What is the event horizon of a black hole?

One of the terms that are mentioned the most when talking about black holes is the event horizon. This is one of the most important parts that form a black hole, and it represents the limit after which gravity becomes so intense that it becomes impossible to escape from the black hole, even for light. Below we will explain in more detail what is the event horizon.

Escape velocity, and why light can’t escape from black holes

Escape velocity is the necessary speed that needs to acquire an object in order to get out from the gravitational field of another body (such as the Earth, the Sun, or a black hole). For example, the escape velocity of the Earth is about 11 km/s, or 7 miles per second. This is the speed that have to attain rockets to get out from the gravitational field of our planet.

The escape velocity of a body increases the more massive the latter is. For example, the escape velocity of the Sun is about 620 km/s. Moreover, the smaller the distance is from the body in question, the higher will be the speed required to leave its gravitational field: it is not the same to escape from the gravity of the Earth from its surface than being in orbit.

The gravity of black holes is so intense that, from what we call the event horizon, the escape velocity becomes greater than the speed of light, which is approximately 300,000 kilometres per second. According to Einstein’s theory of general relativity, nothing can go faster than light. Consequently, not a single particle or body can escape the black hole once it has crossed the event horizon, not even light. Once the event horizon is crossed, there is no coming back.

This is the reason why it is called the event horizon. Anything that takes place inside the event horizon can’t be seen from the outside: it is a horizon beyond which we cannot see.

The Schwarzschild radius is the radius of the event horizon of a non-rotating black hole. If this was the case, the event horizon would be a perfect sphere (because we live in a 3-dimensions universe).

Any body with mass can be converted into a black hole. Take the Earth as an example. If we compressed it in a sphere of 8.8 millimetres of radius, it would become a black hole. This radius of 8.8 millimetres is the Schwarzschild radius of the Earth. Every body has its own Schwarzschild radius.

Rotating black holes

The Schwarzschild radius only applies to non-rotating black holes. In the case of rotating black holes, also called Kerr black holes, the event horizon has a different shape. Just as in the case of the gravitational field of planets and stars, the event horizon of rotating black holes is flattened in the poles and bulges in the equator.

Even if we have previously talked about non-rotating black holes, studied from a theoretical point of view, it is very unlikely, not to say impossible, that this type of bodies really exist. Black holes form when a massive star collapses under its own weight. As in the beginning the star already rotates, this movement is conserved, and even highly intensified, when the black hole is created. For this reason, the existence of non-rotating black holes is seen as very unlikely.

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