Tagged: Quantum Gravity Toggle Comment Threads | Keyboard Shortcuts

  • richardmitnick 4:07 pm on June 20, 2017 Permalink | Reply
    Tags: , Conjectures about gravity, Cosmic censorship conjecture, , Naked singularity in a four-dimensional universe, , , Quantum Gravity, 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

    1
    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?

    3
    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.

    5
    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.

    6
    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 .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    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 12:43 pm on December 14, 2016 Permalink | Reply
    Tags: , , , Quantum Gravity   

    From Ethan Siegel: “Has LIGO already discovered evidence for quantum gravity?” 

    From Ethan Siegel

    12.13.16

    1
    Two merging black holes. Image credit: SXS, the Simulating eXtreme Spacetimes (SXS) project (http://www.black-holes.org).

    Merging black holes are some of the most extreme events in the Universe. Could a modified event horizon reveal quantum gravity?

    “The bedrock nature of space and time and the unification of cosmos and quantum are surely among science’s great ‘open frontiers.’ These are parts of the intellectual map where we’re still groping for the truth — where, in the fashion of ancient cartographers, we must still inscribe ‘here be dragons.’”
    -Martin Rees

    When Einstein first wrote down the general theory of relativity in 1915, this brand new theory of gravity not only explained phenomena that Newton’s old one couldn’t, it predicted a whole host of new ones. In strong gravitational fields, clocks would run slower, light would shift its frequency, particle trajectories would bend, and accelerating masses would emit a new type of radiation: gravitational waves. While a great many of Einstein’s predictions had been borne out and verified over the years, it took until 2015 for the first gravitational wave signals to be directly detected by humanity. There were two that had enough significance to be announced as “discoveries,” while one other remains a strong candidate. But perhaps these events — created by merging black holes — will do us one better than Einstein: perhaps they’ve already given us our first hints of quantum gravity. In a new paper by theoretical physicists Jahed Abedi, Hannah Dykaar and Niayesh Afshordi, they claim the first evidence of gravitational effects beyond general relativity in the data of these mergers.

    The reason it’s so difficult to go beyond general relativity is because the scale at which quantum effects should become important happen at extreme scales. Not extreme like at the LHC or in the center of the Sun, but at energies well beyond anything the Universe has seen since the Big Bang, or at distance scales some 10¹⁸ times smaller than a proton’s width. While quantum effects show up for the other forces at much more accessible scales and energies, part of why a theory of quantum gravity has been so elusive is that we have no experiments to guide us. The only hopes we have, realistically, are to look in two places:

    1. At the echoes of cosmic inflation, the ultra-high-energy state of spacetime prior to the Big Bang.
    2. At and around the event horizons of black holes during catastrophic events, where quantum effects will be strongest.

    3
    Gravitational waves can only be generated from inflation if gravity is an inherently quantum theory. Image credit: BICEP2 Collaboration.

    Bicep 2 Collaboration Steffen Richter Harvard
    Bicep 2 Collaboration Steffen Richter Harvard

    For the first one, there are teams looking for particular polarization signals of the Big Bang’s leftover glow. If that signal shows up in the data with a particular pattern on a variety of angular scales, it will be an unambiguous verification of inflation, plus the first direct evidence that gravity is quantum in nature. While many things in the Universe produce gravitational waves, some of these processes are classical (like inspiraling black holes), while others are purely quantum. The quantum ones rely on the fact that gravitation, like the other forces, should exhibit quantum fluctuations in space and time, along with the inherent uncertainty that quantum physics brings with it. In cosmic inflation, those fluctuations get stretched across the Universe, and can imprint in the Big Bang’s leftover glow. While the initial report of such a detection a few years ago, by BICEP2, was shown to be false, the prospects remain enticing.

    Gravitational Wave Background from BICEP 2
    Gravitational Wave Background from BICEP 2 proven to be false

    4
    Gravitational wave signals and their origins, including what detectors will be sensitive to them. Image credit: NASA Goddard Space Flight Center.

    But there’s another approach: to look for quantum effects that show up along with the classical ones in the strongest gravitational wave signals this Universe generates. LIGO’s announcements earlier this year gave the scientific community a celebratory jolt, as the first and second gravitational wave events from merging black holes were unambiguously detected.

    LIGO bloc new
    Caltech/MIT Advanced aLigo Hanford, WA, USA installation
    Caltech/MIT Advanced aLigo Hanford, WA, USA installation
    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA
    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA
    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib
    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

    A third probably detection was also released, but was just below the significance threshold for discovery. While LIGO has just recently fired back up at increased sensitivity, a new idea gives us something important to look for: quantum corrections that show up in the mergers.

    6
    The LIGO signal (blue line) for gravitational waves emitted by the first-ever detected merger may have quantum corrections (black), which could alter the total signal (yellow) that shows up in the detector. Image credit: Abedi, Dykaar and Afshordi, 2016, via https://arxiv.org/abs/1612.00266.

    According to Einstein, a black hole’s event horizon should have specific properties, determined by its mass, charge and angular momentum. In most ideas of what quantum gravity would look like, that event horizon would be no different. Some models, however, predict notably different event horizons, and it’s those departure models that offer a glimmer of hope for quantum gravity. If we see a difference from what Einstein’s theory predicts, perhaps we can uncover not only that gravity must be a quantum theory, but what properties quantum gravity actually has.

    7
    The inspiral and merger gravitational wave signal extracted from the event on December 26, 2015. Image credit: Figure 1 from B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), Phys. Rev. Lett. 116, 241103 — Published 15 June 2016.

    The templates for LIGO generated by teams working with numerical relativity fit the merger events extremely well. After all, that’s how they were able to tease the signal out of such spectacular noise; they knew exactly what they were looking for and how to find it. If there’s a secondary, sub-dominant signal in there, arising from quantum gravity, a similar approach should be able to uncover it. The key — if these are quantum gravitational effects — is that they should occur at the Planck scale: at energies of 10¹⁹ GeV or distance scales of around 10^-33 meters. This is exactly the type of signal that Abedi, Dykaar and Afshordi decided to look for.

    8
    While Einstein’s theory makes explicit predictions for a black hole’s event horizon and the spacetime just outside, quantum corrections could alter that significantly. Image credit: NASA.

    In classical (Einstein’s) general relativity, there are a few problems that arise from black holes: that there ought to be a firewall at the event horizon; that information about what falls into the black hole appears to be destroyed; how you reconcile a black hole-containing Universe with one that has a non-zero, positive cosmological constant. Some of the proposed quantum gravitational resolutions modify the event horizon of a black hole. When two black holes merge under these scenarios, the differences in the event horizons from Einstein’s theory should lead to “echoes” visible in the merging gravitational wave signal. They’ll be dominated by the main, Einsteinian prediction, but with good enough data and good enough algorithms, we should be able to tease that signal out, too.

    9
    Spacetime depiction of gravitational wave echoes from a membrane/firewall on the stretched horizon, following a black hole merger event. Image credit: Abedi, Dykaar and Afshordi, 2016, via https://arxiv.org/abs/1612.00266.

    In particular, there should be an echoing timescale, defined solely by the masses of the merging black holes and the frequencies at which they are merging or inspiraling. There should be these periodic echoes as the signals from the two event horizons interact, and it should exhibit “after-echoes” that continue for some time after the merger is complete.

    9
    LIGO original template for GW150914, along with their best fit template for the echoes. Image credit: Abedi, Dykaar and Afshordi, 2016, via https://arxiv.org/abs/1612.00266.

    Interestingly, when they compare it to the data from all three mergers, they arrive at a prediction for what they ought to see: it ought to exhibit these extra waves on timescales related to the echo period and the merger/inspiral period. The most unambiguous and easy-to-detect signal, from GW150914, contains the greatest information and significance: it shows evidence for this signal at almost exactly the predicted frequency, with only a 0.54% offset. (And they searched over a range with a ±5% offset.) If you then add in the signals for the other two black hole mergers using those same parameters, the statistical significance increases from 95% (about a 1-in-20 chance of random fluctuations) to 99.6% (about a 1-in-270 chance).

    11
    The signal and its significance from GW150914 (red) and from all three waves combined (black). Image credit: Abedi, Dykaar and Afshordi, 2016, via https://arxiv.org/abs/1612.00266.

    On the one hand, this is incredible. There are very few prospects for detecting a signal from quantum gravity because of the fact that we don’t have a working theory of quantum gravity; all we have are models and approximations. Yet some classes of models make some actual, testable predictions, albeit with uncertainties, and one of those predictions is that merging black holes, in some models, should emit additional echoes of particular frequencies and amplitudes.

    12
    Under General Relativity alone, gravitational waves should make a particular patterns and signal. If some models of quantum gravity are correct, there should be an additional signal superimposed over the main, Einsteinian one. Image credit: NASA/Ames Research Center/C. Henze.

    But on the other hand, there are reasons to doubt that this effect is real.

    Only the first gravitational wave signal, GW150914, exhibits enough significance to have this additional signal stand out against the background on its own. The other two are undetectable without assuming the prior results from GW150914.
    There is an additional signal offset by -2.8% from the predicted frequency at nearly 95% confidence when all three gravitational wave signals are included, and three more at greater than 80% confidence.
    And perhaps most damningly, we have known for months that there are additional signals, likely from external sources, superimposed on the LIGO data at a 3.2-sigma (99.9%) confidence level.

    In other words, there may or may not be a real signal there, and it may have nothing to do with quantum gravity at all even if it is real.

    But this new paper is remarkable for the fact that it makes an explicit prediction for what a quantum gravitational signature in the LIGO data will look like. It takes advantage of the actual LIGO data to show that there is the hint of a signal already there, and it explicitly tells the LIGO team what signatures they should look for in future events to see if this model of quantum gravity has it right. As LIGO is now operational once again at even greater sensitivity than during its prior run, we have every reason to expect that more black hole mergers are coming. The smart money is still on this signal not being real (or if it is, that it’s due to an external source rather than quantum gravity), but science never advanced without looking for an out-of-the-mainstream possibility. This time, the technology is already in place, and the next 24 months should be critical in revealing whether quantum gravity shows itself in the physics of merging black holes!

    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 7:05 pm on November 30, 2016 Permalink | Reply
    Tags: , , , , , , , Quantum Gravity   

    From Quanta: “The Case Against Dark Matter” 

    Quanta Magazine
    Quanta Magazine

    November 29, 2016
    Natalie Wolchover

    1
    Erik Verlinde
    Ilvy Njiokiktjien for Quanta Magazine

    For 80 years, scientists have puzzled over the way galaxies and other cosmic structures appear to gravitate toward something they cannot see. This hypothetical “dark matter” seems to outweigh all visible matter by a startling ratio of five to one, suggesting that we barely know our own universe. Thousands of physicists are doggedly searching for these invisible particles.

    But the dark matter hypothesis assumes scientists know how matter in the sky ought to move in the first place. This month, a series of developments has revived a long-disfavored argument that dark matter doesn’t exist after all. In this view, no missing matter is needed to explain the errant motions of the heavenly bodies; rather, on cosmic scales, gravity itself works in a different way than either Isaac Newton or Albert Einstein predicted.

    The latest attempt to explain away dark matter is a much-discussed proposal by Erik Verlinde, a theoretical physicist at the University of Amsterdam who is known for bold and prescient, if sometimes imperfect, ideas. In a dense 51-page paper posted online on Nov. 7, Verlinde casts gravity as a byproduct of quantum interactions and suggests that the extra gravity attributed to dark matter is an effect of “dark energy” — the background energy woven into the space-time fabric of the universe.

    Instead of hordes of invisible particles, “dark matter is an interplay between ordinary matter and dark energy,” Verlinde said.

    To make his case, Verlinde has adopted a radical perspective on the origin of gravity that is currently in vogue among leading theoretical physicists. Einstein defined gravity as the effect of curves in space-time created by the presence of matter. According to the new approach, gravity is an emergent phenomenon. Space-time and the matter within it are treated as a hologram that arises from an underlying network of quantum bits (called “qubits”), much as the three-dimensional environment of a computer game is encoded in classical bits on a silicon chip. Working within this framework, Verlinde traces dark energy to a property of these underlying qubits that supposedly encode the universe. On large scales in the hologram, he argues, dark energy interacts with matter in just the right way to create the illusion of dark matter.

    In his calculations, Verlinde rediscovered the equations of “modified Newtonian dynamics,” or MOND. This 30-year-old theory makes an ad hoc tweak to the famous “inverse-square” law of gravity in Newton’s and Einstein’s theories in order to explain some of the phenomena attributed to dark matter. That this ugly fix works at all has long puzzled physicists. “I have a way of understanding the MOND success from a more fundamental perspective,” Verlinde said.

    Many experts have called Verlinde’s paper compelling but hard to follow. While it remains to be seen whether his arguments will hold up to scrutiny, the timing is fortuitous. In a new analysis of galaxies published on Nov. 9 in Physical Review Letters, three astrophysicists led by Stacy McGaugh of Case Western Reserve University in Cleveland, Ohio, have strengthened MOND’s case against dark matter.

    The researchers analyzed a diverse set of 153 galaxies, and for each one they compared the rotation speed of visible matter at any given distance from the galaxy’s center with the amount of visible matter contained within that galactic radius. Remarkably, these two variables were tightly linked in all the galaxies by a universal law, dubbed the “radial acceleration relation.” This makes perfect sense in the MOND paradigm, since visible matter is the exclusive source of the gravity driving the galaxy’s rotation (even if that gravity does not take the form prescribed by Newton or Einstein). With such a tight relationship between gravity felt by visible matter and gravity given by visible matter, there would seem to be no room, or need, for dark matter.

    Even as dark matter proponents rise to its defense, a third challenge has materialized. In new research that has been presented at seminars and is under review by the Monthly Notices of the Royal Astronomical Society, a team of Dutch astronomers have conducted what they call the first test of Verlinde’s theory: In comparing his formulas to data from more than 30,000 galaxies, Margot Brouwer of Leiden University in the Netherlands and her colleagues found that Verlinde correctly predicts the gravitational distortion or “lensing” of light from the galaxies — another phenomenon that is normally attributed to dark matter. This is somewhat to be expected, as MOND’s original developer, the Israeli astrophysicist Mordehai Milgrom, showed years ago that MOND accounts for gravitational lensing data. Verlinde’s theory will need to succeed at reproducing dark matter phenomena in cases where the old MOND failed.

    Kathryn Zurek, a dark matter theorist at Lawrence Berkeley National Laboratory, said Verlinde’s proposal at least demonstrates how something like MOND might be right after all. “One of the challenges with modified gravity is that there was no sensible theory that gives rise to this behavior,” she said. “If [Verlinde’s] paper ends up giving that framework, then that by itself could be enough to breathe more life into looking at [MOND] more seriously.”

    The New MOND

    In Newton’s and Einstein’s theories, the gravitational attraction of a massive object drops in proportion to the square of the distance away from it. This means stars orbiting around a galaxy should feel less gravitational pull — and orbit more slowly — the farther they are from the galactic center. Stars’ velocities do drop as predicted by the inverse-square law in the inner galaxy, but instead of continuing to drop as they get farther away, their velocities level off beyond a certain point. The “flattening” of galaxy rotation speeds, discovered by the astronomer Vera Rubin in the 1970s, is widely considered to be Exhibit A in the case for dark matter — explained, in that paradigm, by dark matter clouds or “halos” that surround galaxies and give an extra gravitational acceleration to their outlying stars.

    Searches for dark matter particles have proliferated — with hypothetical “weakly interacting massive particles” (WIMPs) and lighter-weight “axions” serving as prime candidates — but so far, experiments have found nothing.

    2
    Lucy Reading-Ikkanda for Quanta Magazine

    Meanwhile, in the 1970s and 1980s, some researchers, including Milgrom, took a different tack. Many early attempts at tweaking gravity were easy to rule out, but Milgrom found a winning formula: When the gravitational acceleration felt by a star drops below a certain level — precisely 0.00000000012 meters per second per second, or 100 billion times weaker than we feel on the surface of the Earth — he postulated that gravity somehow switches from an inverse-square law to something close to an inverse-distance law. “There’s this magic scale,” McGaugh said. “Above this scale, everything is normal and Newtonian. Below this scale is where things get strange. But the theory does not really specify how you get from one regime to the other.”

    Physicists do not like magic; when other cosmological observations seemed far easier to explain with dark matter than with MOND, they left the approach for dead. Verlinde’s theory revitalizes MOND by attempting to reveal the method behind the magic.

    Verlinde, ruddy and fluffy-haired at 54 and lauded for highly technical string theory calculations, first jotted down a back-of-the-envelope version of his idea in 2010. It built on a famous paper he had written months earlier, in which he boldly declared that gravity does not really exist. By weaving together numerous concepts and conjectures at the vanguard of physics, he had concluded that gravity is an emergent thermodynamic effect, related to increasing entropy (or disorder). Then, as now, experts were uncertain what to make of the paper, though it inspired fruitful discussions.

    The particular brand of emergent gravity in Verlinde’s paper turned out not to be quite right, but he was tapping into the same intuition that led other theorists to develop the modern holographic description of emergent gravity and space-time — an approach that Verlinde has now absorbed into his new work.

    In this framework, bendy, curvy space-time and everything in it is a geometric representation of pure quantum information — that is, data stored in qubits. Unlike classical bits, qubits can exist simultaneously in two states (0 and 1) with varying degrees of probability, and they become “entangled” with each other, such that the state of one qubit determines the state of the other, and vice versa, no matter how far apart they are. Physicists have begun to work out the rules by which the entanglement structure of qubits mathematically translates into an associated space-time geometry. An array of qubits entangled with their nearest neighbors might encode flat space, for instance, while more complicated patterns of entanglement give rise to matter particles such as quarks and electrons, whose mass causes the space-time to be curved, producing gravity. “The best way we understand quantum gravity currently is this holographic approach,” said Mark Van Raamsdonk, a physicist at the University of British Columbia in Vancouver who has done influential work on the subject.

    The mathematical translations are rapidly being worked out for holographic universes with an Escher-esque space-time geometry known as anti-de Sitter (AdS) space, but universes like ours, which have de Sitter geometries, have proved far more difficult. In his new paper, Verlinde speculates that it’s exactly the de Sitter property of our native space-time that leads to the dark matter illusion.

    De Sitter space-times like ours stretch as you look far into the distance. For this to happen, space-time must be infused with a tiny amount of background energy — often called dark energy — which drives space-time apart from itself. Verlinde models dark energy as a thermal energy, as if our universe has been heated to an excited state. (AdS space, by contrast, is like a system in its ground state.) Verlinde associates this thermal energy with long-range entanglement between the underlying qubits, as if they have been shaken up, driving entangled pairs far apart. He argues that this long-range entanglement is disrupted by the presence of matter, which essentially removes dark energy from the region of space-time that it occupied. The dark energy then tries to move back into this space, exerting a kind of elastic response on the matter that is equivalent to a gravitational attraction.

    Because of the long-range nature of the entanglement, the elastic response becomes increasingly important in larger volumes of space-time. Verlinde calculates that it will cause galaxy rotation curves to start deviating from Newton’s inverse-square law at exactly the magic acceleration scale pinpointed by Milgrom in his original MOND theory.

    Van Raamsdonk calls Verlinde’s idea “definitely an important direction.” But he says it’s too soon to tell whether everything in the paper — which draws from quantum information theory, thermodynamics, condensed matter physics, holography and astrophysics — hangs together. Either way, Van Raamsdonk said, “I do find the premise interesting, and feel like the effort to understand whether something like that could be right could be enlightening.”

    One problem, said Brian Swingle of Harvard and Brandeis universities, who also works in holography, is that Verlinde lacks a concrete model universe like the ones researchers can construct in AdS space, giving him more wiggle room for making unproven speculations. “To be fair, we’ve gotten further by working in a more limited context, one which is less relevant for our own gravitational universe,” Swingle said, referring to work in AdS space. “We do need to address universes more like our own, so I hold out some hope that his new paper will provide some additional clues or ideas going forward.”


    Access mp4 video here .

    The Case for Dark Matter

    Verlinde could be capturing the zeitgeist the way his 2010 entropic-gravity paper did. Or he could be flat-out wrong. The question is whether his new and improved MOND can reproduce phenomena that foiled the old MOND and bolstered belief in dark matter.

    One such phenomenon is the Bullet cluster, a galaxy cluster in the process of colliding with another.

    4
    X-ray photo by Chandra X-ray Observatory of the Bullet Cluster (1E0657-56). Exposure time was 0.5 million seconds (~140 hours) and the scale is shown in megaparsecs. Redshift (z) = 0.3, meaning its light has wavelengths stretched by a factor of 1.3. Based on today’s theories this shows the cluster to be about 4 billion light years away.
    In this photograph, a rapidly moving galaxy cluster with a shock wave trailing behind it seems to have hit another cluster at high speed. The gases collide, and gravitational fields of the stars and galalxies interact. When the galaxies collided, based on black-body temperture readings, the temperature reached 160 million degrees and X-rays were emitted in great intensity, claiming title of the hottest known galactic cluster.
    Studies of the Bullet cluster, announced in August 2006, provide the best evidence to date for the existence of dark matter.
    http://cxc.harvard.edu/symposium_2005/proceedings/files/markevitch_maxim.pdf
    User:Mac_Davis

    5
    Superimposed mass density contours, caused by gravitational lensing of dark matter. Photograph taken with Hubble Space Telescope.
    Date 22 August 2006
    http://cxc.harvard.edu/symposium_2005/proceedings/files/markevitch_maxim.pdf
    User:Mac_Davis

    The visible matter in the two clusters crashes together, but gravitational lensing suggests that a large amount of dark matter, which does not interact with visible matter, has passed right through the crash site. Some physicists consider this indisputable proof of dark matter. However, Verlinde thinks his theory will be able to handle the Bullet cluster observations just fine. He says dark energy’s gravitational effect is embedded in space-time and is less deformable than matter itself, which would have allowed the two to separate during the cluster collision.

    But the crowning achievement for Verlinde’s theory would be to account for the suspected imprints of dark matter in the cosmic microwave background (CMB), ancient light that offers a snapshot of the infant universe.

    CMB per ESA/Planck
    CMB per ESA/Planck

    The snapshot reveals the way matter at the time repeatedly contracted due to its gravitational attraction and then expanded due to self-collisions, producing a series of peaks and troughs in the CMB data. Because dark matter does not interact, it would only have contracted without ever expanding, and this would modulate the amplitudes of the CMB peaks in exactly the way that scientists observe. One of the biggest strikes against the old MOND was its failure to predict this modulation and match the peaks’ amplitudes. Verlinde expects that his version will work — once again, because matter and the gravitational effect of dark energy can separate from each other and exhibit different behaviors. “Having said this,” he said, “I have not calculated this all through.”

    While Verlinde confronts these and a handful of other challenges, proponents of the dark matter hypothesis have some explaining of their own to do when it comes to McGaugh and his colleagues’ recent findings about the universal relationship between galaxy rotation speeds and their visible matter content.

    In October, responding to a preprint of the paper by McGaugh and his colleagues, two teams of astrophysicists independently argued that the dark matter hypothesis can account for the observations. They say the amount of dark matter in a galaxy’s halo would have precisely determined the amount of visible matter the galaxy ended up with when it formed. In that case, galaxies’ rotation speeds, even though they’re set by dark matter and visible matter combined, will exactly correlate with either their dark matter content or their visible matter content (since the two are not independent). However, computer simulations of galaxy formation do not currently indicate that galaxies’ dark and visible matter contents will always track each other. Experts are busy tweaking the simulations, but Arthur Kosowsky of the University of Pittsburgh, one of the researchers working on them, says it’s too early to tell if the simulations will be able to match all 153 examples of the universal law in McGaugh and his colleagues’ galaxy data set. If not, then the standard dark matter paradigm is in big trouble. “Obviously this is something that the community needs to look at more carefully,” Zurek said.

    Even if the simulations can be made to match the data, McGaugh, for one, considers it an implausible coincidence that dark matter and visible matter would conspire to exactly mimic the predictions of MOND at every location in every galaxy. “If somebody were to come to you and say, ‘The solar system doesn’t work on an inverse-square law, really it’s an inverse-cube law, but there’s dark matter that’s arranged just so that it always looks inverse-square,’ you would say that person is insane,” he said. “But that’s basically what we’re asking to be the case with dark matter here.”

    Given the considerable indirect evidence and near consensus among physicists that dark matter exists, it still probably does, Zurek said. “That said, you should always check that you’re not on a bandwagon,” she added. “Even though this paradigm explains everything, you should always check that there isn’t something else going on.”

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    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 3:27 pm on May 30, 2016 Permalink | Reply
    Tags: , , , Quantum Gravity, ,   

    From PI: “Bridging Two Roads of Physics” Women in Science 

    Perimeter Institute
    Perimeter Institute

    May 30, 2016
    Rose Simone

    Recent Perimeter research based on the holographic principle seeks new connections between general relativity and quantum field theory.

    Imagine driving along a road that traverses a beautiful landscape. Around every corner, there is a new vista of natural beauty to explore. Suddenly you come to a chasm.

    You can see a road on the other side, but how do you get there to complete the journey? You need a bridge.

    That’s the state of physics today, and Bianca Dittrich, Perimeter Institute researcher in mathematical physics and quantum gravity, is one of the people trying to build that bridge.

    1
    Bianca Dittrich

    On one side of the chasm is the road built by Albert Einstein’s theory of general relativity. It describes the force of gravity as the warping of spacetime by large masses such as planets and stars.

    On the other side is quantum field theory, our best description of interacting particles and the three other forces (the strong and weak nuclear forces and electromagnetism) operating at minuscule subatomic distances.

    The theories are incredibly successful in their respective realms, yet they are so different, both in formulation and conceptually, that it is difficult to bridge them.

    “Basically, we are trying to bridge all of the scales that we know,” Dittrich says. “That is what physics is about, but it is very hard. You need to bridge all of these scales by modelling the tiny scales, and show that this model actually does indeed describe reality as we know it at macroscopic scales.”

    In general relativity, spacetime is smooth and continuous. If you were to zoom in with a microscope to arbitrarily small distances, it should look the same as it does when you zoom out for the larger view. Quantum field theory, on the other hand, describes particles and forces that come as discrete “packets,” and spacetime would also have to be discrete and granular, like the pixels in a photograph.

    Scientists need a theory to describe the force of gravity at the quantum scale, and it must be consistent with the larger picture of general relativity. Building the bridge to a theory of quantum gravity is what occupies many physicists around the world today.

    It is easier said than done. If general relativity is scaled down to the quantum size, you start to get nonsensical “infinities” in the calculations. “Quantizing gravity sounds simple, in that it should be just the quantization of another force, besides the three forces (the non-gravitational forces) that were quantized decades ago,” Dittrich says. “But in fact it is a very hard and open problem.”

    There are many approaches to this longstanding problem. In loop quantum gravity, for example, physicists speak in terms of “spacetime atoms” linked together in a network like a fine mesh. This provides a model of what spacetime itself is made of.

    But in a recent paper*, “3D Holography: From Discretum to Continuum,” Dittrich and co-author Valentin Bonzom, now an assistant professor at Université Paris 13 who was previously a postdoctoral researcher at Perimeter Institute, tested a different approach, based on the holographic principle.

    The holographic principle says everything that happens in a given space can be explained in terms of information stored on the boundary of that space. (The principle takes its name from holograms, in which two-dimensional surfaces contain all the information needed to project a three-dimensional image.)

    A popular mathematical framework based on the holographic principle is known as the AdS/CFT correspondence. AdS is short for anti-de Sitter space, which describes a particular kind of geometry. Just like a bowling ball will stretch a rubber sheet, the elliptical shape of anti-de Sitter space can also stretch or contract, thus allowing it to describe gravity.

    CFT, meanwhile, is short for conformal field theory. Field theories are the language of quantum mechanics and can describe, for example, how an electrical field might change over space and time.

    The holographic principle applies because the AdS/CFT correspondence basically states that for every conformal field theory, there is a corresponding theory of gravity with one more dimension. So a two-dimensional CFT would correspond to a three-dimensional theory of gravity, for instance.

    But the holographic principle applies to infinitely large boundaries, and Dittrich and Bonzom wanted to see if it could also hold for finite boundaries, and for other types of geometries apart from AdS. This would then provide a more manageable way of describing a piece of spacetime, and understanding the microscopic details as they reconstruct the spacetime bulk.

    Working with a boundary without worrying too much about the bulk “very much simplifies the construction of a theory of quantum gravity,” Dittrich explains.

    They tested this in three spacetime dimensions, and “it turned out that the holographic principle indeed holds for finite boundaries, and we also obtained a very simple description of how to translate the boundary data into the geometry of the bulk,” she says.

    That this could be done in 3D was not too surprising, but the more challenging part will be extending this work into 4D space, Dittrich adds.

    Most theories of quantum gravity require the force of gravity to also be mediated by hypothetical particles called gravitons. If Dittrich can get her model to work in 4D, then she will have successfully taken it into a realm where gravitons exist. “Gravity can propagate through that spacetime,” Dittrich says.

    Dittrich has been on the physics road for some time. She grew up in Germany, reading a lot of popular books about science, as well as history and literature, and when she finished high school she considered various options, including areas such as geo-ecology.

    But she realized it was physics that could take her on the journey to the most complete understanding of nature. “If you want to understand why something works, the answer is in physics,” she says.

    Now, she is designing another bridge that will span that chasm between the two great roads and carry physicists to that more complete understanding of nature.

    *Science paper:
    3D holography: from discretum to continuum

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    About Perimeter

    Perimeter Institute is a leading centre for scientific research, training and educational outreach in foundational theoretical physics. Founded in 1999 in Waterloo, Ontario, Canada, its mission is to advance our understanding of the universe at the most fundamental level, stimulating the breakthroughs that could transform our future. Perimeter also trains the next generation of physicists through innovative programs, and shares the excitement and wonder of science with students, teachers and the general public.

     
  • richardmitnick 10:58 am on March 19, 2016 Permalink | Reply
    Tags: , , , Quantum Gravity   

    From NOVA: “Why Quantize Gravity?” 

    PBS NOVA

    NOVA

    24 Feb 2016
    Sabine Hossenfelder

    A good question is one that you know has an answer—if only you could find it. What’s her name again? Where are my keys? How do I quantize gravity? We’ve all been there.

    Science is the art of asking questions. Scientists often have questions to which they would like an answer, yet aren’t sure there is one. For instance, why is the mass of the proton 1.67 x 10-27 kilograms? Maybe there is an answer—but then again, maybe the masses of elementary particles are what they are, without deeper explanation. And maybe the four known forces are independent of each other, not aspects of one unified “Theory of Everything.”

    Spacetime with Gravity Probe B. NASA
    Spacetime with Gravity Probe B. NASA

    The quest for a theory of quantum gravity is different. To remove contradictions in the known laws of nature, physicists need a theory that can resolve the clash between the laws of gravity and those of quantum mechanics. Gravity and quantum mechanics have been developed and confirmed separately in countless experiments over the last century, but when applied together they produce nonsense. A working theory of quantum gravity would resolve these contradictions by applying the rules of quantum mechanics to gravity, thereby endowing the gravitational field with the irreducible randomness and uncertainty characteristic of quantization. We know there must be a way: if only we could find it.

    Take the double-slit experiment: In quantum mechanics an electron is able to pass through two slits at once, creating a wave-like interference pattern on a detection screen. Yet the electron is neither a wave nor a particle. Instead, it is described by a wave-function—a mathematical in-between of particle and wave—that allows it to act like a particle in some respects and a wave in others. This way, the electron can exist in a quantum superposition: It can be in two different places at once and go through both the right and the left slit. It remains in a superposition until a measurement forces it to “decide” on one location. This behavior of the electron, unintuitive as it seems, has been tested and verified over and over again. Strange or not, we know it’s real.

    But what about the electron’s gravitational field? Electrons have mass, and mass creates a gravitational field. So if the electron goes through both the left and the right slit, its gravitational field should go through both slits, too. But in general relativity the gravitational field cannot do this: General relativity is no quantum theory, and the gravitational field cannot behave like a wave-function. Unlike the electron itself, the electron’s gravitational field must be either here or there, which means that electrons don’t always have their gravitational pull in the right place. We must conclude then that the existing theories just cannot describe what the gravitational field does when the electron goes through a double-slit. There has to be an answer to this, but what?

    At first theorists thought there would be a simple fix: Just modify general relativity to allow the gravitational field to be in two places at once. Physicists Bryce DeWitt and Richard Feynman [collaborated] on just such a theory in the 1960s, but they quickly realized that it worked only at small energies, whereas at high energies, when space-time becomes strongly curved, it produces nonsensical infinite results. This straightforward quantization, it turned out, is only an approximation to a more complete theory, one which should not suffer from the problem of infinities. It is this complete, still unknown, theory that physicists refer to as “quantum gravity.”

    These first attempts at quantization break down when the gravitational force becomes very strong. This happens when large amounts of energy are compressed into a small regions of [spacetime]. Without a full theory of quantum gravity, thus, physicists cannot understand what happens in the early universe or inside black holes.

    Indeed, the black hole information loss problem is another strong indication that we need a theory of quantum gravity. As Stephen Hawking demonstrated in 1974, quantum fluctuations of matter fields close to a black hole’s horizon lead to the production of particles, now called Hawking radiation, that make the black hole lose mass and shrink until nothing is left. Today, the amount of radiation leaking out of the black holes in the Milky Way other galaxies is minuscule; they gain more mass from swallowing matter and gas around them than they can lose by Hawking radiation. But once the universe has cooled down sufficiently, which will inevitably happen, black holes will begin to evaporate. It will take hundreds of billions of years, but eventually they will be gone, leaving behind nothing but radiation.

    This radiation does not carry any information besides its temperature. All the information about what fell into the black hole is irretrievably destroyed during the evaporation. The problem? In quantum mechanics, all processes are reversible, at least in principle, and information about the initial state of any process can always be retrieved. The information might be very scrambled and unrecognizable, such as when you burn a book and are left with smoke and ashes, but in principle the remains still contain the book’s information. Not so for a black hole. A book that crosses the horizon is gone for good, which conflicts with quantum mechanics, which demands that information always be conserved. The information loss problem is not a practical concern that affects observational predictions, but it is a deep conceptual worry about the soundness of our theories. It’s the kind of problem that keeps physicists up at night, and it shows once again that leaving gravity unquantized results in a conundrum which has to be resolved by quantum gravity.

    Black holes and the Big Bang pose another problem for unquantized gravity because they lead to singularities, locations in [spacetime] with a seemingly infinite energy density. Similar singularities appear in other theories too, and in these cases physicists understand that singularities signal the theories’ breakdown. The equations of fluid dynamics, for example, can have singularities. But these equations are no longer useful on distances below the size of atoms, where they must be corrected by a more fundamental theory. Physicists therefore interpret the singularities in general relativity as signs that the theory is no longer applicable and must be corrected.

    Many physicists believe that a theory of quantum gravity will also shed light on other puzzles, such as the nature of dark energy or the unification of the other three known forces, the strong, electromagnetic, and weak [interaction].

    Given that thousands of the brightest minds have tried their hands at it, 80 years seems a long time for a question to remain unanswered. But physicists are not giving up. They know there must be an answer—if only they could find it.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    NOVA is the highest rated science series on television and the most watched documentary series on public television. It is also one of television’s most acclaimed series, having won every major television award, most of them many times over.

     
  • richardmitnick 2:48 pm on February 2, 2016 Permalink | Reply
    Tags: , , , Quantum Gravity   

    From FNAL: “Quantum Gravity” video with Don Lincoln 

    FNAL II photo

    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    2.2.16
    FNAL Don Lincoln
    Don Lincoln

    While there are many challenges facing modern particle physics, perhaps the ultimate one (and certainly among the most difficult) is to describe the nature of gravity in the quantum realm. Despite a century of effort, scientists have had only the most cursory of success. In this video, Fermilab’s Dr. Don Lincoln talks about the idea of quantum gravity and sketches out the need for this difficult advance.


    Download the mp4 video here .

    Watch, enjoy, learn.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Fermilab Campus

    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics. Fermilab is America’s premier laboratory for particle physics and accelerator research, funded by the U.S. Department of Energy. Thousands of scientists from universities and laboratories around the world
    collaborate at Fermilab on experiments at the frontiers of discovery.

     
  • richardmitnick 7:43 pm on November 19, 2015 Permalink | Reply
    Tags: , , Quantum Gravity, ,   

    From SPACE.com: “Einstein’s Unfinished Dream: Marrying Relativity to the Quantum World” 

    space-dot-com logo

    SPACE.com

    November 18, 2015
    FNAL Don Lincoln
    Don Lincoln, Senior Scientist, Fermi National Accelerator Laboratory; Adjunct Professor of Physics, University of Notre Dame

    1
    This artist’s illustration depicts how the foamy structure of space-time may appear, showing tiny bubbles quadrillions of times smaller than the nucleus of an atom that are constantly fluctuating and last for only infinitesimal fractions of a second. Credit: NASA/CXC/M.Weiss

    This November marks the centennial of Albert Einstein’s theory of general relativity. This theory was the crowning achievement of Einstein’s extraordinary scientific life. It taught us that space itself is malleable, bending and stretching under the influence of matter and energy. His ideas revolutionized humanity’s vision of the universe and added such mind-blowing concepts as black holes and wormholes to our imagination.

    Einstein’s theory of general relativity describes a broad range of phenomena, from nearly the moment of creation to the end of time, and even a journey spiraling from the deepest space down into a ravenous black hole, passing through the point of no return of the event horizon, down, down, down, to nearly the center, where the singularity lurks.

    Deep into a quantum world

    If you were reading that last paragraph carefully, you’ll note that I used the word “nearly” twice. And that wasn’t an accident. Einstein’s theory has been brilliantly demonstrated at large size scales. It deftly explains the behavior of orbiting binary pulsars and the orbit of Mercury. It is a crucial component of the GPS system that helps many of us navigate in our cars every day.

    But the beginning of the universe and the region near the center of a black hole are very different worlds — quantum worlds. The size scales involved in those environments are subatomic. And that’s where the trouble starts.

    Einstein’s heyday coincided with the birth of quantum mechanics, and the stories of his debates with physicist Niels Bohr over the theory’s counterintuitive and probabilistic predictions are legendary. “God does not play dice with the universe,” he is famously reported to have said.

    However, regardless of his disdain for the theory of quantum mechanics, Einstein was well aware of the need to understand the quantum realm. And, in his quest to understand and explain general relativity, he sought to understand how of gravity performed in his epic theory when it was applied to the world of the supersmall. The result can be summarized in three words: It failed badly.


    download the mp4 video here.

    Bridging the quantum world to relativity

    Einstein spent the rest of his life, without success, pursuing ways to integrate his theory of general relativity with quantum mechanics. While it is tempting to describe the history of this attempt, the effort is of interest primarily to historians. After all, he didn’t succeed, nor did anyone in the decades that followed.

    Instead, it is more interesting to get a sense of the fundamental problems associated with wedding these two pivotal theories of the early 20th century. The initial issue was a systemic one: General relativity uses a set of differential equations that describe what mathematicians call a smooth and differentiable space. In layman’s terms, this means that the mathematics of general relativity is smooth, without any sharp edges.

    In contrast, quantum mechanics describes a quantized world, e.g. a world in which matter comes in discrete chunks. This means that there is an object here, but not there. Sharp edges abound.

    The water analogy

    In order to clarify these different mathematical formulations, one need think a bit more deeply than usual about a very familiar substance we know quite well: liquid water. Without knowing it, you already hold two different ideas about water that illustrate the tension between differential equations and discrete mathematics.

    For example, when you think of the familiar experience of running your hand through water, you think of water as a continuous substance. The water near your hand is similar to the water a foot away. That distant water might be hotter or colder or moving at a different speed, but the essence of water is the same. As you consider different volumes of water that get closer and closer to your hand, your experience is the same. Even if you think about two volumes of water separated by just a millimeter or half a millimeter, the space between them consists of more water. In fact, the mathematics of fluid flow and turbulence assumes that there is no smallest, indivisible bit of water. Between any two arbitrarily-close distances, there will be water. The mathematics that describes this situation is differential equations. Digging down to its very essence, you find that differential equations assume that there is no smallest distance.

    But you also know that this isn’t true. You know about water molecules. If you consider distances smaller than about three angstroms (the size of a water molecule), everything changes. You can’t get smaller than that, because when you probe even smaller distances, water is no longer a sensible concept. At that point, you’re beginning to probe the empty space inside atoms, in which electrons swirl around a small and dense nucleus. In fact, quantum mechanics is built around the idea that there are smallest objects and discrete distances and energies. This is the reason that a heated gas emits light at specific wavelengths: the electrons orbit at specific energies, with no orbits between the prescribed few.

    Thus a proper quantum theory of water has to take into account the fact that there are individual molecules. There is a smallest distance for which the idea of “water” has any meaning.

    Thus, at the very core, the mathematics of the two theories (e.g. the differential equations of general relativity and the discrete mathematics of quantum mechanics) are fundamentally at odds.


    download the mp4 video here.

    Can the theories merge?

    This is not, in and of itself, an insurmountable difficulty. After all, parts of quantum mechanics are well described by differential equations. But a related problem is that when one tries to merge the two theories, infinities abound; and when an infinity arises in a calculation, this is a red flag that you have somehow done something wrong.

    As an example, suppose you treat an electron as a classical object with no size and calculate how much energy it takes to bring two electrons together. If you did that, you’d find that the energy is infinite. And infinite to a mathematician is a serious business. That’s more energy than all of the energy emitted by all of the stars in the visible universe. While that energy is mind-boggling in its scale, it isn’t infinite. Imagining the energy of the entire universe concentrated in a single point is just unbelievable, and infinite energy is much more than that.

    Therefore, infinities in real calculations are a clear sign that you’ve pushed your model beyond the realm of applicability and you need to start looking to find some new physical principles that you’ve overlooked in your simplified model.

    In the modern day, scientists have tried to solve the same conundrum that so flummoxed Einstein. And the reason is simple: The goal of science is to explain all of physical reality, from the smallest possible objects to the grand vista of the cosmos.

    The hope is to show that all matter originates from a small number of building blocks (perhaps only one) and a single underlying force from which the forces we currently recognize originates. Of the four known fundamental forces of nature, we have been able to devise quantum theories of three: electromagnetism, the strong nuclear force, and the weak nuclear forces. However, a quantum theory of gravity has eluded us.

    General relativity is no doubt an important advance, but until we can devise a quantum theory of gravity, there is no hope of devising a unified theory of everything. While there is no consensus in the scientific community on the right direction in which to proceed, there have been some ideas that have had limited success.

    Superstring theory

    The best-known theory that can describe gravity in the microworld is called superstring theory. In this theory, the smallest known particles should not be thought of as little balls, but rather tiny strings, kind of like an incredibly small stick of uncooked spaghetti or a micro-miniature Hula-Hoop. The basic idea is that these tiny strings (which are smaller compared to a proton than a proton is compared to you) vibrate, and each vibration presents a different fundamental particle.

    Employing a musical metaphor, an electron might be an A-sharp, while a photon could be a D-flat. In the same way that a single violin string can have many overtones, the vibrations of a single superstring can be different particles. The beauty of superstring theory is that it allows for one of the vibrations to be a graviton, which is a particle that has never been discovered but is thought to be the particle that causes gravity.

    It should be noted that superstring theory is not generally accepted, and indeed, some in the scientific community don’t even consider it to be a scientific theory at all. The reason is that, in order for a theory to be scientific, it must be able to be tested, and have the potential to be proven wrong. However, the very small scale of these theoretical strings makes it difficult to imagine any tests that could be done in the foreseeable future. And, some say, if you can’t realistically do a test, it isn’t science.

    Personally, I think that is an extreme opinion, as one can imagine doing such a test when technology advances. But that time will be far in the future.

    Another idea for explaining quantum gravity is called loop quantum gravity. This theory actually quantizes space-time itself. In other words, this model says that there is a smallest bit of space and a shortest time. This provocative idea suggests, among other things, that the speed of light might be different for different wavelengths. However, this effect, if it exists, is small and requires that light travel for great distances before such differences could be observed. Toward that end, scientists are looking at gamma-ray bursts, explosions so bright that they can be seen across billions of light-years — an example of the cosmic helping scientists study the microscopic.

    The simple fact is that we don’t yet have a good and generally accepted theory of quantum gravity. The question is simply just too difficult, for now. The microworld of the quantum and the macroworld of gravity have long resisted a life of wedded bliss and, at least for the moment, they continue to resist. However, scientists continue to find the linkage that blends the two. In the meantime, a theory of quantum gravity remains one of the most ambitious goals of modern science — the hope that we will one day fulfill Einstein’s unfinished dream.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

     
  • richardmitnick 7:24 am on October 24, 2015 Permalink | Reply
    Tags: , , , Quantum Gravity   

    From livescience: “What Is the Smallest Thing in the Universe?” 2012 but Worth Your Time 

    Livescience

    September 17, 2012
    Clara Moskowitz

    1
    One contender for the smallest thing in the universe is the singularity at the center of a black hole. (Shown here, an artist’s drawing of a black hole pulling gas away from a companion star.
    Credit: NASA E/PO, Sonoma State University, Aurore Simonnet

    The answer to the enduring question of the smallest thing in the universe has evolved along with humanity. People once thought grains of sand were the building blocks of what we see around us. Then the atom was discovered, and it was thought indivisible, until it was split to reveal protons, neutrons and electrons inside. These too, seemed like fundamental particles, before scientists discovered that protons and neutrons are made of three quarks each.

    “This time we haven’t been able to see any evidence at all that there’s anything inside quarks,” said physicist Andy Parker. “Have we reached the most fundamental layer of matter?”

    And even if quarks and electrons are indivisible, Parker said, scientists don’t know if they are the smallest bits of matter in existence, or if the universe contains objects that are even more minute.

    Parker, a professor of high-energy physics at England’s Cambridge University, recently hosted a television special on the U.K.’s BBC Two channel called Horizon: How Small is the Universe?

    Strings or points?

    In experiments, teensy, tiny particles like quarks and electrons seem to act like single points of matter with no spatial distribution. But point-like objects complicate the laws of physics. Because you can get infinitely close to a point, the forces acting on it can become infinitely large, and scientists hate infinities.

    An idea called superstring theory could solve this issue. The theory posits that all particles, instead of being point-like, are actually little loops of string. Nothing can get infinitely close to a loop of string, because it will always be slightly closer to one part than another. That “loophole” appears to solve some of these problems of infinities, making the idea appealing to physicists. Yet scientists still have no experimental evidence that string theory is correct.

    Another way of solving the point problem is to say that space itself isn’t continuous and smooth, but is actually made of discrete pixels, or grains, sometimes referred to as space-time foam. In that case, two particles wouldn’t be able to come infinitely close to each other because they would always have to be separated by the minimum size of a grain of space.

    A singularity

    Another contender for the title of smallest thing in the universe is the singularity at the center of a black hole. Black holes are formed when matter is condensed in a small enough space that gravity takes over, causing the matter to pull inward and inward, ultimately condensing into a single point of infinite density. At least, according to the current laws of physics.

    But most experts don’t think black holes are really infinitely dense. They think this infinity is the product of an inherent conflict between two reigning theories — general relativity and quantum mechanics — and that when a theory of quantum gravity can be formulated, the true nature of black holes will be revealed.

    “My guess is that [black hole singularities] are quite a lot smaller than a quark, but I don’t believe they’re of infinite density,” Parker told LiveScience. “Most likely they are maybe a million million times or even more than that smaller than the distances we’ve seen so far.”

    That would make singularities roughly the size of superstrings, if they exist.

    The Planck length

    Superstrings, singularities, and even grains of the universe could all turn out to be about the size of the “Planck length.”

    A Planck length is 1.6 x 10^-35 meters (the number 16 preceded by 34 zeroes and a decimal point) — an incomprehensibly small scale that is implicated in various aspects of physics.

    The Planck length is far and away too small for any instrument to measure, but beyond that, it is thought to represent the theoretical limit of the shortest measureable length. According to the uncertainty principle, no instrument should ever be able to measure anything smaller, because at that range, the universe is probabilistic and indeterminate.

    This scale is also thought to be the demarcating line between general relativity and quantum mechanics.

    “It corresponds to the distance where the gravitational field is so strong that it can start to do things like make black holes out of the energy of the field,” Parker said. “At the Planck length we expect quantum gravity takes over.”

    Perhaps all of the universe’s smallest things are roughly the size of the Planck length.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

     
  • richardmitnick 7:41 am on October 6, 2015 Permalink | Reply
    Tags: , , Quantum Gravity,   

    From NOVA: “Are Space and Time Discrete or Continuous?” 

    PBS NOVA

    NOVA

    01 Oct 2015
    Sabine Hossenfelder

    Split a mile in half, you get half a mile. Split the half mile, you get a quarter, and on and on, until you’ve carved out a length far smaller than the diameter of an atom. Can this slicing continue indefinitely, or will you eventually reach a limit: a smallest hatch mark on the universal ruler?

    The success of some contemporary theories of quantum gravity may hinge on the answer to this question. But the puzzle goes back at least 2500 years, to the paradoxes thought up by the Greek philosopher Zeno of Elea, which remained mysterious from the 5th century BC until the early 1800s. Though the paradoxes have now been solved, the question they posed—is there a smallest unit of length, beyond which you can’t divide any further?—persists.

    1
    Credit: Flickr user Ian Muttoo, adapted under a Creative Commons license.

    The most famous of Zeno’s paradoxes is that of Achilles and the Tortoise in a race. The tortoise gets a head start on the faster-running Achilles. Achilles should quickly catch up—at least that’s what would happen in a real-world footrace. But Zeno argued that Achilles will never pass over the tortoise, because in the time it takes for Achilles to reach the tortoise’s starting point, the tortoise too will have moved forward. While Achilles pursues the tortoise to cover this additional distance, the tortoise moves yet another bit. Try as he might, Achilles only ever reaches the tortoise’s position after the animal has already left it, and he never catches up.

    Obviously, in real life, Achilles wins the race. So, Zeno argued, the assumptions underlying the scenario must be wrong. Specifically, Zeno believed that space is not indefinitely divisible but has a smallest possible unit of length. This allows Achilles to make a final step surpassing the distance to the tortoise, thereby resolving the paradox.

    It took more than two thousand years to develop the necessary mathematics, but today we know that Zeno’s argument was plainly wrong. After mathematicians understood how to sum an infinite number of progressively smaller steps, they calculated the exact moment Achilles surpasses the tortoise, proving that it does not take forever, even if space is indefinitely divisible.

    Zeno’s paradox is solved, but the question of whether there is a smallest unit of length hasn’t gone away. Today, some physicists think that the existence of an absolute minimum length could help avoid another kind of logical nonsense; the infinities that arise when physicists make attempts at a quantum version of [Albert]Einstein’s General Relativity, that is, a theory of “quantum gravity.” When physicists attempted to calculate probabilities in the new theory, the integrals just returned infinity, a result that couldn’t be more useless. In this case, the infinities were not mistakes but demonstrably a consequence of applying the rules of quantum theory to gravity. But by positing a smallest unit of length, just like Zeno did, theorists can reduce the infinities to manageable finite numbers. And one way to get a finite length is to chop up space and time into chunks, thereby making it discrete: Zeno would be pleased.

    He would also be confused. While almost all approaches to quantum gravity bring in a minimal length one way or the other, not all approaches do so by means of “discretization”—that is, by “chunking” space and time. In some theories of quantum gravity, the minimal length emerges from a “resolution limit,” without the need of discreteness. Think of studying samples with a microscope, for example. Magnify too much, and you encounter a resolution-limit beyond which images remain blurry. And if you zoom into a digital photo, you eventually see single pixels: further zooming will not reveal any more detail. In both cases there is a limit to resolution, but only in the latter case is it due to discretization.

    In these examples the limits could be overcome with better imaging technology; they are not fundamental. But a resolution-limit due to quantum behavior of space-time would be fundamental. It could not be overcome with better technology.

    So, a resolution-limit seems necessary to avoid the problem with infinities in the development of quantum gravity. But does space-time remain smooth and continuous even on the shortest distance scales, or does it become coarse and grainy? Researchers cannot agree.

    3
    Artist concept of Gravity Probe B orbiting the Earth to measure space-time, a four-dimensional description of the universe including height, width, length, and time.
    Date 18 May 2008
    Source http://www.nasa.gov/mission_pages/gpb/gpb_012.html
    Author NASA

    In string theory, for example, resolution is limited by the extension of the strings (roughly speaking, the size of the ball that you could fit the string inside), not because there is anything discrete. In a competing theory called loop quantum gravity, on the other hand, space and time are broken into discrete blocks, which gives rise to a smallest possible length (expressed in units of the Planck length, about 10-35 meters), area and volume of space-time—the fundamental building blocks of our universe. Another approach to quantum gravity, “asymptotically safe gravity,” has a resolution-limit but no discretization. Yet another approach, “causal sets,” explicitly relies on discretization.

    And that’s not all. Einstein taught us that space and time are joined in one entity: space-time. Most physicists honor Einstein’s insight, and so most approaches to quantum gravity take space and time to either both be continuous or both be discrete. But some dissidents argue that only space or only time should be discrete.

    So how can physicists find out whether space-time is discrete or continuous? Directly measuring the discrete structure is impossible because it is too tiny. But according to some models, the discreteness should affect how particles move through space. It is a miniscule effect, but it adds up for particles that travel over very long distances. If true, this would distort images from far-away stellar objects, either by smearing out the image or by tearing apart the arrival times of particles that were emitted simultaneously and would otherwise arrive on Earth simultaneously. Astrophysicists have looked for both of these signals, but they haven’t found the slightest evidence for graininess.

    Even if the direct effects on particle motion are unmeasurable, defects in the discrete structure could still be observable. Think of space-time like a diamond. Even rare imperfections in atomic lattices spoil a crystal’s ability to transport light in an orderly way, which will ruin a diamond’s clarity. And if the price tags at your jewelry store tell you one thing, it’s that perfection is exceedingly rare. It’s the same with space-time. If space-time is discrete, there should be imperfections. And even if rare, these imperfections will affect the passage of light through space. No one has looked for this yet, and I’m planning to start such a search in the coming months.

    Next to guiding the development of a theory of quantum gravity, finding evidence for space-time discreteness—or ruling it out!—would also be a big step towards solving a modern-day paradox: the black hole information loss problem, posed by Stephen Hawking in 1974. We know that black holes can only store so much information, which is another indication for a resolution-limit. But we do not know exactly how black holes encode the information of what fell inside. A discrete structure would provide us with elementary storage units.

    Black hole information loss is a vexing paradox that Zeno would have appreciated. Let us hope we will not have to wait 2000 years for a solution.

    Editor and author’s picks for further reading

    arXiv: Minimal Length Scale Scenarios for Quantum Gravity

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    NOVA is the highest rated science series on television and the most watched documentary series on public television. It is also one of television’s most acclaimed series, having won every major television award, most of them many times over.

     
  • richardmitnick 11:15 am on October 15, 2014 Permalink | Reply
    Tags: , , , , Quantum Gravity, , , White Holes   

    From NOVA: “Are White Holes Real?” 

    PBS NOVA

    NOVA

    Tue, 19 Aug 2014
    Maggie McKee

    Sailors have their krakens and their sea serpents. Physicists have white holes: cosmic creatures that straddle the line between tall tale and reality. Yet to be seen in the wild, white holes may be only mathematical monsters. But new research suggests that, if a speculative theory called loop quantum gravity is right, white holes could be real—and we might have already observed them.

    wh

    A white whole is, roughly speaking, the opposite of a black hole. “A black hole is a place where you can go in but you can never escape; a white hole is a place where you can leave but you can never go back,” says Caltech physicist Sean Carroll. “Otherwise, [both share] exactly the same mathematics, exactly the same geometry.” That boils down to a few essential features: a singularity, where mass is squeezed into a point of infinite density, and an event horizon, the invisible “point of no return” first described mathematically by the German physicist Karl Schwarzschild in 1916. For a black hole, the event horizon represents a one-way entrance; for a white hole, it’s exit-only.

    There is excellent evidence that black holes really exist, and astrophysicists have a robust understanding of what it takes to make one. To imagine how a white hole might form, though, we have to go out on a bit of an astronomical limb. One possibility involves a spinning black hole. According to [Albert] Einstein’s general theory of relativity, the rotation smears the singularity into a ring, making it possible in theory to travel through the swirling black hole without being crushed. General relativity’s equations suggest that someone falling into such a black hole could fall through a tunnel in space-time called a wormhole and emerge from a white hole that spits its contents into a different region of space or period of time.

    Though mathematical solutions to those equations exist for white holes, “they’re not realistic,” says Andrew Hamilton, an astrophysicist at the University of Colorado at Boulder. That is because they describe universes that contain only black holes, white holes and wormholes—no matter, radiation or energy. Indeed, previous research, including Hamilton’s, suggests that anything that falls into a spinning black hole will essentially plug up the wormhole, preventing the formation of a passage to a white hole.

    But there’s a light at the end of the wormhole, so to speak. General relativity, from which Hamilton draws his predictions, breaks down at a black hole’s singularity. “The energy density and the curvature become so large that classical gravity is not a good description of what’s happening there,” says Stephen Hsu, a physicist at Michigan State University in East Lansing. Perhaps a more complete model of gravity—one that works as well on the quantum scale as it does on large ones—would negate the instability and allow for white holes, he says.

    Indeed, a unified theory that merges gravity and quantum mechanics is one of the holy grails of contemporary physics. Applying one such theory, loop quantum gravity, to black holes, theorists Hal Haggard and Carlo Rovelli of Aix-Marseille University in France have shown that black holes could metamorphose into white holes via a quantum process. In July, they published their work online.

    Loop quantum gravity proposes that space-time is made up of fundamental building blocks shaped like loops. According to Haggard and Rovelli, the loops’ finite size prevents a dying star from collapsing all the way down into a point of infinite density, and the shrinking object rebounds into a white hole instead. This process may take just a few thousandths of a second, but thanks to the intense gravity involved, the effects of relativity make the transformation appear to take much, much longer to anyone watching from afar. That means that minuscule black holes born in the infant universe could “now be ready to pop off like firecrackers,” forming white holes, according to a report in Nature. Some of the explosions astronomers thought were supernovae may actually be the wails of newborn white holes.

    The black-to-white conversion could resolve a nettlesome conundrum known as the black hole information paradox. The notion that information can be destroyed is anathema in physics, and general relativity says that anything, including information, that falls into a black hole can never escape. These two statements are not at odds if black holes simply act as locked safes for any information they slurp up, but Stephen Hawking showed 40 years ago that black holes actually evaporate over time. That led to the disturbing possibility that the information contained within them could be lost too, triggering a debate that rages to this day.

    But if a black hole instead turns into a white hole, then “all the information is recovered,” says Haggard. “We are quite excited about this mechanism because it avoids so many of the thorny issues that surround this discussion.”

    The new work is preliminary, however, and it is far from clear whether loop quantum gravity is an accurate description of reality. The only glimpse we get of white holes might turn out to be those we model in labs and kitchen sinks. But Carroll says that’s okay. Just thinking about these possibly mythical cosmic creatures can improve physicists’ intuition, “even if the real world is messy and not like those exact situations,” he says. “That’s the way in which white holes are very useful.”

    See the full article here.

    NOVA is the highest rated science series on television and the most watched documentary series on public television. It is also one of television’s most acclaimed series, having won every major television award, most of them many times over.

    ScienceSprings relies on technology from

    MAINGEAR computers

    Lenovo
    Lenovo

    Dell
    Dell

     
c
Compose new post
j
Next post/Next comment
k
Previous post/Previous comment
r
Reply
e
Edit
o
Show/Hide comments
t
Go to top
l
Go to login
h
Show/Hide help
shift + esc
Cancel
%d bloggers like this: