He now suggests an ‘apparent horizon’ — a surface along which light rays attempting to rush away from the black hole’s core would be suspended — would be a black hole’s boundary.

Acclaimed theoretical physicist Stephen Hawking, one of the creators of modern theory on black holes, has now questioned the existence of the phenomenon known as event horizon around them.
In a paper posted online on University of Cambridge, Britain, Prof. Hawking has proposed a new theory called ‘apparent horizon’ — a surface along which light rays attempting to rush away from the black hole’s core would be suspended.
Prof. Hawking’s new suggestion is that the ‘apparent horizon’ is the real boundary.
“The absence of event horizons mean that there are no black holes — in the sense of regimes from which light can not escape to infinity,” Prof. Hawking, director of research at the centre for theoretical cosmology, was quoted as saying in a report by the journal Nature.
An event horizon is termed as a boundary in space time beyond which events cannot affect an outside observer. In layman’s terms, it is defined as ‘the point of no return’.
“A full explanation of the process would require a theory that successfully merges gravity with the other fundamental forces of nature. The correct treatment, however, remains a mystery,” added Prof. Hawking, author of A Brief History of Time.
Titled ‘Information Preservation and Weather Forecasting for Black Holes’, the paper is an attempt to solve the black hole firewall paradox — discovered by theoretical physicist Joe Polchinski at the Kavli Institute for Theoretical Physics in Santa Barbara, California.
According to the Nature report, Prof. Polchinski’s team found that the laws of quantum mechanics, which govern particles on small scales, change the situation completely.
Quantum theory, they said, dictates that the event horizon must actually be a transformed into a highly energetic region, or ‘firewall’, that would burn the astronaut to a crisp.
This was alarming because, while the firewall obeyed quantum rules, it flouted Einstein’s general theory of relativity.
“The picture Hawking gives sounds reasonable,” Don Page, an expert on black holes at the University of Alberta in Edmonton, Canada, was quoted as saying.
“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,” Prof. Page added.
Dr. 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 said.

Source : The Hindu