Black Holes: the Event Horizon and Schwarzschild Radius

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  • 0:01 Schwarzschild Black Holes
  • 0:58 Black Holes: A Brief Review
  • 1:55 Event Horizon &…
  • 4:01 Black Holes & Gravity
  • 5:52 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson describes the event horizon and the Schwarzschild radius, including their implications on the gravitational attraction a black hole exerts on the universe around it.

Schwarzschild Black Holes

I'm sure you know that Albert Einstein and Stephen Hawking are two of the world's most famous 20th century physicists. For good reason, too. Einstein came up with a theory of space and time that is known as the general theory of relativity. In essence, he considered the two concepts of space and time as one, space-time, and gravity as a curvature of space-time.

Astronomer Karl Schwarzschild then used this information to describe an electrically neutral, non-rotating black hole, known as a Schwarzschild black hole. Hawking and Roy Kerr used even more advanced mathematics to describe rotating and charged black holes, but to keep things simple, we'll pretend all black holes are actually Schwarzschild black holes and will use his concepts to describe the event horizon, the Schwarzschild radius, and their implications.

Black Holes: A Brief Review

Just in case you missed the lesson that defines black holes and singularity, I'll briefly go over everything. Also, a refresher never hurts, even if you did see that lesson. A black hole is a spherical region or volume of space that has, at its center, the singularity. A singularity is an object, a mass, which was once a gigantic star that collapsed its matter into a point of zero radius and infinite density.

The black hole can also be seen as an extremely strong gravitational field. Because of this, the escape velocity is so high that not even light can escape a black hole. And because nothing can travel faster than the speed of flight, nothing else can escape it, either. Therefore, no information can ever be received about the singularity or the space around it since no light is emitted by it and, therefore, that region of space is called a black hole.

The Event Horizon & Schwarzschild Radius

Surrounding the black hole is something called the event horizon, the boundary between the black hole and the rest of the universe where the escape velocity is just equal to the speed of light. Nothing that occurs within the event horizon is visible to us.

The event horizon is like the edge of a really deep, dark, black well. If you cross it, you fall in and you're gone. Of course, in space, the event horizon isn't something physical you can touch; it's just a region of space. The distance from the center of a non-rotating black hole to the event horizon is known as the Schwarzschild radius. This is like taking a ruler and measuring the distance from the dark well's edge to its center. The Schwarzschild radius depends only on the mass of the object that creates the black hole.

So, let's pretend our deep, dark well was made by a really heavy rock that sank deep into the earth. A small rock sinks and creates a well with a small radius, but, logically, a big rock sinks and creates a large well with a large radius. The earth has a Schwarzschild radius of one centimeter. This means that if the earth shrunk to a radius of less than one centimeter, it would become a black hole. But, that's not possible; Earth cannot collapse to such a small size because its supportive structure of rock and metal is strong enough to hold up its own weight.

But stars whose stellar cores are greater than three solar masses are large enough to collapse into a black hole because there is no force that can hold up their own weight. As the most massive stars collapse, they eventually shrink inside their own Schwarzschild radius, giving rise to a black hole, and disappear from view forever. Thereafter, they presumably contract all of their matter into the singularity.

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