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The black-hole information paradox
1976· Open question
Open questionNobody knows the answer.
If black holes evaporate into featureless heat, what happens to the information about everything that fell in?
See it on the Science map →How it works
Quantum theory insists information is never destroyed: in principle, the past can always be reconstructed. Hawking's 1976 calculation said black hole radiation is random heat that depends only on mass, spin and charge. Burn an encyclopedia or a brick, and once the black hole is gone the two leave identical leftovers. Either quantum theory fails, the information escapes in a subtle way, or something stays behind.
Why it matters
It is the sharpest clash between general relativity and quantum mechanics, and a key testing ground for any theory of quantum gravity.
Think of it like
Like a title chain that suddenly breaks: the deed exists, but nobody can trace who owned it before.
How we know
No experiment can test it yet. Most physicists now believe information survives, based on theory: the 1997 AdS/CFT correspondence and 2019 Page-curve calculations.
Still a mystery
Exactly how the information gets out, and whether anything strange happens at the horizon, is unresolved.
Connections (7)
▸The wave function forbids losing information, creating The black-hole information paradoxOpen question
The relationship
In quantum mechanics, a system's wave function evolves by a rule that is always reversible, called unitarity. In principle, the present state always determines the past, so information is never truly lost. Hawking's calculation said black holes turn any input into the same featureless heat. That would erase information, violating the rule. The clash between these two claims is the paradox.
How the link was found
Hawking stated the conflict in 1976 in "Breakdown of predictability in gravitational collapse," arguing quantum mechanics must be modified.
Is it proven?
Reversibility is tested in every quantum experiment to date. Whether black holes respect it cannot be tested directly; most physicists now think they do, based on theoretical results such as AdS/CFT.
Open questions and mysteries
If information is preserved, where exactly is it stored and how does it get into the radiation?
▸Stephen Hawking posed The black-hole information paradoxOpen question
The relationship
Hawking showed his radiation is exactly thermal, carrying no record of what fell in. Once a black hole evaporates completely, the information seems gone, breaking the quantum rule that it is always preserved. He argued quantum mechanics must change. Many physicists disagreed, and the dispute became the black-hole information paradox, a decades-long contest of ideas.
How the link was found
Hawking, 1976. In 1997 Hawking and Kip Thorne bet John Preskill that information is lost. Hawking conceded in 2004, giving Preskill a baseball encyclopedia.
Is it proven?
The paradox is a genuine contradiction between two well-tested theories. Its resolution is not proven; theoretical work since 1997 points strongly toward information surviving.
Open questions and mysteries
Hawking's 2004 concession and later "soft hair" work (2016) did not settle exactly how the information escapes.
▸Hawking radiation: black holes evaporate creates The black-hole information paradoxOpen question
The relationship
Hawking radiation's properties depend only on the black hole's mass, spin and charge, not on what fell in. As the black hole radiates, it shrinks; eventually it may vanish completely, leaving only this generic heat. If so, the details of everything that fell in are gone, which quantum theory forbids. The radiation is what turns a hidden puzzle into a real contradiction.
How the link was found
Stephen Hawking, 1976, as a direct consequence of his 1974 discovery.
Is it proven?
The contradiction is a solid theoretical result. Hawking's original calculation is approximate, and later work (Page 1993, islands 2019) suggests subtle correlations in the radiation do carry information out.
Open questions and mysteries
Where do those correlations come from physically, and could even an ideal observer ever decode them?
▸The black-hole information paradox is a test case for Quantum gravity: the missing linkOpen question
The relationship
The paradox arises exactly where gravity and quantum theory both matter, so solving it means knowing, at least in part, how they combine. Any proposed theory of quantum gravity must explain what happens to information in an evaporating black hole. It works as a stress test: a thought experiment precise enough to rule out candidate ideas that answer it wrongly.
How the link was found
Clear from Hawking's 1976 paper; Juan Maldacena's 1997 AdS/CFT correspondence made it a concrete test, offering a setting where gravity is equivalent to ordinary quantum theory.
Is it proven?
AdS/CFT strongly indicates information is preserved, at least in simplified model universes. That is mathematical evidence, not an experiment, and our universe differs from those models.
Open questions and mysteries
Does the AdS/CFT lesson carry over to our expanding universe? What does it say spacetime is made of?
▸The black-hole information paradox has a proposed answer in Black-hole complementaritySpeculative
The relationship
Complementarity tries to dissolve the paradox by saying the outside and infalling descriptions are both correct but never compared. Outside, information is absorbed by a hot layer just above the horizon and later radiated out. Inside, it falls through smoothly. Because no single observer can check both versions, quantum theory's ban on copying information is never visibly broken.
How the link was found
Leonard Susskind, Lárus Thorlacius and John Uglum, 1993, building on Gerard 't Hooft's ideas from the late 1980s.
Is it proven?
Not proven or testable. In 2012 Almheiri, Marolf, Polchinski and Sully ("AMPS") argued it leads to a contradiction unless a "firewall" burns infallers at the horizon.
Open questions and mysteries
Is there a firewall? Can complementarity be repaired, for instance by the "ER = EPR" link between entanglement and wormholes?
▸The black-hole information paradox has recent progress in The Page curve and "islands": information escapesSpeculative
The relationship
If information escapes, the radiation's entanglement with the black hole must follow the Page curve: rising early, then falling back to zero as evaporation completes. Hawking's method gave only a rising line. Newer calculations count a region inside the black hole, the "island," as part of the radiation and recover the falling curve, showing information can leave.
How the link was found
Don Page predicted the curve in 1993. In 2019 Geoff Penington, and separately Almheiri, Engelhardt, Marolf and Maxfield, reproduced it; "replica wormhole" papers (2019–2020) explained why.
Is it proven?
These are mathematical results, mostly in simplified model universes, using well-understood gravity. They are strong evidence that information survives, but not experimental proof.
Open questions and mysteries
The calculation shows the bookkeeping balances but not the physical mechanism that carries information out.
▸Information is physical makes losing it a crisis in The black-hole information paradoxOpen question
The relationship
If information were just an abstract description, losing it inside a black hole might not matter. But information is physical, and quantum theory requires that it is never truly destroyed, only scrambled. So Hawking's claim that evaporating black holes erase information threatened a core law of physics, not a bookkeeping detail.
How the link was found
Stephen Hawking argued in 1976 that black holes destroy information. Gerard 't Hooft and Leonard Susskind led the pushback through the 1980s-90s, treating information conservation as non-negotiable.
Is it proven?
Information conservation, called unitarity, holds in every quantum experiment ever done. Landauer's principle, measured in 2012, confirms information carries physical cost. Black holes themselves have not been tested.
Open questions and mysteries
Exactly how information escapes an evaporating black hole, and in what form, is still being worked out.