The boundary of a black hole- Event Horizon

 

Welcome back to AstroMe!

I guess it would be perfect to call” The black hole series”. So in this series, we have already looked into what is black hole, how is it formed and what happens after a star dies. If you haven’t looked into it yet, click on AstroMe.

This time we are going to see about event horizon (the boundary of a black hole).

Black holes have a boundary called the event horizon; it is the boundary of no escape. It is the place where the light particles where about to escape but ultimately it couldn’t.

For anything including humans to escape the event horizon, we must have velocity that exceeds the speed of light which is impossible.

What happens in the event horizon? The light particles travel parallel i.e. they move away from the event horizon.

Think what will happen if the light particles move towards the black hole. In this case, event horizon contracts. But this does not happen. The light particles travel away from the black hole. Hence, the event horizon has a non decreasing behavior. It stays the same or it increases.

Event Horizon is the measure of entropy in them. Entropy is the measure of degree of disorder (the amount of energy that is unavailable for doing useful work). 

 According to the 2^nd law of thermodynamics , the entropy of a place always increases or stays the same it does not decrease this holds good for the universe as well. This law also states that entropy emits light particles.

We now know that black holes emit light particles due to its entropy. How is it possible? We learnt that nothing can escape a black hole then how does a black hole emit light? 

Many astronomers spent years coming up with the answer. Stephen Hawking said that the black holes do not emit light particles at all; it is the empty space around them which does. Empty space is not clearly empty; it consists of energy matter, gamma rays and even debris.

Stephen Hawking

These gamma rays are the one which emit radiations. For an observer it may look as if the black hole is emitting the radiation but it is not, it's just the empty space around it. This light radiation produced by the empty space is known as Hawking Radiation.

Hawking Radiation

This can also answer another question of how we can detect a black hole when it is completely black. It is the rays produced by the empty space which show some changes near a black hole i.e.it acts differently nearby and also they can find by detecting their effect on other matters nearby. In case of stars or planets it may look red and dimmer. 

Event horizon is also known as Schwarzschild radius. It is the radius of the event horizon found by Karl Schwarzschild. Look at the picture below. We will be learning about it in detail in my future posts, just know what it is.

  

Note:

Singularity: That's the place that matter goes when it falls through the event horizon. It's located at the center of the black hole, and it has an enormous density. You will be crushed into quarks long before you get there!  

So, we now know what is event horizon and at what point we will get trapped into the black hole. See you in my next post! Bye for now.