Quantum Correlations Could Solve the Black Hole Information Paradox

The black hole information paradox has puzzled physicists for decades. New research shows how quantum connections in spacetime itself may resolve the paradox, and in the process leave behind a subtle signature in gravitational waves.

For a long time we thought black holes, as mysterious as they were, didn’t cause any trouble. Information can’t be created or destroyed, but when objects fall below the event horizons, the information they carry with them is forever locked from view. Crucially, it’s not destroyed, just hidden.

But then Stephen Hawking discovered that black holes aren’t entirely black. They emit a small amount of radiation and eventually evaporate, disappearing from the cosmic scene entirely. But that radiation doesn’t carry any information with it, which created the famous paradox: when the black hole dies, where does all its information go?

One solution to this paradox is known as non-violent nonlocality. This takes advantage of a broader version of quantum entanglement, the “spooky action at a distance” that can tie together particles. But in the broader picture, aspects of spacetime itself become entangled with each other. This means that whatever happens inside the black hole is tied to the structure of spacetime outside of it.

Usually spacetime is only altered during violent processes, like black hole mergers or stellar explosions. But this effect is much quieter, just a subtle fingerprint on the spacetime surrounding an event horizon.

If this hypothesis is true, the spacetime around black holes carries tiny little perturbations that aren’t entirely random; instead, the variations would be correlated with the information inside the black hole. Then when the black hole disappears, the information is preserved outside of it, resolving the paradox.

In a recent paper appearing in the journal preprint server arXiv, but not yet peer-reviewed, a pair of researchers at Caltech investigated this intriguing hypothesis to explore how we might be able to test it.

The researchers found that these signatures in spacetime also leave an imprint in the gravitational waves when black holes merge. These imprints are incredibly tiny, so small that we are not yet able to detect them with existing gravitational wave experiments. But they do have a very unique structure that stands on top of the usual wave pattern, making them potentially observable.

The next generation of gravitational wave detectors, which aim to come online in the next decade, might have enough sensitivity to tease out this signal. If they see it, it would be tremendous, as it would finally point to a clear solution of the troubling paradox, and open up a new understanding of both the structure of spacetime and the nature of quantum nonlocality.