What Is Inside a Black Hole?

Einstein's general relativity our best theory of gravity makes a specific prediction about black hole interiors: a singularity. This is not a physical object but a mathematical point where spacetime curvature becomes infinite. All the mass of the black hole is compressed to zero volume. Time as a dimension ceases to function normally. The equations stop producing meaningful answers.

Before reaching the singularity, anything falling into a black hole crosses the event horizon the point of no return. Interestingly, from the perspective of the falling observer, crossing the event horizon isn't an event at all: for a large enough black hole, the tidal forces at the horizon are gentle, and you'd cross it without noticing, only later finding you couldn't escape. For an outside observer, you'd appear to slow and freeze at the horizon due to extreme gravitational time dilation, eventually fading as Hawking radiation.

Between the event horizon and the singularity lies the interior a region where, according to relativity, space and time swap roles. Movement toward the singularity becomes as inevitable as movement forward in time. You can no more turn away from the singularity than you can turn backward in time.

Here's the crisis: Stephen Hawking showed in 1974 that black holes slowly evaporate. They emit 'Hawking radiation' quantum-mechanical thermal radiation that carries away the black hole's mass until nothing remains. This is a triumph of combining general relativity with quantum theory.

The problem: quantum mechanics has a fundamental law that information cannot be destroyed. The physical state of any system at one moment completely determines its state at any other moment past or future. But Hawking radiation appears to carry no information about what fell into the black hole. When the black hole evaporates completely, that information seems to be gone. This violates quantum mechanics' deepest principle.

Hawking himself changed his position on this multiple times. The current leading resolution involves the concept of 'black hole complementarity' and more recently the 'firewall paradox' and insights from holography the idea that all the information is somehow encoded on the event horizon's surface, not destroyed inside. But no complete, agreed-upon solution exists.