Season 17, Episode 6: The Black Hole Information Paradox Explained

Hey StarTalkians! In Season 17, Episode 6, Neil and Chuck tackle another collection of Cosmic Queries, with a focus on black holes. One question they received touched the black hole information paradox. 

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The Queries episodes are great for covering a lot of ground, but it’s easy to get a little lost during the brief explanations. Neil covered the main points broadly, but here’s a more detailed explanation.

Hawking Radiation and Information in a Black Hole

Stephen Hawking made a huge stir in the 1970s with his research on how quantum mechanics works on the edge of a black hole (Breakdown of predictability in gravitational collapse).

The basic idea depends on pair production. It’s like the universe has an energy bank account and after a quick loan, a pair of one particle and one anti-particle can emerge. But that loan has to be paid back: they combine, annihilate and the energy debt is repaid.

Hawking wondered, what happens if this happens on the very edge of a black hole? One particle might fall in while the other escapes into space.

If this happens, who pays the energy debt? The black hole. He concluded that black holes must evaporate by emitting radiation – called Hawking radiation – to repay this debt.

The problem is that you can’t get the original particle’s information from the radiation that comes out. It’s like trying to reconstruct an ancient stone by studying the grains of sand on the beach.

But if the information is lost, it’s not possible to determine what happened in the past by studying the present. It throws the whole project of physics into question.

The Black Hole Information Paradox

Hawking’s student, Don Page, came back in the 1990s with an ingenious insight (Information in black hole radiation). He realized that the radiation would carry information from the black hole via quantum entanglement.

Page showed that the “entanglement entropy” – the information contained in the entangled state – would increase as the black hole evaporated, but would decline back to zero as the black hole disappears.

The only problem is that nobody knew how the entropy trend could just reverse like this. Physicists were left with a paradox: either weird new physics was needed way sooner than expected, or Hawking was right and information is lost forever.