Event Horizon: Definition & Overview

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

This lesson will explain the meaning of the term 'event horizon', and discuss the two main types: a black hole event horizon, and the observable universe event horizon. A short quiz will follow.

Definition: The Point of No Return

When an event happens, how do you know?

Let's say your mom drops a vase and it smashes on the floor. Maybe your eyes told you it happened, because you could see it fall and break into pieces. Maybe you heard the smashing noise. Maybe you could smell the roses that were inside it. But what if it happened so far away that nobody could possibly know for certain the vase fell? You might say that the event is beyond the horizon.

An event horizon is a boundary within spacetime itself beyond which events that happen can't affect you. Events beyond this boundary are either so far away, or so disconnected from you, that you cannot see them, hear them, or receive any kind of knowledge that they happened.

That may sound very abstract and hard to picture, so let's go through a couple of examples.

Black Holes

A black hole is an object so heavy and dense, with such strong gravity, that nothing can escape from it -- not even light! Black holes were first predicted as part of Einstein's general theory of relativity, though he didn't realize it at the time. It was only through later scientists analyzing his equations, starting with Karl Schwarzschild in 1916, that we realized black holes could really exist.

What color is a black hole? Surprisingly, the answer isn't 'black'. Surrounding most black holes is a bright accretion disc. As molecules fall into the black hole, their gravitational potential energy is changed into kinetic energy, increasing the temperature of the material. As it gets hotter, it gets brighter, in the same way that the Sun is bright. But how can we see the light, if nothing can escape from a black hole?

A Black Hole and Accretion Disc
A Black Hole and Accretion Disc

Well, that's where the 'event horizon' comes in. Light from material that travels beyond the event horizon of the black hole cannot ever escape. Small amounts of light from molecules that are outside of the event horizon can escape, and this is why we can see the accretion disc.

So the event horizon of the black hole is the point where it really does become black. It's known as the Schwarzschild radius.

Only the very center of a black hole is black!
Only the very center of a black hole is black!

The Observable Universe

The observable universe also has an event horizon.

When we look at the sky, we don't see how things are right now, we see what they were like when the light left those objects on its journey to our eyes. The further away an object, the more of a lag there is on the things we see. So when astronomers look at objects really far away, it's like looking back in time... in some cases all the way to the start of the universe!

Panoramic Near-Infrared View of the Sky
Panoramic Near-Infrared View of the Sky

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