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For obvious reasons, we do not know what the inside of a black hole looks like. But thanks to theoretical physics, we can ask what the inside should look like if Einstein's theory of gravity and the rules of quantum mechanics are both true. A new study in the journal ÌÇÐÄÊÓƵical Review Letters has done exactly this by concentrating on two black holes that are deeply entangled (linked together by quantum rules).

Mapping the interior

The research by scientists from the U.S. and Argentina theoretically mapped the shared inner space between the two objects—the wormhole connecting them. They found that for a typical, messy entangled pair, the interior isn't the smooth tunnel of science fiction.

Instead, it's a long, lumpy structure they called the "Einstein-Rosen caterpillar." It's named after the Einstein-Rosen Bridge, the mathematical structure that connects two regions of spacetime, and "caterpillar" because of its bumpy, segmented shape. This discovery is a significant step toward proving that the bizarre rules of quantum mechanics can control the shape of spacetime inside a black hole.

To map the complex interior, the researchers started with a simple theoretical model of a perfect, smooth wormhole that has an ordered quantum state. Then, to mimic a chaotic black hole pair, the team used a computer simulation to scramble the quantum connection between them. Finally, they calculated the wormhole's resulting geometry. To keep the system stable during this chaos, the wormhole had to be long and bumpy.

This finding revealed a direct mathematical link between the quantum chaos and the size of the wormhole. The more random and chaotic the quantum state of the black holes, the more complex the physical wormhole connecting them becomes. "The ensemble of ER caterpillars of average length ℓ and matter correlation scale ℓ_Δ forms an ε-approximate quantum state k design of the black holes for k ~ (ℓ—ℓ_ε)/ ℓ_Δ," wrote the researchers.

Challenging the firewall paradox

The implications of finding this long, stable caterpillar wormhole could be huge for a major conflict in physics known as the firewall paradox. Some theories suggest that the interior of a typical black hole should not be smooth or stable. Instead, spacetime could be violently broken at the edge of the black hole by a curtain of energy called a "firewall." In the models studied by the researchers, even when quantum entanglement is messy and random, the wormhole remains a predictable, stable tunnel where the classical laws of gravity still hold.

This result supports the idea that two of the strangest concepts in physics (quantum entanglement and wormholes) are equivalent, or two sides of the same coin—the ER=EPR conjecture. The authors say, "The construction and main result of this Letter support a vastly more general form of ER = EPR and seem to be in some tension with arguments against semiclassicality of typical interiors."

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