An event horizon has been defined as an invisible cloak covering a black hole that allows nothing -- not even light -- to escape. The option proposed by Hawking's paper is that black holes simply don't have an event horizon, and the notion of an event horizon, from which nothing can escape, is incompatible with quantum theory.
According to Nature, this claim is based on the idea that quantum effects around the black hole cause space-time to fluctuate too inconsistently to maintain any kind of sharp border.
Hawking's proposed alternative to the event horizon is an "apparent horizon," which temporarily holds matter hostage and mangles it before releasing it.
Nature reports that, for an unchanging black hole, these two horizons are identical in general relativity because light trying to escape can reach only as far as the event horizon and will be held there.
Can the two horizons be distinguished? In principle, yes ? Nature explains that the event horizon will swell and grow larger than the apparent horizon if more matter gets swallowed by the black hole.
Another big difference: the apparent horizon can eventually dissolve, unlike the event horizon.
"There is no escape from a black hole in classical theory," Hawking told Nature. But quantum theory "enables energy and information to escape from a black hole," he said.
Hawking admits to Nature that a full explanation would require a theory that successfully merges gravity with the other fundamental forces of nature -- a goal that physicists have had for nearly a century.
"The correct treatment remains a mystery," Hawking said.
Nature reports that, 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."
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