Ask Ethan: What Does A Black Hole’s Size Tell Us?

“When we talk about the size of a black hole, are we talking about the radius of the event horizon or the actual size of the ‘infinitely’ compressed matter? Or is a black hole an actual singularity: i.e., a dot?”

Have you ever wondered how big a black hole is? Is it better to describe it by the singularity, or by the size of the event horizon? Does the event horizon tells us the size of a black hole’s shadow, or the distance at which a particle can orbit and not fall in, or the size of a sphere of photons surrounding it, or something else entirely?

What we mean by “size” depends very much on the type of question we’re trying to answer. Here’s what a black hole’s size tells us.