The term “black hole” was first coined by John Arichbald Wheeler in 1967. In theoretical physics, a black hole refers to a region in space whereby the gravitational potential GM/R is more than the speed of light. Generally, a blackhole is an astronomical body whose escape velocity exceeds the speed of light where light is the ultimate speed limit of the universe. This implies that while gravity is the determining force that keeps human beings on earth, and the escape velocity of every single object depends on its specific gravity. Black holes can be described tri-dimensionally. They have mass, angular momentum which implies that they can rotate and they also have electric charges. There is no limit in terms of size for a black hole. They can be very light or heavier than a billion suns. The sun contains more gravity, as such, its escape velocity is higher than that of the earth which is 600 km/s (380 miles/s) as compared to the earth’s (7 miles/second) (Kallosh & Orazi, 2016).
For instance, if one throws a rock up in the air, it drops as fast as the earth’s gravity is able to stop it while the harder the rock is thrown the harder it is for the earth to stop it hence if one could be able to throw the rock hard enough, it would supersede the earth’s velocity and be able to escape the earth. Scientist argue that the black hole has such an intense gravitational pull that nothing can be able to escape it, even light. The reason behind this argument is that the black is massive, more massive than the Sun and the stars and no amount of nuclear fusion can be able to collapse it.
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Black holes can be detected through examining the effects that they have on the surrounding stars. Black holes are said to swallow gas hence modify the trajectory of the stars that surround them hence the stars are able to give information about the black holes surrounding them. These kinds of observations are made from earth and space by use of wavelength using radio and gamma rays. The effects of the black holes to their surrounding have been effectively observed by the use of ESO telescopes that can decipher the ultraviolet light and the infrared and optical wavelengths emitted around them (Kallosh & Orazi, 2016).
Theoretical Background
The concept that blackholes are massive and inescapable emerged in the 18 th century inspired by Newton’s law of gravity. According to Newton, the escape velocity v from a distance r from the center of gravity of a heavy object is described by the mass of the object m hence a body with large mass can be compressed so much so that the escape velocity from its surface could exceed that of light. The missing link was the idea or question whether such objects whose velocity is greater than the speed of light. This is the question that John Mitchell and Pierre Simon de Laplace attempted to answer. The big dilemma was whether the rays of light would fall from the surface of such an object and escape to infinity. They discovered that, due to the nature of light, it might be able to escape. Because of the complexity of this considerations, it is paramount of consider Einstein’s theory of special relativity and general relativity. As such, the ideas were extrapolated by Albert Einstein’s general relativity theory in 1915. Einstein’s theory claimed that matter can curve spacetime and that the larger the matter then the bigger the curvature. He also posited that energy and mass are equal and that a massless phenomenon such as light can influence gravity. Karl Schwarzschild was later to prove Einstein’s theory in 1916 when he used Einstein’s formula to prove that black holes work. Since then, the physics of blackholes has encountered major breakthroughs. In 1784, John Michell proposed the idea that black stars exist. His discovery was rejoined by Pierre-Simon de Laplace in 1796. They believed that the black starts could exert a gravitational force that was strong enough to lock in light. This was followed by Einstein’s paper of “General Relativity” that was published in 1915 where he postulated that black holes are a result of general relativity although he did not believe in their existence (Seminar et.al., 1998).
In 1916, Karl Schwarzschild was able to prove the solution of a non-rotating black hole while a mathematician form New Zealand named Roy Kerr was able to find the theoretical solution to rotating black holes in 1963. Additionally, Subrahmanyan Chandrasekhar, an Indian American astrophysicist observed that massive stars may not be able to cool down hence they collapse into white dwarfs and neutron starts which are denser. Nine years later in 1939, American physicist Hartland Snyder and Robert Oppenheimer speculated that blackholes could indeed form from the collapse of massive stars. In 1974, Stephen Hawking, a British physicist, was able to consider the quantum effects of black holes and he discovered that quantum black holes are not as dark as the regular black holes and they are able to emit thermal radiation (Seminar et.al., 1998).
Another assumption is that in the vicinity of a black hole, light is deflected and the amount of deflection is such that the it increases as one gets closer to the black hole. When the distance is say, D = 1.5 Rs, light would go around in a circular orbit and a ray that is emitted tangentially in the horizon at a distance < 1.5 Rs would be captured. In order to escape it would have to be emitted at an angle < 1800. As the radius is decreased from 1.5 Rs to Rs, the range of angles from which light can escape decreases, to become 00 at the horizon, at which point light can no longer escape.
Types of Black Holes
There are two basic types of black holes: stellar black holes and supermassive blackholes. The major difference in black holes is the mass and association where stellar black holes are a bit smaller and sometimes scattered while supermassive black holes are huge and clustered.
Stellar Black Holes
There are some billion-small sized black holes in the Galaxy and the Milky Way according to the galactic evolutionary models. They are about 3 to 20 solar masses, some isolated and others as close as starts. The coupled black holes are referred to as black hole X-ray binaries, or microquasars. Microquasars. Are a little different because they emit jets or x-ray emissions. the emissions emanate from a disk of gas that is suctioned from the stars and spiraled to the blackholes like water into a plughole.
Super Massive Black Holes
Massive black holes are often found in the middle of the galaxy. This is because the core of the galaxy has numerous wavelengths as is the most luminous. These regions of the galaxy where the massive black holes are found is referred to as the Active Galactic Nuclei (AGN). The AGN is extended a few light minutes or light days away and are a hundred times larger than the entire galaxy. The theory behind the AGN is that they are powered by one massive black hole (Abramowicz, Gunnlaugur, & Pringle, 1998).
Unlike the stellar back holes, these massive black holes are not isolated and they sit at the same place in the core of the galaxies and attract matter using their super strong gravitational fields. The attraction of matter to the black hole is called accretion. Here the gas rotates towards the horizon building a disc. As the gas spirals in, the energy is converted into heat and an electromagnetic spectrum is formed. Most of the matter is gulped by this strong electromagnetic spectrum while the little that escapes are swept away in form of collimated radio jets that are found at the inner edge of the disc. In active galaxies, the black hole is starved because of the lack of matter around it (Frolov&Zelnikov, 2011).
How Black Holes are formed
The most basic way a black hole is formed is through an exploding star. This happens when a star is 25 times the mass of the sun and it explodes. The outer part of that star disintegrates outwards in high speed while the inner part collapses. If the mass is enough, the gravity of the collapsing core would compress radically to form a black hole. When that happens, the black hole will be a few masses heavier than the sun forming a stellar mass black hole. The huge supermassive black holes on the other hand are formed by the collapse of a huge star which accumulates more mass from surrounding matter including other black holes and form ultra-dense stellar cores that merge to form short gamma-ray explosions. These gamma-rays are sued to mark the birth of a black hole and serve as one of the detectable aspects of a black hole (Christodoulou, 2009).
Proof of Existence
The best proof of existence of black holes is the million solar masses called the (Sagittarius A star), which is at the center of the earth’s galaxy and has about 27 000 light years from the earth. The motion of the starts as captured by ESO telescopes is the best empirical evidence of a super massive black hole (eso0846). In the past few years, there has been a correlation between the mass of the galaxy and its core which is the “house” of the black hole. There have been a number of orders, nine to be specific, in terms of magnitude of scale that measure the sphere of influence of a black hole against the environment of the galaxy. These kinds of correlations are alive to the fact that black holes exits and the cosmological distance of light years between the central objects of the galaxy is determinable (Chakrabarti, 1999).
References
Abramowicz, M., Gunnlaugur, B., & Pringle, J. E. (1998). Theory of black hole accretion disks . Cambridge, UK: Cambridge University Press.
Chakrabarti, S. K. (1999). Observational Evidence for Black Holes in the Universe: Proceedings of a Conference held in Calcutta, India, January 10-17, 1998 . Dordrecht: Springer Netherlands.
Christodoulou, D. (2009). The formation of Black Holes in General Relativity . Zürich: European Mathematical Society Publ. House.
Frolov, V. P., &Zelnikov, A. (2011). Introduction to black hole physics . Oxford: Oxford University Press, USA.
In Kallosh, R., & In Orazi, E. (2016). Theoretical Frontiers in Black Holes and Cosmology: Theoretical Perspective in High Energy Physics .
W.E. Heraeus Seminar, Hehl, F. W., Kiefer, C., & Metzler, R. J. K. (1998). Black holes: Theory and observation : proceedings of the 179th W.E. Heraeus Seminar, held at Bad Honnef, Germany, 18-22 August 1997 . Berlin: Springer.
