In place of the event horizon, Hawking invokes an “apparent horizon”, a surface along which light rays attempting to rush away from the black hole’s core will be suspended. In general relativity, for an unchanging black hole, these two horizons are identical, because light trying to 

escape from inside a black hole can reach only as far as the event horizon and will be held there, as though stuck on a treadmill. However, the two horizons can, in principle, be distinguished. If more matter gets swallowed by the black hole, its event horizon will swell and grow larger than the apparent horizon.

Conversely, in the 1970s, Hawking also showed that black holes can slowly shrink, spewing out 'Hawking radiation'. In that case, the event horizon would, in theory, become smaller than the apparent horizon. Hawking’s new suggestion is that the apparent horizon is the real 

boundary. “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,” Hawking writes.

“The picture Hawking gives sounds reasonable,” says Don Page, a physicist and expert on black holes at the University of Alberta in Edmonton, Canada, who collaborated with Hawking in the 1970s. “You 

could say that it is radical to propose there’s no event horizon. But these are highly quantum conditions, and there’s ambiguity about what space-time even is, let alone whether there is a definite region that can be marked as an event horizon.”

Although Page accepts Hawking’s proposal that a black hole could exist without an event horizon, he questions whether that alone is enough to get past the firewall paradox. The presence of even an ephemeral apparent horizon, he cautions, could well cause the same problems as does an event horizon.

Unlike the event horizon, the apparent horizon can eventually dissolve. Page notes that Hawking is opening the door to a scenario so extreme “that anything in principle can get out of a black hole”. Although Hawking does not specify in his paper exactly how an apparent horizon 

would disappear, Page speculates that when it has shrunk to a certain size, at which the effects of both quantum mechanics and gravity combine, it is plausible that it could vanish. At that point, whatever was once trapped within the black hole would be released (although not in good shape).

If Hawking is correct, there could even be no singularity at the core of the black hole. Instead, matter would be only temporarily held behind the apparent horizon, which would gradually move inward owing to the 

pull of the black hole, but would never quite crunch down to the centre. Information about this matter would not destroyed, but would be highly scrambled so that, as it is released through Hawking radiation, it would 
be in a vastly different form, making it almost impossible to work out what the swallowed objects once were.

“It would be worse than trying to reconstruct a book that you burned from its ashes,” says Page. In his paper, Hawking compares it to trying to forecast the weather ahead of time: in theory it is possible, but in practice it is too difficult to do with much accuracy.

Polchinski, however, is sceptical that black holes without an event horizon could exist in nature. The kind of violent fluctuations needed to erase it are too rare in the Universe, he says. “In Einstein’s gravity, the black-hole horizon is not so different from any other part of space,” says Polchinski. “We never see space-time fluctuate in our own neighbourhood: it is just too rare on large scales.”

Raphael Bousso, a theoretical physicist at the University of California, Berkeley, and a former student of Hawking's, says that this latest contribution highlights how “abhorrent” physicists find the potential existence of firewalls. However, he is also cautious about Hawking’s solution. “The idea that there are no points from which you cannot escape a black hole is in some ways an even more radical and problematic suggestion than the existence of firewalls,” he says. "But the fact that we’re still discussing such questions 40 years after Hawking’s first papers on black holes and information is testament to their enormous significance."