When Stephen Hawking contradicts his own theory, that's a paradox worth noting.

'It’s saying there’s no ‘black’ in black hole'- Amanda Peet, University of Toronto physicist

Like the black holes he described 40 years ago, the British cosmologist’s reassessment last week of the concept of "event horizons" is pulling in chatter from astrophysicists around the world.

What is an event horizon?

An event horizon is the invisible surface at which point nothing can escape being drawn into a black hole. Hawking previously stated that nothing could escape this black hole, but he has since revised that theory.

It turns out that the black holes Hawking wrote about in 1974 — those places in the space-time continuum that can devour galaxies and even trap light forever — may not exist in the way that he proposed decades ago.

In a new online paper titled Information Preservation and Weather Forecasting for Black Holes, the 72-year-old Hawking says, "There are no black holes — in the sense from which light can't escape to infinity."

The U-turn from Hawking, one of the pioneers of modern black hole theory, surprised his colleagues, said Amanda Peet, a theoretical physicist and associate professor at the University of Toronto who specializes in black hole paradoxes and string theory.

'Radical departure'

"It’s majorly provocative. It’s saying there’s no ‘black’ in black hole. He’s sort of saying the hole is grey instead, not just absorbing everything," she said. "It’s a very radical departure from past ideas."

In other words, Hawking now admits that light can probably pass through what he once defined as a point-of-no-return void, and that black holes can leak "information" in the form of matter.

This reversal proposes that matter and energy can be temporarily held before being released back into space in a scrambled form.

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Stephen Hawking's new paper modifies his views on the properties of a black hole. (Kimberly White/Reuters)

Peet said that although Hawking’s paper has been described as "cryptic," running just four pages long and lacking any calculations, physicists are rightly paying close attention to what he says.

"This is the mind that originated the whole black hole information paradox in the first place," she said.

Black holes occur when a dead star "literally runs out of gas" for nuclear fusion and begins to collapse on itself under the force of gravity, Peet explained. If there’s enough mass, this creates a "runaway collapse" so powerful that no other force can resist. All nearby matter would get drawn in and crushed.

'An event horizon is this hidden thing, a place where if you fall beyond that surface, you can never get out.…You end up getting squished like a bug.'— Amanda Peet, University of Toronto physicist

Hawking's new revelation means he has ruled out what Peet called one of the classical "signatures" of black holes, the notion of boundaries in space-time known as event horizons.

"An event horizon is this hidden thing, a place where if you fall beyond that surface, you can never get out," Peet explained. "If you fall into it, you end up getting squished like a bug. These event horizons, as imagined in the classical black hole literature, are what Hawking is now backtracking on."

What is a black hole?

According to NASA, black holes can form when a star is dying. Some stars, which can be up to 20 times more than the mass of our sun, emerge when a large star "collapses" or falls in on itself. This creates a strong gravitational pull that draws in everything, including, light. The term "black hole" was coined by physicist John Wheeler, who died in 2008.

Harald Pfeiffer, an expert on gravitational waves who teaches at U of T's Canadian Institute for Theoretical Astrophysics, said it appears that Hawking changed his mind because event horizons are incompatible with quantum theory.

"The picture before last week was whenever you throw things into a black hole, all the information about the properties of what you have thrown in is completely lost," he said.

But Pfeiffer said that led to problems, because it violates a law of quantum mechanics known as "unitarity," which says information can't simply be eaten up and destroyed like that.

“The conflict is that with quantum mechanics, you’re never destroying information,” he said. “If information gets completely lost and falls into a black hole, there's no way of reversing this. So either Einstein’s theory of relativity is incomplete, or quantum mechanics is incomplete.”

Hawking now believes physicists should be thinking in terms of "apparent horizons" — boundaries that can contain matter and light for a period of time — rather than the classical event horizon.

In his paper, Hawking writes: "This suggests that black holes should be redefined as metastable bound states of the gravitational field."

Chaos mangles information

Based on the respected physicist’s latest assumptions, Peet said, the reason it’s so hard to recover information out of black holes would be because this apparent horizon behaves pretty much like a true event horizon would, except that "chaos would be driving the scrambling of information rather than some mysterious property of quantum gravity."

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NASA's Spitzer Space Telescope has imaged this coiled galaxy. The 'eye' at the centre of the galaxy is actually a monstrous black hole surrounded by a ring of stars. (NASA/Reuters)

So what might have changed Hawking’s view?

Peet noted there has been much modern development in the field, with physicists Andrew Strominger and Cumrun Vafa demonstrating in 1996 it’s even possible to calculate the entropy of some types of black holes from first principles.

Pfeiffer said Hawking has always known that his views clashed with quantum physics, but he seems to have been relenting in recent years.

In 2004, for example, he conceded a bet against physics theorist John Preskill about black holes. Hawking admitted that information is probably not swallowed forever in a black hole, as he had previously thought.

“He’s only recently came around,” Pfeiffer said.

"Stephen writes extremely few papers, so … this might very well be the very few occasions that Stephen actually puts these ideas in writing. As such, it might very well be quite an important event," he added.

At his 70th birthday two years ago, Hawking claimed his theory about black holes was his "biggest blunder."

Peet said scientists will be waiting patiently for a possible followup paper from him.

"The usual style if you’re famous enough to put out a paper like this, is you follow it up with more details,” she said. "That’s what physicists like me will be waiting on, because it matters what he thinks."