Friday, November 16, 2007

Black Hole

Black holes are objects so dense that not even light can escape their gravity, and since nothing can travel faster than light, nothing can escape from inside a black hole. On the other hand, a black hole exerts the same force on something far away from it as any other object of the same mass would. For example, if our Sun was magically crushed until it was about 1 mile in size, it would become a black hole, but the Earth would remain in its same orbit.

John Michell, a British geologist and astronomer, designed the experiment made by Henry Cavendish to measure the mass of the earth. Cavendish published the results of the experiment in 1798.

In 1783 Michell published his work, that showed that a star, that has the same density of the sun, but 500 time as big, would have such a gravity, that "All light emitted from such a body would be made to return towards it". He said we wouldn't be able to see such a body, but we sure will feel its gravitational pull.

Pierre-Simon Laplace, got to the same conclusion in 1795, and explained it by saying that "It is therefore possible that the greatest luminous bodies in the universe are on this very account invisible". Michell took to account a body that has the density of the sun, which equals to the density of water, while Laplace took to account a body that has the density of the earth, which is 5.5 more dense that water. To such bodies, there was invented in 1967 the name "Black-Holes"- a black hole in Space-Time. But it seems that over the years Laplace thought of this as a crazy idea, and he didn't work on this subject any more (over the 19th century more and more people believed the wave theory, and not the particle theory).

The people who believed light was composed of only small particles, compared it to a cannon shell, and said that if a cannon shell was pulled after some time to the earth, so would the light. But this comparison isn't completely true, because a cannon shell was also slowed down, while the light's speed is stable.

The first really main theory that dealt with gravity's effect on light was Einstein's General Theory of Relativity in 1905. Even then, it took time until it was used to see the effect of big stars on light.

The Indian research-student, Subrahmanyan Chandrasekhar, based his calculations on the life cycle of a star, while sailing to study in Cambridge with Arthur Eddington, an expert for the General Theory of Relativity, as his professor. He tried to calculate how massive a star can be, and still be in a stable condition, in spite of its gravitational pull, after the star has cooled down.
Black Hole

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