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  • richardmitnick 11:57 am on February 4, 2016 Permalink | Reply
    Tags: , , Black Holes, Galaxy mergers,   

    From Nautilus: “Why It’s Hard for Black Holes to Get Together” 

    Nautilus

    Nautilus

    February 4, 2016
    Kate Becker
    Illustration by David Plunkert

    Black hole in color

    It begins like a classic romance: Two black holes meet. The attraction is practically instant. They dance around each other, swirling closer and closer, until …

    Until what? As with any love affair, this is where things get messy.

    First predicted by [Albert] Einstein’s theory of general relativity, black holes are bottomless pits in the fabric of spacetimegravitational wells so deep that nothing, not even light, can escape. Little black holes, trapping the mass of just a few suns, litter the universe like silent land mines, while the biggest ones, the supermassive black holes, occupy the center of nearly every galaxy, hoovering up torrents of infalling matter. These giants contain as much mass as hundreds of millions of suns; astronomers think that they formed from a long chain of galactic mergers—dozens, or perhaps even hundreds, of happy unions stretching back to the early days of the universe.

    “Our whole picture of the formation of the structure of the universe involves this hierarchical process of small galaxies merging together to form large galaxies, and large galaxies merging to become even larger galaxies,” says physicist Robert Owen, who studies black-hole collisions at Oberlin College, in Ohio, as part of the Simulating eXtreme Spacetimes collaboration. Each merger takes hundreds of millions of years or more—too long to see in action—but theorists can use simulations to recreate the entire affair in computer code.

    And here’s where the trouble starts. When physicists run their simulations, the two central black holes in a pair of colliding galaxies get stuck. Rarely, if ever, do black holes crash head-on. Instead, because they are typically traveling along separate, unaligned paths when they meet, their conserved angular momentum causes them to spiral toward each other. They corskscrew ever closer, captives of their mutual attraction, until they are orbiting at arms length—on the order of three light-years, or one parsec, apart. Then, like bashful lovers, they go no farther.

    Why? Owen offers an analogy: Imagine that your hand is one of these black holes, and you’re swirling it in a bucket of water that represents the swarm of merging galactic matter. At first the water pushes against your hand, slowing it down. In space, this gravitational interaction, known as dynamical friction, decreases the angular momentum of a black hole in orbit, causing it to drift toward its partner. But over time, the water in the bucket starts flowing in the same direction as your hand, so you feel less resistance. Likewise, in a simulated galactic merger, stars and other objects align their motion with the paths of the two twirling black holes. And as dynamical friction gradually peters out, the black holes settle into stable orbits.

    This is the fabric of spacetime itself moving.

    If physicists’ story about the formation of the universe is correct, such paired black holes should eventually collide and consume each other, becoming one. But to do this, they must somehow lose enough energy to resume their inward spiral past the final parsec. Once they get very close—just billions of miles apart (about 0.001 parsec)—general relativity says that they will jettison the last of their orbital momentum in a great crescendo of gravitational waves, ripples in spacetime that ring out from a gravitational disturbance. This final outburst of energy plunges the black holes together, finishing the job in a matter of hours, days, or years, depending on how massive the black holes are.

    What drives this fatal embrace? The question, known as the “final parsec problem,” isn’t just a matter of curiosity. The answer could change our understanding of how the universe built up its elaborate structure, and of the nature of gravity itself. Which is why, as physicists tinker with their simulations, astronomers are searching the skies for clues to how black holes solve the final parsec problem in the wild—if they do at all.

    LO95 0313-192 is the large, edge-on spiral galaxy left of centre in this image
    L095-0313-192 is the large edge on spiral galaxy left of centre in this image of two galaxies which may merge in the future.

    Over the past 30 years, astronomers have collected snapshots of hundreds of galaxies with dual supermassive black holes in various stages of collision. But even the most intimate portraits don’t reveal pairs circling nearer than a few thousand parsecs. “Looking for ones which are much closer to merger, on the parsec scale or smaller, is much harder,” says computational scientist Matthew Graham at the California Institute of Technology. Even the biggest telescopes on Earth can’t zoom in enough to resolve an image of two black holes in such a tight orbit.

    So Graham and his colleagues are instead searching by an indirect route: flickering quasar light. Quasars are tremendously bright cores of massive, ancient galaxies. As matter swirls toward the supermassive black holes at their centers, it accumulates into a disc whose angular momentum converts some of this mass into radiation that outshines the galaxy itself. Because gas and dust don’t flow into the disc in a smooth stream, quasar light varies, typically in a random pattern.

    But in late 2013 came a quasar that “stood out like a sore thumb,” Graham says. Using 10 years worth of data from a collaboration called the Catalina Real-Time Transient Survey, he and his colleagues picked up a strangely predictable signal: Some 3.5 billion light-years from Earth, quasar PG 1302-102 appeared to be getting steadily brighter and dimmer every five and a half years, as if someone were slowly toggling some cosmic dimmer switch.

    What could be producing this cycle? “We came up with four or five different physical scenarios,” Graham says. The revolutions of a second supermassive black hole, for instance, could be routinely redirecting the quasar’s radiation jets like searchlight beams. Or perhaps this extra black hole was contorting the disc of whirling matter, thus brightening and dimming the quasar on a regular schedule. All of the researchers’ explanations had one thing in common: They made sense only if the black hole at the center of PG 1302- 102 was actually two black holes.

    If there really is a black-hole binary at the center of PG 1302-102, Graham and his team estimate their separation at just 0.01 parsec. Another analysis, by a team at Columbia University, puts the pair even closer, at 0.001 parsec, or roughly the diameter of our solar system—about the point at which the black holes should be shedding gravitational waves like layers of clothing, plunging them into one another’s arms. Either way, if researchers are reading the signals from PG 1302-102 correctly, the moral is the same: Nature has solved the final parsec problem.

    Graham and other researchers have so far identified more than 100 quasars in the Catalina data set that they think could contain black-hole binaries, all easily within the final parsec. If they can confirm their suspicion, these candidates could give them a peek at the grand finale of the collision saga, which nature has kept so well-hidden.

    The big break in the final parsec problem, however—the revelation of how black holes unlock themselves from a stable orbit to complete their union—may come from looking at the universe in an entirely new way. “We’re really just fumbling around with electromagnetic waves,” Owen says, describing efforts to find tight black-hole binaries using traditional telescopes. Theoretically, a black-hole merger should release 100 million times as much energy as a supernova explosion, but all of that energy comes in the form of gravitational waves, not light. “We’re trying to hear with our eyes—it’s like inferring that a drum is oscillating just by looking at it, without being able to hear it.”

    Observing black-hole collisions via gravitational waves could give astronomers a much clearer view. “The light coming from the centers of galaxies is often absorbed, re-emitted, or scattered by clouds of gas and dust,” producing a dim and distorted picture, explains Chiara Mingarelli, an astrophysicist at Caltech and the Max Planck Institute for Radio Astronomy. “[Gravitational] ripples don’t care if there’s gas and dust—they move through it, undisturbed. This is the fabric of spacetime itself moving.”

    Spotting these ripples, however, won’t be easy: Gravitational-wave astronomy is a fledgling science, yet to return a single detection. What’s more, state-of-the-art laser-based observatories such as LIGO aren’t sensitive to the slowly oscillating waves that astronomers suspect are pumping out of intimate black-hole binaries like PG 1302-102.

    Caltech Ligo
    MIT/Caltech Advanced aLIGO

    Researchers hope instead to pick up these disturbances using “telescopes” provided by nature: millisecond pulsars. These dense, spinning corpses of exploded stars dot the cosmos like buoys on an ocean, sweeping beams of radio waves past Earth with atomic-clock accuracy. By monitoring the tick-tock of dozens of millisecond pulsars (a pulsar timing array) in our own galaxy, the Milky Way, astronomers can look for telltale deviations that reveal the surge of gravitational waves from two black holes crossing through the final parsec in a distant galaxy.

    The spectral signature of these waves—from rapid flutters to slow swells, and everything in between—would provide data against which physicists can test new or revised models of the unification process. “Pulsar timing arrays are the only instrument we have to tell us what’s happening on this last-parsec scale—what’s really driving the final stages of binary black-hole mergers,” says Joseph Simon, a graduate student studying these collisions at the University of Wisconsin-Milwaukee.

    The absence of gravitational waves could also provide an important clue. After almost a decade of timekeeping, Simon says, pulsar timing arrays “are finally sensitive enough that even non-detection tells us about what’s happening.” The fact that these arrays haven’t yet picked up the scent of gravitational waves could mean that theorists’ understanding of what happens to colliding black holes once they cross the final parsec isn’t quite right. Rather than erupt as gravitational radiation, some of the energy lost in that final plunge may instead bleed away through some yet unknown interaction with nearby stars and gas. Maybe the black holes fling away some stars that veer toward them, for instance. Or maybe their gravitational pull torques the disc of dust and gas around them. If physicists can work out this energy-sapping mechanism, it might explain how merging black holes cross the final parsec in the first place.

    Their calculations will take them to the edges of Einstein’s predictions. “We talk about general relativity like it is an extremely well-confirmed theory, and by some measures it is the most precisely confirmed theory in physics,” Owen says. But scientists have never tested it in extreme gravitational events, such as a black hole merger, where physics dramatically diverges from the laws laid out by Isaac Newton more than three centuries ago; where familiar concepts like energy, momentum, and mass lose their meaning. If it turns out that gravitational outbursts from black-hole unions are indeed weaker than general relativity says they should be, it may be time for a tweak.

    Ultimately, completing the black-hole love story will tell us what kind of ride we’re on here on Earth—whether we’re rolling along on a deluge of gravitational waves, or just a trickle. “This really is the difference between a very calm extragalactic sea of spacetime and a very violent sea of spacetime,” Owen says.

    See the full article here .

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    Welcome to Nautilus. We are delighted you joined us. We are here to tell you about science and its endless connections to our lives. Each month we choose a single topic. And each Thursday we publish a new chapter on that topic online. Each issue combines the sciences, culture and philosophy into a single story told by the world’s leading thinkers and writers. We follow the story wherever it leads us. Read our essays, investigative reports, and blogs. Fiction, too. Take in our games, videos, and graphic stories. Stop in for a minute, or an hour. Nautilus lets science spill over its usual borders. We are science, connected.

     
  • richardmitnick 3:53 pm on February 2, 2016 Permalink | Reply
    Tags: , , Black Holes, Far Away, , Pictor A: Blast from Black Hole in a Galaxy Far   

    From Chandra: “Pictor A: Blast from Black Hole in a Galaxy Far, Far Away” 

    NASA Chandra

    February 2, 2016
    Pictor A Blast from Black Hole in a Galaxy Far, Far Away
    Credit X-ray: NASA/CXC/Univ of Hertfordshire/M.Hardcastle et al., Radio: CSIRO/ATNF/ATCA
    Release Date February 2, 2016

    A giant jet spanning continuously for over 300,000 light years is seen blasting out of the galaxy Pictor A.

    A new composite image shows this jet in X-rays (blue) and radio waves (red). [see the original full article for more images.]

    In addition to the main jet, there is evidence for a jet moving in the opposite direction.

    Chandra observations at various times over a 15-year period provide new details of this impressive system.

    The Star Wars franchise has featured the fictitious “Death Star,” which can shoot powerful beams of radiation across space. The Universe, however, produces phenomena that often surpass what science fiction can conjure.

    The Pictor A galaxy is one such impressive object. This galaxy, located nearly 500 million light years from Earth, contains a supermassive black hole at its center. A huge amount of gravitational energy is released as material swirls towards the event horizon, the point of no return for infalling material. This energy produces an enormous beam, or jet, of particles traveling at nearly the speed of light into intergalactic space.

    To obtain images of this jet, scientists used NASA’s Chandra X-ray Observatory at various times over 15 years. Chandra’s X-ray data (blue) have been combined with radio data from the Australia Telescope Compact Array (red) in this new composite image.

    CSIRO Australia Compact Array
    Australia Telescope Compact Array

    By studying the details of the structure seen in both X-rays and radio waves, scientists seek to gain a deeper understanding of these huge collimated blasts.

    The jet [to the right] in Pictor A is the one that is closest to us. It displays continuous X-ray emission over a distance of 300,000 light years. By comparison, the entire Milky Way is about 100,000 light years in diameter. Because of its relative proximity and Chandra’s ability to make detailed X-ray images, scientists can look at detailed features in the jet and test ideas of how the X-ray emission is produced.

    In addition to the prominent jet seen pointing to the right in the image, researchers report evidence for another jet pointing in the opposite direction, known as a “counterjet”. While tentative evidence for this counterjet had been previously reported, these new Chandra data confirm its existence. The relative faintness of the counterjet compared to the jet is likely due to the motion of the counterjet away from the line of sight to the Earth.

    The labeled image shows the location of the supermassive black hole, the jet and the counterjet. Also labeled is a “radio lobe” where the jet is pushing into surrounding gas and a “hotspot” caused by shock waves – akin to sonic booms from a supersonic aircraft – near the tip of the jet.

    The detailed properties of the jet and counterjet observed with Chandra show that their X-ray emission likely comes from electrons spiraling around magnetic field lines, a process called synchrotron emission. In this case, the electrons must be continuously re-accelerated as they move out along the jet. How this occurs is not well understood

    The researchers ruled out a different mechanism for producing the jet’s X-ray emission. In that scenario, electrons flying away from the black hole in the jet at near the speed of light move through the sea of cosmic background radiation (CMB) left over from the hot early phase of the Universe after the Big Bang3.

    Cosmic Background Radiation Planck
    CMB per ESA/Planck

    ESA Planck
    ESA/Planck

    When a fast-moving electron collides with one of these CMB photons, it can boost the photon’s energy up into the X-ray band.

    The X-ray brightness of the jet depends on the power in the beam of electrons and the intensity of the background radiation. The relative brightness of the X-rays coming from the jet and counterjet in Pictor A do not match what is expected in this process involving the CMB, and effectively eliminate it as the source of the X-ray production in the jet.

    A paper describing these results will be published in the Monthly Notices of the Royal Astronomical Society and is available online. The authors are Martin Hardcastle from the University of Hertfordshire in the UK, Emil Lenc from the University of Sydney in Australia, Mark Birkinshaw from the University of Bristol in the UK, Judith Croston from the University of Southampton in the UK, Joanna Goodger from the University of Hertfordshire, Herman Marshall from the Massachusetts Institute of Technology in Cambridge, MA, Eric Perlman from the Florida Institute of Technology, Aneta Siemiginowska from the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA, Lukasz Stawarz from Jagiellonian University in Poland and Diana Worrall from the University of Bristol.

    See the full article here .

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    NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra’s science and flight operations from Cambridge, Mass.

     
  • richardmitnick 7:02 pm on January 27, 2016 Permalink | Reply
    Tags: , , Black Holes, , ,   

    From WIRED.com: “The Death of General Relativity Lurks in a Black Hole’s Shadow” 

    Wired logo

    Wired

    01.27.16
    Lizzie Wade

    Black hole in color
    Chi-kwan Chan, Feryal Ozel, and Dimitrios Psaltis

    Nothing gets out of a black hole—not even light. Once a star, a planet, a piece of dust, or even a single photon crosses the limit known as the event horizon, it’s not coming out again. Pulled into the crushing gravity of the singularity at the black hole’s heart, it vanishes from the universe.

    That’s a big problem if what you really want from a black hole is a photograph. By definition, it’s impossible. All light getting sucked in means no light reflects back—so a black hole is invisible, across the spectrum. And, duh, invisible objects don’t show up in photographs.

    But thanks to a new telescope, Tim Johannsen, an astrophysicist at the Perimeter Institute and the University of Waterloo in Ontario, Canada, may be able to get a black hole pic after all. A loophole in physics means he might be able to see not the black hole itself, but its shadow. And, no big deal, but that photo just might overturn Albert Einstein’s theory of general relativity.

    So…wait. Black holes have shadows? Sort of. As gas and dust and other cosmic material approaches a black hole, “that stuff heats up, like millions and millions of degrees,” Johannsen says. That superheated matter swirls around the black hole in what’s called an accretion disk; because it’s so hot, the accretion disk emits a lot of light.

    Some of those photons zoom out towards Earthbound telescopes, while others cross the event horizon and fall into the void. So when astronomers look at a black hole, what they expect to see is a ring of bright light—the accretion disk—surrounding a circle of nothingness. That circle of nothingness is the shadow. (The black hole itself is just a single point within.) You can see a model of that here:


    Download mp4 video here .

    At least, that’s the idea. No one has ever actually seen a black hole’s shadow. “Despite their enormous mass, black holes are also exceedingly small,” says Avery Broderick, Johannsen’s colleague at the Perimeter Institute and the University of Waterloo. Seen from Earth, the shadow of Sagittarius A*, the supermassive black hole at the center of the Milky Way (also known as Sgr A*, which astrophysicists pronounce “Saj-A-star”) is just 1/35,000,000th the width of the Moon, or 50 microarcseconds wide.

    Sag A prime
    Sgr A*

    Here’s where that new telescope comes in. Maybe. Johannsen, Broderick, and their colleagues hope the Event Horizon Telescope will be able to resolve Sgr A*’s shadow. The EHT is actually nine [radio] telescopes (and counting), all working together and each located in a different spot on Earth.

    Event Horizon Telescope map
    EHT map

    Telescopes of the EHT

    ALMA

    ALMA Array
    APEX

    ESO APEX

    Arizona Radio Observatory (U. of Arizona)

    Arizona Radio Observatory

    Caltech Submillimeter Observatory

    Caltech Submillimeter Observatory

    CARMA

    CARMA Array

    Harvard Smithsonian Center for Astrophysics Submillimeter Array

    SMA Submillimeter Array

    Institut de Radioastronomie Millimetrique (IRAM) 30m

    IRAM

    James Clerk Maxwell Telescope (JCMT)

    The Large Millimeter Telescope Alfonso Serrano (LMT)

    Plateau de Bure interferometer

    IRAM Interferometer Submillimeter Array of Radio Telescopes

    South Pole Telescope]

    South Pole Telescope

    Coordinating those telescopes’ observations allows them to work as one big telescope that is, in essence, as big as the planet. The bigger your telescope, the higher your resolution. “The Event Horizon Telescope has the capability to produce the highest-resolution images in the history of astronomy”, Broderick says. “That means, for the first time, we can see what happens right down in the immediate vicinity of black hole event horizons.”

    Scientists working on the EHT hope to see images in the spring of 2017. But they already have some ideas of what they’ll get. General relativity describes gravity not as a force drawing two objects together, but rather as the warped spacetime that governs each of those objects movements.

    Spacetime with Gravity Probe B
    Artist concept of Gravity Probe B orbiting the Earth to measure space-time, a four-dimensional description of the universe including height, width, length, and time. NASA

    Concentrate a big enough mass in a small enough region of spacetime, and its gravity will be inescapably huge—voila, you’ve got a black hole. If that sounds weird to you, well, it took 50 years for astronomers to discover that black holes were real objects, not just a quirk of general relativity’s math.

    The problem is, general relativity is really good at describing giant things like stars, but breaks down utterly when it comes to really teeny tiny things like photons and quarks. To talk about those, you need a different theory: quantum mechanics. The central problem in physics today is that the theories are fundamentally incompatible. To figure that out, physicists are keen to find places where the theories overlap or break down—like, for example, the event horizon of a black hole.

    General relativity doesn’t just predict the existence of black holes. It also precisely describes the size and shape of their shadows. Sgr A*’s shadow is supposed to be perfectly circular and 50 microarcseconds wide. “What would it look like if general relativity were wrong?” wonders Broderick (and just about every other astrophysicist on the planet). There are two possibilities. “The shadow could be more egg shaped,” says Johannsen. “That would be a smoking gun for a GR violation.” It might also be slightly smaller or bigger than general relativity predicts. All he needs to figure it out is the picture from the EHT. (Johannsen and Broderick just published a paper outlining their strategy in Physical Review Letters.)

    And what if Sgr A*’s shadow doesn’t look the way general relativity says it should? Well, that would be great. If the results held up, physicists could start looking for alternative theories of gravity that did predict the shadow’s size and shape. Success wouldn’t mean the new theory would automatically be the successor to general relativity, of course. But it’s a good way to figure out which theories might be on the right track, so you can give their other predictions a closer look.

    Johannsen’s favorite possibility involves extra dimensions. A shortcoming of general relativity is that it doesn’t explain why gravity is so much weaker than the other fundamental forces. “Let’s assume there is another space dimension. Gravity would immediately penetrate that and become kind of diluted,” Johannsen says. In other words, gravity isn’t weak, it’s just working across more dimensions than the other forces. Amazingly, theories that predict those extra dimensions also predict a different size for Sgr A*’s shadow. In a couple years, finally proving—or falsifying—this weird new physics could “literally be as ‘easy’ as putting a ruler across the image,” Johannsen says.

    “We’re getting this amazing opportunity to finally put Einstein to the test around the most enigmatic and striking predictions of this theory,” Broderick says. If Einstein is wrong, general relativity won’t go away—it’s too good at what it does. It just won’t be the whole story anymore. Isaac Newton was plenty right about how gravity worked here on Earth; Einstein revolutionized our understanding of the universe. But the universe is big enough to have room for someone to come along and do it again.

    See the full article here .

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  • richardmitnick 11:02 am on January 17, 2016 Permalink | Reply
    Tags: , , Black Holes, ,   

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

    Livescience

    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 2:04 pm on January 12, 2016 Permalink | Reply
    Tags: , , Black Holes, ,   

    From Symmetry: “Black holes” 

    Symmetry

    01/12/16
    Ali Sundermier

    Let yourself be pulled into the weird world of black holes.

    Temp 1

    Imagine, somewhere in the galaxy, the corpse of a star so dense that it punctures the fabric of space and time. So dense that it devours any surrounding matter that gets too close, pulling it into a riptide of gravity that nothing, not even light, can escape.

    And once matter crosses over the point of no return, the event horizon, it spirals helplessly toward an almost infinitely small point, a point where spacetime is so curved that all our theories break down: the singularity. No one gets out alive.

    Black holes sound too strange to be real. But they are actually pretty common in space. There are dozens known and probably millions more in the Milky Way and a billion times that lurking outside. Scientists also believe there could be a supermassive black hole at the center of nearly every galaxy, including our own. The makings and dynamics of these monstrous warpings of spacetime have been confounding scientists for centuries.

    A history of black holes

    It all started in England in 1665, when an apple broke from the branch of a tree and fell to the ground. Watching from his garden at Woolsthorpe Manor, Isaac Newton began thinking about the apple’s descent: a line of thought that, two decades later, ended with his conclusion that there must be some sort of universal force governing the motion of apples and cannonballs and even planetary bodies. He called it gravity.

    Newton realized that any object with mass would have a gravitational pull. He found that as mass increases, gravity increases. To escape an object’s gravity, you would need to reach its escape velocity. To escape the gravity of Earth, you would need to travel at a rate of roughly 11 kilometers per second.

    It was Newton’s discovery of the laws of gravity and motion that, 100 years later, led Reverend John Michell, a British polymath, to the conclusion that if there were a star much more massive or much more compressed than the sun, its escape velocity could surpass even the speed of light. He called these objects “dark stars.” Twelve years later, French scientist and mathematician Pierre Simon de Laplace arrived at the same conclusion and offered mathematical proof for the existence of what we now know as black holes.

    In 1915, Albert Einstein set forth the revolutionary theory of general relativity, which regarded space and time as a curved four-dimensional object. Rather than viewing gravity as a force, Einstein saw it as a warping of space and time itself. A massive object, such as the sun, would create a dent in spacetime, a gravitational well, causing any surrounding objects, such as the planets in our solar system, to follow a curved path around it.

    A month after Einstein published this theory, German physicist Karl Schwarzschild discovered something fascinating in Einstein’s equations. Schwarzschild found a solution that led scientists to the conclusion that a region of space could become so warped that it would create a gravitational well that no object could escape.

    Up until 1967, these mysterious regions of spacetime had not been granted a universal title. Scientists tossed around terms like “collapsar” or “frozen star” when discussing the dark plots of inescapable gravity. At a conference in New York, physicist John Wheeler popularized the term “black hole.”

    How to find a black hole

    During star formation, gravity compresses matter until it is stopped by the star’s internal pressure. If the internal pressure does not stop the compression, it can result in the formation of a black hole.

    Some black holes are formed when massive stars collapse. Others, scientists believe, were formed very early in the universe, a billion years after the big bang.

    There is no limit to how immense a black hole can be, sometimes more than a billion times the mass of the sun. According to general relativity, there is also no limit to how small they can be (although quantum mechanics suggests otherwise). Black holes grow in mass as they continue to devour their surrounding matter. Smaller black holes accrete matter from a companion star while the larger ones feed off of any matter that gets too close.

    Black holes contain an event horizon, beyond which not even light can escape. Because no light can get out, it is impossible to see beyond this surface of a black hole. But just because you can’t see a black hole, doesn’t mean you can’t detect one.

    Scientists can detect black holes by looking at the motion of stars and gas nearby as well as matter accreted from its surroundings. This matter spins around the black hole, creating a flat disk called an accretion disk. The whirling matter loses energy and gives off radiation in the form of X-rays and other electromagnetic radiation before it eventually passes the event horizon.

    This is how astronomers identified Cygnus X-1 in 1971. Cygnus X-1 was found as part of a binary star system in which an extremely hot and bright star called a blue supergiant formed an accretion disk around an invisible object. The binary star system was emitting X-rays, which are not usually produced by blue supergiants. By calculating how far and fast the visible star was moving, astronomers were able to calculate the mass of the unseen object. Although it was compressed into a volume smaller than the Earth, the object’s mass was more than six times as heavy as our sun.

    Several different experiments study black holes. The Event Horizon Telescope [EHT] will look at black holes in the nucleus of our galaxy and a nearby galaxy, M87. Its resolution is high enough to image flowing gas around the event horizon.

    Event Horizon Telescope map
    EHT

    Scientists can also do reverberation mapping, which uses X-ray telescopes to look for time differences between emissions from various locations near the black hole to understand the orbits of gas and photons around the black hole.

    The Laser Interferometer Gravitational-Wave Observatory, or LIGO, seeks to identify the merger of two black holes, which would emit gravitational radiation, or gravitational waves, as the two black holes merge.

    Caltech Ligo
    MIT/Caltech Advanced LIGO

    In addition to accretion disks, black holes also have winds and incredibly bright jets erupting from them along their rotation axis, shooting out matter and radiation at nearly the speed of light. Scientists are still working to understand how these jets form.

    What we don’t know

    Scientists have learned that black holes are not as black as they once thought them to be. Some information might escape them. In 1974, Stephen Hawking published results that showed that black holes should radiate energy, or Hawking radiation.

    Matter-antimatter pairs are constantly being produced throughout the universe, even outside the event horizon of a black hole. Quantum theory predicts that one particle might be dragged in before the pair has a chance to annihilate, and the other might escape in the form of Hawking radiation. This contradicts the picture general relativity paints of a black hole from which nothing can escape.

    But as a black hole radiates Hawking radiation, it slowly evaporates until it eventually vanishes. So what happens to all the information encoded on its horizon? Does it disappear, which would violate quantum mechanics? Or is it preserved, as quantum mechanics would predict? One theory is that the Hawking radiation contains all of that information. When the black hole evaporates and disappears, it has already preserved the information of everything that fell into it, radiating it out into the universe.

    Black holes give scientists an opportunity to test general relativity in very extreme gravitational fields. They see black holes as an opportunity to answer one of the biggest questions in particle physics theory: Why can’t we square quantum mechanics with general relativity?

    Beyond the event horizon, black holes curve into one of the darkest mysteries in physics. Scientists can’t explain what happens when objects cross the event horizon and spiral towards the singularity. General relativity and quantum mechanics collide and Einstein’s equations explode into infinities. Black holes might even house gateways to other universes called wormholes and violent fountains of energy and matter called white holes, though it seems very unlikely that nature would allow these structures to exist.

    Sometimes reality is stranger than fiction.

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 10:57 am on January 7, 2016 Permalink | Reply
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    From Space.com: “Visible Light from a Black Hole Spotted by Telescope, a First” 

    space-dot-com logo

    SPACE.com

    January 06, 2016
    Charles Q. Choi

    For the first time, astronomers have seen dim flickers of visible light from near a black hole, researchers with an international science team said. In fact, the light could be visible to anyone with a moderate-size telescope.

    These dramatically variable fluctuations of light are yielding insights onto the complex ways in which matter can swirl into black holes, scientists added. The researchers also released a video of the black hole’s light seen by a telescope. In a statement, they added that such light from an active black hole could be spotted by an observer with a 20-cm telescope.

    Temp 1
    This image still from a video by scientists studying the black hole V404 Cygni located about 7,800 light-years from Earth shows visible light that could be viewable by stargazers with a medium-size telescope. Credit: Michael Richmond/Rochester Institute Of Technology

    Anything falling into black holes cannot escape, not even light, earning black holes their name. However, as disks of gas and dust fall or accrete onto black holes — say, as black holes rip apart nearby stars — friction within these accretion disks can superheat them to 18 million degrees Fahrenheit (10 million degrees Celsius) or more, making them glow extraordinarily brightly.

    Scientists discovered accreting black holes in the Milky Way more than 40 years ago. Previous research suggested that the accretion disks of black holes can have dramatic effects on galaxies. For instance, streams of plasma known as relativistic jets that spew out from accreting black holes at near the speed of light can travel across an entire galaxy, potentially shaping its evolution. However, much remains unknown about how accretion works, since matter can behave in very complex ways as it spirals into black holes, said study lead author Mariko Kimura, an astronomer at Kyoto University in Japan, and her colleagues.

    To learn more about the mysterious process of accretion, researchers in the new study analyzed V404 Cygni, a binary system composed of a black hole about nine times the mass of the sun and a companion star slightly less massive than the sun. Located about 7,800 light-years away from Earth in the constellation Cygnus, the swan, V404 Cygni possesses one of the black holes closest to Earth.

    After 26 years during which the system was dormant, astronomers detected an outburst of X-rays from V404 Cygni in 2015 that lasted for about two weeks. This activity from the accretion disk of V404 Cygni’s black hole briefly made it one of the brightest sources of X-rays seen in the universe.

    Following this outburst, the researchers detected flickering visible light from V404 Cygni, whose fluctuations varied over timescales of 100 seconds to 150 minutes. Normally, astronomers monitor black holes by looking for X-rays or gamma-rays.

    “We find that activity in the vicinity of a black hole can be observed in optical light at low luminosity for the first time,” Kimura told Space.com. “These findings suggest that we can study physical phenomena that occur in the vicinity of the black hole using moderate optical telescopes without high-spec X-ray or gamma-ray telescopes.”

    Similar variable flickering was seen in the X-ray emissions from another black hole system, GRS 1915+105, located about 35,900 light-years away from Earth in the constellation Aquila, the eagle. GRS 1915+105 experiences high levels of accretion. As such, researchers previously suggested the system’s variable flickering was due to instabilities that can occur in accretion disks when they get very massive.

    However, the accretion rates at V404 Cygni are at least 10 times lower than those seen at other black hole systems that have similar oscillations. This suggests that high accretion rates are not the main factor behind this variable flickering, the researchers said.

    Instead, the scientists noted that in both V404 Cygni and GRS 1915+105, the black holes and their companion stars are relatively far apart, which permits a large accretion disk to form. In such large disks, matter from the outer disk might not flow in a steady manner to the inner disk near the black hole, the researchers said. As such, the researchers suggest that accretion onto these black holes can become unstable and fluctuate wildly. This sporadic activity, they said, could then explain the oscillating patterns of light from these black holes.

    The scientists said they hope that worldwide coordination will permit future research to better understand the nature of these extreme events.

    “Thanks to international cooperation, we could get extensive optical observational data in our research with 35 telescopes at 26 locations,” Kimura said. “We would like more people to join in optical observations of black-hole binaries.”

    Kimura and her colleagues detailed their findings in the Jan. 7 issue of the journal Nature.

    See the full article here . In the Nature article you can find the science team.

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  • richardmitnick 2:44 pm on January 6, 2016 Permalink | Reply
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    From Discovery: “Black Holes Set the Clock for Life on Earth” 

    Discovery News
    Discovery News

    Jan 6, 2016
    Larry O’Hanlon

    Temp 1
    ESO/M. Kornmesser

    There is a chance — just a chance — that if black holes rule the universe, they could have “switched on” habitable planets, such as Earth, allowing them to support complex life.

    It’s an unavoidable implication of the work of astrophysicist Paul Mason, who is examining the role of the super high-energy particles from black holes and exploding stars in the advent of habitable planets.

    Before life started on Earth, the planet was bathed in deadly radiation from the younger, angrier sun as well as a high tide of energetic particles — a.k.a. cosmic rays — being blasted around the galaxy and universe by exploding stars and giant black holes at the centers of galaxies. At some point the cosmic ray flux dropped enough so that life on Earth — and on any Earth-like planet anywhere in the universe — had a chance to flourish.

    “It has taken the universe a while for the cosmic ray density and the frequency of bad events to decrease enough for life to handle it,” Mason told Discovery News. Mason is a professor at New Mexico State University in Las Cruces and presented his work on Wednesday at the meeting of the American Astronomical Society in Kissimmee, Fla.

    Those bad events include supernovas — the explosive deaths of very large and short-lived stars — which were much more common in the early universe, when the rates of stars births was far higher, said Mason. Other very bad events were the storms of radiation that might have blown from the gigantic central black holes of galaxies when they gulped down matter. Such feeding frenzies — and the harsh, sterilizing radiation they released — were also more common in the past, as astronomers have learned by looking at more distant, and therefore more ancient, galaxies.

    Compounding the early universe’s problem with life is the fact that everything was much closer together. The small young universe was packed thick with sterilizing cosmic rays. It took billions of years for the expanding universe to pull things apart and help thin that deadly soup.

    It implies that the expansion of the universe is important for life,” Mason said, regarding this cosmic ray perspective on the universe.

    See the full article here .

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  • richardmitnick 7:00 pm on December 23, 2015 Permalink | Reply
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    From AAAS: “Physicists figure out how to retrieve information from a black hole” 

    AAAS

    AAAS

    23 December 2015
    Adrian Cho

    Temp 1
    It would take technologies beyond our wildest dreams to extract the tiniest amount of quantum information from a black hole like this one. NASA; M. Weiss/Chandra X-Ray Center

    Black holes earn their name because their gravity is so strong not even light can escape from them. Oddly, though, physicists have come up with a bit of theoretical sleight of hand to retrieve a speck of information that’s been dropped into a black hole. The calculation touches on one of the biggest mysteries in physics: how all of the information trapped in a black hole leaks out as the black hole “evaporates.” Many theorists think that must happen, but they don’t know how.

    Unfortunately for them, the new scheme may do more to underscore the difficulty of the larger “black hole information problem” than to solve it. “Maybe others will be able to go further with this, but it’s not obvious to me that it will help,” says Don Page, a theorist at the University of Alberta in Edmonton, Canada, who was not involved in the work.

    You can shred your tax returns, but you shouldn’t be able to destroy information by tossing it into a black hole. That’s because, even though quantum mechanics deals in probabilities—such as the likelihood of an electron being in one location or another—the quantum waves that give those probabilities must still evolve predictably, so that if you know a wave’s shape at one moment you can predict it exactly at any future time. Without such “unitarity” quantum theory would produce nonsensical results such as probabilities that don’t add up to 100%.

    But suppose you toss some quantum particles into a black hole. At first blush, the particles and the information they encode is lost. That’s a problem, as now part of the quantum state describing the combined black hole-particles system has been obliterated, making it impossible to predict its exact evolution and violating unitarity.

    Physicists think they have a way out. In 1974, British theorist Stephen Hawking argued that black holes can radiate particles and energy. Thanks to quantum uncertainty, empty space roils with pairs of particles flitting in and out of existence. Hawking realized that if a pair of particles from the vacuum popped into existence straddling the black hole’s boundary then one particle could fly into space, while the other would fall into the black hole. Carrying away energy from the black hole, the exiting Hawking radiation should cause a black hole to slowly evaporate. Some theorists suspect information reemerges from the black hole encoded in the radiation—although how remains unclear as the radiation is supposedly random.

    Now, Aidan Chatwin-Davies, Adam Jermyn, and Sean Carroll of the California Institute of Technology in Pasadena have found an explicit way to retrieve information from one quantum particle lost in a black hole, using Hawking radiation and the weird concept of quantum teleportation.

    Quantum teleportation enables two partners, Alice and Bob, to transfer the delicate quantum state of one particle such as an electron to another. In quantum theory, an electron can spin one way (up), the other way (down), or literally both ways at once. In fact, its state can be described by a point on a globe in which north pole signifies up and the south pole signifies down. Lines of latitude denote different mixtures of up and down, and lines of longitude denote the “phase,” or how the up and down parts mesh. However, if Alice tries to measure that state, it will “collapse” one way or the other, up or down, squashing information such as the phase. So she can’t measure the state and send the information to Bob, but must transfer it intact.

    To do that Alice and Bob can share an additional pair of electrons connected by a special quantum link called entanglement. The state of either particle in the entangled pair is uncertain—it simultaneously points everywhere on the globe—but the states are correlated so that if Alice measures her particle from the pair and finds it spinning, say, up, she’ll know instantly that Bob’s electron is spinning down. So Alice has two electrons—the one whose state she wants to teleport and her half of the entangled pair. Bob has just the one from the entangled pair.

    To perform the teleportation, Alice takes advantage of one more strange property of quantum mechanics: that measurement not only reveals something about a system, it also changes its state. So Alice takes her two unentangled electrons and performs a measurement that “projects” them into an entangled state. That measurement breaks the entanglement between the pair of electrons that she and Bob share. But at the same time, it forces Bob’s electron into the state that her to-be-teleported electron was in. It’s as if, with the right measurement, Alice squeezes the quantum information from one side of the system to the other.

    Chatwin-Davies and colleagues realized that they could teleport the information about the state of an electron out of a black hole, too. Suppose that Alice is floating outside the black hole with her electron. She captures one photon from a pair born from Hawking radiation. Much like an electron, the photon can spin in either of two directions, and it will be entangled with its partner photon that has fallen into the black hole. Next, Alice measures the total angular momentum, or spin, of the black hole—both its magnitude and, roughly speaking, how much it lines up with a particular axis. With those two bits of information in hand, she then tosses in her electron, losing it forever.

    But Alice can still recover the information about the state of that electron, the team reports in a paper in press at Physical Review Letters. All she has to do is once again measure the spin and orientation of the black hole. Those measurements then entangle the black hole and the in-falling photon. They also teleport the state of the electron to the photon that Alice captured. Thus, the information from the lost electron is dragged back into the observable universe.

    Chatwin-Davies stresses that the scheme is not a plan for a practical experiment. After all, it would require Alice to almost instantly measure the spin of a black hole as massive as the sun to within a single atom’s spin. “We like to joke around that Alice is the most advanced scientist in the universe,” he says.

    The scheme also has major limitations. In particular, as the authors note, it works for one quantum particle, but not for two or more. That’s because the recipe exploits the fact that the black hole conserves angular momentum, so that its final spin is equal to its initial spin plus that of the electron. That trick enables Alice to get out exactly two bits of information—the total spin and its projection along one axis—and that’s just enough information to specify the latitude and longitude of quantum state of one particle. But it’s not nearly enough to recapture all the information trapped in a black hole, which typically forms when a star collapses upon itself.

    To really tackle the black hole information problem, theorists would also have to account for the complex states of the black hole’s interior, says Stefan Leichenauer, a theorist at the University of California, Berkeley. “Unfortunately, all of the big questions we have about black holes are precisely about these internal workings,” he says. “So, this protocol, though interesting in its own right, will probably not teach us much about the black hole information problem in general.”

    However, delving into the interior of black holes would require a quantum mechanical theory of gravity. Of course, developing such a theory is perhaps the grandest goal in all of theoretical physics, one that has eluded physicists for decades.

    See the full article here .

    The American Association for the Advancement of Science is an international non-profit organization dedicated to advancing science for the benefit of all people.

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  • richardmitnick 4:28 pm on December 12, 2015 Permalink | Reply
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    From Ethan Siegel: “How Do Black Holes Really Evaporate?” 

    Starts with a bang
    Starts with a Bang

    12.12.15
    Ethan Siegel

    1
    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.

    2
    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!

    3
    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.

    4

    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

    5
    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.

    6
    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.

    7
    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.

    8
    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.

    9
    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 10:19 am on December 10, 2015 Permalink | Reply
    Tags: , , Black Holes,   

    From New Scientist: “Black holes have a size limit of 50 billion suns” 

    NewScientist

    New Scientist

    10 December 2015
    Joshua Sokol

    1
    Image credit: Science Picture Co/Science Photo Library

    Even gluttons can’t eat forever. When black holes at the hearts of galaxies swell to 50 billion times the mass of our sun, they may lose the discs of gas they use as cosmic feedlots.

    Most galaxies host a supermassive black hole at their centre. Around this is a region of space where gas settles into an orbiting disc. The gas can lose energy and fall inwards, feeding the black hole. But these discs are known to be unstable and prone to crumbling into stars.

    Theoretically, a black hole could grow so big that it swallows up the stable part of the disc and destroys it. However, most people thought that black holes would not actually achieve that. “It didn’t occur to us to worry about it, because the mass required was so large,” says Andrew King of the University of Leicester, UK.

    But there were observational hints that such a limit should exist. In 2008, an independent group led by Priya Natarajan of Yale University and Ezequiel Treister of the University of Concepcion in Chile considered how much black holes feasted in the early universe and the free gas available for them to swallow in recent times.

    Given how much black holes have eaten since the dawn of the universe, they argued, the greediest ones could have grown to a size of about 50 billion solar masses.

    Mega find

    It was the discovery of mega black holes within the last few years that prompted King to return to the subject. The heaviest black holes we’ve now seen have a mass of up to 40 billion times that of our sun, which led King to calculate how big a black hole would have to be for its outer edge to keep a disc from forming. He also came up with a figure of 50 billion solar masses, firming up the previous findings.

    Without a disc, the black hole would stop growing, making this the upper limit. The only way it could grow larger would be if a star fell straight in or another black hole merged with it. But neither process would fatten it up as efficiently as a gas disc. “Unless you merge with another monster, you’ll make almost no difference to the black hole mass,” King says.

    Although Natarajan came up with a similar limit, she thinks King’s approach might be a bit of an oversimplification. King’s calculations focus on the gas disc’s stability, but Natarajan argues that you can’t ignore the amount of gas around the black hole, either.

    As hot gas spirals into a black hole, it blasts the rest of the disc with X-rays that clear out the environment – meaning a black hole that feeds too quickly can choke on its meal so much that it clears the table by ejecting the gas. The amount of gas available helps to determine when this will happen.

    “You have to take into account the central galaxy environment in which the black hole is embedded,” Natarajan says. “It’s not enough to look only at gravitational stability.”

    Journal reference: arxiv.org/abs/1511.08502

    See the full article here .

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