black_hole_milkyway

http://www.pbs.org/wgbh/nova/next/physics/are-black-holes-real/

What is a black hole ?

The term “ black hole” was invented by the American physicist John Wheeler in 1967. A black hole is, first and foremost, characterised by the existence of an horizon : it is the spherical surface from which even the light won’t come out, which in that case is inexorably attracted towards a central singularity. The light is affected by the presence of gravity : the stronger the gravitational field is, the weaker the photons which slip from the light and the more displaced they are to a bigger wavelength. Nothing can escape from a black hole and it can be proved by a mathematical application. The equation of the escape velocity ‘v’ is v where G is the gravitational constant and G = 6,67 x10-11m3/kg.s², M is the mass of the object and R is the radius. The escape velocity of the Earth is around 11km/s, the escape velocity for a black hole would be superior to the light speed, and we know that nothing can go faster than the light speed.

Only very large stars, called supergiants can become a black hole. The star must be one and a half times the mass of the sun or larger to turn into a black hole. The limit is called the Chandrasekhar limit, so if the star is less heavy than the number it will turn into a smaller type of star but not into a black hole.

Example : 

If a friend dives in the black hole, the closer he’s going to be from this one, the slower he is going to fall. When he is going to reach the Schwarzschild radius (the blackhole’s horizon), this movement will take an undefined time. His picture will stay frozen to infinity. His picture will also turn red and then disappear.

However for the person who is coming closer to  the black hole, the time is not going to change. but when he looks at your watch, an hour will be accomplished in a second, a millisecond, a microsecond… He will be able to observe the entire future history of the Universe and maybe its end. In a black hole the gravity is stronger and stronger while we go down into it. So the gravity isn’t the same in one black hole. If a friend went in the black hole his feet first, the lower he would get, the more different the gravity would be between his feet and his head, so his body would be quartered (that’s why the name given is “spaghetti effect”). We commonly speak about tidal forces to describe the differences of the gravitational forces with the different distances. The formula of this gravitational attraction is : v-1  and when the distance to center is different, then the force is different and the “spaghetti effect” happens.