Archive for the ‘Stephen Hawking’ Category

Stephen Hawking's black hole theory
Notion of an ‘event horizon’, from which nothing can escape, is incompatible with quantum theory, physicist claims.

by Zeeya Merali

Most physicists foolhardy enough to write a paper claiming that “there are no black holes” — at least not in the sense we usually imagine — would probably be dismissed as cranks. But when the call to redefine these cosmic crunchers comes from Stephen Hawking, it’s worth taking notice. In a paper posted online, the physicist, based at the University of Cambridge, UK, and one of the creators of modern black-hole theory, does away with the notion of an event horizon, the invisible boundary thought to shroud every black hole, beyond which nothing, not even light, can escape.

In its stead, Hawking’s radical proposal is a much more benign “apparent horizon”, which only temporarily holds matter and energy prisoner before eventually releasing them, albeit in a more garbled form.

“There is no escape from a black hole in classical theory,” Hawking told Nature. Quantum theory, however, “enables energy and information to escape from a black hole”. A full explanation of the process, the physicist admits, would require a theory that successfully merges gravity with the other fundamental forces of nature. But that is a goal that has eluded physicists for nearly a century. “The correct treatment,” Hawking says, “remains a mystery.”

Hawking posted his paper on the arXiv preprint server on 22 January1. He titled it, whimsically, ‘Information preservation and weather forecasting for black holes’, and it has yet to pass peer review. The paper was based on a talk he gave via Skype at a meeting at the Kavli Institute for Theoretical Physics in Santa Barbara, California, in August 2013.

Hawking’s new work is an attempt to solve what is known as the black-hole firewall paradox, which has been vexing physicists for almost two years, after it was discovered by theoretical physicist Joseph Polchinski of the Kavli Institute and his colleagues.

In a thought experiment, the researchers asked what would happen to an astronaut unlucky enough to fall into a black hole. Event horizons are mathematically simple consequences of Einstein’s general theory of relativity that were first pointed out by the German astronomer Karl Schwarzschild in a letter he wrote to Einstein in late 1915, less than a month after the publication of the theory. 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.

But on analysing the situation in detail, Polchinski’s team came to the startling realization 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 transformed into a highly energetic region, or ‘firewall’, that would burn the astronaut to a crisp.

This was alarming because, although the firewall obeyed quantum rules, it flouted Einstein’s general theory of relativity. 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.

Now Hawking proposes a third, tantalizingly simple, option. Quantum mechanics and general relativity remain intact, but black holes simply do not have an event horizon to catch fire. The key to his claim is that quantum effects around the black hole cause space-time to fluctuate too wildly for a sharp boundary surface to exist.

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.”

http://www.nature.com/news/stephen-hawking-there-are-no-black-holes-1.14583?WT.mc-id=GPL_NatureNews

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“Our hypothesis is that the inside of a black hole — it may not be there. Probably that’s the end of space itself. There’s no inside at all.”
– Joe Polchinski, physicist

It could rightly be called the most massive debate of the year: Physicists are locked in an argument over what happens if you fall into a black hole.

On one side are those who support the traditional view from Albert Einstein. On the other, backers of a radical new theory that preserves the very core of modern physics by destroying space itself.

Regardless of who’s right, the new take on black holes could lead to a better understanding of the universe, says Leonard Susskind, a physicist at Stanford University. “This is the kind of thing where progress comes from.”

Black holes are regions of space so dense that nothing, not even light, can escape.

There’s a long-standing view about what would happen if you fell into one of these holes. At first, you’re not going to notice much of anything — but the black hole’s gravity is getting stronger and stronger. And eventually you pass a point of no return.

“It’s kind of like you’re rowing on Niagara Falls, and you pass the point [where] you can’t row fast enough to escape the current,” Susskind says. “Well, you’re doomed at that point. But passing the point of no return — you wouldn’t even notice it.”

Now you can’t get out. And gravity from the black hole is starting to pull on your feet more than your head. “The gravity wants to sort of stretch you in one direction and squeeze you in another,” says Joe Polchinski, a physicist at the University of California, Santa Barbara. He says the technical term for this stretching is spaghettification.

“It’d be kind of medieval,” says Polchinkski. “It’d be like something on Game of Thrones.”

In Einstein’s version of events, that’s the end. But Polchinski has a new version of things: “Our hypothesis is that the inside of a black hole — it may not be there,” he says.

So what’s inside the black hole? Nothing, Polchinski says. Actually even less than that. “Probably that’s the end of space itself; there’s no inside at all.”
This “no inside” idea may sound outrageous, but it’s actually a stab at solving an even bigger problem with black holes.

According to the dominant theory of physics — quantum mechanics — information can never disappear from the universe. Put another way, the atoms in your body are configured in a particular way. They can be rearranged (radically if you happen to slip inside a black hole). But it should always be possible, at least in theory, to look at all those rearranged atoms and work out that they were once part of a human of your dimensions and personality.

This rule is absolutely fundamental. “Everything is built on it,” says Susskind. “If it were violated, everything falls apart.”

For a long time, black holes stretched this rule, but they didn’t break it. People thought that if you fell into a black hole, your spaghettified remains would always be in there, trapped beyond the point of no return.

That is, until the famous physicist Stephen Hawking came along. In the 1970s, Hawking showed that, according to quantum mechanics, a black hole evaporates — very slowly, it vanishes. And that breaks the fundamental rule because all that information that was once in your spaghettified remains vanishes with it.

This didn’t seem to bother Hawking. (“I’m not a psychiatrist, and I can’t psychoanalyze him,” Susskind says.) But it has bothered a lot of other physicists since.

And in the intervening years, work by another theorist — Juan Maldacena, with Princeton’s Institute for Advanced Study — seems to show that Hawking was wrong. Information has to get out of the black hole … somehow. But nobody knows how.

So Polchinski took another look. “We took Hawking’s original argument,” he says, “and very carefully ran it backwards.”

And Polchinski and his colleagues found one way to keep things from vanishing when they fall inside a black hole — they got rid of the inside. By tearing apart the fabric of space beyond the point of no return, the group was able to preserve the information rule of quantum mechanics.

In this version, anything falling into a black hole is instantly vaporized at the point of no return, in a fiery storm of quantum particles. Particles coming from the hole collectively carry away any and all information about the object that’s falling in.

So in Polchinski’s version, when you fall into a black hole, you don’t disappear. Instead, you smack into the end of the universe.

“You just come to the end of space, and there’s nothing beyond it. Terminated,” Susskind says. All the information once contained in your atoms is re-radiated in a quantum mechanical fire.

This new version seems too radical to Susskind. “I don’t think this is true,” he says. “In fact, I think almost nobody thinks this is true — that space falls apart inside a black hole.”

Even Polchinski still feels that black holes should have insides. “My gut believes that the black hole has an interior,” he says. But, he adds, nobody’s been able to disprove his hypothesis that it doesn’t.

“Every counterargument I’ve seen is flawed,” Polchinski says.

Susskind agrees: “Nobody quite knows exactly what’s wrong with their argument — and that’s what makes this so important and interesting.”

And as crazy as it sounds, this is progress. In the year ahead, Susskind hopes someone can find the flaw in Polchinski’s argument, just the way Polchinski found a flaw in Stephen Hawking’s argument. But it will be awhile before we understand black holes inside and out.

http://www.npr.org/2013/12/27/256897343/stretch-or-splat-how-a-black-hole-kills-you-matters-a-lot

Heidi Hankins has become one of the society’s youngest members after her  intellect was measured at 59 points above the average child’s, and only one point below Stephen Hawking.

‘We always thought Heidi was pretty bright because she was reading early,’ said her father, Matthew, 46.

‘I got her the complete set of the Oxford Reading Tree books when she was two  and she read through the whole 30 in about an hour. It’s what you would expect a  seven-year-old to do.’

University lecturer Mr Hankins and wife Sophy, 42, tested Heidi and sent the  results to Mensa after nursery staff said they had no activities to challenge  her.

‘I specialise in measuring IQs in children, and I was curious about her and  the results were off the scale,’ added Mr Hankins of Winchester, Hampshire.

‘The thing is she is not precocious, she is just a little girl who likes her  Barbies and Lego but then you will find her sitting down and reading a book.’

Although she is yet to start school, Heidi can already do addition and  subtraction, write in clear sentences and could count to 40 when she was just  two.

Mensa’s youngest ever member, Elise Tan Roberts, from Edmonton, north London,  was two when she joined in 2009.

Read more: http://www.metro.co.uk/news/896011-genius-four-year-old-heidi-hankins-joins-mensa-with-iq-of-159#ixzz1rtJZywzA