The Event Horizon.

So, what is the event horizon…?

       Black holes are still one of the mysterious things that exist and attract everything inside them that crosses the boundary. This boundary is known as the "event horizon. An event horizon, a term associated with a black hole, is a point from which the gravitational attraction is so strong that even light can't escape through it. An object which crosses this boundary or approaches towards it is observed to be moving slower and slower and never appears to pass through the event horizon. The object appears stretched and, due to redshift, appears redder as it moves towards the black hole. This point is also called a point of no return, as from here if an object wants to return, its escape velocity needs to be greater than the speed of light. That is, it needs to travel against the pull of a black hole in the opposite direction at a speed greater than the speed of light.

What does the past say...?

  So, in 1784, the Newtonian theory of gravitation and the particle theory of light were dominant, and they proposed that if the required escape speed was greater than the speed of light, light originating inside or from such a distance of super massive objects could escape temporarily but would return. But, at the same time, John Michell proposed that near super massive objects, the pull of gravitation would increase to such an extent that not even light could escape. After a long time, in 1958, David Finkelstein, using General Relativity, proposed that the boundary, i.e., the event horizon, is the final boundary of a black hole, beyond which nothing can escape the attraction of black holes.

The future and past for the event horizon….

Black holes are generally final in nature; that is, we need to know all the future space-time of a black hole to identify its correct and current location, which is practically impossible. Generally, the model suggested earlier for black holes has

It does not tell us that black holes rotate and have a fixed point singularity, but now the present situation is different and proposes that the black hole has a spin. Because of this, the singularity does not remain as a point, but now it is a collection of compact discs. Because of this  the event horizons of rotating black holes appear uneven and also squashed at the poles and bulging at their equators. As presented by today's situation, a rotating black hole is surrounded by a region of space-time in which it is impossible to stand erect in the same position, called the ergo sphere. This is due to a process known as frame-dragging, which says that any rotating mass tends to drag slightly along the space-time surrounding it. But still, space-time in it is technically pulled a bit faster than the speed of light.

The flow of time in the event horizon.

Due to extreme gravitational forces, it has some effect on time. Time inside the boundary of the event horizon slows down as gravitation is inversely proportional to time. This phenomenon is known as gravitational time dilation, relative to observers outside the field. So, for a person inside the event horizon and a person outside it, time moves differently. The person inside the event horizon is never seen to be moving by the person outside because the time difference is huge. So, the denser the black hole is, the greater the pull of gravitational attraction and the slower time moves. As shown in the film Interstellar, the main character discovers different planets around a black hole, and the time spent on each planet differs significantly from the time spent on Earth.

The final conclusion to sum up.

  Some virtual particles exist at the boundary of the event horizon. These particles are continuously divided. That means half of the particles are drawn inside the black hole and half of them move out of the black hole and become real particles. The particles that move inside the black hole have negative energy, which combines with the positive energy of the black hole, resulting in the loss of energy. This process is slow at the start, but as the size of the black hole becomes smaller, the process catches up in speed, but still, with respect to time, this process is very slow.

  Schwarzschild's radius is the radius to which an object needs to be squeezed, keeping its mass constant so that it collapses under its own gravitational field. As an example, to convert the earth into a black hole, we need to squeeze it to the size of a marble with an inch diameter. The radius which it defines is the radius of the event horizon from the singularity to the boundary. The objects far away from the event horizon move according to their own space-time, but as they approach the event horizon, the black hole sucks up all the space-time and the motion of the object is inevitable the same as time. It can now move only in one direction, and that is inside the black hole. But the observer never sees it cross the event horizon because as we get closer to the black hole, the gravitational force increases and time slows down, so it takes an infinite amount of time for the object to cross the event horizon.