What is Event Horizon: Explained in Simple Language

What is event horizon cover image

We’ve seen black holes in movies, TV shows, read about them in books. But there is one crucial component of black holes that as attracted many curious minds; What is the event horizon and why is it such a crucial component of a black hole? We explain it in this article in the easiest way possible, both for spinning and non-spinning black holes. 

An event horizon can be described as a boundary around a black hole where gravity is so strong that even light cannot escape from it. I am sure you must have heard of this definition a hundred times in movies and other articles. But what this means in simpler terms? And what is the significance of it? 

Understanding the event horizon in simple terms 

When you throw something from a great height, the reason the fall becomes more dangerous as the height increases is because of something called terminal velocity. This is the maximum velocity attained by a free-falling object due to the acceleration by gravity. 

This acceleration is because gravity is strongest at the center of the body. The gravitational pull increases with a decrease in distance from the body. The very same thing happens with black holes, but the consequences are more complicated than falling into it. 

Escape velocity 

Escape velocity is the speed required for a body to escape the gravitational hold of the earth. Space is not that far from your house. You can reach the exosphere (where space begins) within a few miles. It’s like driving to the beach or any store near your house. The distance does not matter, but the speed does. 

Earth’s escape velocity is 7 miles per second or 25,000 miles per hour. This means you’ll have to travel at that speed to reach the orbit. Anything lower than that, and you’ll fall to earth, possibly killing yourself. A very similar thing happens inside the this boundary, but instead of any object, light falls back inside the black hole. 

Illustration showing how Earth's escape velocity works
The white area surrounding earth is its gravitational influence. The Red path shows a craft traveling slower than the escape velocity while the green path showing speed of more than the escape velocity. One falls and the other escapes.

Why light cannot escape 

The more the gravitational pull of the body, the higher is the escape velocity. Jupiter has an escape velocity of 37 miles per second. But what about black holes? This is what creates the event horizon. 

The gravity of a black hole is so strong, so powerful that even the fastest thing in this universe, light is unable to get out of it. So imagine light as a rocket that tries to take off from the center of the black hole. It rises up but as it goes up, the speed decreases. And before it could cross the event horizon (which is a circular boundary), it falls. 

Light has a speed of around 186,282 miles per second and since light cannot escape this boundary of no return, the escape velocity needed to get out of it has to be more than the speed of light. But nothing is known to man that is faster than light, hence, nothing can escape the event horizon of a black hole: Once you cross the event horizon, there is no coming back.

This is why it is also called an event horizon. Any event that happened outside this horizon is an event that can be seen, recorded, or cause an effect. Any event that happens inside the event horizon will never (according to the current understanding of physics) affect anything outside the event horizon. This means once you get inside the event horizon, anything you do will not affect the outside universe.

Time and space reversal 

Another confusion concept about black holes is how time and space change near a black hole. We won’t get into the relativistic aspects of the process since we promised to explain it in simple terms. What we will use is the example of a calendar. 

The day you are reading this could be any day, Thursday, Wednesday, Friday (yes!), Monday (no!). But whatever day it is, you know the order and no matter what you do, Tuesday will come after Monday. This is known. But where will you go today? 

Your movement is decided by you. Friday and you might hit the club, Monday and you’ll have to go to work. But you are free to move. What I am getting at is you can move in space in all 6 directions (forward, backward, sides, up and down) but in time in only one direction, forward. This changes inside a blackhole, once you enter the event horizon. 

As you enter the event horizon, now you cannot escape (since nothing is faster than light). This means the direction of your motion will be towards the center of the black hole. Space is acting like time now (moving forward inevitably). Time stops inside a black hole since the gravity is so strong. This means that time becomes stationary, like space. This is, on a surface level, the reversal of space and time.

So now you understand the significance of the event horizon. But there are a few things that need a brief explanation to understand the event horizon. We’ll stick to a brief description because that’s what’s needed to understand the event horizon. 

The event horizon of a black hole (Illustration)
Our terrible attempt at Illustrating the event horizon of a black hole

Schwarzschild radius

This is the radius of the event horizon of a non-rotating black hole. Remember, a non-rotating black hole. But this isn’t an easy understanding. Let us give you one. 

There’s a relation between the Schwarzschild radius of a black hole (or any body with mass, even you!) and a black hole. Anything with mass can be converted into black holes (even energy can be converted into black holes but let’s stick to mass).

Let’s assume our sun’s mass. If you took the sun and compressed it into a sphere of a radius of 2.954 kilometers or 1.61 miles, you’ll turn the sun into a black hole. This radius of 2.95 is the Schwarzschild radius of the sun. Different masses have different Schwarzschild radii. 

You can calculate the Schwarzschild radius of any mass on this online calculator. Click to visit.

Rotating black holes event horizon

The Schwarzschild radius is only for non-rotating black holes. For rotating black holes, which are also called Kerr black holes, the entire event horizon is different. In a rotating black hole, the event horizon has a different shape. As the gravitational field of planets, the event horizon bulges towards the equator and flattens at the poles.

This was all about the event horizon and why it is such a crucial component of a black hole. It is the point of no return. It is the boundary which once crossed, changes the fate of your motion: You will fall into the singularity. 

To escape out of the event horizon, your speed must (escape velocity) must be faster than the speed of light. This, as we know and proven, is impossible: Nothing can travel faster than light. So nothing can escape the event horizon. What to take from this? If you ever reach a black hole, keep away from the event horizon. 

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