Referat Black Holes by David Braunstein

Black Holes

by David Braunstein

The black hole is a very widely discussed phenomenon in science. The first thing one might want to know, is that nobody has ever seen a black hole. The only reason to believe in them, is the mathematical effect of its gravitational field. The first (known) person to talk about black holes was John Michell, from Cambridge, in 1783. He suggested that a star that had a greater gravitation than light could not be seen by anyone, because the light itself would never be able to leave that star. Later on this idea was expanded to a star that had collapsed because of its own gravity, which could not be upheld by the star forever. Eventually it would loose its nuclear fuel, and therefor not be able to hold up the needed pressure, which would make it collapse. This collapsed star then would suck everything near it with a small enough mass and velocity into itself. As soon as its escape velocity had reached a radius of over 300,000 kilometers, light would not be able to escape anymore. With all the new matter being part of this black hole, its gravity would become bigger and pull larger objects into its gravitational field. What at first would only be particles and space dust would eventually become whole stars and even suns. The black hole would grow bigger, and its gravity would have effects on other planets and suns that would change their orbits because of the differed gravitational field that surrounds them. This is what can be mathematically proven, and therefor also proves the existence of black holes.

Many science-fiction authors used the phenomenon of a black hole as basis for their stories. They used them to explain time travel or moving from one galaxy to another. This is not possible. Though particles are theoretically able to move between universes, and even travel backwards through time, it is not possible for an object to cross the black hole without being torn apart. There is no material that could withstand the extreme forces of a black hole, and when once passed a certain distance, no object no matter how fast it can go is able to escape. This distance in which the gravity of a black hole is too strong to escape, is called event horizon. At this point the gravity of the black hole is so big, that not even light can leave the gravitational field produced by the black hole anymore. And in the regard of the special theory of relativity nothing exists that is able to travel faster than light, and therefor nothing could escape this gravity anymore.

Explanation: The center of nearby giant galaxy M87 is a dense and violent place. In this 1994 photograph by the Hubble Space Telescope, a disk of hot gas was found to be orbiting at the center of this massive elliptical galaxy. The disk is evident on the lower left of the picture. The rotation speed of gas in this disk indicates the mass of the object the gas is orbiting, while the size of the disk indicates an approximate volume of the central object. These observations yield a central density so high that the only hypothesized object that could live there is a black hole.

This picture and its explanation are from the following web-page (Credit: NASA): https://antwrp.gsfc.nasa.gov/apod/ap960305.html

With pictures like this, mostly taken by the Hubble Space Telescope, the existence of black holes has been proven and investigated. Scientists believe that there probably are black holes in our own galaxy, but there is no evidence of their positions, yet.

It is just logical to assume, that a black hole would never emit any objects or particles, after it once had reached that certain size which made it a 'black' hole. But through investigation and research scientists found particles that came out of the black hole. This fact kept them busy, until the idea emerged that those particles were not coming out of the hole, but were travelling 'backwards' in time. Therefor they were really sucked into the hole, but from our view of time they seemed to come out of it. This idea once more was based on Einstein's theory of relativity.

Black holes do not 'live' forever, they explode after a certain amount of time. This explosion has been estimated to equal the power of 10 million 1-megaton hydrogen bombs, if based on the theory, that there are only six different varieties of quarks, which make up particles. If based on another theory, the explosion could be 100,000 times as big. These theories probably can not be proven until scientists are able to watch a black hole explode.

The explosion of a black hole is also the basis for the big-bang theory. One big black hole, swallowing other black holes, which finally explodes and spreads the particles all over the universe. It is commonly believed that the observation of black holes would enable scientists to research and analyze the whole idea of quantum mechanics, which would not only lead to a better understanding of the universe, but also of our own planet and the structures that make it up.

[Sources: The Quantum Mechanics of Black Holes and Black Holes and Baby Universes by Stephen Hawkins, NASA Webpage]