Physics XI Content
Karl Schwarzschild in 1916, derived an expression for the critical velocity from Einstein’s general theory of relativity, known as Schwarzschild radius Rs. This is given as
If a spherical, non-rotating body of mass M has a radius smaller than Rs, then nothing –not even light can escape from the surface of the body. The body is then a black hole. Any other body within a distance Rs from the centre of the black hole is trapped by the gravitational attraction of the black hole and cannot escape from it.
The surface of sphere with radius ‘Rs’, surrounding a black hole is called event horizon. We cannot see events occurring inside it. All that can be known about a black hole is its mass (from its gravitational force on other bodies), its electric charge (from electric forces on other charged bodies).
At points far from a black hole, its gravitational effects are the same as those of any normal body having same mass. So, if the sun collapsed to form a black hole, the orbits of the planets would not be affected, but things get dramatically different close to the black hole.
If you go inside the black hole carrying a radio transmitter to send the signals to the outside observers, they would have to return their receiver continuously to lower and lower frequencies; this effect is called the gravitational red shift. Consistent with this shift, the outside observers would observe that the clock would appear to run more and more slowly; this effect is called time dilation. Actually, the observers would never see you due to horizon.
As you fell with your feet first into the black hole, the gravitational pull on your feet would be greater than that on your head. The differences in gravitational force on different parts of your body would be greater enough to stretch you along the direction towards the black hole and compress your perpendicular to it. These effects, called tidal forces, would rip you to atoms, and then rip your atoms apart before you reached the event horizon.