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# Introduction

## What are black holes?

 We can describe light not only as electromagnetic waves, but also as particles (called photons). What happens if a particle flies by a very massive star? - You're right, it is pulled in by the star's gravitation. Does this happen to light too? Is light affected by gravitation? The scientists already thought about this problem way back in the 18th century. We can already find one of those thoughts in "The Philosophical Transactions of The Royal Society of London" (Vol. XV) in a work called "Distance and Magnitude of the Stars" by John Michell (written 1784): "Let us now suppose the particles of light to be attracted in the same manner as all other bodies with which we are acquainted; that is, by forces bearing the same proportion to their vis interiae; of which there can be no reasonable doubt, gravitation being, as far as we know, or have any reason to believe, a universal law of nature." (Page 466) Nowadays it is proven that light is indeed attracted by gravitation. We can describe the attraction as a result from Einstein's famous formula E=mc². This formula tells us that every energy is equivalent to a mass. This mass is attracted with m=E/c². For light being Energy it is also attracted by gravitation. What do you think happens if a shining body has an enormous mass? You might have the same thoughts as Peter Simon La Place*: "A shining star with the same density as the earth, which diameter would be 250 times larger than the diameter of the sun, would let none of its light come to us due to its attraction. It is therefore possible that the biggest shining bodies of the universe are not visible for this reason." This was probably the first description of a black hole. A black hole artwork. Original work of C007571, ThinkQuest

A black hole is a body which has such a great mass that not even light can escape from it due to it's gravitation. This strong gravitation creates another problem because it also acts on the black hole itself. The gravitational force is strong enough that even the subatomic particles cannot withstand the pressure. The black hole collapses into one singular point. This point contains all the mass of the black hole. It is called the singularity.

The radius within which the Gravitation is so strong that nothing can escape the black hole is called the Schwarzschild radius. In the theory of relativity, time stops at this distance. Because nothing can reach us from beyond the Schwarzschild radius, it's also called the event horizon.

Normally a black hole is created when a huge star has burned all of its fuel and the heat cannot withstand the pressure of gravitation any longer. Tiny black holes might have been created during the big bang.

 It is suspected that a massive black hole is loacated at the center of galaxy NGC4261. Image no: STScI-PRC95-47 H. Ford and L. Ferrarese (JHU), NASA December 4, 1995 Courtesy of NIX NASA Image Exchange.

The Hole Man

* `Translated from the ````German version into English. The German version was translated from the French original by Johann Karl Friedrich Hanff in 1797. The Quote is from: P. S. La Place. Darstellung des Weltsystems, translated by J. K. F. Hanff (1797), Volume 4, page 333. Back```

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``````"Black holes aren't black - After Hawking they shine!"
Presented by Angie, Matthias and Thorsten
Team C007571,ThinkQuest Internet Challenge 2000.