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  • richardmitnick 4:07 pm on June 20, 2017 Permalink | Reply
    Tags: Black Holes, Conjectures about gravity, Cosmic censorship conjecture, , Naked singularity in a four-dimensional universe, , , , Singularities, , Then Stephen said ‘You want to bet?’, Weak gravity   

    From Quanta: “Where Gravity Is Weak and Naked Singularities Are Verboten’ 

    Quanta Magazine
    Quanta Magazine

    June 20, 2017
    Natalie Wolchover

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    Mike Zeng for Quanta Magazine

    Physicists have wondered for decades whether infinitely dense points known as singularities can ever exist outside black holes, which would expose the mysteries of quantum gravity for all to see. Singularities — snags in the otherwise smooth fabric of space and time where Albert Einstein’s classical gravity theory breaks down and the unknown quantum theory of gravity is needed — seem to always come cloaked in darkness, hiding from view behind the event horizons of black holes. The British physicist and mathematician Sir Roger Penrose conjectured in 1969 that visible or “naked” singularities are actually forbidden from forming in nature, in a kind of cosmic censorship. But why should quantum gravity censor itself?

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    Roger Penrose in Berkeley, California, in 1978, nine years after proposing the cosmic censorship conjecture. George M. Bergman, Berkeley. Source: Archives of the Mathematisches Forschungsinstitut Oberwolfach

    Now, new theoretical calculations provide a possible explanation for why naked singularities do not exist — in a particular model universe, at least. The findings indicate that a second, newer conjecture about gravity, if it is true, reinforces Penrose’s cosmic censorship conjecture by preventing naked singularities from forming in this model universe. Some experts say the mutually supportive relationship between the two conjectures increases the chances that both are correct. And while this would mean singularities do stay frustratingly hidden, it would also reveal an important feature of the quantum gravity theory that eludes us.

    “It’s pleasing that there’s a connection” between the two conjectures, said John Preskill of the California Institute of Technology, who in 1991 bet Stephen Hawking that the cosmic censorship conjecture would fail (though he actually thinks it’s probably true).

    The new work, reported in May in Physical Review Letters by Jorge Santos and his student Toby Crisford at the University of Cambridge and relying on a key insight by Cumrun Vafa of Harvard University, unexpectedly ties cosmic censorship to the 2006 weak gravity conjecture [JHEP], which asserts that gravity must always be the weakest force in any viable universe, as it is in ours. (Gravity is by far the weakest of the four fundamental forces; two electrons electrically repel each other 1 million trillion trillion trillion times more strongly than they gravitationally attract each other.) Santos and Crisford were able to simulate the formation of a naked singularity in a four-dimensional universe with a different space-time geometry than ours. But they found that if another force exists in that universe that affects particles more strongly than gravity, the singularity becomes cloaked in a black hole. In other words, where a perverse pinprick would otherwise form in the space-time fabric, naked for all the world to see, the relative weakness of gravity prevents it.

    Santos and Crisford are running simulations now to test whether cosmic censorship is saved at exactly the limit where gravity becomes the weakest force in the model universe, as initial calculations suggest. Such an alliance with the better-established cosmic censorship conjecture would reflect very well on the weak gravity conjecture. And if weak gravity is right, it points to a deep relationship between gravity and the other quantum forces, potentially lending support to string theory over a rival theory called loop quantum gravity. The “unification” of the forces happens naturally in string theory, where gravity is one vibrational mode of strings and forces like electromagnetism are other modes. But unification is less obvious in loop quantum gravity, where space-time is quantized in tiny volumetric packets that bear no direct connection to the other particles and forces. “If the weak gravity conjecture is right, loop quantum gravity is definitely wrong,” said Nima Arkani-Hamed, a professor at the Institute for Advanced Study who co-discovered the weak gravity conjecture.

    The new work “does tell us about quantum gravity,” said Gary Horowitz, a theoretical physicist at the University of California, Santa Barbara.

    The Naked Singularities

    In 1991, Preskill and Kip Thorne, both theoretical physicists at Caltech, visited Stephen Hawking at Cambridge. Hawking had spent decades exploring the possibilities packed into the Einstein equation, which defines how space-time bends in the presence of matter, giving rise to gravity. Like Penrose and everyone else, he had yet to find a mechanism by which a naked singularity could form in a universe like ours. Always, singularities lay at the centers of black holes — sinkholes in space-time that are so steep that no light can climb out. He told his visitors that he believed in cosmic censorship. Preskill and Thorne, both experts in quantum gravity and black holes (Thorne was one of three physicists who founded the black-hole-detecting LIGO experiment), said they felt it might be possible to detect naked singularities and quantum gravity effects. “There was a long pause,” Preskill recalled. “Then Stephen said, ‘You want to bet?’”

    The bet had to be settled on a technicality and renegotiated in 1997, after the first ambiguous exception cropped up. Matt Choptuik, a physicist at the University of British Columbia who uses numerical simulations to study Einstein’s theory, showed that a naked singularity can form in a four-dimensional universe like ours when you perfectly fine-tune its initial conditions. Nudge the initial data by any amount, and you lose it — a black hole forms around the singularity, censoring the scene. This exceptional case doesn’t disprove cosmic censorship as Penrose meant it, because it doesn’t suggest naked singularities might actually form. Nonetheless, Hawking conceded the original bet and paid his debt per the stipulations, “with clothing to cover the winner’s nakedness.” He embarrassed Preskill by making him wear a T-shirt featuring a nearly-naked lady while giving a talk to 1,000 people at Caltech. The clothing was supposed to be “embroidered with a suitable concessionary message,” but Hawking’s read like a challenge: “Nature Abhors a Naked Singularity.”

    The physicists posted a new bet online, with language to clarify that only non-exceptional counterexamples to cosmic censorship would count. And this time, they agreed, “The clothing is to be embroidered with a suitable, truly concessionary message.”

    The wager still stands 20 years later, but not without coming under threat. In 2010, the physicists Frans Pretorius and Luis Lehner discovered a mechanism [Physical Review Letters]for producing naked singularities in hypothetical universes with five or more dimensions. And in their May paper, Santos and Crisford reported a naked singularity in a classical universe with four space-time dimensions, like our own, but with a radically different geometry. This latest one is “in between the ‘technical’ counterexample of the 1990s and a true counterexample,” Horowitz said. Preskill agrees that it doesn’t settle the bet. But it does change the story.

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    Lucy Reading-Ikkanda/Quanta Magazine

    The Tin Can Universe

    The new discovery began to unfold in 2014, when Horowitz, Santos and Benson Way found that naked singularities could exist in a pretend 4-D universe called “anti-de Sitter” (AdS) space whose space-time geometry is shaped like a tin can. This universe has a boundary — the can’s side — which makes it a convenient testing ground for ideas about quantum gravity: Physicists can treat bendy space-time in the can’s interior like a hologram that projects off of the can’s surface, where there is no gravity. In universes like our own, which is closer to a “de Sitter” (dS) geometry, the only boundary is the infinite future, essentially the end of time. Timeless infinity doesn’t make a very good surface for projecting a hologram of a living, breathing universe.

    Despite their differences, the interiors of both AdS and dS universes obey Einstein’s classical gravity theory — everywhere outside singularities, that is. If cosmic censorship holds in one of the two arenas, some experts say you might expect it to hold up in both.

    Horowitz, Santos and Way were studying what happens when an electric field and a gravitational field coexist in an AdS universe. Their calculations suggested that cranking up the energy of the electric field on the surface of the tin can universe will cause space-time to curve more and more sharply around a corresponding point inside, eventually forming a naked singularity. In their recent paper, Santos and Crisford verified the earlier calculations with numerical simulations.

    But why would naked singularities exist in 5-D and in 4-D when you change the geometry, but never in a flat 4-D universe like ours? “It’s like, what the heck!” Santos said. “It’s so weird you should work on it, right? There has to be something here.”

    Weak Gravity to the Rescue

    In 2015, Horowitz mentioned the evidence for a naked singularity in 4-D AdS space to Cumrun Vafa, a Harvard string theorist and quantum gravity theorist who stopped by Horowitz’s office. Vafa had been working to rule out large swaths of the 10^500 different possible universes that string theory naively allows. He did this by identifying “swamplands”: failed universes that are too logically inconsistent to exist. By understanding patterns of land and swamp, he hoped to get an overall picture of quantum gravity.

    Working with Arkani-Hamed, Luboš Motl and Alberto Nicolis in 2006, Vafa proposed the weak gravity conjecture as a swamplands test. The researchers found that universes only seemed to make sense when particles were affected by gravity less than they were by at least one other force. Dial down the other forces of nature too much, and violations of causality and other problems arise. “Things were going wrong just when you started violating gravity as the weakest force,” Arkani-Hamed said. The weak-gravity requirement drowns huge regions of the quantum gravity landscape in swamplands.

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    Jorge Santos (left) and Toby Crisford of the University of Cambridge have found an unexpected link between two conjectures about gravity.
    Courtesy of Jorge Santos

    Weak gravity and cosmic censorship seem to describe different things, but in chatting with Horowitz that day in 2015, Vafa realized that they might be linked. Horowitz had explained Santos and Crisford’s simulated naked singularity: When the researchers cranked up the strength of the electric field on the boundary of their tin-can universe, they assumed that the interior was classical — perfectly smooth, with no particles quantum mechanically fluctuating in and out of existence. But Vafa reasoned that, if such particles existed, and if, in accordance with the weak gravity conjecture, they were more strongly coupled to the electric field than to gravity, then cranking up the electric field on the AdS boundary would cause sufficient numbers of particles to arise in the corresponding region in the interior to gravitationally collapse the region into a black hole, preventing the naked singularity.

    Subsequent calculations by Santos and Crisford supported Vafa’s hunch; the simulations they’re running now could verify that naked singularities become cloaked in black holes right at the point where gravity becomes the weakest force. “We don’t know exactly why, but it seems to be true,” Vafa said. “These two reinforce each other.”

    Quantum Gravity

    The full implications of the new work, and of the two conjectures, will take time to sink in. Cosmic censorship imposes an odd disconnect between quantum gravity at the centers of black holes and classical gravity throughout the rest of the universe. Weak gravity appears to bridge the gap, linking quantum gravity to the other quantum forces that govern particles in the universe, and possibly favoring a stringy approach over a loopy one. Preskill said, “I think it’s something you would put on your list of arguments or reasons for believing in unification of the forces.”

    However, Lee Smolin of the Perimeter Institute, one of the developers of loop quantum gravity, has pushed back, arguing that if weak gravity is true, there might be a loopy reason for it. And he contends that there is a path to unification [J.Phys.A] of the forces within his theory — a path that would need to be pursued all the more vigorously if the weak gravity conjecture holds.

    Given the apparent absence of naked singularities in our universe, physicists will take hints about quantum gravity wherever they can find them. They’re as lost now in the endless landscape of possible quantum gravity theories as they were in the 1990s, with no prospects for determining through experiments which underlying theory describes our world. “It is thus paramount to find generic properties that such quantum gravity theories must have in order to be viable,” Santos said, echoing the swamplands philosophy.

    Weak gravity might be one such property — a necessary condition for quantum gravity’s consistency that spills out and affects the world beyond black holes. These may be some of the only clues available to help researchers feel their way into the darkness.

    See the full article here .

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    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

     
  • richardmitnick 9:56 am on June 8, 2017 Permalink | Reply
    Tags: , , , Black Holes, , , , , Nautlius, , , Sean Carroll at Caltech, , Will Quantum Mechanics Swallow Relativity   

    From Nautilus: “Will Quantum Mechanics Swallow Relativity?” 

    Nautilus

    Nautilus

    June 8, 2017
    By Corey S. Powell
    Illustration by Nicholas Garber

    The contest between gravity and quantum physics takes a new turn.

    It is the biggest of problems, it is the smallest of problems.

    At present physicists have two separate rulebooks explaining how nature works. There is general relativity, which beautifully accounts for gravity and all of the things it dominates: orbiting planets, colliding galaxies, the dynamics of the expanding universe as a whole. That’s big. Then there is quantum mechanics, which handles the other three forces—electromagnetism and the two nuclear forces. Quantum theory is extremely adept at describing what happens when a uranium atom decays, or when individual particles of light hit a solar cell. That’s small.

    Now for the problem: Relativity and quantum mechanics are fundamentally different theories that have different formulations. It is not just a matter of scientific terminology; it is a clash of genuinely incompatible descriptions of reality.

    The conflict between the two halves of physics has been brewing for more than a century—sparked by a pair of 1905 papers by Einstein, one outlining relativity and the other introducing the quantum—but recently it has entered an intriguing, unpredictable new phase. Two notable physicists have staked out extreme positions in their camps, conducting experiments that could finally settle which approach is paramount.

    Basically you can think of the division between the relativity and quantum systems as “smooth” versus “chunky.” In general relativity, events are continuous and deterministic, meaning that every cause matches up to a specific, local effect. In quantum mechanics, events produced by the interaction of subatomic particles happen in jumps (yes, quantum leaps), with probabilistic rather than definite outcomes. Quantum rules allow connections forbidden by classical physics. This was demonstrated in a much-discussed recent experiment, in which Dutch researchers defied the local effect. They showed two particles—in this case, electrons—could influence each other instantly, even though they were a mile apart. When you try to interpret smooth relativistic laws in a chunky quantum style, or vice versa, things go dreadfully wrong.

    Relativity gives nonsensical answers when you try to scale it down to quantum size, eventually descending to infinite values in its description of gravity. Likewise, quantum mechanics runs into serious trouble when you blow it up to cosmic dimensions. Quantum fields carry a certain amount of energy, even in seemingly empty space, and the amount of energy gets bigger as the fields get bigger. According to Einstein, energy and mass are equivalent (that’s the message of e=mc2), so piling up energy is exactly like piling up mass. Go big enough, and the amount of energy in the quantum fields becomes so great that it creates a black hole that causes the universe to fold in on itself. Oops.

    Craig Hogan, a theoretical astrophysicist at the University of Chicago and the director of the Center for Particle Astrophysics at Fermilab, is reinterpreting the quantum side with a novel theory in which the quantum units of space itself might be large enough to be studied directly. Meanwhile, Lee Smolin, a founding member of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, is seeking to push physics forward by returning back to Einstein’s philosophical roots and extending them in an exciting direction.

    To understand what is at stake, look back at the precedents. When Einstein unveiled general relativity, he not only superseded Isaac Newton’s theory of gravity; he also unleashed a new way of looking at physics that led to the modern conception of the Big Bang and black holes, not to mention atomic bombs and the time adjustments essential to your phone’s GPS. Likewise, quantum mechanics did much more than reformulate James Clerk Maxwell’s textbook equations of electricity, magnetism, and light. It provided the conceptual tools for the Large Hadron Collider, solar cells, all of modern microelectronics.

    What emerges from the dustup could be nothing less than a third revolution in modern physics, with staggering implications. It could tell us where the laws of nature came from, and whether the cosmos is built on uncertainty or whether it is fundamentally deterministic, with every event linked definitively to a cause.

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    THE MAN WITH THE HOLOMETER: Craig Hogan, a theoretical astrophysicist at Fermilab, has built a device to measure what he sees as the exceedingly fine graininess of space. “I’m hoping for an experimental result that forces people to focus the theoretical thinking in a different direction,” Hogan says.The Department of Astronomy and Astrophysics, the University of Chicago

    A Chunky Cosmos

    Hogan, champion of the quantum view, is what you might call a lamp-post physicist: Rather than groping about in the dark, he prefers to focus his efforts where the light is bright, because that’s where you are most likely to be able to see something interesting. That’s the guiding principle behind his current research. The clash between relativity and quantum mechanics happens when you try to analyze what gravity is doing over extremely short distances, he notes, so he has decided to get a really good look at what is happening right there. “I’m betting there’s an experiment we can do that might be able to see something about what’s going on, about that interface that we still don’t understand,” he says.

    A basic assumption in Einstein’s physics—an assumption going all the way back to Aristotle, really—is that space is continuous and infinitely divisible, so that any distance could be chopped up into even smaller distances. But Hogan questions whether that is really true. Just as a pixel is the smallest unit of an image on your screen and a photon is the smallest unit of light, he argues, so there might be an unbreakable smallest unit of distance: a quantum of space.

    In Hogan’s scenario, it would be meaningless to ask how gravity behaves at distances smaller than a single chunk of space. There would be no way for gravity to function at the smallest scales because no such scale would exist. Or put another way, general relativity would be forced to make peace with quantum physics, because the space in which physicists measure the effects of relativity would itself be divided into unbreakable quantum units. The theater of reality in which gravity acts would take place on a quantum stage.

    Hogan acknowledges that his concept sounds a bit odd, even to a lot of his colleagues on the quantum side of things. Since the late 1960s, a group of physicists and mathematicians have been developing a framework called string theory to help reconcile general relativity with quantum mechanics; over the years, it has evolved into the default mainstream theory, even as it has failed to deliver on much of its early promise. Like the chunky-space solution, string theory assumes a fundamental structure to space, but from there the two diverge. String theory posits that every object in the universe consists of vibrating strings of energy. Like chunky space, string theory averts gravitational catastrophe by introducing a finite, smallest scale to the universe, although the unit strings are drastically smaller even than the spatial structures Hogan is trying to find.

    Chunky space does not neatly align with the ideas in string theory—or in any other proposed physics model, for that matter. “It’s a new idea. It’s not in the textbooks; it’s not a prediction of any standard theory,” Hogan says, sounding not the least bit concerned. “But there isn’t any standard theory right?”

    If he is right about the chunkiness of space, that would knock out a lot of the current formulations of string theory and inspire a fresh approach to reformulating general relativity in quantum terms. It would suggest new ways to understand the inherent nature of space and time. And weirdest of all, perhaps, it would bolster an au courant notion that our seemingly three-dimensional reality is composed of more basic, two-dimensional units. Hogan takes the “pixel” metaphor seriously: Just as a TV picture can create the impression of depth from a bunch of flat pixels, he suggests, so space itself might emerge from a collection of elements that act as if they inhabit only two dimensions.

    Like many ideas from the far edge of today’s theoretical physics, Hogan’s speculations can sound suspiciously like late-night philosophizing in the freshman dorm. What makes them drastically different is that he plans to put them to a hard experimental test. As in, right now.

    Starting in 2007, Hogan began thinking about how to build a device that could measure the exceedingly fine graininess of space. As it turns out, his colleagues had plenty of ideas about how to do that, drawing on technology developed to search for gravitational waves. Within two years Hogan had put together a proposal and was working with collaborators at Fermilab, the University of Chicago, and other institutions to build a chunk-detecting machine, which he more elegantly calls a “holometer.” (The name is an esoteric pun, referencing both a 17th-century surveying instrument and the theory that 2-D space could appear three-dimensional, analogous to a hologram.)

    Beneath its layers of conceptual complexity, the holometer is technologically little more than a laser beam, a half-reflective mirror to split the laser into two perpendicular beams, and two other mirrors to bounce those beams back along a pair of 40-meter-long tunnels. The beams are calibrated to register the precise locations of the mirrors. If space is chunky, the locations of the mirrors would constantly wander about (strictly speaking, space itself is doing the wandering), creating a constant, random variation in their separation. When the two beams are recombined, they’d be slightly out of sync, and the amount of the discrepancy would reveal the scale of the chunks of space.

    For the scale of chunkiness that Hogan hopes to find, he needs to measure distances to an accuracy of 10-18 meters, about 100 million times smaller than a hydrogen atom, and collect data at a rate of about 100 million readings per second. Amazingly, such an experiment is not only possible, but practical. “We were able to do it pretty cheaply because of advances in photonics, a lot of off the shelf parts, fast electronics, and things like that,” Hogan says. “It’s a pretty speculative experiment, so you wouldn’t have done it unless it was cheap.” The holometer is currently humming away, collecting data at the target accuracy; he expects to have preliminary readings by the end of the year.

    Hogan has his share of fierce skeptics, including many within the theoretical physics community. The reason for the disagreement is easy to appreciate: A success for the holometer would mean failure for a lot of the work being done in string theory. Despite this superficial sparring, though, Hogan and most of his theorist colleagues share a deep core conviction: They broadly agree that general relativity will ultimately prove subordinate to quantum mechanics. The other three laws of physics follow quantum rules, so it makes sense that gravity must as well.

    For most of today’s theorists, though, belief in the primacy of quantum mechanics runs deeper still. At a philosophical—epistemological—level, they regard the large-scale reality of classical physics as a kind of illusion, an approximation that emerges from the more “true” aspects of the quantum world operating at an extremely small scale. Chunky space certainly aligns with that worldview.

    Hogan likens his project to the landmark Michelson-Morley experiment of the 19th century, which searched for the aether—the hypothetical substance of space that, according to the leading theory of the time, transmitted light waves through a vacuum. The experiment found nothing; that perplexing null result helped inspire Einstein’s special theory of relativity, which in turn spawned the general theory of relativity and eventually turned the entire world of physics upside down. Adding to the historical connection, the Michelson-Morley experiment also measured the structure of space using mirrors and a split beam of light, following a setup remarkably similar to Hogan’s.

    “We’re doing the holometer in that kind of spirit. If we don’t see something or we do see something, either way it’s interesting. The reason to do the experiment is just to see whether we can find something to guide the theory,” Hogan says. “You find out what your theorist colleagues are made of by how they react to this idea. There’s a world of very mathematical thinking out there. I’m hoping for an experimental result that forces people to focus the theoretical thinking in a different direction.”

    Whether or not he finds his quantum structure of space, Hogan is confident the holometer will help physics address its big-small problem. It will show the right way (or rule out the wrong way) to understand the underlying quantum structure of space and how that affects the relativistic laws of gravity flowing through it.

    _______________________________________________________________________

    The Black Hole Resolution

    Here on Earth, the clash between the top-down and bottom-up views of physics is playing out in academic journals and in a handful of complicated experimental apparatuses. Theorists on both sides concede that neither pure thought nor technologically feasible tests may be enough to break the deadlock, however. Fortunately, there are other places to look for a more definitive resolution. One of the most improbable of these is also one of the most promising—an idea embraced by physicists almost regardless of where they stand ideologically.

    “Black hole physics gives us a clean experimental target to look for,” says Craig Hogan, a theoretical astrophysicist at the University of Chicago and the director of the Center for Particle Astrophysics at Fermilab. “The issues around quantum black holes are important,” agrees Lee Smolin, a founding member of the Perimeter Institute for Theoretical Physics in Waterloo, Canada.

    Black holes? Really? Granted, these objects are more commonly associated with questions than with answers. They are not things you can create in the laboratory, or poke and prod with instruments, or even study up close with a space probe. Nevertheless, they are the only places in the universe where Hogan’s ideas unavoidably smash into Smolin’s and, more importantly, where the whole of quantum physics collides with general relativity in a way that is impossible to ignore.

    At the outer boundary of the black hole—the event horizon—gravity is so extreme that even light cannot escape, making it an extreme test of how general relativity behaves. At the event horizon, atomic-scale events become enormously stretched out and slowed down; the horizon also divides the physical world into two distinct zones, inside and outside. And there is a very interesting meeting place in terms of the size of a black hole. A stellar-mass black hole is about the size of Los Angeles; a black hole with the mass of the Earth would be roughly the size of a marble. Black holes literally bring the big-small problem in physics home to the human scale.

    The importance of black holes for resolving that problem is the reason why Stephen Hawking and his cohorts debate about them so often and so vigorously. It turns out that we don’t actually need to cozy up close to black holes in order to run experiments with them. Quantum theory implies that a single particle could potentially exist both inside and outside the event horizon, which makes no sense. There is also the question of what happens to information about things that fall into a black hole; the information seems to vanish, even though theory says that information cannot be destroyed. Addressing these contradictions is forcing theoretical physicists to grapple more vigorously than ever before with the interplay of quantum mechanics and general relativity.

    Best of all, the answers will not be confined to the world of theory. Astrophysicists have increasingly sophisticated ways to study the region just outside the event horizon by monitoring the hot, brilliant clouds of particles that swirl around some black holes. An even greater breakthrough is just around the corner: the Event Horizon Telescope. This project is in the process of linking together about a dozen radio dishes from around the world, creating an enormous networked telescope so powerful that it will be able to get a clear look at Sagittarius A*, the massive black hole that resides in the center of our galaxy. Soon, possibly by 2020, the Event Horizon Telescope should deliver its first good portraits. What they show will help constrain the theories of black holes, and so offer telling clues about how to solve the big-small problem.

    Human researchers using football stadium-size radio telescopes, linked together into a planet-size instrument, to study a star-size black hole, to reconcile the subatomic-and-cosmic-level enigma at the heart of physics … if it works, the scale of the achievement will be truly unprecedented.

    Event Horizon Telescope Array

    Event Horizon Telescope map

    The locations of the radio dishes that will be part of the Event Horizon Telescope array. Image credit: Event Horizon Telescope sites, via University of Arizona at https://www.as.arizona.edu/event-horizon-telescope.

    Arizona Radio Observatory
    Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT)

    ESO/APEX
    Atacama Pathfinder EXperiment (APEX)

    CARMA Array no longer in service
    Combined Array for Research in Millimeter-wave Astronomy (CARMA)

    Atacama Submillimeter Telescope Experiment (ASTE)
    Atacama Submillimeter Telescope Experiment (ASTE)

    Caltech Submillimeter Observatory
    Caltech Submillimeter Observatory (CSO)

    IRAM NOEMA interferometer
    Institut de Radioastronomie Millimetrique (IRAM) 30m

    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA
    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA

    Large Millimeter Telescope Alfonso Serrano
    Large Millimeter Telescope Alfonso Serrano

    CfA Submillimeter Array Hawaii SAO
    Submillimeter Array Hawaii SAO

    ESO/NRAO/NAOJ ALMA Array
    ESO/NRAO/NAOJ ALMA Array, Chile

    Future Array/Telescopes

    Plateau de Bure interferometer
    Plateau de Bure interferometer

    South Pole Telescope SPTPOL
    South Pole Telescope SPTPOL

    _______________________________________________________________________

    3
    THE SYNTHESIZER: Black holes are the only place where the whole of quantum physics collides with general relativity in a way that is impossible to ignore. An artist’s impression shows the surroundings of the supermassive black hole at the heart of the active galaxy in the southern constellation of Centaurus. Observations at a European Southern Observatory in Chile have revealed not only the torus of hot dust around the black hole but also a wind of cool material in the polar regions. ESO/M. Kornmesser

    A Really, Really Big Show

    If you are looking for a totally different direction, Smolin of the Perimeter Institute is your man. Where Hogan goes gently against the grain, Smolin is a full-on dissenter: “There’s a thing that Richard Feynman told me when I was a graduate student. He said, approximately, ‘If all your colleagues have tried to demonstrate that something’s true and failed, it might be because that thing is not true.’ Well, string theory has been going for 40 or 50 years without definitive progress.”

    And that is just the start of a broader critique. Smolin thinks the small-scale approach to physics is inherently incomplete. Current versions of quantum field theory do a fine job explaining how individual particles or small systems of particles behave, but they fail to take into account what is needed to have a sensible theory of the cosmos as a whole. They don’t explain why reality is like this, and not like something else. In Smolin’s terms, quantum mechanics is merely “a theory of subsystems of the universe.”

    A more fruitful path forward, he suggests, is to consider the universe as a single enormous system, and to build a new kind of theory that can apply to the whole thing. And we already have a theory that provides a framework for that approach: general relativity. Unlike the quantum framework, general relativity allows no place for an outside observer or external clock, because there is no “outside.” Instead, all of reality is described in terms of relationships between objects and between different regions of space. Even something as basic as inertia (the resistance of your car to move until forced to by the engine, and its tendency to keep moving after you take your foot off the accelerator) can be thought of as connected to the gravitational field of every other particle in the universe.

    That last statement is strange enough that it’s worth pausing for a moment to consider it more closely. Consider a thought problem, closely related to the one that originally led Einstein to this idea in 1907. What if the universe were entirely empty except for two astronauts. One of them is spinning, the other is stationary. The spinning one feels dizzy, doing cartwheels in space. But which one of the two is spinning? From either astronaut’s perspective, the other is the one spinning. Without any external reference, Einstein argued, there is no way to say which one is correct, and no reason why one should feel an effect different from what the other experiences.

    The distinction between the two astronauts makes sense only when you reintroduce the rest of the universe. In the classic interpretation of general relativity, then, inertia exists only because you can measure it against the entire cosmic gravitational field. What holds true in that thought problem holds true for every object in the real world: The behavior of each part is inextricably related to that of every other part. If you’ve ever felt like you wanted to be a part of something big, well, this is the right kind of physics for you. It is also, Smolin thinks, a promising way to obtain bigger answers about how nature really works, across all scales.

    “General relativity is not a description of subsystems. It is a description of the whole universe as a closed system,” he says. When physicists are trying to resolve the clash between relativity and quantum mechanics, therefore, it seems like a smart strategy for them to follow Einstein’s lead and go as big as they possibly can.

    Smolin is keenly aware that he is pushing against the prevailing devotion to small-scale, quantum-style thinking. “I don’t mean to stir things up, it just kind of happens that way. My role is to think clearly about these difficult issues, put my conclusions out there, and let the dust settle,” he says genially. “I hope people will engage with the arguments, but I really hope that the arguments lead to testable predictions.”

    At first blush, Smolin’s ideas sound like a formidable starting point for concrete experimentation. Much as all of the parts of the universe are linked across space, they may also be linked across time, he suggests. His arguments led him to hypothesize that the laws of physics evolve over the history of the universe. Over the years, he has developed two detailed proposals for how this might happen. His theory of cosmological natural selection, which he hammered out in the 1990s, envisions black holes as cosmic eggs that hatch new universes. More recently, he has developed a provocative hypothesis about the emergence of the laws of quantum mechanics, called the principle of precedence—and this one seems much more readily put to the test.

    Smolin’s principle of precedence arises as an answer to the question of why physical phenomena are reproducible. If you perform an experiment that has been performed before, you expect the outcome will be the same as in the past. (Strike a match and it bursts into flame; strike another match the same way and … you get the idea.) Reproducibility is such a familiar part of life that we typically don’t even think about it. We simply attribute consistent outcomes to the action of a natural “law” that acts the same way at all times. Smolin hypothesizes that those laws actually may emerge over time, as quantum systems copy the behavior of similar systems in the past.

    One possible way to catch emergence in the act is by running an experiment that has never been done before, so there is no past version (that is, no precedent) for it to copy. Such an experiment might involve the creation of a highly complex quantum system, containing many components that exist in a novel entangled state. If the principle of precedence is correct, the initial response of the system will be essentially random. As the experiment is repeated, however, precedence builds up and the response should become predictable … in theory. “A system by which the universe is building up precedent would be hard to distinguish from the noises of experimental practice,” Smolin concedes, “but it’s not impossible.”

    Although precedence can play out at the atomic scale, its influence would be system-wide, cosmic. It ties back to Smolin’s idea that small-scale, reductionist thinking seems like the wrong way to solve the big puzzles. Getting the two classes of physics theories to work together, though important, is not enough, either. What he wants to know—what we all want to know—is why the universe is the way it is. Why does time move forward and not backward? How did we end up here, with these laws and this universe, not some others?

    The present lack of any meaningful answer to those questions reveals that “there’s something deeply wrong with our understanding of quantum field theory,” Smolin says. Like Hogan, he is less concerned about the outcome of any one experiment than he is with the larger program of seeking fundamental truths. For Smolin, that means being able to tell a complete, coherent story about the universe; it means being able to predict experiments, but also to explain the unique properties that made atoms, planets, rainbows, and people. Here again he draws inspiration from Einstein.

    “The lesson of general relativity, again and again, is the triumph of relationalism,” Smolin says. The most likely way to get the big answers is to engage with the universe as a whole.

    And the Winner Is …

    If you wanted to pick a referee in the big-small debate, you could hardly do better than Sean Carroll, an expert in cosmology, field theory, and gravitational physics at Caltech. He knows his way around relativity, he knows his way around quantum mechanics, and he has a healthy sense of the absurd: He calls his personal blog Preposterous Universe.

    Right off the bat, Carroll awards most of the points to the quantum side. “Most of us in this game believe that quantum mechanics is much more fundamental than general relativity is,” he says. That has been the prevailing view ever since the 1920s, when Einstein tried and repeatedly failed to find flaws in the counterintuitive predictions of quantum theory. The recent Dutch experiment demonstrating an instantaneous quantum connection between two widely separated particles—the kind of event that Einstein derided as “spooky action at a distance”—only underscores the strength of the evidence.

    Taking a larger view, the real issue is not general relativity versus quantum field theory, Carroll explains, but classical dynamics versus quantum dynamics. Relativity, despite its perceived strangeness, is classical in how it regards cause and effect; quantum mechanics most definitely is not. Einstein was optimistic that some deeper discoveries would uncover a classical, deterministic reality hiding beneath quantum mechanics, but no such order has yet been found. The demonstrated reality of spooky action at a distance argues that such order does not exist.

    “If anything, people under-appreciate the extent to which quantum mechanics just completely throws away our notions of space and locality [the notion that a physical event can affect only its immediate surroundings]. Those things simply are not there in quantum mechanics,” Carroll says. They may be large-scale impressions that emerge from very different small-scale phenomena, like Hogan’s argument about 3-D reality emerging from 2-D quantum units of space.

    Despite that seeming endorsement, Carroll regards Hogan’s holometer as a long shot, though he admits it is removed from his area of research. At the other end, he doesn’t think much of Smolin’s efforts to start with space as a fundamental thing; he regards the notion as absurd as trying to argue that air is more fundamental than atoms. As for what kind of quantum system might take physics to the next level, Carroll remains broadly optimistic about string theory, which he says “seems to be a very natural extension of quantum field theory.” In all these ways, he is true to the mainstream, quantum-based thinking in modern physics.

    Yet Carroll’s ruling, while almost entirely pro-quantum, is not purely an endorsement of small-scale thinking. There are still huge gaps in what quantum theory can explain. “Our inability to figure out the correct version of quantum mechanics is embarrassing,” he says. “And our current way of thinking about quantum mechanics is simply a complete failure when you try to think about cosmology or the whole universe. We don’t even know what time is.” Both Hogan and Smolin endorse this sentiment, although they disagree about what to do in response. Carroll favors a bottom-up explanation in which time emerges from small-scale quantum interactions, but declares himself “entirely agnostic” about Smolin’s competing suggestion that time is more universal and fundamental. In the case of time, then, the jury is still out.

    No matter how the theories shake out, the large scale is inescapably important, because it is the world we inhabit and observe. In essence, the universe as a whole is the answer, and the challenge to physicists is to find ways to make it pop out of their equations. Even if Hogan is right, his space-chunks have to average out to the smooth reality we experience every day. Even if Smolin is wrong, there is an entire cosmos out there with unique properties that need to be explained—something that, for now at least, quantum physics alone cannot do.

    By pushing at the bounds of understanding, Hogan and Smolin are helping the field of physics make that connection. They are nudging it not just toward reconciliation between quantum mechanics and general relativity, but between idea and perception. The next great theory of physics will undoubtedly lead to beautiful new mathematics and unimaginable new technologies. But the best thing it can do is create deeper meaning that connects back to us, the observers, who get to define ourselves as the fundamental scale of the universe.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    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 1:40 pm on June 1, 2017 Permalink | Reply
    Tags: , , , Black Holes, , , Pan-STARRS1,   

    From Universe Today: “What Exactly Should We See When a Star Splashes into a Black Hole Event Horizon?” 

    universe-today

    Universe Today

    1 June , 2017
    Evan Gough

    1
    This artist’s impression shows the surroundings of the supermassive black hole at the heart of the active galaxy NGC 3783. Credit: ESO/M. Kornmesser

    At the center of our Milky Way galaxy dwells a behemoth.

    Sag A* NASA Chandra X-Ray Observatory 23 July 2014, the supermassive black hole at the center of the Milky Way

    An object so massive that nothing can escape its gravitational pull, not even light. In fact, we think most galaxies have one of them. They are, of course, supermassive black holes.

    Supermassive black holes are stars that have collapsed into a singularity. Einstein’s General Theory of Relativity predicted their existence. And these black holes are surrounded by what’s known as an event horizon, which is kind of like the point of no return for anything getting too close to the black hole. But nobody has actually proven the existence of the event horizon yet.

    Some theorists think that something else might lie at the center of galaxies, a supermassive object event stranger than a supermassive black hole. Theorists think these objects have somehow avoided a black hole’s fate, and have not collapsed into a singularity. They would have no event horizon, and would have a solid surface instead.

    “Our whole point here is to turn this idea of an event horizon into an experimental science, and find out if event horizons really do exist or not,” – Pawan Kumar Professor of Astrophysics, University of Texas at Austin.

    A team of researchers at the University of Texas at Austin and Harvard University have tackled the problem. Wenbin Lu, Pawan Kumar, and Ramesh Narayan wanted to shed some light onto the event horizon problem.

    They wondered about the solid surface object, and what would happen when an object like a star collided with it. They published their results in the Monthly Notices of the Royal Astronomical Society.

    The trio predicted that in the 3.5 year time-frame captured by the Pan-STAARS survey, 10 of these collisions would occur and should be represented in the data.

    Pan-STARRS1 located on Haleakala, Maui, HI, USA

    The team found none of the flare-ups they expected to see if the hard-surface theory is true.

    2
    Artist’s conception of the event horizon of a black hole. Credit: Victor de Schwanberg/Science Photo Library

    They’re hoping to improve their test with the upcoming Large Synoptic Survey Telescope (LSST) being built in Chile.


    LSST Camera, built at SLAC



    LSST telescope, currently under construction at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

    The LSST is a wide field telescope that will capture images of the night sky every 20 seconds over a ten-year span. Every few nights, the LSST will give us an image of the entire available night sky. This will make the study of transient objects much easier and effective.

    See the full article here .

    Please help promote STEM in your local schools.

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  • richardmitnick 3:04 pm on May 20, 2017 Permalink | Reply
    Tags: , , , Black Holes, , ,   

    From Ethan Siegel: “Ask Ethan: What Happens When A Black Hole’s Singularity Evaporates?” 

    Ethan Siegel
    May 20, 2017

    1
    The event horizon of a black hole is a spherical or spheroidal region from which nothing, not even light, can escape. But outside the event horizon, the black hole is predicted to emit radiation. NASA; Jörn Wilms (Tübingen) et al.; ESA

    It’s hard to imagine, given the full diversity of forms that matter takes in this Universe, that for millions of years, there were only neutral atoms of hydrogen and helium gas. It’s perhaps equally hard to imagine that someday, quadrillions of years from now, all the stars will have gone dark. Only the remnants of our now-vibrant Universe will be left, including some of the most spectacular objects of all: black holes. But even they won’t last forever. David Weber wants to know how that happens for this week’s Ask Ethan, inquiring:

    What happens when a black hole has lost enough energy due to hawking radiation that its energy density no longer supports a singularity with an event horizon? Put another way, what happens when a black hole ceases to be a black hole due to hawking radiation?

    In order to answer this question, it’s important to understand what a black hole actually is.

    2
    The anatomy of a very massive star throughout its life, culminating in a Type II Supernova when the core runs out of nuclear fuel. Nicole Rager Fuller/NSF

    Black holes generally form during the collapse of a massive star’s core, where the spent nuclear fuel ceases to fuse into heavier elements. As fusion slows and ceases, the core experiences a severe drop in radiation pressure, which was the only thing holding the star up against gravitational collapse. While the outer layers often experience a runaway fusion reaction, blowing the progenitor star apart in a supernova, the core first collapses into a single atomic nucleus — a neutron star — but if the mass is too great, the neutrons themselves compress and collapse to such a dense state that a black hole forms. (A black hole can also form if a neutron star accretes enough mass from a companion star, crossing the threshold necessary to become a black hole.)

    3
    When a neutron star accretes enough matter, it can collapse to a black hole. When a black hole accretes matter, it grows an accretion disk and will increase its mass as matter gets funneled into the event horizon. NASA/ESA Hubble Space Telescope collaboration

    NASA/ESA Hubble Telescope

    From a gravitational point of view, all it takes to become a black hole is to gather enough mass in a small enough volume of space that light cannot escape from within a certain region. Every mass, including planet Earth, has an escape velocity: the speed you’d need to achieve to completely escape from the gravitational pull at a given distance (e.g., the distance from Earth’s center to its surface) from its center-of-mass. But if there’s enough mass so that the speed you’d need to achieve at a certain distance from the center of mass is the speed of light or greater, then nothing can escape from it, since nothing can exceed the speed of light.

    4
    Cornell SXS team; Bohn et al 2015

    That distance from the center of mass where the escape velocity equals the speed of light — let’s call it R — defines the size of the black hole’s event horizon. But the fact that there’s matter inside under these conditions has another consequence that’s less-well appreciated: this matter must collapse down to a singularity. You might think there could be a state of matter that’s stable and has a finite volume within the event horizon, but that’s not physically possible.

    In order to exert an outward force, an interior particle would have to send a force-carrying particle away from the center-of-mass and closer to the event horizon. But that force-carrying particle is also limited by the speed of light, and no matter where you are inside the event horizon, all light-like curves wind up at the center. The situation is even worse for slower, massive particles. Once you form a black hole with an event horizon, all the matter inside gets crunched into a singularity.

    5
    http://www.speed-light.info/miracles_of_quran/singularity.htm

    6
    The exterior spacetime to a Schwarzschild black hole, known as Flamm’s Paraboloid, is easily calculable. But inside an event horizons, all geodesics lead to the central singularity. Wikimedia Commons user AllenMcC

    And since nothing can escape, you might think a black hole would remain a black hole forever. If it weren’t for quantum physics, this would be exactly what happens. But in quantum physics, there’s a non-zero amount of energy inherent to space itself: the quantum vacuum. In curved space, the quantum vacuum takes on slightly different properties than in flat space, and there are no regions where the curvature is greater than near the singularity of a black hole. Combining these two laws of nature — quantum physics and the General Relativistic spacetime around a black hole — gives us the phenomenon of Hawking radiation.

    6
    Ethan Siegel

    8
    A visualization of QCD illustrates how particle/antiparticle pairs pop out of the quantum vacuum for very small amounts of time as a consequence of Heisenberg uncertainty. Derek B. Leinweber

    Performing the quantum field theory calculation in curved space yields a surprising solution: that thermal, blackbody radiation is emitted in the space surrounding a black hole’s event horizon. And the smaller the event horizon is, the greater the curvature of space near the event horizon is, and thus the greater the rate of Hawking radiation. If our Sun were a black hole, the temperature of the Hawking radiation would be about 62 nanokelvin; if you took the black hole at the center of our galaxy, 4,000,000 times as massive, the temperature would be about 15 femtokelvin, or just 0.000025% the temperature of the less massive one.

    9
    An X-ray / Infrared composite image of the black hole at the center of our galaxy: Sagittarius A*. It has a mass of about four million Suns, and is found surrounded by hot, X-ray emitting gas. However, it also emits (undetectable) Hawking radiation, at much, much lower temperatures. X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI

    NASA/Chandra Telescope

    NASA Infrared Telescope facility Mauna Kea, Hawaii, USA

    This means the smallest black holes decay the fastest, and the largest ones live the longest. Doing the math, a solar mass black hole would live for about 10^67 years before evaporating, but the black hole at the center of our galaxy would live for 10^20 times as long before decaying. The crazy thing about it all is that right up until the final fraction-of-a-second, the black hole still has an event horizon. Once you form a singularity, you remain a singularity — and you retain an event horizon — right up until the moment your mass goes to zero.

    That final second of a black hole’s life, however, will result in a very specific and very large release of energy. When the mass drops down to 228 metric tonnes, that’s the signal that exactly one second remains. The event horizon size at the time will be 340 yoctometers, or 3.4 × 10^-22 meters: the size of one wavelength of a photon with an energy greater than any particle the LHC has ever produced. But in that final second, a total of 2.05 × 10^22 Joules of energy, the equivalent of five million megatons of TNT, will be released. It’s as though a million nuclear fusion bombs went off all at once in a tiny region of space; that’s the final stage of black hole evaporation.

    10
    As a black hole shrinks in mass and radius, the Hawking radiation emanating from it becomes greater and greater in temperature and power. NASA

    What’s left? Just outgoing radiation. Whereas previously, there was a singularity in space where mass, and possibly charge and angular momentum existed in an infinitesimally small volume, now there is none. Space has been restored to its previously non-singular state, after what must have seemed like an eternity: enough time for the Universe to have done all it’s done to date trillions upon trillions of times over. There will be no other stars or sources of light left when this occurs for the first time in our Universe; there will be no one to witness this spectacular explosion. But there’s no “threshold” where this occurs. Rather, the black hole needs to evaporate completely. When it does, to the best of our knowledge, there will be nothing left behind at all but outgoing radiation.

    11
    Against a seemingly eternal backdrop of everlasting darkness, a single flash of light will emerge: the evaporation of the final black hole in the Universe. ortega-pictures / pixabay

    In other words, if you were to watch the last black hole in our Universe evaporate, you would see an empty void of space, that displayed no light or signs of activity for perhaps 10^100 years or more. All of a sudden, a tremendous outrush of radiation of a very particular spectrum and magnitude would appear, leaving a single point in space at 300,000 km/s. For the last time in our observable Universe, an event would have occurred to bathe the Universe in radiation. The last black hole evaporation of all would, in a poetic way, be the final time that the Universe would ever say, “Let there be light!”

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    “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 1:59 pm on April 29, 2017 Permalink | Reply
    Tags: Ask Ethan: What should a black hole’s event horizon look like?, Black Holes, , , , ,   

    From Ethan Siegel: “Ask Ethan: What should a black hole’s event horizon look like?” 

    Ethan Siegel
    Apr 29, 2017

    1
    An illustration of a black hole. Despite how dark it is, all black holes are thought to have formed from normal matter alone, but illustrations like these are only partially accurate. Image credit: NASA / JPL-Caltech.

    You might think that it should be all black, but then how would we see it?

    “It is conceptually interesting, if not astrophysically very important, to calculate the precise apparent shape of the black hole… Unfortunately, there seems to be no hope of observing this effect.” -Jim Bardeen

    Earlier this month, telescopes from all around the world took data, simultaneously, of the Milky Way’s central black hole.

    Here is the Event Horizon Telescope Array

    Event Horizon Telescope map

    The locations of the radio dishes that will be part of the Event Horizon Telescope array. Image credit: Event Horizon Telescope sites, via University of Arizona at https://www.as.arizona.edu/event-horizon-telescope.

    Arizona Radio Observatory
    Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT)

    ESO/APEX
    Atacama Pathfinder EXperiment (APEX)

    CARMA Array no longer in service
    Combined Array for Research in Millimeter-wave Astronomy (CARMA)

    Atacama Submillimeter Telescope Experiment (ASTE)
    Atacama Submillimeter Telescope Experiment (ASTE)

    Caltech Submillimeter Observatory
    Caltech Submillimeter Observatory (CSO)

    IRAM NOEMA interferometer
    Institut de Radioastronomie Millimetrique (IRAM) 30m

    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA
    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA

    Large Millimeter Telescope Alfonso Serrano
    Large Millimeter Telescope Alfonso Serrano

    CfA Submillimeter Array Hawaii SAO
    Submillimeter Array Hawaii SAO

    ESO/NRAO/NAOJ ALMA Array
    ESO/NRAO/NAOJ ALMA Array, Chile

    Future Array/Telescopes

    Plateau de Bure interferometer
    Plateau de Bure interferometer

    South Pole Telescope SPTPOL
    South Pole Telescope SPTPOL

    Also involved:

    European VLBI

    Of all the black holes that are known in the Universe, the one at our galactic center — Sagittarius A* — is special.

    Sag A* NASA Chandra X-Ray Observatory 23 July 2014, the supermassive black hole at the center of the Milky Way

    From our point of view, its event horizon is the largest of all black holes. It’s so large that telescopes positioned at different locations on Earth should be able to directly image it, if they all viewed it simultaneously. While it will take months to combine and analyze the data from all the different telescopes, we should get our first image of an event horizon by the end of 2017. So what will it looks like? That’s the question of Dan Barrett, who’s seen some illustrations and is a bit puzzled:

    Shouldn’t the event horizon completely surround the black hole like an egg shell? All the artist renderings of a black hole are like slicing a hard boiled egg in half and showing that image. How is it that the event horizon does not completely surround the black hole?

    There are a few different classes of illustrations floating around, to be sure. But which ones, if any, are correct?

    2
    Artwork illustrating a simple black circle, perhaps with a ring around it, is an oversimplified picture of what an event horizon looks like. Image credit: Victor de Schwanberg.

    The oldest type of illustration is simply a circular, black disk, blocking out all the background light from behind it. This makes sense if you think about what a black hole actually is: a collection of mass that’s so great and so compact that the escape velocity from its surface is greater than the speed of light! Since nothing can move that quickly, not even the forces or interactions between the particles inside the black hole, the inside of a black hole collapses to a singularity, and an event horizon is created around the black hole. From this spherical region of space, no light can escape, and so it should appear as a black circle, from any perspective, superimposed on the background of the Universe.

    3
    A black hole isn’t just a mass superimposed over an isolated background, but will exhibit gravitational effects that stretch, magnify and distort background light due to gravitational lensing. Image credit: Ute Kraus, Physics education group Kraus / Axel Mellinger.

    But there’s more to the story than that. Because of their gravity, black holes will magnify and distort any background light, due to the effect of gravitational lensing. This is a more detailed and accurate illustration of what a black hole looks like, as it also possesses an apparent event horizon sized appropriately with the curvature of space in General Relativity.

    Unfortunately, these illustrations are flawed, too: they fail to account for foreground material and for accretion around the black hole. Some illustrations, though, do successfully add these in.

    4
    An illustration of an active black hole, one that accretes matter and accelerates a portion of it outwards in two perpendicular jets, may describe the black hole at the center of our galaxy in many regards. Image credit: Mark A. Garlick.

    Because of their tremendous gravitational effects, black holes will form accretion disks in the presence of other sources of matter. Asteroids, gas clouds, or even entire stars will be torn apart by the tidal forces coming from an object as massive as a black hole. Due to the conservation of angular momentum, and of collisions between the various infalling particles, a disk-like object will emerge around the black hole, which will heat up and emit radiation. In the innermost regions, particles occasionally fall in, adding to the mass of the black hole, while the material in front of the black hole will obscure part of the sphere/circle you’d otherwise see.

    But the event horizon itself isn’t transparent, and you shouldn’t be able to see the matter behind it.

    4
    The black hole, as illustrated in the movie Interstellar, shows an event horizon fairly accurately for a very specific class of rotating black holes. Image credit: Interstellar / R. Hurt / Caltech.

    It might seem surprising that a Hollywood film — Interstellar — has a more accurate illustration of a black hole than many of the professional pieces of artwork created for/by NASA, but misconceptions abound, even among professionals, when it comes to black holes. Black holes don’t suck matter in; they simply gravitate. Black holes don’t tear things apart because of any extra force; it’s simply tidal forces — where one part of the infalling object is closer to the center than another — that does it. And most importantly, black holes rarely exist in a “naked” state, but rather exist in the vicinity of other matter, such as at the center of our galaxy.

    5
    An X-ray / Infrared composite image of the black hole at the center of our galaxy: Sagittarius A*. It has a mass of about four million Suns, and is found surrounded by hot, X-ray emitting gas. Image credit: X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI.

    So with all of that in mind, what are the hard-boiled-egg images that have been going around? Remember, we can’t image the black hole itself, because it doesn’t emit light! All we can do is look at a particular wavelength, and see a combination of the emitting light that comes from around, behind and in front of the black hole itself. The expected signal, indeed, does resemble a split hard-boiled egg.

    6
    Some of the possible profile signals of the black hole’s event horizon as simulations of the Event Horizon Telescope indicate. Image credit: High-Angular-Resolution and High-Sensitivity Science Enabled by Beamformed ALMA, V. Fish et al., arXiv:1309.3519

    ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres

    This has to do with what it is we’re imaging. We can’t look in X-rays, because there are simply too few X-ray photons overall. We can’t look in visible light, because the galactic center is opaque it it. And we can’t look in the infrared, because the atmosphere blocks infrared light. But what we can do is look in the radio, and we can do it all over the world, simulataneously, to get the optimal resolution possible.

    The black hole at the galactic center has an angular size of about 37 micro-arc-seconds, while the resolution of this telescope array is around 15 micro-arc-seconds, so we should be able to see it! At radio frequencies, the overwhelming majority of that radiation comes from charged matter particles being accelerated around the black hole. We don’t know how the disk will be oriented, whether there will be multiple disks, whether it will be more like a swarm of bees or more like a compact disk. We also don’t know whether it will prefer one “side” of the black hole, as viewed from our perspective, over another.

    6
    Five different simulations in general relativity, using a magnetohydrodynamic model of the black hole’s accretion disk, and how the radio signal will look as a result. Image credit: GRMHD simulations of visibility amplitude variability for Event Horizon Telescope images of Sgr A*, L. Medeiros et al., arXiv:1601.06799.

    We fully expect the event horizon to be real, to be of a specific size, and to block all the light coming from behind it. But we also expect that there will be some signal in front of it, that the signal will be messy due to the messy environment around the black hole, and that the orientation of the disk with respect to the black hole will play an important role in determining what we see.

    One side is brighter as the disk rotates towards us; one side is fainter as the disk rotates away. The entire “outline” of the event horizon may be visible as well, thanks to the effect of gravitational lensing. Perhaps most importantly, whether the disk is seen “edge-on” or “face-on” with respect to us will drastically alter the signal, as the 1st and 3rd panels below illustrate.

    7
    The orientation of the accretion disk as either face-on (left two panels) or edge-on (right two panels) can vastly alter how the black hole appears to us. Image credit: ‘Toward the event horizon — the supermassive black hole in the Galactic Center’, Class. Quantum Grav., Falcke & Markoff (2013).

    There are other effects we can test for, including:

    whether the black hole has the right size as predicted by general relativity,
    whether the event horizon is circular (as predicted), or oblate or prolate instead,
    whether the radio emissions extend farther than we thought,

    or whether there are any other deviations from the expected behavior. This is a brand new frontier in physics, and we’re poised to actually test it directly. One thing’s for certain: no matter what it is that the Event Horizon Telescope sees, we’re bound to learn something new and wonderful about some of the most extreme objects and conditions in the Universe!

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    “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 1:11 pm on April 15, 2017 Permalink | Reply
    Tags: , , , Black Holes, , , ,   

    From ESO: “Taking the First Picture of a Black Hole” 

    ESO 50 Large

    European Southern Observatory

    30.3.2017

    1. What are the Event Horizon Telescope and the Global mm-VLBI Array?

    At the centre of our galaxy lurks a cosmic monster: a supermassive black hole called Sagittarius A* with a mass about four million times that of the Sun.

    Sag A* NASA Chandra X-Ray Observatory 23 July 2014, the supermassive black hole at the center of the Milky Way

    Its gravity is so intense that not even light can escape its pull, but if it wasn’t for its strong gravitational influence on the stars and gas around it, we would have no idea that it was there! Now, an ambitious new endeavour is underway to take a never-seen-before image, of the event horizon of the black hole itself.

    Two international collaborations of radio telescopes have linked up to create Earth-sized virtual telescopes: the Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA), working at different wavelengths.

    1
    This infographic details the locations of the participating telescopes of the Event Horizon Telescope and the Global mm-VLBI Array. Credit: ESO/O. Furtak

    Global mm-VLBI Array

    Event Horizon Telescope Array

    Event Horizon Telescope map

    The locations of the radio dishes that will be part of the Event Horizon Telescope array. Image credit: Event Horizon Telescope sites, via University of Arizona at https://www.as.arizona.edu/event-horizon-telescope.

    Arizona Radio Observatory
    Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT)

    ESO/APEX
    Atacama Pathfinder EXperiment (APEX)

    CARMA Array no longer in service
    Combined Array for Research in Millimeter-wave Astronomy (CARMA)

    Atacama Submillimeter Telescope Experiment (ASTE)
    Atacama Submillimeter Telescope Experiment (ASTE)

    Caltech Submillimeter Observatory
    Caltech Submillimeter Observatory (CSO)

    IRAM NOEMA interferometer
    Institut de Radioastronomie Millimetrique (IRAM) 30m

    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA
    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA

    Large Millimeter Telescope Alfonso Serrano
    Large Millimeter Telescope Alfonso Serrano

    CfA Submillimeter Array Hawaii SAO
    Submillimeter Array Hawaii SAO

    Future Array/Telescopes

    ESO/NRAO/NAOJ ALMA Array
    ESO/NRAO/NAOJ ALMA Array, Chile

    Plateau de Bure interferometer
    Plateau de Bure interferometer

    South Pole Telescope SPTPOL
    South Pole Telescope SPTPOL

    The impressive line-up of telescopes, which stretch across the globe from the South Pole to Hawaii to Europe, will work together to target the supermassive black hole at the heart of the Milky Way.

    To do this, astronomers will exploit a technique known as Very-long-baseline Interferometry (VLBI), where telescopes thousands of kilometres apart can link together and act as one.

    European VLBI

    This cooperative technique can achieve a far higher resolution than any single facility could obtain on its own — a resolution 2000 times that of the NASA/ESA Hubble Space Telescope! This super-high resolution is crucial for detecting the black hole, which — despite being about 20 times bigger than the Sun — lies a long way away, over 26 000 light-years from Earth.

    The plan to image a black hole has been in the works for years, but it’s only recently that technology has brought the ambitious endeavour within reach. Plus, a radio telescope heavyweight has just joined the team: the Atacama Large Millimeter/submillimeter Array (ALMA).

    Located high up on the Chajnantor plateau in Chile’s Atacama Desert, ALMA’s 66 antennas and exquisite receivers make it the largest and most sensitive component of the EHT/GMVA collaboration, increasing the overall sensitivity by a factor of 10. Despite being a state-of-the-art facility, ALMA has undergone several upgrades to take part in the collaboration. Specialist equipment has been installed, including new hard drives that are necessary to store the sheer amount of data produced by the observations, as well as an extremely accurate atomic clock, which is critical to link ALMA to the entire VLBI network.

    The first groundbreaking observations will be made in April 2017: observations at 3 millimetre wavelengths will be made with the GMVA from 1–4 April 2017, and with the EHT at 1.3 millimetre wavelengths from 5–14 April 2017. The GMVA will investigate the properties of the accretion and outflow around the Galactic Centre, while the EHT will attempt to image, for the very first time, the shadow of the black hole’s event horizon.

    There is a long, hard road ahead to process the massive amounts of data that will be acquired during the observation periods, and results are expected to become available towards the end of 2017.

    The outcome of these observations is eagerly awaited by the astronomy community worldwide, as their scientific potential is incredibly exciting and the collaboration are pursuing some awesome goals. These could include testing Einstein’s theory of general relativity, which predicts a roughly circular “shadow” around the black hole. Other goals include learning about how material accretes around black holes, as well as the formation of extremely fast jets of gas that blast out from them.

    3
    Simulated images of the shadow of a black hole: General relativity predicts that the shadow should be circular (middle), but a black hole could potentially also have a prolate (left) or oblate (right) shadow. Future EHT images will test this prediction. Credit: D. Psaltis and A. Broderick.

    This is the first post of a blog series that will take you along for the astronomical ride, giving insight into how cutting-edge research is done and what risks are involved.

    In the following posts, we’ll explore questions such as: What makes black holes so interesting? How do radio telescopes see the Universe? And what do we really know about the supermassive monster lurking at the centre of the Milky Way?

    11.4.2017

    2. What is a black hole?

    Right now, astronomers are attempting to take the first image of the event horizon of the supermassive black hole at the centre of the Milky Way — but what exactly are black holes?

    Black holes are some of the most bizarre and fascinating objects in the Universe. Essentially, they’re reality-bending concentrations of matter squeezed into a very tiny space, creating an object with an immense gravitational pull. Around a black hole is a boundary called an event horizon — the surface beyond which nothing can escape the black hole’s clutches, not even light.

    Take a tour of the anatomy of a black hole with our handy infographic:

    4
    Credit: ESO, ESA/Hubble, M. Kornmesser/N. Bartmann

    Since no light can escape from a black hole, we can’t see them directly. But their huge gravitational influence gives away their presence. Black holes are often orbited by stars, gas and other material in tight paths that become more crowded and frantic as they’re dragged closer to the event horizon. This creates a superheated accretion disc around the black hole, which emits vast amounts of radiation of different wavelengths.

    By observing this radiation from the activity around black holes, astronomers have determined that there are two main types: stellar mass and supermassive.

    A stellar mass black hole is the corpse of a star more than about 30 times as massive as our Sun. At the end of its life, such stars violently collapse and don’t stopped collapsing until all of their constituent matter has condensed down into an unimaginably tiny space. It’s easiest to discover stellar mass black holes that are part of an X-ray binary system, where the black hole is guzzling down material from its companion star.

    5
    Artist’s impression of the formation of a stellar black hole in a binary system. Credit: ESO/L. Calçada/M.Kornmesser

    The second type is called a supermassive black hole. These gargantuan black holes are up to billions of times more massive than an average star, and how they formed is much less clear and is a matter of ongoing study. One theory proposes they formed from enormous clouds of matter that collapsed when galaxies first formed; another theory suggests that colliding stellar mass black holes can merge into one enormous object.

    Today, these supermassive monsters reside at the centres of almost every galaxy — including our own Milky Way. They exert tremendous influence on their home galaxies, especially when they gorge on gas and stars.

    6
    Artist’s impression of a gas cloud after a close approach to the black hole at the centre of the Milky Way. The star orbiting the black hole are shown, along with blue lines that mark their fast, tight orbits. Credit: ESO/MPE/Marc Schartmann

    26 000 light-years away from Earth, Sagittarius A* (Sgr A* for short) is the supermassive black hole in the hot, violent centre of the Milky Way. It’s over 4 million times more massive than our Sun, over 20 million kilometres across, and is spinning at a large fraction of the speed of light. It’s shrouded from optical telescopes by dense clouds of dust and gas, so observatories that can observe different wavelengths — either longer (such as ALMA) or shorter (X-ray telescopes) — are essential to study its properties.

    Soon, through the combined power of ALMA and other millimetre-wavelength telescopes across the globe, we may become much better acquainted with the monstrous heart of our galaxy. The Global mm-VLBI Array is currently investigating the process of how gas, dust and other material accrete onto supermassive black holes, as well as the formation of the extremely fast gas jets that flow from them. The Event Horizon Telescope, on the other hand, is working towards a different goal: imaging the shadow of the event horizon, the point of no return.

    This is the second post of a blog series following the EHT and GMVA projects. Stay tuned to find out more about why the event horizon of a black hole is so interesting!

    Event Horizon Telescope website
    GMVA website
    BlackHoleCam — an EU-funded project to finally image, measure and understand astrophysical black holes
    Read more about ALMA
    Find out more about ALMA’s VLBI capabilities

    See the full article here .

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    ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

    ESO LaSilla
    ESO/Cerro LaSilla 600 km north of Santiago de Chile at an altitude of 2400 metres

    ESO VLT
    VLT at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level

    ESO Vista Telescope
    ESO/Vista Telescope at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level

    ESO NTT
    ESO/NTT at Cerro LaSilla 600 km north of Santiago de Chile at an altitude of 2400 metres

    ESO VLT Survey telescope
    VLT Survey Telescope at Cerro Paranal with an elevation of 2,635 metres (8,645 ft) above sea level

    ALMA Array
    ALMA on the Chajnantor plateau at 5,000 metres

    ESO E-ELT
    ESO/E-ELT to be built at Cerro Armazones at 3,060 m

    ESO APEX
    APEX Atacama Pathfinder 5,100 meters above sea level, at the Llano de Chajnantor Observatory in the Atacama desert

     
    • Nikola Milovic 8:48 am on April 16, 2017 Permalink | Reply

      I wish I could make contact with scientists who are trying to learn something more about black holes. The main reason for this contact is my view that science still does not know what a black hole is and how and why it occurs.
      It is true that this is a place where even light can not escape. But, one must know the limits of long acting black hole and what happens within those boundaries towards the center of the black hole and outside those borders where both matter and light still “confused” and do not know which way to go. To be deciphered. What if scientists to “see” with new telescopes, is again out of the black hole and its limits where the “forbidden transition both sides of the border.
      It is true that this is an enormous amount of gravity, but how and why this occurs, science can not know if you do not know the structure of the universe.
      The black hole has a spherical shape and is situated so that all sides around this sphere can “suck every form of matter. The fact that science sees as the horizon, not what is in reality, neither of the black hole can form any kind of matter, nor can they be two black holes collide.

      Like

  • richardmitnick 8:18 am on March 31, 2017 Permalink | Reply
    Tags: , , , Black Holes, , , direct collapse black hole ala Avi Loeb   

    From COSMOS: “When giants warped the universe” 

    Cosmos Magazine bloc

    COSMOS

    31 March 2017
    Graham Phillips

    1
    They don’t make them like they used to: supermassive black holes emerged billions of years earlier than thought. Getty Images

    They gobble stars, bend space, warp time and may even provide gateways to other universes.

    Black holes fire the imagination of scientists and science-fiction aficionados alike. But at least one thing about them wasn’t all that mind-bending: we’ve long understood black holes to be the end point in the life of a big star, when it runs out of fuel and collapses on itself.

    However, in recent times astronomers have been confronted with a paradox: gigantic black holes that existed when the universe was less than a billion years old.

    Since average-sized black holes take many billions of years to form, astrophysicists have been scratching their heads to figure out how these monsters could have arisen so early. It now seems that rather than being the end game in the evolution of stars and galaxies, supermassive black holes were around at their beginnings and played a major role in shaping them.

    Recommended reading: The bright side of black holes

    It was the little known English clergyman and scientist John Michell who, in 1783, first articulated the idea of “dark stars” whose gravity was so great they would prevent light from escaping them. The concept was astonishingly prescient even if parts of his theory – particularly those based on Newton’s idea that light particles had mass – were flawed.

    The first accurate description of black holes came in 1916 from German physicist and astronomer Karl Schwarzschild. Schwarzschild was serving in the German Army at the time, despite already being over 40 years of age.

    After seeing action on both the western and eastern fronts, Schwarzschild was sent home due to a serious auto-immune skin disease, pemphigus.

    It was late 1915 and Einstein’s theory of General Relativity had just been published. Inspired, Schwarzschild lost no time writing a paper that predicted the existence of black holes; it was published just months before he succumbed to his disease in May 1916.

    According to Einstein’s theory, the force of gravity was the result of a mass distorting the fabric of space-time. In the same way that a bowling ball dimples the fabric of a trampoline, a star’s mass dimpled the space-time fabric of its system, keeping planets circling around it.

    The theory was underpinned by equations laying out the interaction of energy, mass, space and time. Schwarzschild’s achievement was to apply Einstein’s equations to a simplified scenario: a perfectly spherical star. One of the things that jumped out of his mathematical musings was an object with such a strong gravitational pull that not even light could escape it.

    While Schwarzschild’s idea made sense in the theoretical realm of mathematics, most physicists did not expect to find an exemplar in the real universe.

    By the 1960s, however, expectations were changing. Astronomers discovered the existence of extremely dense objects known as neutron stars. Detected by their unusual pulsing of electromagnetic radiation, they were the dense corpses of large stars that had exhausted their fuel. Without the force of the burning fuel pushing against their own gravity, they collapsed, compressing their matter until only the pressure of neutron against neutron halted the crush.

    Neutron stars got astrophysicists thinking back to Schwarzschild’s idea. What happens when really big suns with even stronger gravity cave in? All the matter would be squeezed down to a point with an extraordinarily strong gravitational field.

    Sometime in the 1960s, physicists coined the term “black hole”, and the hunt for something more than just a mathematical artefact was on.

    The first evidence that black holes weren’t just theoretical came in 1964, when a rocket decked with sensitive instruments was shot into sub-orbital space. It detected suspicious X-rays emanating from the constellation of Cygnus (the swan).

    The X-ray source became known as Cygnus X-1. By the early 1970s most astronomers inferred the X-rays were radiated by super-heated matter being sucked into the gravitational field of the black hole. It would take decades more, however, before the first conclusive evidence that black holes exist and obey Einstein’s equations of general relativity.

    This came in September 2015 with the detection of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States.



    Caltech/MIT Advanced aLigo Hanford, WA, USA installation

    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA


    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

    These ripples in the fabric of space-time had been generated by two black holes colliding 1.3 billion years ago. Theorists had predicted that if such a titanic event occurred somewhere in our galaxy, the reverberations should be measurable on Earth.


    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

    LIGO’s detection of gravitational waves thus also confirmed the existence of black holes. Yet even as the evidence that black holes truly exist has firmed up, our understanding of how they arise seems to be crumbling.

    The cracks in the theory grew gradually as astronomers accumulated evidence for the existence of a very different kind of black hole. While most black holes have a mass that is equivalent to 10-100 times that of our Sun, these monsters were equivalent to a million or a billion solar masses. With typical prosaicness, astronomers dubbed them supermassive black holes.

    Unlike smaller black holes, they also resided at the centres of galaxies. Most surprising of all, far-reaching telescopes like the European Southern Observatory’s Very Large Telescope detected them in extremely distant galaxies.


    ESO/VLT at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level

    Because of the extreme length of time it takes for their light to reach Earth, these galaxies provide snapshots of the universe in its infancy.

    “A billion years after the big bang you have black holes that are as massive as the biggest black holes we find around us today,” says Avi Loeb, an astrophysicist at Harvard University.

    That simply doesn’t make sense according to the accepted understanding that black holes come only at the end of a star’s life. “It’s sort of like going to the delivery room in a hospital and finding giant babies.”

    Were these monster babies the result of many black holes colliding? Or did they arise from moderately sized black holed that ballooned by feeding on gas and other stars? Neither of these scenarios sits well with astrophysicists.

    “Getting from even a hundred solar masses up to several billion solar masses in less than a billion years is quite challenging,” says Mitch Begelman, an astrophysicist from the University of Colorado. “Black holes are not vacuum cleaners. That’s a popular misconception. It’s very difficult to get a black hole to swallow lots of stuff [in a short period of time].”

    Loeb, who has been captivated by supermassive black holes since he got into astrophysics, thinks he might have a solution to the mystery: in 1994, he came up with the idea that a different kind of process gave birth to black holes in the early universe.

    In the modern universe, a black hole takes billions of years to form. The black hole’s precursor star (which must be greater than 10 solar masses to muster the required gravitational force) must first burn through its fuel, then explode as a supernova before it collapses.

    But while the biggest stars today reach the size of 300 solar masses, the early universe might have blazed with stars equivalent to as much as a million solar masses. Such a super star, according to Loeb’s calculations, would burn so feverishly it would use up its fuel in just a million years.

    Then it would collapse directly into a black hole a million times the mass of the Sun – what Loeb calls “a direct collapse black hole”.

    According to Loeb, the reason super stars were formed only in the embryonic universe, is because back then stars were made of simpler stuff: “The gas was pristine. It came from the big bang and had only hydrogen and helium,” he explains.

    Lacking heavier elements to radiate heat, the clouds stayed relatively warm. That allowed them to grow without fragmenting, forming super stars.

    By contrast, in today’s universe star dust contains heavy atoms like carbon, silicon and oxygen – forged in the nuclear furnaces of the first generation of stars and blown throughout the cosmos when those stars exploded.

    As result, modern-day dust clouds can cool to extremely low temperatures and fragment, mostly forming stars about the size of the Sun.

    If Loeb is right, early super stars gave rise to the direct collapsers, which gave rise to supermassive black holes. These monsters have had an enormous influence on how the universe evolved. They shaped galaxies in two ways.

    First, they gobbled up clouds and stars in their immediate vicinity. Second, like some cosmic air blower, they beamed out jets of energy that propelled dust and gas out of their galaxy.

    “Within tens of millions of years the black holes can remove the gas from the host galaxy,” Loeb says. By cleaning the galaxy of the raw material for star creation and growth, the black holes have capped the size of galaxies.

    If not for the supermassive black hole at the centre of the Milky Way, Loeb estimates, our galaxy could have grown a thousand times bigger than it is today. That would be some night sky to look up at.

    “The growth of black holes seems to be a crucial element in galaxy formation,” Begelman agrees. “Galaxies would look very different if there weren’t these black holes.”

    Of course, the absolute proof that direct collapse black holes exist will come when one is observed.

    In the past year astronomers have seen some tantalising clues. One is a galaxy known as CR7, which hosts a source of light much brighter than its stars – perhaps the radiation caused by a black hole sucking in gas.

    “You see evidence for a galaxy that has mainly hydrogen and helium,” Loeb says. “That could potentially be the birthplace of a direct collapse black hole.”

    See the full article here .

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  • richardmitnick 1:31 pm on March 28, 2017 Permalink | Reply
    Tags: , Black Holes, , , , , SUPERRADIANCE   

    From PI via GIZMODO: “Mind-Blowing New Theory Connects Black Holes, Dark Matter, and Gravitational Waves” 

    Perimeter Institute
    Perimeter Institute

    GIZMODO

    3.28.17
    Ryan F. Mandelbaum

    The past few years have been incredible for physics discoveries. Scientists spotted the Higgs boson, a particle they’d been hunting for almost 50 years, in 2012, and gravitational waves, which were theorized 100 years ago, in 2016. This year, they’re slated to take a picture of a black hole. So, thought some theorists, why not combine all of the craziest physics ideas into one, a physics turducken? What if we, say, try to spot the dark matter radiating off of black holes through their gravitational waves?

    It’s really not that strange of an idea. Now that scientists have detected gravitational waves, ripples in spacetime spawned by the most violent physical events, they want to use their discovery to make real physics observations. They think they have a way to spot all-new particles that might make up dark matter, an unknown substance that accounts for over 80 percent of all of the gravity in the universe.

    The basic idea is that we’re trying to use black holes… the densest, most compact objects in the universe, to search for new kinds of particles,” Masha Baryakhtar, postdoctoral researcher at the Perimeter Institute for Theoretical Physics in Canada, told Gizmodo. Especially one particle: “The axion. People have been looking for it for 40 years.”

    Black holes are the universe’s sinkholes, so strong that light can’t escape their pull once it’s entered. They’ve got such powerful gravitational fields that they produce gravitational waves when they collide with each other. Dark matter might not be made from particles (specks of mass and energy), but if it was, we might observe it as axions, particles around one quintillion (a billion billion) times lighter than an electron, hanging around black holes. Now that you understand all the terms, here’s how the theory works.

    Baryakhtar and her teammates think that black holes are more than just bear traps for light, but nuclei at the center of a sort of gravitational atom. The axions would be the electrons, so to speak. If you already know about black holes, you know they have incredibly hot, high-energy discs of gas orbiting them, produced by the friction between particles accelerated by the black hole’s gravity. This theory ignores that stuff, since axions wouldn’t interact via friction.

    Keeping with the atom analogy, the axions can jump around the black hole, gaining and losing energy the same way that electrons do. But electrons interact via electromagnetism, so they let out electromagnetic waves, or light waves. Axions interact via gravity, so they let out gravitational waves. But like I said earlier, axions are tiny. Unlike a tiny atom, the black hole in these “gravity atoms” rotates, supercharging the space around it and coaxing it into producing more axions. Despite the axion’s tiny mass, this so-called superradiance process could generate 10^80 axions, the same number of atoms in the entire universe, around a single black hole. Are you still with me? Crazy spinning blob makes lots of crazy stuff.

    Craziest of all, we should be able to hear a gravitational wave hum from these axions moving around and releasing gravitational waves in our detectors, similar to the way you see spectral lines coming off of electrons in atoms in chemistry class. “You’d see this at a particular frequency which would be roughly twice the axion mass,” said Baryakhtar.

    There are giant gravitational wave detectors scattered around the world; presently there’s one called LIGO (Laser Interferometer Gravitational Wave Observatory) in Washington State, another LIGO in Louisiana, and one called Virgo in Italy that are sensitive enough to detect gravitational waves, and with upgrades, to detect axions and prove their theory right.



    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA



    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    Scientists would essentially need to record data, play it back, and tune their analysis like a radio to pick up the signal at just the right frequency.

    There are other ways the team thinks it could spot this superradiance effect, by measuring the spins in sets of colliding black holes. If black holes really do produce axions, scientists would see very few quickly-spinning black holes in collisions, since the superradiance effects would slow down some of the colliding black holes and create a visible effect in the data, according to the research published this month in the journal Physical Review D. The black hole spins would have a specific pattern which we should be able to spot in the gravitational wave detector data.

    Other scientists were immediately excited about this paper. “I’m always super excited about new ways to detect my favorite pet particle, the axion! Also, SUPERRADIANCE!” Dr. Chanda Prescod-Weinstein, the University of Washington axion wrangler, told Gizmodo in an email. “It’s so cool, and I haven’t read a paper that talked about [superradiance] in years. So it was really fun to see superradiance and axions in one paper.”

    There are a few drawbacks, as there are with any theory. These theorized black hole atoms would have to produce axions of a certain mass, but that mass isn’t an ideal one for the axion to be a dark matter particle, said Prescod-Weinstein. Plus, the second detection idea, the one that looks at the spin rate of colliding black holes, might not work. “They say [in the paper] that they don’t take into account the potential influence of another black hole” in the colliding pair, Dr. Lionel London, a research associate at Cardiff University School of Physics and Astronomy specializing in gravitational wave modeling, told Gizmodo. “If this does turn out to be a significant effect and they’re not including it, this could cast doubt on their results.” But there’s hope. “There’s good reason to believe the effect of a companion [black hole] won’t be large.”

    When would we spot these kinds of events? As of now, the LIGO and Virgo gravitational wave detectors probably aren’t ready. “With the current sensitivity we’re on the edge” of detecting axions, said Baryakhtar. “But LIGO will continue improving their instruments and at design sensitivity we might be able to see as many as 1000s of these axion signals coming in,” she said. Thousands of hums from these black hole-atoms.

    So, if you’ve gotten all the way to this point of the story and still don’t understand what’s going on, a recap: We’ve got these gravitational wave detectors that cost hundreds of millions of dollars each, that are good at spotting really crazy things going on in the universe. Theorists have come up with an interesting way to use them to solve one of the most important interstellar mysteries: What the heck is dark matter? As with most new ideas in theoretical physics, this is something cool to think about and isn’t ready for the big time… yet.

    “I think that timescale is always a concern, but we’re just getting started with LIGO discoveries,” said Prescod-Weinstein. “So who knows what’s around the corner over the next 10 years.”

    See the full article here .

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    About Perimeter

    Perimeter Institute is the world’s largest research hub devoted to theoretical physics. The independent Institute was founded in 1999 to foster breakthroughs in the fundamental understanding of our universe, from the smallest particles to the entire cosmos. Research at Perimeter is motivated by the understanding that fundamental science advances human knowledge and catalyzes innovation, and that today’s theoretical physics is tomorrow’s technology. Located in the Region of Waterloo, the not-for-profit Institute is a unique public-private endeavour, including the Governments of Ontario and Canada, that enables cutting-edge research, trains the next generation of scientific pioneers, and shares the power of physics through award-winning educational outreach and public engagement.

     
  • richardmitnick 10:26 am on March 16, 2017 Permalink | Reply
    Tags: , , , Black Holes, ,   

    From MIT: “Scientists identify a black hole choking on stardust” 

    MIT News

    MIT Widget

    MIT News

    March 14, 2017
    Jennifer Chu

    1
    In this artist’s rendering, a thick accretion disk has formed around a supermassive black hole following the tidal disruption of a star that wandered too close. Stellar debris has fallen toward the black hole and collected into a thick chaotic disk of hot gas. Flashes of X-ray light near the center of the disk result in light echoes that allow astronomers to map the structure of the funnel-like flow, revealing for the first time strong gravity effects around a normally quiescent black hole.
    Image: NASA/Swift/Aurore Simonnet, Sonoma State University

    Data suggest black holes swallow stellar debris in bursts.

    In the center of a distant galaxy, almost 300 million light years from Earth, scientists have discovered a supermassive black hole that is “choking” on a sudden influx of stellar debris.

    In a paper published today in Astrophysical Journal Letters, researchers from MIT, NASA’s Goddard Space Flight Center, and elsewhere report on a “tidal disruption flare” — a dramatic burst of electromagnetic activity that occurs when a black hole obliterates a nearby star. The flare was first discovered on Nov. 11, 2014, and scientists have since trained a variety of telescopes on the event to learn more about how black holes grow and evolve.

    The MIT-led team looked through data collected by two different telescopes and identified a curious pattern in the energy emitted by the flare: As the obliterated star’s dust fell into the black hole, the researchers observed small fluctuations in the optical and ultraviolet (UV) bands of the electromagnetic spectrum. This very same pattern repeated itself 32 days later, this time in the X-ray band.

    The researchers used simulations of the event performed by others to infer that such energy “echoes” were produced from the following scenario: As a star migrated close to the black hole, it was quickly ripped apart by the black hole’s gravitational energy. The resulting stellar debris, swirling ever closer to the black hole, collided with itself, giving off bursts of optical and UV light at the collision sites. As it was pulled further in, the colliding debris heated up, producing X-ray flares, in the same pattern as the optical bursts, just before the debris fell into the black hole.

    “In essence, this black hole has not had much to feed on for a while, and suddenly along comes an unlucky star full of matter,” says Dheeraj Pasham, the paper’s first author and a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research. “What we’re seeing is, this stellar material is not just continuously being fed onto the black hole, but it’s interacting with itself — stopping and going, stopping and going. This is telling us that the black hole is ‘choking’ on this sudden supply of stellar debris.”

    Pasham’s co-authors include MIT Kavli postdoc Aleksander Sadowski and researchers from NASA’s Goddard Space Flight Center, the University of Maryland, the Harvard-Smithsonian Center for Astrophysics, Columbia University, and Johns Hopkins University.

    See the full article here .

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  • richardmitnick 9:02 am on March 2, 2017 Permalink | Reply
    Tags: , , Black Holes, , , Rapid changes point to origin of ultra-fast black hole winds   

    From ESA: “Rapid changes point to origin of ultra-fast black hole winds” 

    ESA Space For Europe Banner

    European Space Agency

    1 March 2017
    Markus Bauer








    ESA Science and Robotic Exploration Communication Officer









    Tel: +31 71 565 6799









    Mob: +31 61 594 3 954









    Email: markus.bauer@esa.int

    Michael Parker
    Institute of Astronomy, Cambridge, UK
    Email: mlparker@ast.cam.ac.uk

    Andrew Fabian
    Institute of Astronomy, Cambridge, UK
    Email: acf@ast.cam.ac.uk

    Norbert Schartel
    XMM-Newton project scientist
    Email: Norbert.Schartel@esa.int

    1
    Black hole with ultrafast winds. No image credit

    ESA and NASA space telescopes have made the most detailed observation of an ultra-fast wind flowing from the vicinity of a black hole at nearly a quarter of the speed of light.

    Outflowing gas is a common feature of the supermassive black holes that reside in the centre of large galaxies. Millions to billions of times more massive than the Sun, these black holes feed off the surrounding gas that swirls around them. Space telescopes see this as bright emissions, including X-rays, from the innermost part of the disc around the black hole.

    Occasionally, the black holes eat too much and burp out an ultra-fast wind. These winds are an important characteristic to study because they could have a strong influence on regulating the growth of the host galaxy by clearing the surrounding gas away and therefore suppressing the birth of stars.

    Using ESA’s XMM-Newton and NASA’s NuStar telescopes, scientists have now made the most detailed observation yet of such an outflow, coming from an active galaxy identified as IRAS 13224–3809.

    ESA/XMM Newton
    ESA/XMM Newton

    NASA/NuSTAR
    NASA/NuSTAR

    The winds recorded from the black hole reach 71 000 km/s – 0.24 times the speed of light – putting it in the top 5% of fastest known black hole winds.

    XMM-Newton focused on the black hole for 17 days straight, revealing the extremely variable nature of the winds.

    “We often only have one observation of a particular object, then several months or even years later we observe it again and see if there’s been a change,” says Michael Parker of the Institute of Astronomy at Cambridge, UK, lead author of the paper published in Nature this week that describes the new result.

    “Thanks to this long observation campaign, we observed changes in the winds on a timescale of less than an hour for the first time.”

    The changes were seen in the increasing temperature of the winds, a signature of their response to greater X-ray emission from the disc right next to the black hole.

    Furthermore, the observations also revealed changes to the chemical fingerprints of the outflowing gas: as the X-ray emission increased, it stripped electrons in the wind from their atoms, erasing the wind signatures seen in the data.

    “The chemical fingerprints of the wind changed with the strength of the X-rays in less than an hour, hundreds of times faster than ever seen before,” says co-author Andrew Fabian, also from the Institute of Astronomy and principal investigator of the project.

    “It allows us to link the X-ray emission arising from the infalling material into the black hole, to the variability of the outflowing wind farther away.”

    “Finding such variability, and finding evidence for this link, is a key step in understanding how black hole winds are launched and accelerated, which in turn is an essential part of understanding their ability to moderate star formation in the host galaxy,” adds Norbert Schartel, ESA’s XMM-Newton project scientist.

    The response of relativistic outflowing gas to the inner accretion disk of a black hole,” by M. Parker et al. is published in Nature.

    See the full article here .

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    The European Space Agency (ESA), established in 1975, is an intergovernmental organization dedicated to the exploration of space, currently with 19 member states. Headquartered in Paris, ESA has a staff of more than 2,000. ESA’s space flight program includes human spaceflight, mainly through the participation in the International Space Station program, the launch and operations of unmanned exploration missions to other planets and the Moon, Earth observation, science, telecommunication as well as maintaining a major spaceport, the Guiana Space Centre at Kourou, French Guiana, and designing launch vehicles. ESA science missions are based at ESTEC in Noordwijk, Netherlands, Earth Observation missions at ESRIN in Frascati, Italy, ESA Mission Control (ESOC) is in Darmstadt, Germany, the European Astronaut Centre (EAC) that trains astronauts for future missions is situated in Cologne, Germany, and the European Space Astronomy Centre is located in Villanueva de la Cañada, Spain.

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