Despite its ability to bend both minds and space, an Einsteinian black hole looks so simple a child could draw it. There’s a point in the center, a perfectly spherical boundary a bit farther out, and that’s it.
The point is the singularity, an infinitely dense, unimaginably small dot contorting space so radically that anything nearby falls straight in, leaving behind a vacuum. The spherical boundary marks the event horizon, the point of no return between the vacuum and the rest of the universe. But according to Einstein’s theory of gravity, the event horizon isn’t anything that an unlucky astronaut would immediately notice if she were to cross it. “It’s like the horizon outside your window,” said Samir Mathur, a physicist at Ohio State University. “If you actually walked over there, there’s nothing.”
In 2012, however, this placid picture went up in flames. A team of four physicists took a puzzle first put forward by Stephen Hawking about what happens to all the information that falls into the black hole, and turned it on its head. Rather than insisting that an astronaut (often named Alice) pass smoothly over the event horizon, they prioritized a key postulate of quantum mechanics: Information, like matter and energy, must never be destroyed. That change ended up promoting the event horizon from mathematical boundary to physical object, one they colorfully named the wall of fire.