The black hole
Have you ever wondered, while watching one of those science fiction movies, why the astronauts are afraid of a black hole?What is a black hole? How is it formed? What are its characteristics? Is it really that dangerous?The simplest way to describe the black hole is to start with the classical analysis.
Suppose we wished to launch a rocket to a far distant place in the outer space, how fast must the rocket travel to be able to escape the gravitational field of the earth?It has to travel with a speed (v) called the escape velocity of the earth where:
V = the square root of (2GM/R)G: the universal gravitational constant.
M: the mass of the earth.
R: the radius of the earth.
Now if the radius of the earth is contracted to be smaller than 8.85x10-3 m (about 9 mm), the escape velocity will be greater than the velocity of light.
Then, the earth becomes a black hole. And nothing, not even the light, can escape from its gravitational field.
If we took the sun as an example, if its radius is contracted to be smaller than 2.95x103 m (about 3 Km), it will be a black hole.
The black hole is formed when the equilibrium of the forces inside a star disappears (a star can be imagined as a massive nuclear reactor, billions of nuclear bombs blow off at the same time).
The reason for the name "black hole" comes from the idea that if we look at an object in space, such as a star, we see the light coming from this star. But if it became a black hole, no light would come out of it. Then, we will see nothing but blackness.
Till now, the analysis is a classical one.
Black holes and General Relativity
As we have seen, Einstein's theory of general relativity says that "mass warps spacetime".
So, the greater the mass of the gravitating body the greater the warping of spacetime.
Shortly after "Einstein" stated the principle of the general relativity, a scientist called "K.Shwarzschild" solved Einstein's equations of the gravitational field and stated that:
"For the spacetime to be warped by a star, the velocity of the light reaches zero".
Then, no light can come out.
Confusing, isn't it?
Raceman…
posted by scientific @ 5:20 AM
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