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  • richardmitnick 10:01 am on January 26, 2017 Permalink | Reply
    Tags: Accelerating mirror, , Black hole paradox, , , Plasma wakefield accelerator, , Shooting electron waves through plasma could reveal if black holes permanently destroy information, Stephen Hawking   

    From Science Alert: “Shooting electron waves through plasma could reveal if black holes permanently destroy information” 


    Science Alert

    25 JAN 2017

    Interstellar/Paramount Pictures

    Without having to enter a black hole ourselves…

    One of the greatest dilemmas in astrophysics is the black hole paradox – if black holes really do destroy every scrap of information that enters them.

    Now, physicists might have finally come up with a way to test the paradox once and for all, by accelerating a wave of negatively charged electrons through a cloud of plasma.

    As far as objects in space go, black holes need little introduction. Get too close, and their concentrated mass will swallow you, never to return.

    But in the 1970s, physicists including Stephen Hawking proposed that black holes weren’t necessarily forever.

    Thanks to the peculiarities of quantum mechanics, particles did indeed radiate away from black holes, Hawking hypothesised, which means, theoretically, black holes could slowly evaporate away over time.

    This poses the paradox. Information – the fundamental coding of stuff in the Universe – can’t just disappear. That’s a big rule. But when a black hole evaporates away, where does its bellyful of information go?

    A clue might be found in the nature of the radiation Hawking described. This form of radiation arises when a pair of virtual particles pops into existence right up against a black hole’s line of no return – the ‘event horizon’.

    Usually, such paired particles cancel each other out, and the Universe is none the wiser. But in the case of Hawking radiation, one of these particles falls across the horizon into the gravitational grip of the black hole. The other barely escapes off into the Universe as a bona fide particle.

    Physicists have theorised that this escaped particle preserves the information of its twin thanks to the quirks of quantum dynamics. In this case, the phenomenon of entanglement would allow the particles to continue share a connection, even separated by time and space, leaving a lasting legacy of whatever was devoured by the black hole.

    To demonstrate this, physicists could catch a particle that has escaped a black hole’s event horizon, and then wait for the black hole to spill its guts in many, many years, to test if there’s indeed a correlation between one of the photons and its entangled twin. Which, let’s face it, isn’t exactly practical.

    Now, Pisin Chen from the National Taiwan University and Gerard Mourou from École Polytechnique in France have described a slightly easier method.

    They suggest that a high-tech ‘accelerating mirror’ should provide the same opportunity of separating entangled particles.

    That sounds strange, but as a pair of particles zips into existence in this hypothetical experiment, one would reflect from the accelerating mirror as the other became trapped at the boundary. Just as it might happen in a black hole.

    Once the mirror stopped moving, the ‘trapped’ photon would be freed, just as the energy would be released from a dying black hole.

    Chen’s and Mourou’s mirror would be made by pulsing an X-ray laser through a cloud of ionised gas in a plasma wakefield accelerator. The pulse would leave a trail of negatively charged electrons, which would serve nicely as a mirror.

    By altering the density of the plasma on a small enough scale, the ‘mirror’ would accelerate away from the laser pulse.

    As clever as the concept is, the experiment is still in its ‘thought bubble ‘stage. Even with established methods and trusted equipment, entanglement is tricky business to measure.

    And Hawking radiation itself has yet to be observed as an actual thing.

    Yet Chen’s and Mourou’s model could feasibly be built using existing technology, and as the researchers point out in their paper, could also serve to test other hypotheses on the physics of black holes.

    It sounds far more appealing than waiting until the end of time in front of a black hole, at least.

    This research was published in Physical Review Letters.

    See the full article here .

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  • richardmitnick 1:35 pm on June 27, 2016 Permalink | Reply
    Tags: 3D map of the known uviverse planned, , , Stephen Hawking,   

    From U Cambridge via Cambridge News: “Professor Stephen Hawking set to reveal plans to map known universe” 

    U Cambridge bloc

    Cambridge University

    Cambridge News

    June 27, 2016
    Hannah Mirsky

    Professor Hawking in 2013

    Physicist Professor Stephen Hawking is set to reveal plans to map the entire known universe at a conference held in his honour.

    Prof Hawking, a cosmology professor at Cambridge University, will detail how a supercomputing centre he founded in the city will use images of radiation to create the map.

    He is set to discuss the plans at the Starmus science conference – this year themed as a “tribute to Stephen Hawking” – which begins today in Tenerife.

    The COSMOS supercomputing centre was founded in Cambridge in 1997 by a group of scientists brought together by Prof Hawking.

    SGI COSMOS supercomputing centre. The COSMOS facility, which is located in the Stephen Hawking Centre for Theoretical Cosmology (CTC) at the University, is dedicated to research in cosmology, astrophysics and particle physics. It was switched on in 2012.

    Cosmologists at the centre are now working to create a 3D map of the universe by plotting the position of billions of cosmic structures, including supernovas, black holes, and galaxies.

    Professor Paul Shellard, director of the COSMOS computing centre, said that the computer would create a map of the early universe using images of radiation from the Big Bang, which have been captured by the European Space Agency’s Planck satellite.

    Cosmic Background Radiation per Planck
    Cosmic Background Radiation per Planck


    Prof Shellard told The Sunday Times: “Planck gives us an amazing picture of the early distribution of matter and how that led to the structure of the modern universe.”

    The map of the universe will also be created using data from the Dark Energy Survey, which has a telescope with a 13ft diameter in Chile.

    Dark Energy Icon
    Dark Energy Camera,  built at FNAL
    NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile
    Dark Energy Survey; DECam, built at FNAL, USA, and the NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile

    It is hoped the cosmologists’ work will reveal the nature of the dark energy which is causing the universe to expand more rapidly.

    The European Space Agency is set to launch a probe called Euclid in 2020, and Prof Shellard said that this would also help the Cambridge scientists create a picture of the universe.

    ESA/Euclid spacecraft

    The probe is set to map 10 billion galaxies.

    Prof Shellard said: “Hawking is a great theorist but he always wants to test his theories against observations. What will emerge is a 3D map of the universe with the positions of billions of galaxies.”

    At the Starmus festival, Prof Hawking will also present the first ever Stephen Hawking Medal for Science Communication.

    He is set to give the gong to composer Hans Zimmer for his work creating the soundtrack to the film Interstellar.

    When the new medal was unveiled, Prof Hawking said: “When I wrote A Brief Theory Of Everything, I was told nobody would want to read a hardback book about physics.

    “Luckily for me that turned out not to be true. The people wanted to know, they wanted to understand.

    “Science communicators put science right at the heart of daily life. Bringing science to the people brings the people to science.”

    See the full article here .

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    U Cambridge Campus

    The University of Cambridge[note 1] (abbreviated as Cantab in post-nominal letters[note 2]) is a collegiate public research university in Cambridge, England. Founded in 1209, Cambridge is the second-oldest university in the English-speaking world and the world’s fourth-oldest surviving university.[6] It grew out of an association of scholars who left the University of Oxford after a dispute with townsfolk.[7] The two ancient universities share many common features and are often jointly referred to as “Oxbridge”.

    Cambridge is formed from a variety of institutions which include 31 constituent colleges and over 100 academic departments organised into six schools.[8] The university occupies buildings throughout the town, many of which are of historical importance. The colleges are self-governing institutions founded as integral parts of the university. In the year ended 31 July 2014, the university had a total income of £1.51 billion, of which £371 million was from research grants and contracts. The central university and colleges have a combined endowment of around £4.9 billion, the largest of any university outside the United States.[9] Cambridge is a member of many associations and forms part of the “golden triangle” of leading English universities and Cambridge University Health Partners, an academic health science centre. The university is closely linked with the development of the high-tech business cluster known as “Silicon Fen”.

  • richardmitnick 6:53 am on June 7, 2016 Permalink | Reply
    Tags: , , , Stephen Hawking   

    From Science Alert: “Stephen Hawking’s finally published a solution to the black hole information paradox” 


    Science Alert

    7 JUN 2016

    ESA/V. Beckmann (NASA-GSFC)


    Stephen Hawking made headlines back in January when he told the world he’d found a possible solution to his black hole information paradox – or in other words, he’d come up with a potential explanation for how black holes can simultaneously erase information and retain it.

    Back then, he put his paper up on pre-print site arXiv.org, so the rest of the physics community could poke holes in it, and now, almost six months later, the research has finally been published in a peer-reviewed journal – and it suggests that we might actually be getting closer to figuring out this problem once and for all.

    To understand why this is such a big deal, and what the black hole information paradox really is, we need to go back to where it all started.

    Our original understanding of black holes, according to Einstein’s generally theory of relativity, is that everything that crosses the event horizon – the boundary of a black hole – is lost forever. Even light can’t escape its clutches, which is why black holes are called black holes (and also why it’s impossible for us to actually see one).

    But then in the 1970s, Hawking proposed that radiation actually can escape from a black hole, because of the laws of quantum mechanics. Put very simply, he suggested that when a black hole swallows one half of a particle-antiparticle pair, the other particle radiates away into space, stealing a little energy from the black hole as it leaves.

    Because of this, eventually, black holes can disappear, and the only remaining trace would be the electromagnetic radiation they emitted – which is known as ‘Hawking radiation’.

    The problem is that, according to Hawking’s best calculations, that radiation would contain no useful information about what the black hole ate – the information swallowed up would have been lost forever. And that doesn’t gel with our understanding of modern physics, which states that it’s always possible to reverse time. In theory, at least, processes in the Universe will look the same if they’re running forwards or backwards.

    As Dennis Overbye explains over at The New York Times:

    “The Universe, like a kind of supercomputer, is supposed to be able to keep track of whether one car was a green pickup truck and the other was a red Porsche, or whether one was made of matter and the other antimatter. These things may be destroyed, but their ‘information’ – their essential physical attributes – should live forever.”

    Hence the paradox. And it’s actually a big deal not just for astrophysicists, because if the rules of quantum mechanics don’t hold up for black holes, then what’s to say they apply to the rest of us?

    But Hawking thinks he finally has a solution to the problem – black holes might actually have a halo of ‘soft hair’ surrounding them, which are capable of storing information.

    That ‘hair’ isn’t actually hair – as you might have already assumed – but is actually low-energy quantum excitations that carry with them a signature pattern of everything that’s been swallowed up by the black hole, long after it evaporates.

    “That pattern, like the pixels on your iPhone or the wavy grooves in a vinyl record, contains information about what has passed through the horizon and disappeared,” writes Overbye.

    To come to this conclusion, Hawking identified two underlying problems with his original assumptions, which is why he says his original calculations – which suggested that the information inside a black hole would be lost forever – were wrong.

    Those two assumptions were that the vacuum in quantum gravity is unique, and that black holes have no quantum ‘hair’. That’s getting a little complex, but what you need to know is that Hawking has since revised his calculations, and is fairly sure that black holes have ‘soft hair’ haloed around them.

    This hypothesis has now been peer-reviewed and published in Physical Review Letters, and researchers are claiming that, while there’s more work to be done, it’s a promising step towards solving the information paradox.

    “It is important to note that this paper does not solve the black hole information problem,” writes physicist Gary Horowitz from the University of California, Santa Barbara, in an accompanying commentary.

    “First, the analysis must be repeated for gravity, rather than just electromagnetic fields. The authors are currently pursuing this task, and their preliminary calculations indicate that the purely gravitational case will be similar,” he adds. “More importantly, the soft hair they introduce is probably not enough to capture all the information about what falls into a black hole.”

    His criticism is that it’s still unclear whether all the information swallowed up by a black hole really can be transferred to the soft hair – rather than just an energy signature of everything that’s been lost.

    But he admits: “It is certainly possible that, following the path indicated by this work, further investigation will uncover more hair of this type, and perhaps eventually lead to a resolution of the black hole information problem.”

    And that would certainly be a red-letter day in physics. Because we’d be one step closer to understanding some of the biggest enigmas in the known Universe – the weirdness that are black holes.

    What does that mean for the rest of us? As Hawking explained in a talk last year: “[Black holes] are not the eternal prisons they were once thought. If you feel you are trapped in a black hole, don’t give up. There is a way out.”

    And there might just be a little trace of you lingering on the outside, too.

    See the full article here .

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  • richardmitnick 12:28 pm on May 30, 2016 Permalink | Reply
    Tags: , , , Stephen Hawking   

    From Science Vibe: “Black holes are a passage to another universe, says Stephen Hawking” 

    Science Vibe bloc


    August 26, 2015
    No writer credit

    Illustration of Cygnus X-1, another stellar-mass black hole located 6070 ly (chandra.harvard.edu)

    According to a new radical theory proposed by Stephen Hawking, humans could escape from black holes, rather than getting stuck in them, or crushed to smithereens. Unfortunate those lucky space travelers won’t bragging about their close call on return to Earth, as they will end up in a different universe.

    “The existence of alternative histories with black holes suggests this might be possible,” Hawking said, while speaking at Stockholm University. “The hole would need to be large and if it was rotating it might have a passage to another universe. But you couldn’t come back to our universe. So although I’m keen on space flight, I’m not going to try that. If you feel you are in a black hole, don’t give up,” he told the audience, with his classic sense of humor. “There’s a way out.”

    A paradox has puzzled physicists for years is what happens to things when they go beyond the event horizon of a black hole, (in layman’s terms, “the point of no return”) where even light can’t get back. Scientists believe that the information about an object has to be preserved, even if the thing itself is swallowed up. Hawking suggests that the information is stored right on the boundary, exactly at the event horizon, which would mean that since it never makes its way into the black hole, it does not need to find its way out again either.

    With this line of thinking humans might not disappear if they fall into a black hole. They’d either become a “hologram” on the edge, or fall out into another universe.

    Well, even so, I think I’ll pass on that black hole trip.

    See the full article here .

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  • richardmitnick 9:29 pm on January 27, 2016 Permalink | Reply
    Tags: , , , Stephen Hawking   

    From SA: “Hawking’s Latest Black Hole Paper Splits Physicists” 

    Scientific American

    Scientific American

    January 27, 2016
    Davide Castelvecchi, Nature magazine

    Black hole in color
    Black hole in color.

    Almost a month after Stephen Hawking and his colleagues posted a paper about black holes online, physicists still cannot agree on what it means.

    Some support the preprint’s claim—that it provides a promising way to tackle a conundrum known as the black hole information paradox, which Hawking identified more than 40 years ago. “I think there is a general sense of excitement that we have a new way of looking at things that may get us out of the logjam,” says Andrew Strominger, a physicist at Harvard University in Cambridge, Massachusetts, and a co-author of the latest paper.

    Strominger presented the results on January 18 at a crowded talk at the University of Cambridge, UK, where Hawking is based.

    Others are not so sure that the approach can solve the paradox, although some say that the work illuminates various problems in physics. In the mid-1970s, Hawking discovered that black holes are not truly black, and in fact emit some radiation. According to quantum physics, pairs of particles must appear out of quantum fluctuations just outside the event horizon—the black hole’s point of no return. Some of these particles escape the pull of the black hole but take a portion of its mass with them, causing the black hole to slowly shrink and eventually disappear.

    In a paper published in 1976, Hawking pointed out that the outflowing particles—now known as Hawking radiation—would have completely random properties. As a result, once the black hole was gone, the information carried by anything that had previously fallen into the hole would be lost to the Universe. But this result clashes with laws of physics that say that information, like energy, is conserved, creating the paradox. “That paper was responsible for more sleepless nights among theoretical physicists than any paper in history,” Strominger said during his talk.

    The mistake, Strominger explained, was to ignore the potential for the empty space to carry information. In their paper, he and Hawking, along with their third co-author Malcolm Perry, also at the University of Cambridge, turn to soft particles. These are low-energy versions of photons, hypothetical particles known as gravitons and other particles. Until recently, these were mainly used to make calculations in particle physics. But the authors note that the vacuum in which a black hole sits need not be devoid of particles—only energy—and therefore that soft particles are present there in a zero-energy state.

    It follows, they write, that anything falling into a black hole would leave an imprint on these particles. “If you’re in one vacuum and you breathe on it—or do anything to it—you stir up a lot of soft gravitons,” said Strominger. After this disturbance, the vacuum around the black hole has changed, and the information has been preserved after all.

    The paper goes on to suggest a mechanism for transferring that information to the black hole—which would have to happen for the paradox to be solved. The authors do this by calculating how to encode the data in a quantum description of the event horizon, known whimsically as black hole hair.

    Tricky transfer

    Still, the work is incomplete. Abhay Ashtekar, who studies gravitation at Pennsylvania State University in University Park, says that he finds the way that the authors transfer the information to the black hole—which they call ‘soft hair’—unconvincing. And the authors acknowledge that they do not yet know how the information would subsequently transfer to the Hawking radiation, a further necessary step.

    Steven Avery, a theoretical physicist at Brown University in Providence, Rhode Island, is sceptical that the approach will solve the paradox, but is excited by the way it broadens the significance of soft particles. He notes that Strominger has found that soft particles reveal subtle symmetries of the known forces of nature, “some of which we knew and some of which are new”.

    Other physicists are more optimistic about the method’s prospects for solving the information paradox, including Sabine Hossenfelder of the Frankfurt Institute for Advanced Studies in Germany. She says that the results on soft hair, together with some of her own work, seem to settle a more-recent controversy over black holes, known as the firewall problem. This is the question of whether the formation of Hawking radiation makes the event horizon a very hot place. That would contradict Albert Einstein’s general theory of relativity, in which an observer falling through the horizon would see no sudden changes in the environment.

    “If the vacuum has different states,” Hossenfelder says, “then you can transfer information into the radiation without having to put any kind of energy at the horizon. Consequently, there’s no firewall.”

    See the full article here .

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  • richardmitnick 11:02 am on January 17, 2016 Permalink | Reply
    Tags: , , , , Stephen Hawking   

    From livescience: “Stephen Hawking: Black Holes Have ‘Hair'” 


    January 14, 2016
    Tia Ghose

    Temp 1
    This artist’s concept shows a black hole’s surroundings, including its accretion disk, jet and magnetic field. Credit: ESO/L. Calçada

    Black holes may sport a luxurious head of “hair” made up of ghostly, zero-energy particles, says a new hypothesis proposed by Stephen Hawking and other physicists.

    Temp 5
    Dr. Stephen Hawking

    The new paper, which was published online Jan. 5 in the preprint journal arXiv, proposes that at least some of the information devoured by a black hole is stored in these electric hairs.

    Still, the new proposal doesn’t prove that all the information that enters a black hole is preserved.

    “The million dollar question is whether all the information is stored in this way, and we have made no claims about that,” said study author Andrew Strominger, a physicist at Harvard University in Massachusetts. “It seems unlikely that the kind of hair that we described is rich enough to store all the information.”

    Black holes

    According to [Albert] Einstein’s theory of general relativity, black holes are extremely dense celestial objects that warp space-time so strongly that no light or matter can escape their clutches. Some primordial black holes formed soon after the Big Bang and may be the size of a single atom yet as massive as a mountain, according to NASA. Others form as gigantic stars collapse in on themselves, while supermassive black holes lie at the hearts of almost all galaxies.

    In the 1960s, physicist John Wheeler and colleagues proposed that black holes “have no hair,” a metaphor meaning that black holes were shorn of all complicated particularities. In Wheeler’s formulation, all black holes were identical except for their spin, angular momentum and mass.

    Then, in the 1970s, Stephen Hawking proposed the notion now called Hawking radiation. In this formulation, all black holes “leak” mass in the form of ghostly quantum particles that escape over time. Eventually, Hawking radiation causes black holes to evaporate altogether, leaving a single, unique vacuum. The vacuums left by these black holes, according to the original theory, would be identical, and thus incapable of storing information about the objects from which they were formed, Strominger said.

    Since the Hawking radiation leaking from a black hole is completely random, that would mean black holes lose information over time, and there would be no way of knowing much about the celestial objects that formed the black holes. Yet that notion creates a paradox, because on the smallest scale, the laws of physics are completely reversible, meaning information that existed in the past should be theoretically recoverable. In recent years, Hawking has walked back the notion of information loss and conceded that black holes do store information after all.

    Black hole “snowflakes”

    In the past several years, Strominger has been dismantling some of these notions. First, he asked the question: What happens if you add a “soft” photon, or a particle of light with no energy, to the vacuum left behind after a black hole evaporates?

    Though most people have never heard of soft photons, the particles are ubiquitous, Strominger said. (Other particles, called soft gravitons, are hypothetical quantum particles that transmit gravity. Though they have never been detected, most physicists believe these particles exist and are also incredibly abundant, Strominger said).

    “Every collision at the Large Hadron Collider produces an infinite number of soft photons and soft gravitons,” Strominger said. “We’re swimming in them all the time.”

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    After working through the equations, he — together with Hawking and Malcolm Perry, who are both physicists at the University of Cambridge in England — found that the black hole vacuum would have the same energy but different angular momentum after the addition of a soft photon. That meant the vacuum state of an evaporated black hole is a kind of celestial snowflake, with its individual properties dependent on its origin and history.

    “Far from being a simple, vanilla object, it’s like a large hard drive which can store essentially an infinite amount of information in the form of these zero-energy photons and gravitons,” Strominger told Live Science.

    The new work is an extension of a short paper Hawking put out in 2014, which argued that the event horizon, or the point of no return before an object would get swallowed into a black hole forever, may not be a fixed boundary. The new paper posits that hairs of soft photons and gravitons fringe a black holes’ event horizon.

    Information paradox stands

    The problem is that this information is “incredibly scrambled up,” so retrieving it from a black hole is akin to determining what someone tossed into a bonfire after it has burned up, Strominger said. Essentially, the new work is the black hole equivalent of using smoke and fire to figure out the identity of the original object that was burnt, he added.

    “It’s not a final answer to the information problem, but it does seem like a step in the right direction,” said Aidan Chatwin-Davies, a physicist at the California Institute of Technology, who was not involved in the study.

    While some of the information in a black hole may be contained in its hairy halo of soft photons and gravitons, not all of it necessarily resides there, he said.

    “If anything, it puts forward some new ideas for us to think about which could prove very helpful in understanding black holes and how they encode information,” Chatwin-Davies told Live Science.

    See the full article here .

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  • richardmitnick 3:32 pm on January 8, 2016 Permalink | Reply
    Tags: , , , Stephen Hawking   

    From SA: “Stephen Hawking’s New Black-Hole Paper, Translated: An Interview with Co-Author Andrew Strominger” 

    Scientific American

    Scientific American

    January 8, 2016
    Seth Fletcher

    Temp 1
    Penrose diagram for a semiclassical evaporating black hole, as presented in “Soft Hair on Black Holes.” CREDIT: Prahar Mitra

    In the mid 1970s, Stephen Hawking made a string of unnerving discoveries about black holes—that they could evaporate, even explode, and destroy all information about what had fallen in. Physicists spent the next 40 years sorting through the wreckage. Then last year, at a conference in Stockholm, Hawking said that he and some collaborators were close to a solution to the so-called black-hole information paradox. Details, however, would have to wait.

    Now the details are here—at least some of them. This week Hawking, the University of Cambridge physicist Malcolm J. Perry, and the Harvard University string theorist Andrew Strominger posted a paper online in which the authors claim to make real progress toward solving the black-hole information paradox. Despite the inviting title—Soft Hair on Black Holes—the paper is mercilessly technical, so I asked Strominger to walk me through it. An edited transcript of our conversation follows.

    Seth Fletcher: Physicists are comfortable with all sorts of insane-sounding ideas, but the idea that black holes destroy information is not one of them. Why is this something that they cannot abide?

    Andrew Strominger: Black holes destroying information means that the world is not deterministic. That is, the present doesn’t predict the future perfectly, and it also can’t be used to reconstruct the past. That’s sort of the essence of what a physical law is. Going way back to Galileo or earlier, the idea of a physical law is that you start out with bodies in some state of motion and interacting, and you use the physical laws to determine either where they will be in the future or where they must have come from. So it’s a very big thing if black holes destroy information. It’s a very big thing to say that we cannot use physical laws in the way that we’ve been accustomed to for thousands of years to describe the world around us.

    Now just because it’s a very big thing doesn’t mean that it’s impossible. In a way, the history of physics is the history of learning that things that we thought had to be true weren’t true. We used to think that space and time were absolute. We used to think the Earth is the center of the universe. All of these things seemed completely obvious and well defined. And one by one they went by the wayside. That could happen to determinism, too. The very fact that the universe has a beginning seems to be in contradiction with determinism, because if you have nothing and then there’s something, that’s not deterministic. So determinism should be on the table. And indeed when Hawking first came out with his argument [that black holes destroyed information], it seemed like such a good argument that many or even most of the people who listened to it believed that determinism was over.

    But three things happened that have changed that. The first is that you can’t just throw up your hands and say we can’t describe the universe. You need some kind of alternative—some sort of probabilistic laws or something. And Hawking and other people put out some formalism that enables you to have probabilistic laws, and so on, but it was rather quickly shown to be internally self-inconsistent.

    The second thing was that experimentally it’s not plausible to say that determinism breaks down only when you make a big black hole and let it collapse because according to quantum mechanics and the uncertainty principle, you would have little black holes popping in and out of the vacuum. And so you would have to violate determinism everywhere. And the experimental bounds on that are truly extraordinary. So experimentally there are very serious consequences if there are even teeny, tiny violations of determinism.

    SF: What are some of those consequences?

    AS: In order to say that a symmetry implies a conservation law, you need determinism. Otherwise [symmetries] only imply conservation laws on the average. So electric charge would only have to be conserved on the average. Or energy would only have to be conserved on the average. And the experimental bounds on energy conservation are extraordinary. If you added terms to the laws of physics that violated determinism in some form, they would have to have fantastically small coefficients, one part in 101,000 or something.

    So [the black-hole information paradox] is experimentally a problem and it’s theoretically a problem. Those are the first two things. The third thing was string theory. I would say up until the 1990s, the community was kind of split 50-50. But then Cumrun Vafa and I showed that certain string-theoretic black holes were capable of storing the requisite information, and they apparently also have some method of letting the information go in and out. And the fact that that worked—I mean, people had been trying for 25 years to reproduce this Bekenstein-Hawking area entropy law, or in other words, to derive the information content of a black hole from first principles. And nobody had been able to do it. And then we did it with complete accuracy. All the numbers, everything worked perfectly. And it had to be some kind of clue to something. It couldn’t just be an accident.

    Now, we don’t know whether or not string theory describes the world, and we won’t know anytime soon. But this, I think, gave a lot of people, including Hawking, hope that in the real world there would be some mechanism that resembled what happens in string theory and enables information to come out of a black hole.

    SF: And in a new paper that you, Stephen Hawking, and Malcolm Perry posted online this week, you argue that you’ve taken some concrete steps toward explaining how information can get in and out of a black hole. The first step in your argument is to undercut some of the assumptions underlying Hawking’s original argument using “new discoveries about the infrared structure of quantum gravity.” Can you tell us about these discoveries?

    AS: Infrared structure means the behavior of things that vary at the longest wavelengths. I discovered in the last two years what I think are some hugely surprising facts about the long wavelength structure not just of quantum gravity but also of quantum electrodynamics. It was clear that [these facts] had profound implications for the black hole information puzzle. They implied that some of the things that had been assumed in the argument that black holes destroy information were demonstrably wrong. And that’s how this all got started.

    SF: Let’s walk through those two assumptions. One involves the final evaporation state of a black hole, and the other is the no-hair theorem.

    AS: The first one has to do with the vacuum. The lowest energy thing is the vacuum. And it’s always been assumed that the vacuum in quantum gravity or in quantum electrodynamics is unique—that there’s only one zero-energy state. And what I’ve shown in the last couple of years is that that assumption is wrong. There are in fact infinitely many different vacuum states. In a way, what I showed was implicit in things that other people had said. It all started out by showing this equivalence of two different bodies of work that were done in the 1960s by Steve Weinberg and by Bondi, van der Burg, Metzner and Sachs.

    In my earlier papers, I understood that this negated Stephen’s [original black-hole information-loss] argument—that it showed that one of the assumptions was wrong. But I hadn’t started the exploration of them in any detail because I had to fill out that story better. What’s happening now is that we’re starting to look in detail at exactly how this story is implemented when there’s a black hole around.

    SF: Stephen Hawking is an author on this paper, so I take it he agrees that his original argument was flawed in this way.

    AS: Right. I think that’s why he got excited. People have made all kinds of crazy criticisms of his argument, and to the best of my impressions, he’s correctly objected to all of them. But this one, he heard it and he seemed to immediately agree that this was the key. In fact, as you’ve learned from what happened at Stockholm, he’s more certain than I am that this is the missing link in understanding black hole information. I’ve been surprised so many times in my career about how things turn out that I’m not making any predictions. But there is a logical stream that we are following through now, and we’re going to see what its implications are. I’m sure there are going to be more surprises. But this is a first step in working out those implications.

    SF: The next step in the paper seems crucial: you say that the no-hair theorem is unfounded, and that, in fact, black holes have “soft hair.”

    AS: Right. In my earlier work I said that just by these conservation laws that I discovered, black holes must have some kind of hair. But I didn’t really know how it could be described in equations. And that’s what we understood here: how to describe it and how to do calculations.

    SF: In the new paper, “soft hair” refers to “soft” photons and gravitons. What does “soft” mean in this context?

    AS: Soft means not very much energy, or zero energy. That usage has been around since maybe the 1960s. The crucial subtlety is if you take the vacuum and you add to it a photon with some energy E you get a new state. It’s a different quantum state as energy E, and it has a different angular momentum because the photon has a spin. But now suppose you consider a limit where that energy goes to zero. Then you’re adding to the vacuum something that has no energy. So it’s still a zero energy state, but you’ve changed its angular momentum. Is that a new state, or the same state, or what? How are we supposed to think about that?

    The first thing you have to do is to be very precise about what you mean about two states being different. And what I did, in a way that I think the world of theoretical physicists agrees with, is I made all of this very precise. And I showed that it is in fact a different state, and that the different states are related by symmetry. And associated with this symmetry there are conservation laws. I think there’s general acceptance that these papers are correct.

    So that’s what a soft particle is. It’s a particle that has zero energy. And when the energy goes to zero, because the energy is [proportional to the] wavelength, it’s also spread over an infinitely large distance. If you like, it’s spread over the whole universe. It somehow runs off to the boundary. What we learn from that is that if you add a zero-energy particle to the vacuum, you get a new state. And so there are infinitely many vacua, which can be thought of as being different from one another by the addition of soft photons or soft gravitons.

    What we showed in this present paper is that this is also true for black holes. And that’s the sense in which black holes have hair: they can have different numbers of soft photons or soft gravitons on them.

    SF: In the paper, you argue that these particles, which together form the soft hair, are deposited on the black hole by something called “supertranslation.” Can you explain that process?

    AS: The horizon of a black hole has the weird feature that it’s a sphere and it’s expanding outward at the speed of light. For every point on the sphere, there’s a light ray. So it’s composed of light rays. But it doesn’t get any bigger and that’s because of the force of gravity and the curvature of space. And, by the way, that’s why nothing that is inside a black hole can get out—because the boundary of the black hole itself is already moving at the speed of light.

    There’s this symmetry of a black hole that we all knew about in which you move uniformly forward and backward in time along all of the light rays. But there’s another symmetry, which is the new thing in this paper (though various forms of it have been discussed elsewhere). It’s a symmetry in which the individual light rays are moved up and down. See, individual light rays can’t talk to each other—if you’re riding on a light ray, causality prevents you from talking to somebody riding on an adjacent light ray. So these light rays are not tethered together. You can slide them up and down relative to one another. That sliding is called a super-translation.

    And in a way, it looks like you’re not doing anything. Think of a bundle of infinitely long straws and you move one up and down relative to the other. Are you doing anything, or not? What we showed is that you are doing something. It turns out that adding a soft graviton has an alternate description as a super-translation in which you move some of these light rays back and forth relative to one another.

    That’s super-translations on black holes. Super-translations were introduced in the 1960s, and they were talking not about the light rays that comprise the boundary of spacetime at the horizon of a black hole but the light rays that comprise the boundary of spacetime out at infinity. The story started by analyzing those supertranslations.

    SF: So the soft photons and gravitons implanted by supertranslations store information in that they are “quantum pixels” on an information-storing “holographic plate.” [Editor’s note: For a quick primer on the holographic principle, watch this video.] In what way do they store information? What does it mean for a zero-energy photon to be “on” the horizon and to hold information about a particle that fell in?

    AS: Let me go back to the soft photons or gravitons in flat space. As the energy of a particle goes to zero, its wavelength spreads out over a larger and larger region. And when its energy is zero, there’s a sense in which you can think of it as living on the boundary of spacetime. Now the horizon of a black hole is a three-dimensional surface. There are the two angular directions around a sphere. And then there is the timelike direction, which is actually lightlike because the horizon is moving at the speed of light. And that lightlike direction has a boundary. If you go to the end of those light rays there’s a boundary. And that boundary is where the hologram lives. So the soft photons or gravitons—when you add them to the black hole—they can be thought of as living at that boundary.

    We show that when a charged particle goes in, it adds a soft photon to the black hole. So it adds hair to the black hole. And more generally if any particle goes in—because all particles carry mass uncoupled to gravity—they always add a soft graviton. So there’s a kind of recording device. These soft photons and gravitons record information about what went into the black hole—infinitely more information than we previously believed is recorded by this mechanism. Now whether all information is recorded by this mechanism… I’m pretty sure the answer to that is no, but there are generalizations of this mechanism and then it’s a lot more confusing.

    SF: Okay, so in-falling particles deposit soft particles, or hair, on the horizon of the black hole. What about at the first infinitely tiny fraction of a second after a black hole forms? Does it have any of these soft particles on its horizon? Are there any there from the beginning saying something about what went in to form the black hole?

    AS: Let me make a statement first just about the vacuum that has an obvious extension to black holes. I can say it more clearly and with more certainty for the vacuum. If you add a soft photon to the vacuum, you get a new state that has one more soft photon than the old state. The relative number of photons by which two different vacuum states differ is a well-defined question. But the absolute number is not. I can say vacuum A has one more soft photon than vacuum B, but I can’t say which one of them has none. That’s a kind of arbitrary convention. So I haven’t really carefully addressed the question in the way you’ve posed it, but I’m guessing that there’s not going to be any meaning to the question “which is the black hole with no soft photons?” You can say some have more or some have less. You can say how many particles you would have to throw in for black hole A to have the same number of soft photons as B. But there’s no absolute notion there.

    This was part of the subtlety, right? Three or four years ago, and even now, people who haven’t been following my papers might say that the vacuum with the soft photon is the same with the vacuum without a soft photon. This thing is spread out to infinity and it doesn’t mean anything. But one of the lessons we’ve learned is that the boundaries of spacetimes at infinity are very important to carefully keep track of, especially when you want to study something like black hole information.

    SF: So the information is stored on the surface. What happens when the black hole evaporates?

    AS: We talk about adding soft photons to the black hole. If you compare two black holes that differ only by the addition of a soft photon that doesn’t change the energy, they’re different black holes. And then you let them evaporate. They should evaporate into something different. And indeed we give an exact formula, which is one of the main results of our paper for the difference in the quantum state resulting from a black hole with or without a soft photon.

    SF: You write in the paper that there is a suggestive relationship between the minimum size of these soft hairs and the Planck length and the Hawking-Bekenstein forumula, which relates the entropy of a black hole to the area of its event horizon.

    AS: The area-entropy law that [Jacob] Bekenstein and Hawking derived 40 years ago makes a prediction. If we have all the ingredients for understanding quantum black-hole dynamics, it makes a prediction for how many holographic pixels there are. That has to come out exactly right. And it won’t come out exactly right until we get all the details right.

    One thing that bothered us about this right from the very beginning is: Why doesn’t this allow an infinite amount of information? We don’t want an infinite amount of information. Ultimately we’d like to somehow use this to recover the Hawking- Bekenstein area entropy law. It looked like we were getting an infinite amount of hair because you seem to be able to have these soft photons that had an angular localization that was arbitrarily small. But there’s no physical way to excite one of those. So those are not physically realizable states of the black hole.

    SF: Those smaller than the Planck length?

    AS: Those smaller than the Planck length. I wouldn’t know how to implant such a hair.

    It’s very important to note that there have been lots of proposals trying to understand black hole entropy which get the area correct—which get the proportionality to the area correct—but don’t get the one-quarter [term in the equation] correct. The real acid test, which we haven’t passed, is getting that one-quarter. That’s what string theory was able to do, and that was what turned the tide on a lot of thinking about this problem. But we haven’t gotten the one-quarter here.

    SF: Is there a clear road ahead?

    AS: I’ve got a list of 35 problems on the board, each of which will take many months. It’s a very nice stage to be in if you’re a theoretical physicist because there are things we don’t understand, but there are calculations that we can do that will definitely shed light on it. I alluded to this briefly, but there’s something much richer and bigger and at the same time more enigmatic than the supertranslations called the superrotations.

    SF: Superrotations?

    AS: They are another kind of symmetry at infinity where you don’t just shift the light rays up and down, but you move them relative to one another. You interchange them. If we can make sense of them, they’re going to be more important. But they’re a much newer thing. Supertranslations were understood in the 1960s. The superrotations are something that people just started to look at about ten years ago. But we’ve learned a lot about the superrotations in the last two years.

    I also think that there is a very vital connection with studies that people have been doing of entanglement entropy. That needs to be incorporated into this general framework. So there are many very concrete things to do at this point.

    See the full article here .

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  • richardmitnick 4:28 pm on December 12, 2015 Permalink | Reply
    Tags: , , , , Stephen Hawking   

    From Ethan Siegel: “How Do Black Holes Really Evaporate?” 

    Starts with a bang
    Starts with a Bang

    Ethan Siegel

    Image credit: BBC, Illus.: T.Reyes, via http://www.universetoday.com/115307/hawking-radiation-replicated-in-a-laboratory/.

    Hawking’s greatest achievement is also the greatest source of misunderstanding.

    “Maybe that is our mistake: maybe there are no particle positions and velocities, but only waves. It is just that we try to fit the waves to our preconceived ideas of positions and velocities. The resulting mismatch is the cause of the apparent unpredictability.” –Stephen Hawking

    Perhaps the greatest thing Stephen Hawking ever discovered — and the reason he’s so renowned among physicists — is that black holes don’t live forever.

    Image credit: NASA/ESA Hubble Space Telescope collaboration.

    NASA Hubble Telescope
    NASA/ESA Hubble

    Rather, they radiate their energy away over extraordinarily long timescales through a process discovered in 1974 that’s now known as Hawking radiation. The big question for this week, that Spencer Müller Diniz wants to know the answer to, is:

    Ever since Stephen Hawking discovered Hawking Radiation, scientific publications describe it as a phenomenon where black holes slowly “evaporate” due to spontaneous creation of quantum entangled particle pairs near the event horizon. It is said the one of the particles gets sucked in to the black hole and the other escapes as Hawking Radiation. Because of Hawking Radiation, black holes slowly lose mass until eventually evaporating completely. The question is, if one particle falls in the black hole and the other is ejected, why is the black hole getting smaller? Shouldn’t it actually be gaining mass?

    This is a big question, and it’s one that’s loaded with misconceptions, many of which are Stephen Hawking’s own fault. So let’s get into it!

    Image credit: Wikimedia Commons user AllenMcC., of Flamm’s Paraboloid, the exterior Schwarzschild solution to spacetime.

    This month marks the 100th anniversary of the very first exact solution ever discovered in General Relativity: the spacetime that describes a massive singularity with an event horizon around it. The discovery was made by Karl Schwarzschild, who immediately realized that this would be a black hole: an object so massive and dense that nothing, not even light itself, could escape from its gravitational pull.

    For a long time, it was recognized that if you got enough mass together in a small enough region of space, gravitational collapse down to a black hole would be inevitable, and that no matter what the original configuration of the mass was, the singularity would be a point, and the event horizon would be a sphere. In fact, the only parameter of interest — the size of that event horizon — was determined exclusively by the black hole’s mass.


    As the black hole swallowed more and more matter over time, its mass would grow, and hence it would increase in size. For a long time, it was thought that this would continue without fail, until there was no more matter left to swallow or the Universe came to an end.

    But something happened to change this picture: the revolution that our Universe was made up of tiny, indivisible particles that obeyed a different set of laws, quantum laws. Particles interacted with one another through a variety of fundamental interactions, each of which could be expressed as a set of quantum fields

    Image credit: Derek B. Leinweber of http://www.physics.adelaide.edu.au/theory/staff/leinweber/VisualQCD/Nobel/index.html.

    Want to know how two electrically charged particles interact, or how photons interact? That’s governed by quantum electrodynamics, or the quantum theory of the electromagnetic interactions. How about the particles that are responsible for the strong nuclear force: the force that binds protons or other atomic nuclei together? That’s quantum chromodynamics, or the quantum theory of the strong interactions. And what of radioactive decays? That’s the quantum theory of the weak nuclear interactions.

    But there are two ingredients missing from this. One’s easy to see: there’s no gravitational interaction listed in the quantum world, because we don’t have a quantum theory of gravity. But another is tougher to see: the three quantum theories we mentioned are normally performed in flat space, or where the gravitational interactions are negligible. (The spacetime that corresponds to this in General Relativity is known as Minkowski space.) However, near a black hole, space is curved and is given by Schwarzschild space, not Minkowski space.

    Image credit: Concept art by NASA; Jörn Wilms (Tübingen) et al.; ESA.

    So what happens to these quantum fields not in empty, flat space, but in curved space, like around a black hole? That was the problem that Hawking tackled in 1974, demonstrating that the presence of these quantum fields in the curved space around a black hole causes the emission of thermal, blackbody radiation at a particular temperature. This temperature (and the flux) is lower the more massive the black hole is, due to the fact that the curvature of space is smaller at the event horizon of larger, more massive black holes.

    In his popular science book, A Brief History Of Time (still Amazon’s #1 best seller in cosmology), Stephen Hawking described the vacuum of space as consisting of particle/antiparticle pairs of virtual particles, popping in-and-out of existence. Around a black hole, he explained, sometimes one of the two components of these virtual pairs falls in to the event horizon, while the other remains outside. When this happens, he states, the “out” member of the pair escapes with real, positive energy, meaning the “in” member must fall in with negative energy, subtracting from the mass of the black hole and causing it to slowly decay.

    Image credit: Ulf Leonhardt of the University of St. Andrews, via http://www.st-andrews.ac.uk/~ulf/fibre.html

    Of course, this picture isn’t right. For starters, the radiation doesn’t come exclusively from the edge of the black hole’s event horizon, but rather throughout the space surrounding it. But the biggest erroneous way of thinking about it like Hawking describes is that the black hole emits photons, not particles and antiparticles, when it comes to this radiation. And in fact, the radiation is of such low energy that it couldn’t produce particle/antiparticle pairs at all.

    I, myself, tried to improve on this explanation by emphasizing that these are virtual particles, or a way of visualizing the quantum fields in nature; these are not real particles at all. But these properties of the field can (and do) conspire to produce real radiation.

    Image credit: E. Siegel, of a better (but still incorrect) picture of Hawking radiation.

    However, this is not quite right, either. It implies that close to the black hole’s event horizon, the radiation is enormous, and only appears small and low in temperature when you’re far away. In reality, the radiation is small at all locations, and only a small percent of the radiation can be traced back to the event horizon at all.

    The real explanation is a lot more complex, and shows that this simplistic picture has its limits. The root of the problem is that different observers have different views and perceptions of particles and the vacuum, and this problem is more complicated in curved space than in flat space. Basically, one observer would see empty space, but an accelerated observer would see particles in that space. The origin of Hawking radiation has everything to do with where that observer is, and what they see as accelerated versus what they see as at rest.

    Image credit: NASA, via http://www.nasa.gov/topics/universe/features/smallest_blackhole.html.

    When you create a black hole where there was none initially, you are accelerating particles from outside the event horizon to, eventually, inside the event horizon. This process is the origin of that radiation, and Hawking’s calculation showed just how tremendously long the timescale for this emission of evaporative radiation is. For a black hole the mass of the Sun, it will take 10⁶⁷ years to evaporate; for the largest, 10 billion solar mass black holes in the Universe, it will be more like 10¹⁰⁰ years. For comparison, the Universe is only around 10¹⁰ years old today, and the rate of evaporation is so small that it will take around 10²⁰ years before black holes begin evaporating faster than the rate of growth due to the occasional collision with an interstellar proton, neutron or electron.

    So the short answer to your question, Spencer, is that Hawking’s picture is totally oversimplified to the point of being wrong. The slightly longer answer is that it’s the infalling of matter itself that causes the radiation, and it’s the extreme curvature of space that causes this radiation to be emitted so slowly, over such long periods of time and over such a large volume of space in the black hole’s vicinity. For even longer, more technical explanations, I recommend (in order of increasing difficulty) Sabine Hossenfelder’s, John Baez’s, and finally Steve Giddings’.

    See the full article here .

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

  • richardmitnick 12:05 pm on September 6, 2015 Permalink | Reply
    Tags: , , , , Stephen Hawking   

    From CNN: “Has Stephen Hawking solved the mystery of black holes?” 


    September 4, 2015
    FNAL Don Lincoln
    Don Lincoln, FNAL

    Black holes have a way of capturing our imagination. That’s why when Stephen Hawking recently talked about them the media went wild.

    Stephen Hawking
    Stephen Hawking

    But what was he really saying? Was it a breakthrough moment?

    At the Hawking Radiation Conference organized by Laura Mersini-Houghton, a professor of physics at the University of North Carolina, 32 eminent physicists gathered to discuss outstanding issues involved with apparent contradictions in our current understanding of the theories of [General] relativity and quantum mechanics. The convergence of the two take us to the inner workings of black holes.

    Black holes are ravenous monsters of the cosmos, constantly reaching out and gobbling nearby mass as they grow larger and larger. The poster child of [Albert] Einstein’s theory of relativity, black holes exert such a strong gravitational force that not even light can escape, and they are able to distort the very fabric of space and slow the passage of time. These are very real objects.

    And yet they embody a very significant mystery. Black holes are said to absorb matter and never let it go. The matter simply disappears inside the black hole. But matter is more than, well, matter. It is information. For instance, if I have a single atom of hydrogen, I have a proton and electron. That’s matter. But there is also information in how they are connected. Are they near one another, or far apart?

    The information component is even more important in, say, a piece of fruit. While I might tell you just how many protons, neutrons and electrons exist in an apple, without the information that tells you how they arranged, it wouldn’t have the apple’s tart taste. In fact, it wouldn’t be an apple at all. Ultimately, it is information that is at the heart of the mystery.

    According to the rules of quantum mechanics, information should never be lost, not even if it gets sucked inside the black hole. This is because of two premises: causality and reversibility. Taken together, it means that effects have causes, and those causes can be undone.

    For example, you can break a glass and then find all the pieces and glue it back together. Yet, these two premises don’t hold for a classical black hole, in which the information is permanently and irreversibly lost as it enters the black hole.

    Note that information being lost isn’t the same as matter being lost. In the 1970s, Hawking postulated what is now called Hawking radiation, which in principle, cause black holes eventually to evaporate as the radiation carries away energy. However, Hawking radiation should be completely independent of the matter absorbed by a black hole. So, information really does appear to be lost, in complete contradiction of quantum theory.

    This is where Hawking’s announcement comes in. He is saying that he can solve the conundrum.

    He is countering the claim that the black hole gobbles and destroys the information by positing that the information never actually falls into the black hole. Instead, the information is held on the black hole’s surface — the event horizon.

    This is an intriguing thought and is analogous to how holograms are made. Holograms are two-dimensional sheets of, for example, plastic that can make three-dimensional images. All of the information of three dimensions is encoded in the two dimensional plastic. (By the way, there are some who hypothesize that our entire universe is a hologram!)

    It is difficult to properly evaluate Hawking’s announcement. The claim as it has been described is not very precise. There is no paper published on the idea, nor has the idea passed peer review. In fact, scientists who attended the conference are still trying to absorb the idea and to cast it in a mathematical language so that the implication can be assessed.

    Hawking developed this concept in collaboration with Malcolm Perry of Cambridge University and Andrew Strominger of Harvard University. They plan to submit a paper in a month or so. That’s when the real evaluation of the proposal can begin.

    While everyone would much prefer to hear about a definitive advancement in science, the actual process of developing scientific ideas can be both intellectually stimulating and thoroughly messy.

    Stephen Hawking’s new ideas are certainly interesting and may point us in the right direction. But we will have to wait a bit longer to solve the enigma of what happens when information confronts a black hole. Sit tight, we’re on a very long journey.

    See the full article here.

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  • richardmitnick 9:37 am on September 3, 2015 Permalink | Reply
    Tags: , , , Stephen Hawking   

    From NOVA: “How big a deal was Stephen Hawking’s big black hole announcement?” 



    02 Sep 2015
    Sarah Scoles

    “Can you hear me?” Stephen Hawking asked as he was about to begin his August 25 talk at the Royal Institute of Technology’s “Hawking Radiation” conference in Stockholm.

    Stephen Hawking

    The 29-person audience, all VIP physicists, was eager to hear his big announcement and could hear him just fine. They knew, from a pre-announcement announcement the previous night, that Hawking was about to explain his solution to a 40-year-old mystery in physics: how information escapes from black holes.

    But while his idea made big headlines, the mere nine minutes of explanation felt vague and confusing to other physicists.

    The iconic image of a black hole. But, of course, we have never really seen a black hole, not even a supermassive black hole. Artist’s concept illustrates a supermassive black hole with millions to billions times the mass of our sun. Credit: NASA/JPL-Caltech

    What is Hawking’s problem?

    Hawking set out to resolve a problem called the “information paradox.” Understanding this snag requires a brush-up on black holes. If light or matter venture past a boundary around the black hole called the “event horizon,” they are done for: The speed required to overcome the black hole’s gravity and escape is greater than the speed of light [c]. So anything that crosses the event horizon—and any information about what it was in its previous life—stays inside the black hole. Whether you, a Snickers bar, or a whole planet fall in, they all end up the same: as anonymous extra mass piled onto the black hole itself.

    Or at least that used to be the idea. Then, in 1974, Stephen Hawking showed that black holes slowly evaporate. They continuously leak radiation (later named “Hawking radiation”), dwindling away until there’s nothing left, on timescales ranging form a few billion years to much longer than the current age of the universe. But, as theoretical physicist Carlo Rovelli of Aix-Marseille University, who attended the talk, explains, “This creates a problem: Where has all the stuff gone that fell inside? Where is the information about what fell in? It cannot be anymore ‘just inside,’ because the black hole has disappeared. So, where is it? Is it really lost?”

    Quantum mechanics says that information about the stuff can’t be lost. Information can neither be created nor destroyed. But black holes seem to destroy it. But they can’t. But they seem to. That’s the “paradox” part of the information paradox.

    Some scientists think the escaping Hawking radiation carries the information out with it, like a set-free hostage who can tell police about the room he just spent five days in.

    Ideas abound about how that radiation might spill the beans, and Hawking’s new revelation is just one contender. “The situation is not that there is a big problem, and here is the solution,” says Rovelli. “The situation is that there is a big problem, and there are a dozen solutions … none totally convincing, and now we have a new one.”

    Hawking’s big idea

    On August 25, as Hawking sat before the esteemed physicists, his voice played through the room’s speakers. “I propose that the information is stored not in the interior of the black hole, as one might expect, but on its boundary, the event horizon,” he said, “in the form of supertranslations of the horizon.”

    Translation: If you passed over the event horizon, you would leave an imprint on it. That imprint takes the form of, essentially, a hologram called a supertranslation—a two-dimensional representation of your three-dimensional parts—etched into the black hole’s exterior geometry. When Hawking radiation bubbles up, the event horizon leaves a similar imprint on that radiation. It’s like cosmic re-gifting. The Hawking radiation then streams back out into the universe, carrying the imprint, and the encoded information, with it. That code, though, is scrambled: If you fell into a black hole, we could not create your clone from it (sorry). But, cold comfort, your informational essence wouldn’t be eternally lost.

    The idea that information could be stamped onto a black hole’s event horizon was first proposed by Nobel Laureate Gerard ‘t Hooft, and supertranslations—as mathematical ideas—come from the 1960s. We don’t yet know enough about Hawking’s idea to detail how new and different it is.

    The problem with Hawking’s solution

    And that’s part of the problem: According to colleagues, the details of his “solution” feel fuzzy. “Two big questions are where the information from infalling stuff gets deposited, and how that information later gets transferred to stuff leaving the black hole,” says Steven Giddings, a physicist at the University of California, Santa Barbara.

    Those are two big questions—the biggest, most fundamental questions. It’s great that Hawking described the what of his idea, but, in science, the how is much more important. “What we need for a more detailed understanding is a more complete description of the mathematics … to see if they’ve really nailed the answer,” says Giddings. Rovelli agrees, stating, “The picture is very preliminary for the moment.”

    They weren’t the only two left scratching their heads. “In the conference, there were many world-class physicists, including Nobel Prize winners,” says Joe Polchinski, Giddings’ colleague at UC Santa Barbara. “I didn’t perceive much enthusiasm about the new idea. Everybody was interested, of course, but I couldn’t detect anybody that appeared convinced.”

    Polchinski, who has previously science-battled Hawking about black hole paradoxes, also pointed out a problem beyond the idea’s fuzziness: In Hawking’s scenario, information stays on the event horizon. But the information (using the previous example, you yourself) also falls into the black hole, meaning two copies of it would exist. “In quantum mechanics, information can’t be in two places,” Polchinski says, although he points out that Hawking may have found a way to evade this problem.

    Because Hawking’s black-hole revelations (as well as his proclamations about aliens and religion) receive public buzz and papers called “AdS/CFT without holography: A hidden dimension on the CFT side and implications for black-hole entropy” don’t, it may seem that his idea is totally novel. But it’s not. “From what we here understand, his suggestion builds on ideas that people have been tossing around recently,” says Giddings. Rovelli and Polchinski also point out its similarity to ’t Hooft’s 1990s ideas, although Hawking has added “some technical steps.”

    Hawking claims he, and co-conspirators Andrew Strominger of Harvard University and Malcolm Perry of Cambridge University, will leak more information in a paper in late September. If that paper throws around some convincing equations—what black-hole theorists require as evidence—the result could be a big deal. Until then, scientists are waiting to reserve judgment. “For the moment the theory is far too sketchy, in the manner it has been presented,” Rovelli says. “Let me put it this way: The big news is Hawking himself: his persona, his popular fame, the wonderful manner in which he communicate to the public and transmits enthusiasm to the public. This is fantastic and is his mastership. His physics is interesting, as many others’ are.”

    See the full article here.

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