The Hawking Paradox: What Happens to a Black Hole When It Eventually Disappears Into Oblivion

A new theory that could solve the Hawking paradox speculates that black holes could have 'hair.' Hair?!? What exactly do the researchers mean by that?
black hole hawking paradox

In 1974, Stephen Hawking proposed that black holes are not black but emit a faint glow of radiation; this is referred to as Hawking radiation

The paradox is that this radiation causes the black hole to evaporate and slowly disappear. 

But what happens to the ‘information’ that was inside the black hole?

According to Hawking, it is lost forever. This contradicts the laws of physics, which state that information can never be lost. In 2004, Hawking proposed a solution to the paradox, called the “information paradox.” He suggested that the information is not lost but encoded in the Hawking radiation.

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1024px Black holes accretion disk
Black hole’s accretion disk by NASA under CC BY-SA 4.0

The information paradox

The information paradox is a puzzle that arises from studying black holes. Classically, black holes are thought to be objects from which nothing, not even light, can escape. This means that once something falls into a black hole, it is lost forever. 

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Stephen Hawking by Victor R. Ruiz under CC BY 2.0

However, quantum mechanics tells us that information cannot be destroyed. So what happens to the information that falls into a black hole? The information paradox is that there seems to be a contradiction between what classical physics tells us about black holes and what quantum mechanics tells us about information. One way to resolve the paradox is to say that the information that falls into a black hole is not lost. Instead, it is stored in the black hole, in a form we do not yet understand.

A solution to the Hawking information paradox

Robert Lea writing for popularmechanics.com, writes about a review that came out last month in the journal Europhysics Letters that may have brought us closer to a solution. In it, physics researchers at the University of Sussex, Xavier Calmet and Stephen D. H. Hsu, talk about the Hawking paradox and possible ways to solve it.

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The researchers refer to Hawking’s 1974 letter called “Black hole explosions?” published in Nature. Hawking suggested that quantum effects, which are usually ignored in black hole physics, could become important in the mass loss of a black hole.

Calmet says. “If Hawking is right, we would have to admit that one of the well-known theories of physics is wrong. Either quantum mechanics or Einstein’s theory of general relativity needs to be changed.”

To understand the solution to Hawking’s paradox, it helps to know how black holes are made.

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How black holes are made

When big stars reach the end of their lives and run out of fuel for nuclear fusion, they turn into black holes. When nuclear fusion stops, the force that keeps a star from falling apart due to its own gravity is no longer there.

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Black hole formation by NASA

This causes the core to collapse, making a point where spacetime curves indefinitely. This point is called a central singularity, and physics can’t explain it yet. At the edge of this extreme curve is the “event horizon” of the black hole, which is the point where not even light can escape the black hole’s gravity.

An idea that could solve the Hawking paradox

What is known about black holes is that they only have mass, angular momentum, and an electric charge. John Wheeler, a theorist in physics, said, “black holes have no hair.”

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What does Wheeler mean when he says black holes have no hair?

The no-hair theorem is a metaphor that says there aren’t many things that make black holes different from each other. The idea is that black holes can’t be told apart by anything other than their mass, charge, and spin. ‘They don’t even have different hairstyles, cuts, or colors’ is a colloquial analogy invented by Wheeler.

New study finds that black holes could have hair

Calmet and Hsu think that information carried by the matter a black hole eats could be stored in its gravitational field. By figuring out how to make corrections to gravity at the quantum level, they could show that the star’s potential is sensitive to how it is inside. This means that black holes have, for lack of a better term, “quantum hair,” which is made of the same stuff as the star that created it.

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Calmet says, “When this star falls into a black hole, the correction stays, so black holes have a quantum hair.” “In other words, black holes remember something about the star that made them.”

The research concludes that Hawking radiation isn’t just made of heat. Instead, the study has developed the theory that black holes have quantum hair that stores information. Read next about what happens when two mammoth black holes collide.


Featured image credit: NASA

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