365 | Vitor Cardoso on Why Black Holes Are Special

"Black holes are really tiny for the mass they have." That observation from Vitor Cardoso, director of the Center of Gravity at the Niels Bohr Institute, captures a central paradox of modern astrophysics. Black holes are both the simplest and most extreme objects in the universe, yet until very recently, they were purely theoretical. Cardoso joins Sean Carroll during Copenhagen's "Black Hole Week" to explain how the field has been transformed by data, what we are actually seeing when we look at a black hole, and why the most exciting discoveries may still lie ahead.

What a Black Hole Actually Is

The definition of a black hole depends on who you ask. For an observational astrophysicist, Cardoso explains, a black hole is simply "a point-like object, which is very massive and dark." For a theorist like himself, it is something far stranger: "an object that curves spacetime to the extent that time stops at the event horizon." This event horizon is the defining feature—a surface beyond which no information can escape. Everything that happens inside is causally disconnected from the rest of the universe. As Cardoso puts it, a black hole is like a box full of precious materials that can never be opened. All the secrets about quantum gravity and the fate of collapsing stars are hidden from us by definition.