Ask Ethan: What Does A Black Hole’s Size Tell Us?
We have to use the right definition for the specific question we’re asking.
When it comes to the Universe, we frequently characterize objects by examining and reporting on their physical properties. While there might be some quantum effects that play a role for objects that are very small — in their energy, position, lifetime, etc. — there are some properties that remain the same regardless of any uncertainties. Objects that are stable, both microscopically and macroscopically, are described by measurable properties such as mass, volume, electric charge, and spin/angular momentum. But for the most extreme density objects in the Universe, black holes, something like “size” isn’t necessarily well-defined. After all, if all the mass and energy inevitably collapses to a singularity, then what do we even mean by size? That’s what Antoine Merval wants to know, writing in to ask,
“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?”
There’s actually more than one definition for a black hole’s size, and they all have their uses. From the outside in, let’s take a look at what a black hole’s size can tell us.
