity. The theory also states that once these stars run out of energy, they begin to collapse inward with such tremendous force that even the powerful internuclear forces within the atoms of the star aren't sufficient to prevent it from continuing to fall in on itself until the entire mass of the star is concentrated at a point called a singularity. It is within this singularity that matter is infinitely compressed into a region of infinite density, gravity is infinite and spacetime has become infinitely curved. The formation of a black hole begins when a star begins to contract, concentrating itself into an ever-smaller region of space.
As this contraction continues the effects of gravity becomes more and more pronounced, and soon light cannot even escape the pull of gravity itself. This happens when the star reaches the Schwarzschild radius, because the escape velocity has exceeded the speed of light, which we know is absolute, and nothing can go faster than the speed of light. Nothing now can escape the clutches of the black hole. The outer edge of the hole is called event horizon because all events beyond this region are unknown, beyond this lays the photon sphere, in which the gravitational pull of the black hole isn't strong enough to pull light into the event horizon, yet strong enough to prevent it from leaving. We now have showed that black holes exist in theory but the question is have they been found in actual reality. The search for black holes began as soon as the concept began to take a foothold in physics.
The problem was how does one look for an object that is hidden from us? It was the Russian physicist Zel " Dovich who first proposed a method for finding the existence of a black hole in the universe. He suggested that one would have to look for a star whose light spectrum, viewed through a spectrograph, shifted from red to blue to red to blue (which indicates the Doppler shift). This is a sign that the star has a companion, and thus is in a binary system. Measurement of the lights spectra infers the speed of the star around its companion, and from that velocity you can infer the mass of its companion. If the companion is massive and no light can be detected from it then it could be a black hole.
This method was mulled over a few years until a new method for the search was proposed, which like the other method required the black hole to be in a binary system. A black hole with another star beside it should be drawing matter in to itself from the star. The matter then would be spinning round the black hole like water down a plughole and as the stellar matter gets closer to the event horizon it would accelerate at ever-increasing rates. The matter would then heat up and begin to emit photons of electromagnetic radiation, and when these stellar gases are close to extinction they release x-rays of intense energy. It is these x-rays that physicists are looking for, if there part of a binary system. It is now that part of Zel " Dovich method comes into play.
As was described the mass of the companion to the star is inferred from the velocity of the star. With this in mind it can be deduced whether or not the companion to the star is a black hole or not. One candidate for this search was Cygnus X-1, which is nearly 14 million kilometres from the earth. What was found was a binary made of an optically bright and X-ray dark star with its companion being optically dark and X-ray bright. The mass of the black hole is bigger than three solar masses, and is most likely closer to sixteen solar masses. And it appears to be the brightest x-ray source in the universe.
This seemed to fill all of the criteria for the companion to be a black hole, and is thus assumed to be the first black hole found in the universe. So can we conclude now that we have successfully proved the actual existence? Well if one were to put a percentage to the surety of their existence, it would have to be that astrophysicists are at least 95% sure that Cygnus X-1 is a black hole. There is no clear signal that announces that Cygnus X-1 is a black hole, unlike other objects like neutron stars that have clear signal telling us what they are. There is hope though in this research in finding a black hole signature, which is to be found in gravitational radiation. Hopefully soon there will be gravitational wave detectors that will be able to map the locations of black holes.
But black holes have inevitably been accepted as real in the minds of most physicists, and it will only be a matter of time till a firm piece of evidence to show that black holes do exist.