Black hole information paradox

Partly settled. Some of it is understood, some is still argued about.

Stephen Hawking's 1974-1976 calculation showing black holes emit thermal radiation and eventually evaporate seemed to imply that information about whatever fell in would be irretrievably lost, directly contradicting a foundational principle of quantum mechanics. Decades of debate (including a famous bet between Hawking, Kip Thorne, and John Preskill) produced various proposed resolutions (black hole complementarity, the holographic principle, firewalls), but a concrete calculational breakthrough came in 2019, when several groups showed that including 'islands' — regions inside the black hole horizon that get included in the entanglement entropy calculation via quantum extremal surfaces — reproduces the Page curve expected if information is preserved, at least in simplified gravitational models. Extending this rigorously to realistic, four-dimensional evaporating black holes remains an active research frontier as of 2026.

A black hole slowly erasing itself into a mist of radiation forced physicists to ask whether the universe is allowed to truly forget, and a strange geometric trick called an 'entanglement island' may be the answer.

Explanations that circulated

• That the 2019-2020 'islands' calculations fully resolve the paradox for real, physical black holes in our universe — the breakthrough recovers the expected Page curve (showing information is preserved) but so far chiefly in simplified toy models (e.g. two-dimensional gravity, or black holes coupled to a bath), not yet in fully realistic four-dimensional astrophysical black holes

References

• The entropy of bulk quantum fields and the entanglement wedge of an evaporating black hole063), Ahmed Almheiri, Netta Engelhardt, Donald Marolf, Henry Maxfield, Journal of High Energy Physics, 2019
• Entanglement Wedge Reconstruction and the Information Paradox002), Geoffrey Penington, Journal of High Energy Physics, 2020