Still causing black hole controversy (Image: Harold Cunningham/Getty Images) Brief history of black holes

Stephen Hawking has a new mind-bending theory about black holes, the bizarre cosmic objects that cemented his reputation as the world’s most famous living scientist. Rather than getting sucked into a singularity of confusion, read our explainer

What exactly is a black hole?

Good question. According to theoretical physicists, they used to be regions of space-time – the fabric that makes up the universe – that have become so dense that their huge gravity generates an event horizon, from inside which nothing, not even light, can escape. Then in 1974, Hawking added quantum mechanics to the black hole picture and sparked a row that has raged on until the present day (see timeline).


What’s wrong with a bit of quantum?

Quantum mechanics doesn’t get along with the other grand theory of physics, general relativity, making it difficult to understand situations in which both are relevant, such as black holes. Hawking applied quantum theory to black holes and realised they aren’t quite black. Instead, they should emit small amounts of radiation, causing them to shrink and eventually evaporate.

OK, so black holes aren’t immortal. What’s the problem?

The theory of Hawking radiation also suggested that when a black hole dies, it takes everything inside with it, but that is a big quantum no-no. Quantum physics says that information about matter is never destroyed, even when it falls into a black hole. Other theorists suggested solving this “information paradox” by allowing information to escape from the black hole as it evaporated. Hawking disagreed – until 30 years later, when he showed how it might be possible and was forced to concede a seven-year-old wager with another physicist.

Meaning now everyone agrees about black holes?

If only. For the past 18 months the black hole community has been up in arms over a descendant of the information paradox, known as the firewall paradox. A group led by Joseph Polchinski of the University of California in Santa Barbara suggested information leaving a black hole would produce massive amounts of energy, creating a wall of fire at the event horizon that would consume anything falling in. This would break a rule of general relativity that says crossing a black hole’s event horizon should be uneventful – hence the paradox.

It’s yet another quantum versus relativity showdown!

Indeed. Firewalls mean that one of the two theories is wrong, so physicists have been scrambling to find a compromise that doesn’t produce these flaming problems. Now Hawking has waded in and says the solution is to give up the very thing that makes black holes so intriguing – the event horizon.

Wait a minute… does that mean you could actually escape from a black hole?

Potentially, although you would probably need to be travelling at the speed of light. “The absence of event horizons means that there are no black holes – in the sense of regimes from which light can’t escape to infinity,” writes Hawking in his new paper, which he posted online earlier this week. Instead, black holes have “apparent horizons”, surfaces which trap light but can also vary in shape due to quantum fluctuations, leaving the potential for light to escape.

Are the two horizons really that different?

It is unclear. The idea of an apparent horizon isn’t completely new, and Hawking – along with Roger Penrose of the University of Oxford – has previously used general relativity to prove that the two horizons are actually identical. In his most recent paper he is proposing that quantum mechanics might reveal them to be different.

Ah, so is that the new bit?

Not quite. The main contribution of the new paper is an attempt to use these ideas to resolve the firewall paradox. Removing the event horizon also kills off the firewall. That would normally suggest that quantum information must be lost – but Hawking says that needn’t be the case. He proposes that the structure of a black hole just below the horizon is chaotic, making it difficult to understand the information being released. In other words, the information is lost in the sense that it is almost impossible to interpret, but it isn’t actually destroyed. “It will be like weather forecasting on Earth,” he writes. “One can’t predict the weather more than a few days in advance.”

Is he right? Is the paradox solved?

Hawking’s paper is very short, just two pages of text with no calculations, making it difficult to draw any strong conclusions, but there is already some scepticism. “It is not clear what he expects the infalling observer to see,” says Polchinski. “It almost sounds like he is replacing the firewall with a chaos-wall, which could be the same thing.” Samuel Braunstein of the University of York, UK, who has waded into the firewall debate previously, also isn’t convinced: “I don’t see any evidence which really demonstrates that the thing he is talking about doesn’t have a firewall.”

Does it matter if Hawking is right?

If black holes are how he describes, it could lead to a better understanding of quantum mechanics and general relativity. “We might learn some new physics, which may have real implications about the non-trivial structure of the universe,” says Braunstein. But he also points out that we might not.

Does Hawking mind being wrong?

Everyone hates being wrong – and Hawking is human. On his 70th birthday, he told New Scientist that he regards his idea that information was destroyed by black holes, which later turned out to be wrong, as his “biggest blunder” – in science, at least.

New Scientist is in the process of contacting Stephen Hawking for comment on his latest paper.

Journal reference: arxiv.org/abs/1401.5761