Stephen Hawking Challenges Conventional Thinking On Black Holes

redOrbit Staff & Wire Reports – Your Universe Online
If most physicists claimed that the popular notion of black holes as event horizons from which light cannot escape is wrong, they would be almost immediately dismissed (and possibly even scoffed at). However, when Stephen Hawking makes such an assertion, the scientific community stands up and takes notice.
Hawking, the well-known University of Cambridge physicist who was one of the developers of modern black hole theory, has published a new paper in which he says there is no invisible firewall surrounding these black holes, and that matter and energy are only temporarily constrained before they are eventually released in an altered form.
As the professor told Nature‘s Zeeya Merali on Friday, there is “no escape from a black hole in classical theory,” but quantum theory makes it possible for “energy and information to escape from a black hole.” However, a full explanation of the process would require a theory that merges gravity with other fundamental natural forces, which physicists have been unable to find.
According to Mark Prigg of the Daily Mail, the theory advanced by Hawking in his new study is known as the “grey hole theory” and has not yet passed the peer-review process. It is based on a discussion he conducted via Skype during a meeting of the California-based Kavli Institute for Theoretical Physics last August.
Merali refers to the paper as an attempt to solve what is known as the “black hole firewall paradox,” an issue which has been puzzling theoretical physicists for nearly two years. During a thought experiment, researchers at the Kavli Institute asked what would become of an astronaut that fell into a black hole.
“Event horizons are mathematically simple consequences of Einstein’s general theory of relativity that were first pointed out by German physicist Karl Schwarzschild in 1916,” Merali explained. “In that picture, physicists had long assumed, the astronaut would happily pass through the event horizon, unaware of his or her impending doom, before gradually being pulled inwards – stretched out along the way, like spaghetti – and eventually crushed at the ‘singularity’, the black hole’s hypothetical infinitely dense core.”
However, after conducting a detailed analysis of the problem, the Kavli Institute investigators realized that the laws of quantum mechanics governing particles on a smaller scale changed the situation entirely. They claimed quantum theory would dictate that the event horizon would have to be transformed into a “firewall” – an extremely energetic region that would cause the astronaut to burn up completely.
“This was alarming because, while the firewall obeyed quantum rules, it flouted Einstein’s general theory of relativity,” Merali said. “According to that theory, someone in free fall should perceive the laws of physics as being identical everywhere in the Universe – whether they are falling into a black hole or floating in empty intergalactic space. As far as Einstein is concerned, the event horizon should be an unremarkable place.”
Hawking’s new theory takes another approach to the issue. He suggests both quantum mechanics and general relativity remain unchanged, only black holes do not actually have an event horizon to catch fire. Merali explained that this notion is based on the fact that quantum effects surrounding the black hole would cause too many unpredictable spacetime fluctuations around the phenomenon for this kind of boundary surface to exist.
Instead, Hawking suggests that there is an “apparent horizon” around the black holes, and that this surface suspends light rays attempting to flee its core. In terms of general relativity, both types of horizons are identical in an unchanging black hole, because light attempting to escape from the inside would only be able to reach the black hole before it is held there. However, in principle, both horizons can be distinguished from one another. If the black hole swallows more matter, its event horizon would swell and grow larger than the apparent horizon.
As’s Colin Schultz explains it, Hawking says that while there is “something like a black hole… out there,” their grip “is not quite so enduring or destructive as we thought.” Basically, he “wants to do away with the idea of the black hole’s iconic ‘event horizon’ and turn this ultimate gravitational cliff into something a little less steep.”
Former Hawking student and University of California, Berkeley theoretical physicist Raphael Bousso told Prigg that many physicists may find Hawking’s newest work “abhorrent.” However, he added, “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.”

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