Black Holes
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Black holes are one of the most fascinating and unrealistic seeming objects in the world to many people. They seem to be right out of a science fiction novel, along with white holes, and wormholes. So far white holes and wormholes are still only science fiction. A white hole is a reverse of a black hole. And a wormhole is a combination of a white a black hole said to be a place of time travel. However, even quite a long time ago some members of our society realized that the black hole is not science fiction, but is very real and is very interesting. I plan to take you with me on my road to discovering black holes; the history of black holes, what is a black hole, how are they formed, singularity and event horizon, are black holes black and how to detect them, accretion disk, and black holes and evaporation.
About two centuries ago an English geologist named John Michell came to the conclusion that it is tentatively possible for gravity to be so overpowering that nothing, not even light traveling at the speed or 186,000 miles an hour could escape. To produce this much gravity, the source would have to be tremendously massive and exceptionally dense. In that day in time it was thought to be impossible for something such as a “dark star” to exist.
In 1916 the German astronomer tried to theorize how a star could shrink and become what was then typically called a frozen star. He then predicted that our sun would reduce in size two miles in radius. He also predicted that even though the sun had shrunk that its mass and weight would stay the same. This would leave the planets unaffected and continuing on their orbits. There was a question still remaining whether the star would become compacted or not.
In 1934 F. Zwicky and W. Baade calculated that the collapse of a star would cause the strip the atoms of their electrons, thus becoming a neutron star.
Sometime in the 1960s a man name John Archibald Wheeler came up with the term black hole for what had previously been called a frozen star; the term caught and to this day we now call those mysterious masses black holes. But what are they?
How can one really define an object as complicated and abstract as a black hole? To understand the concept behind a black hole one must first understand the notion of Einsteins general Theory of Relativity and the way gravity works. If an object were to be shot towards the sky from a cannon, lets say, and it was shot just hard enough to make it escape earths gravity, that would be called the escape velocity. Escape velocity depends on a planets mass; the greater the mass of a planet means the greater the escape velocity. A black hole to put it in simple terms, is an area of space-time that has so much mass concentrated in it that its gravitational field is so intense that there is no way for a nearby object to escape it. Its escape velocity exceeds the speed of light and since nothing is faster than the speed of light, that means that nothing can escape a black hole.
There are only three properties that we know of that make up black holes. They are net electric charge, total mass, and complete angular momentum. There are four types, or models, of black holes. The one that is used most frequently being the Schwarzchild black hole. This one does not comply with the standards of the first three; the Schwarschild black hole has charge but no angular momentum. The Kerr black hole has angular momentum however it lacks charge. The Kerr-Newman black hole has charge and angular momentum. These are all named after mathematicians who solved Einsteins equations.
Black holes are thought to form from stars when they collapse from their own gravity, thus forming an object whose density is infinite. The star is a huge and intriguing fusion reactor. Because stars are so enormous and are made out of gas, there is always an extreme gravitational field that is trying to collapse the star. The heat in the star is always pushing out and balancing out the force of gravity. When the nuclear fuel becomes exhausted and there is no more burning; there is no heat left to neutralize the force of gravity and the star begins to die. This allows the gravitation to compress the star and its materials inward resulting in the compression of the core. Ultimately the heat becomes so intense that in creates a supernova explosion. This explosion then causes the material and radiation to soar out into space, and what remains is the highly compacted and exceedingly colossal core. This core is concentrated into a single point, which we call a black hole.
This core happens to be the central part of the black hole, which is called the singularity:
At this point both time and space stop. Astrophysicist, Karl Schwarschild, computed the gravitational fields of stars using Einstiens field equation. For this computation Schwarzchild made the assumption that a star was a non-rotating, gravitationally collapsed, and perfectly spherically shaped to make the equation for feasible. The results of these calculations he called the Schwarschild singularity; which lies at the center of the black hole. Within a certain point of the singularity the gravitational pull is so strong that not a thing can break away from it, not even light. This distance is called the event horizon. The more mass this singularity has the larger the event horizon will be. The event horizon is considered the key point because after this point is passed there is no way of escape. This is not a physical border line but is thought to be the point of no return. When referring to a black holes size you are referring to the event horizon. You would think if these objects were so powerful that there would be nothing, and that it would swallow up the whole universe. But, obviously that isnt the case. It is only when you get in a certain distance of the black hole. But if you are not within its so called reach you will not get sucked in. If a person were to manage to get close enough to a black hole, as long as they were outside of the event horizon they would not get sucked in. Once they had passed through, however, nothing they could do could get them back out. They would keep getting sucked in further and further towards the singularity and eventually, it is thought, be ripped apart by the gravitational pull. The point at which that happens depends on the size or mass of the black hole. In many ways a black hole acts in ways similar to earth and other planets, in the way they rotate around the sun. The black hole has the same gravitational force as it did when it was a star, it just extremely compressed.
A thing many people may wonder is if a black hole is truly black, and to answer that question, yes it is. If a person