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  • richardmitnick 6:26 pm on January 19, 2016 Permalink | Reply
    Tags: , , EHT, eLISA, , , Perimeter Institute   

    From PI: “Preparing for a cosmological challenge” 

    Perimeter Institute
    Perimeter Institute

    January 19, 2016
    Rose Simone

    Einstein’s theory of general relativity may soon be put to the ultimate test through measurements of a black hole’s shadow, say a pair of Perimeter researchers.
    __________________________________________________________________________________________________________________________________________
    Even though it is over 100 years old, Albert Einstein’s theory of general relativity is still a formidable prizefighter.

    The theory, which successfully describes gravity as a consequence of the curvature of spacetime itself, has withstood all the experimental tests that physicists have been able to throw at it over the decades.

    So now, to have any hope of challenging general relativity, they need to bring in a heavyweight. Enter the closest challenger: the smallish but still formidable 4.5-million- at the centre of our own Milky Way galaxy.

    The challenge will be assisted by the Event Horizon Telescope (EHT), a radio telescope array as large as the Earth, being configured to take precise images of the silhouette (or the shadow) of that black hole, known as Sagittarius A*.

    1
    Sag A*. This image was taken with NASA’s Chandra X-Ray Observatory. Ellipses indicate light echoes.

    NASA Chandra Telescope
    NASA/Chandra

    Event Horizon Telescope map
    EHT map

    Meanwhile, Tim Johannsen, a postdoctoral fellow at Perimeter Institute and the University of Waterloo, who works with Avery Broderick, an Associate Faculty member at Perimeter Institute jointly appointed at Waterloo, has led a group of researchers in calculating the measurements that will be used to determine whether general relativity really does stand up in the strong gravity regime of that black hole.

    3
    Perimeter postdoctoral researcher Tim Johannsen.

    4
    Perimeter Associate Faculty member Avery Broderick.

    Their paper was recently published in Physical Review Letters, along with an accessible synopsis of the work.

    When the images from the black hole come in and the measurements outlined in the recent paper are actually taken, it will be the first truly broad test of general relativity in the strong gravity regime.

    “That is very exciting and we expect to be able to do that within the next few years,” Johannsen says.

    Black holes are regions of spacetime, where gravity is so strong that not even light can escape once it has passed the threshold of no return − the event horizon. So as the name implies, they are dark.

    But owing to its immense gravity, the black hole pulls in vast quantities of dust and gas from surrounding stars. These accrete into a hot swirling plasma disk that illuminates the silhouette of the black hole. The EHT will be able to capture this, in images that will be historic firsts.

    A lot of physics will be done with the data gleaned from those images, but putting general relativity to the test is perhaps the most exciting challenge.

    General relativity has been fantastically successful. In every experiment that has been done to test how the sun and stars in our cosmos affect spacetime and exert gravitational pull on other objects, its predictions have held up.

    But the question is whether the theory will continue to hold up in a strong gravity environment, such as the surroundings of a black hole.

    Black holes are so massive and compact that the spacetime-warping effects, predicted by general relativity, would be more evident than around the sun or other stars. They are “orders of magnitude” different as gravitational environments go, Broderick says.

    “That means that this is terra incognita and we don’t know what we are going to find,” Broderick says. The EHT provides “an opportunity to begin probing in a critical way the non-linear nature of general relativity in the strong gravity regime.”

    This is important to physicists because even though general relativity has been enormously successful in explaining the cosmos that we can see, there are a number of difficulties with it. “It is not clear, for example, exactly how it should be combined with the quantum theory that we have, and in fact, it is very difficult to reconcile the two in a grand unification scheme,” Johannsen says.

    Moreover, there is the problem of the mysterious “dark energy” driving the accelerated expansion of spacetime, as well as the conundrum about the nature of “dark matter,” unseen mass theorized as an explanation for observed galaxy rotation rates that prevent galaxy clusters from flying apart. Physicists are hoping for some insights about general relativity in the strong gravity regime to make sense of these mysteries.

    Johannsen’s team has developed a way of checking how much the gravitational environment of this black hole might deviate from the theory of general relativity and other gravity theories.

    The paper sets constraints on the parameters of the size of the shadow to fit with general relativity. Other gravity models also propose modifications to the theory of general relativity, such as the Modified Gravity Theory (MOG) and the Randall-Sundrum-type braneworld model (RS2). The paper sets the constraints for the black hole to fit with these gravity models as well.

    “We have made the first realistic estimate of the high precision with which the EHT can detect the size of the shadow,”Johannsen says. “We show that such a measurement can be a precise test of general relativity.”

    A nice bonus from this work is that researchers will also get much more precise measurements of the mass of the black hole and its distance. “Sharpening the precision is great because that will enable us to get even more precise constraints on deviations from general relativity,” Johannsen adds.

    There are already good measurements of how far away Sagittarius A* is and how massive it is, based on other experiments that have looked at the motion of stars as they orbit the black hole, as well as of masers throughout the Milky Way, Johannsen explains. “People have been doing this for about 20 years.”

    This can be used to figure out what it should look like. But once the images from the EHT are available, it will be possible to check: “Do we get what we expect? Or do we get something else?” Johannsen says.

    Getting the measurements is really a matter of drawing a series of lines from the centre of the black hole image to the edge of its shadow. On the image, it looks like a pie shape with slices. Measuring the lines of each slice and calculating an average “gives us the angular radius of the shadow and then we know how big it is,” Johannsen says.

    6
    A reconstructed image of Sgr A* for an EHT observation at 230 GHz with a seven-station array.

    From the measurements of the size of the shadow, it is possible to see how closely the gravity in the black hole environment matches the predictions of general relativity and of other theories of gravity.

    “If general relativity is not correct, there can be significant change in the size. The shadow can also become asymmetric so that it is no longer circular, but egg-shaped, for example,” Johannsen says.

    Getting to the point of making these measurements will take a couple more years because at least seven or eight of the telescopes in the EHT array must be coordinated to get the data at the same time in a massive worldwide collaboration.

    The amount of raw data that has to be gathered to get the images is so enormous, it can’t even be transmitted over the internet.

    “These are humongous data sets. So they literally have to save all this data on hard drives and put them in a box and ship them,” Johannsen says.

    The hard drives get shipped to the MIT Haystack Observatory, which is the headquarters for the EHT. From there, the raw data is analyzed and the images are produced.

    After the images are produced, Johannsen gets to use his measurement technique to find out if general relativity is correct for the strong gravity environment around this black hole.

    This isn’t the only test of general relativity in the strong gravity regime in the works. There are other sophisticated experiments to detect, for example, the gravitational waves that are predicted by general relativity. But the prime experimental candidate to confirm the existence of gravitational waves would be the Evolved Laser Interferometer Space Antenna (eLISA), a space-based telescope with an estimated launch date of 2034.

    LISA graphic
    NASA LISA
    LISA

    The EHT will produce images in the next few years.

    If it turns out that the measurements yield what was expected and general relativity holds up, that would be interesting, “because Einstein had this theory 100 years ago, and then we will know that it is true,” Johannsen says.

    But if the challenger should prevail, and strong gravity does strike a blow to the theory of general relativity, “that would be big,” he adds.

    See the full article here .

    Please help promote STEM in your local schools.

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    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 9:32 pm on January 11, 2016 Permalink | Reply
    Tags: , , Perimeter Institute, The Sky is the Limit   

    From PI: “The sky as a limit” 

    Perimeter Institute
    Perimeter Institute

    January 11, 2016

    Eamon O’Flynn
    Manager, Media Relations
    eoflynn@perimeterinstitute.ca
    (519) 569-7600 x5071

    Perimeter researchers show how the largest possible structure – the curvature of the universe as a whole – can be used as a lens onto the smallest objects observable today, elementary particles.

    1
    Elliot Nelson (left) and Niayesh Afshordi. No image credit found.

    Perimeter Associate Faculty member Niayesh Afshordi and postdoctoral fellow Elliot Nelson recently won a third-place Buchalter Cosmology Prize for uncovering an entirely new way cosmology can shed light on the future of particle physics.

    1
    Hubble Goes to the eXtreme to Assemble Farthest-Ever View of the Universe

    Like photographers assembling a portfolio of best shots, astronomers have assembled a new, improved portrait of mankind’s deepest-ever view of the universe. Called the eXtreme Deep Field, or XDF, the photo was assembled by combining 10 years of NASA Hubble Space Telescope photographs taken of a patch of sky at the center of the original Hubble Ultra Deep Field.

    NASA Hubble Telescope
    NASA/ESA Hubble

    The XDF is a small fraction of the angular diameter of the full moon. The Hubble Ultra Deep Field is an image of a small area of space in the constellation Fornax, created using Hubble Space Telescope data from 2003 and 2004. By collecting faint light over many hours of observation, it revealed thousands of galaxies, both nearby and very distant, making it the deepest image of the universe ever taken at that time. The new full-color XDF image is even more sensitive, and contains about 5,500 galaxies even within its smaller field of view. The faintest galaxies are one ten-billionth the brightness of what the human eye can see. Magnificent spiral galaxies similar in shape to our Milky Way and the neighboring Andromeda Galaxy appear in this image, as do the large, fuzzy red galaxies where the formation of new stars has ceased. These red galaxies are the remnants of dramatic collisions between galaxies and are in their declining years. Peppered across the field are tiny, faint, more distant galaxies that were like the seedlings from which today’s magnificent galaxies grew. The history of galaxies — from soon after the first galaxies were born to the great galaxies of today, like our Milky Way — is laid out in this one remarkable image.

    Hubble pointed at a tiny patch of southern sky in repeat visits (made over the past decade) for a total of 50 days, with a total exposure time of 2 million seconds. More than 2,000 images of the same field were taken with Hubble’s two premier cameras: the Advanced Camera for Surveys [ACS] and the Wide Field Camera 3 [WFC3], which extends Hubble’s vision into near-infrared light.

    NASA Hubble ACS
    ACS

    NASA Hubble WFC3
    WFC3

    “The XDF is the deepest image of the sky ever obtained and reveals the faintest and most distant galaxies ever seen. XDF allows us to explore further back in time than ever before”, said Garth Illingworth of the University of California at Santa Cruz, principal investigator of the Hubble Ultra Deep Field 2009 (HUDF09) program.

    The universe is 13.7 billion years old, and the XDF reveals galaxies that span back 13.2 billion years in time. Most of the galaxies in the XDF are seen when they were young, small, and growing, often violently as they collided and merged together. The early universe was a time of dramatic birth for galaxies containing brilliant blue stars extraordinarily brighter than our sun. The light from those past events is just arriving at Earth now, and so the XDF is a “time tunnel into the distant past.” The youngest galaxy found in the XDF existed just 450 million years after the universe’s birth in the big bang.

    Before Hubble was launched in 1990, astronomers could barely see normal galaxies to 7 billion light-years away, about halfway across the universe. Observations with telescopes on the ground were not able to establish how galaxies formed and evolved in the early universe.

    Hubble gave astronomers their first view of the actual forms and shapes of galaxies when they were young. This provided compelling, direct visual evidence that the universe is truly changing as it ages. Like watching individual frames of a motion picture, the Hubble deep surveys reveal the emergence of structure in the infant universe and the subsequent dynamic stages of galaxy evolution.

    The infrared vision of NASA’s planned James Webb Space Telescope [JWST] will be aimed at the XDF.

    NASA Webb Telescope
    JWST

    The Webb telescope will find even fainter galaxies that existed when the universe was just a few hundred million years old. Because of the expansion of the universe, light from the distant past is stretched into longer, infrared wavelengths. The Webb telescope’s infrared vision is ideally suited to push the XDF even deeper, into a time when the first stars and galaxies formed and filled the early “dark ages” of the universe with light.
    The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA’s Goddard Space Flight Center in Greenbelt, Md., manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Md., conducts Hubble science operations. STScI is operated by the Association of Universities for Research in Astronomy, Inc., in Washington.
    Date 29 June 2012
    Photographer NASA; ESA; G. Illingworth, D. Magee, and P. Oesch, University of California, Santa Cruz; R. Bouwens, Leiden University; and the HUDF09 Team

    2
    Andromeda Galaxy. Adam Evans

    Their work begins with the knowledge that space is flat. While there are local wrinkles, they are wrinkles in a flat space, not wrinkles in curved space. The universe as a whole is within one percent of flat.

    The problem is that it shouldn’t be. The vacuum of space is not empty; it is filled with fields that may be weak but cannot be zero – nothing quantum can ever be zero, because quantum things wiggle. According to general relativity, such fluctuations should cause spacetime to curve. In fact, a straightforward calculation of how much the vacuum should curve predicts a universe so tightly wound that the moon would not fit inside it.

    Cosmologists have typically worked around this problem – that the universe should be curved, but looks flat – by assuming there is some antigravity that exactly offsets the tendency of the vacuum to curve. This set of off-base predictions and unlikely corrections is known as the cosmological constant problem, and it has been dogging cosmology for more than half a century.

    In this paper, Nelson and Afshordi make no attempt to solve it, but where other cosmologists invoked an offsetting constant and moved on, Nelson and Afshordi went on to ask one more question: Does adding such a constant to cancel the vacuum’s energy guarantee a flat spacetime? Their answer: not quite.

    The vacuum is still filled with quantum fields, and it is the nature of quantum fields to fluctuate. Even if they are perfectly offset such that their average value is zero, they will still fluctuate around that zero point. Those fluctuations should (again) cause space to curve – just not as much.

    In this scenario, the amount of curve created by the known fields – the electromagnetic field, for example, or the Higgs field – is too small to be measured, and is therefore allowed. But any unknown field would have to be weak enough that its fluctuations would not cause an observable curve in the universe. This sets a maximum energy for unknown fields.

    A theoretical maximum on a theoretical field may not sound groundbreaking – but the work opens a new window in an unexpected place: particle physics.

    A particle, quantum mechanics teaches us, is just an excitation of a field. A photon is an excitation of the electric field, for example, and the newly discovered Higgs boson is an excitation of Higgs field. It’s roughly similar to the way a wave is an excitation of the ocean. And just as the height of a breaking wave can tell us something about the depth of the water, the mass of a particle depends on the strength of its corresponding field.

    New kinds of quantum fields are often associated with proposals to extend the Standard Model of particle physics.

    5
    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    If Afshordi and Nelson are right, and there can be no such fields whose fluctuations have enough energy to noticeably curve space, there can be no unknown particles with a mass of more than 35 TeV. The authors predict that if there are new fields and particles associated with an extension to the Standard Model, they will be below that range.

    For generations, particle physics has made progress from the bottom up: building more and more powerful colliders to create – then spot and study – heavier and heavier particles. It is as if we started from the ground floor and built up, discovering more particles at higher altitudes as we went. What Nelson and Afshordi have done is lower the sky.

    There is a great deal of debate in particle physics about whether we should build increasingly powerful accelerators to search for heavier unknown particles. Right now, the most powerful accelerator in the world, the Large Hadron Collider [LHC], runs at a top energy of about 14 TeV; a proposed new super accelerator in China would run at about 100 TeV.

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

    As this debate unfolds, this new work could be particularly useful in helping experimentalists decide which energy levels – which skyscraper heights – are the most interesting.

    The sky does indeed have a limit, this research suggests – and we are about to hit it.

    Read the original prize-winning paper from by Afshordi and Nelson

    See the full article here .

    Please help promote STEM in your local schools.

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    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 4:12 pm on January 2, 2016 Permalink | Reply
    Tags: , , , Perimeter Institute,   

    From PI Via Daily Galaxy: “The Big Bang was a Mirage from a Collapsing Higher-Dimensional Star” February 2015 but Very Interesting 

    Daily Galaxy
    The Daily Galaxy

    Perimeter Institute
    Perimeter Institute
    Perimeter Institute bloc

    February 14, 2015 [Just brought forward – again]
    No writer credit

    Temp 1

    Big Bang was a mirage from collapsing higher-dimensional star, theorists propose. While the recent [ESA]Planck results “prove that inflation is correct”, they leave open the question of how inflation happened.

    ESA Planck
    ESA/Planck

    A new The study could help to show how inflation was triggered by the motion of the Universe through a higher-dimensional reality.
    The event horizon of a black hole — the point of no return for anything that falls in — is a spherical surface. In a higher-dimensional universe, a black hole could have a three-dimensional event horizon, which could spawn a whole new universe as it forms.

    It could be time to bid the Big Bang bye-bye. Cosmologists have speculated that the Universe formed from the debris ejected when a four-dimensional star collapsed into a black hole — a scenario that would help to explain why the cosmos seems to be so uniform in all directions.

    Cosmic Background Radiation Planck
    CMB per Planck

    The standard Big Bang model tells us that the Universe exploded out of an infinitely dense point, or singularity. But nobody knows what would have triggered this outburst: the known laws of physics cannot tell us what happened at that moment.

    “For all physicists know, dragons could have come flying out of the singularity,” says Niayesh Afshordi, an astrophysicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada.

    It is also difficult to explain how a violent Big Bang would have left behind a Universe that has an almost completely uniform temperature, because there does not seem to have been enough time since the birth of the cosmos for it to have reached temperature equilibrium.

    To most cosmologists, the most plausible explanation for that uniformity is that, soon after the beginning of time, some unknown form of energy made the young Universe inflate at a rate that was faster than the speed of light. That way, a small patch with roughly uniform temperature would have stretched into the vast cosmos we see today. But Afshordi notes that “the Big Bang was so chaotic, it’s not clear there would have been even a small homogenous patch for inflation to start working on”.

    In a paper posted last week on the arXiv preprint server1, Afshordi and his colleagues turn their attention to a proposal made in 2000 by a team including Gia Dvali, a physicist now at the Ludwig Maximilians University in Munich, Germany. In that model, our three-dimensional (3D) Universe is a membrane, or brane, that floats through a ‘bulk universe’ that has four spatial dimensions.

    Ashfordi’s team realized that if the bulk universe contained its own four-dimensional (4D) stars, some of them could collapse, forming 4D black holes in the same way that massive stars in our Universe do: they explode as supernovae, violently ejecting their outer layers, while their inner layers collapse into a black hole.

    In our Universe, a black hole is bounded by a spherical surface called an event horizon. Whereas in ordinary three-dimensional space it takes a two-dimensional object (a surface) to create a boundary inside a black hole, in the bulk universe the event horizon of a 4D black hole would be a 3D object — a shape called a hypersphere. When Afshordi’s team modelled the death of a 4D star, they found that the ejected material would form a 3D brane surrounding that 3D event horizon, and slowly expand.

    The authors postulate that the 3D Universe we live in might be just such a brane — and that we detect the brane’s growth as cosmic expansion. “Astronomers measured that expansion and extrapolated back that the Universe must have begun with a Big Bang — but that is just a mirage,” says Afshordi.

    The model also naturally explains our Universe’s uniformity. Because the 4D bulk universe could have existed for an infinitely long time in the past, there would have been ample opportunity for different parts of the 4D bulk to reach an equilibrium, which our 3D Universe would have inherited.

    The picture has some problems, however. Earlier this year, the European Space Agency’s Planck space observatory released data that mapped the slight temperature fluctuations in the cosmic microwave background — the relic radiation that carries imprints of the Universe’s early moments. The observed patterns matched predictions made by the standard Big Bang model and inflation, but the black-hole model deviates from Planck’s observations by about 4%. Hoping to resolve the discrepancy, Afshordi says that his is now refining its model.

    Despite the mismatch, Dvali praises the ingenious way in which the team threw out the Big Bang model. “The singularity is the most fundamental problem in cosmology and they have rewritten history so that we never encountered it,” he says. Whereas the Planck results “prove that inflation is correct”, they leave open the question of how inflation happened, Dvali adds. The study could help to show how inflation is triggered by the motion of the Universe through a higher-dimensional reality, he says.

    Nature doi:10.1038/nature.2013.13743

    See the full article here .

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  • richardmitnick 2:59 pm on December 4, 2015 Permalink | Reply
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    From PI: “Unveiling the Turbulent Times of a Dying Star” 

    Perimeter Institute
    Perimeter Institute

    November 26, 2015

    Eamon O’Flynn
    Manager, Media Relations
    eoflynn@perimeterinstitute.ca
    (519) 569-7600 x5071

    Running sophisticated simulations on a powerful supercomputer, an international research team has glimpsed the unique turbulence that fuels stellar explosions.

    2
    Supercomputer visualization of the toroidal magnetic field in a collapsed, massive star, showing how in a span of 10 milliseconds the rapid differential rotation revs up the stars magnetic field to a million billion times that of our sun (yellow is positive, light blue is negative). Red and blue represent weaker positive and negative magnetic fields, respectively. Credit: Robert R. Sisneros (NCSA) and Philipp Mösta.

    When a dying star goes supernova, it explodes with such ferocity that it outshines the entire galaxy in which it lived, spewing material and energy across unimaginable distances at near-light speed.

    In some cases, these cosmic cataclysms defy expectations, blasting not symmetrically in all directions – as an exploding firework might – but instead launching two narrow beams, known as jets, in opposite directions.

    Temp 1

    Understanding how these jets are created is a vexing challenge, but an international research team has recently employed powerful computer simulations to sleuth out some answers.

    The team – led by Philipp Mösta (NASA Einstein Fellow at UC Berkeley), with Caltech researchers Christian Ott, David Radice and Luke Roberts, Perimeter Institute computational scientist Erik Schnetter, and Roland Haas of the Max-Planck Institute for Gravitational Physics – published their findings Nov. 30 in Nature.

    Their work sheds light on an explosive chain reaction that creates jets and, over time, helps create the structure of the universe as we know it.

    “We were looking for the basic mechanism, the core engine, behind how a collapsing star could lead to the formation of jets,” said Schnetter, who designed computer programs for the simulations employed by the research team to model dying stars.

    That core engine, the team discovered, is a highly turbulent place. Any turbulent system – like an aging car with a deteriorating suspension on a bumpy road – is bound to get progressively more chaotic. In certain types of supernovae, that turbulence is caused by what is known as magnetorotational instability – a type of rapid change within the magnetic field of a spinning system, like some stars.


    Supercomputer visualization of the toroidal magnetic field in a collapsed, massive star, showing how in a span of 10 milliseconds the rapid differential rotation revs up the stars magnetic field to a million billion times that of our sun (yellow is positive, light blue is negative). Red and blue represent weaker positive and negative magnetic fields, respectively. Simulations and visualization by Philipp Mösta.

    Prior to the work of Schnetter and colleagues, this instability was believed to be a possible driver of jet-formation in supernovae, but the evidence to support that belief was scant.

    Uncovering such evidence, Schnetter says, required a something of a scientific perfect storm.

    “You need to have the right people, with the right expertise and the right chemistry between them, you need to have the right understanding of physics and mathematics and computer science, and in the end you need the computer hardware that can actually run the experiment.”

    They assembled the right people and found the computational horsepower they needed at the University of Urbana-Champaign in Illinois.

    The team used Blue Waters, one of the world’s most powerful supercomputers, to run simulations of supernovae explosions – simulations so complex that no typical computer could handle the number-crunching required. On Blue Waters, the simulations provided an unprecedented glimpse into the extreme magnetic forces at play in stellar explosions.

    3
    Cray Blue Waters supercomputer

    The 3D simulations revealed an inverse cascade of magnetic energy in the core of spinning stars, which builds up with enough intensity to launch jets from the stellar poles.

    Though the simulations do not take into account every chaotic variable inside a real supernova, they achieve a new level of understanding that will drive follow-up research with more specialized simulations.

    Deepening our understanding of supernova explosions is an ongoing process, Schnetter says, and one that may help us better understand the origins of – to borrow a phrase from Douglas Adams – life, the universe, and everything.

    The formation of galaxies, stars, and even life itself are fundamentally connected to energy and matter blasted outward in exploding stars. Even our own Sun, which supports all life on our planet, is known to be the descendent of earlier supernovae.

    So the study of stellar explosions is, Schnetter says, deeply connected to some of the most fundamental questions humans can ask about the universe. A nice bonus, he adds, is that supernovae are also really awesome explosions.

    “These are some of the most powerful events in the universe,” he says. “Who wouldn’t want to know more about that?”


    Supercomputer visualization of the toroidal magnetic field in a collapsed, massive star, showing how in a span of 10 milliseconds the rapid differential rotation revs up the star’s magnetic field to a million billion times that of our sun (yellow is positive, light blue is negative). Red and blue represent weaker positive and negative magnetic fields, respectively. From left to right are shown: 500m, 200m, 100m, and 50m simulations. Simulations and visualization by Philipp Mösta.

    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 10:56 am on March 24, 2015 Permalink | Reply
    Tags: , Perimeter Institute,   

    From PI: “Quantum Cause and Effect” 

    Perimeter Institute
    Perimeter Institute

    March 23, 2015
    Erin Bow

    Correlation does not imply causation – unless it’s quantum. That’s the message of surprising new work from Perimeter Institute and the Institute for Quantum Computing.

    Does taking a drug and then getting better mean that the drug made you better? Did that tax cut really stimulate the economy or did it recover on its own? The problem of answering such questions – of inferring causal relationships from correlations – reaches across the sciences, and beyond.

    Normally, correlation by itself does not imply causation. But new research from Perimeter Institute and the Institute for Quantum Computing (IQC) has found that in the case of quantum variables, it sometimes can.

    The new work, just published in Nature Physics, is the result of a collaboration between Perimeter Faculty member Robert Spekkens, IQC Faculty member Kevin Resch, PhD student Katja Ried, MSc students Megan Agnew and Lydia Vermeyden, and Max Planck Institute senior research scientist Dominik Janzing.

    As a practical illustration of the difference between correlation and causation, consider a drug trial: some people take a drug, and some of them get better. Even more promising, the doctors find that among people who took the drug, 60 percent recover; among people who didn’t take the drug, only 40 percent recover. What conclusions can the doctors draw?

    At first blush, it may look as if the drug caused the recovery, but the doctors would need more information before drawing that conclusion. It might be that more men than women chose to take the drug, and more men than women tended to spontaneously recover. In that case, a common cause, gender, could potentially explain the correlation.

    This imaginary drug trial shows how tricky it can be to distinguish cause-effect correlations from correlations springing from common causes. That’s why the caution “correlation does not imply causation” is drilled into the heads of every researcher for whom statistics is of even passing importance.

    Over the last century, scientists, mathematicians, and philosophers have developed a powerful toolkit for untangling webs of cause, effect, and correlation in even the most complex evolving system. The case of systems with only two variables – like the drug trial above – turns out to be the hardest one. If you want to avoid introducing assumptions about what’s happening, you need to intervene on variable A – in this case, taking the drug. That’s why a real drug trial would be carefully randomized, assigning some people to take the drug and others to take a placebo. Only active intervention on variable A can establish its causal relationship with variable B.

    But what of quantum variables? This new research shows that certain kinds of quantum correlations do imply causation – even without the kind of active intervention that classical variables require.

    The new research is both theoretical and experimental. Ried, Spekkens, and Janzing worked from the theoretical end. They considered the situation of an observer who has probed two quantum variables – say, the polarization properties of two photons – and found that they are correlated. The measurement is carried out at two points in time, but the observer doesn’t know if she’s looking at the same photon twice (that is, probing a cause-effect relationship) or looking at a pair of photons in an entangled state (that is, probing a common cause relationship).

    The theorists’ crucial insight was that the correlations measured between a photon at one time and the same photon at another time had a different pattern than the correlations measured between two entangled photons. In other words, they discovered that under the right circumstances, they could tell cause-effect from common cause.

    Meanwhile, at the Institute for Quantum Computing, Agnew, Vermeyden, and Resch had the tools to put this remarkable idea to the test. They built an apparatus that could generate two entangled photons, A and B. They measured A, and then sent the pair through a gate that either transmitted photon A, or switched photon A and photon B and transmitted B.

    Crucially, this gate could swap between the two scenarios, choosing one or the other based on the output of a random number generator. On the other side of this gate, the researchers conducted another measurement while blind to which photon they measured. Just as the theorists predicted, they saw two distinct patterns of correlation emerge.

    This means that researchers measuring quantum variables can do something researchers measuring classical variables cannot: tell the difference between cause-effect and common cause in a system with only two variables, without making an active intervention on the first variable.

    This discovery has significance for both quantum information and quantum foundations.

    The work establishes a new class of things that quantum systems can do which classical systems cannot. It’s too early to say how that may play out, but such quantum advantages underpin the promise of quantum technologies: quantum entanglement, for instance, underlies quantum cryptography, and quantum superposition underlies quantum computation.

    The discovery of new quantum advantages has historically led to interesting places, and the researchers are hopeful that this new quantum advantage will follow suit.

    For those interested in quantum foundations, this work provides a new framework to ask basic questions about quantum mechanics. There is a lively and long-standing debate in the field concerning which quantum concepts are about reality, and which are about our knowledge of reality – for instance, whether the quantum uncertainty about (say) the polarization of a photon means that the photon itself has no defined polarization, or if it means that the observer of such a photon has limited knowledge.

    Because correlations are about what observers can infer, while causal relations are about the physical relations among systems, this research opens a new window on such questions.

    The team describes the work as opening the door to many more lines of inquiry, such as: How can these techniques be generalized to scenarios involving more than two systems? Is the menu of possible causal relations between quantum systems larger than between classical systems? And most broadly and excitingly: How should we understand causality in a quantum world?

    See the full article here.

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    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 1:48 pm on March 1, 2015 Permalink | Reply
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    From Perimeter: “Pioneering Women of Physics” 

    Perimeter Institute
    Perimeter Institute

    February 25, 2015

    For more information, contact:
    Lisa Lambert
    Manager, External Relations and Public Affairs
    llambert@perimeterinstitute.ca
    (519) 569-7600 x5051

    They discovered pulsars, found the first evidence of dark matter, pioneered mathematics, radioactivity, nuclear fission, elasticity, and computer programming, and have even stopped light.
    Perimeter celebrates women who made pioneering contributions to physics, often overcoming tremendous challenges to do so.

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    See the full article here.

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    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 9:02 pm on January 8, 2015 Permalink | Reply
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    From Perimeter: “A simpler way to understand ultra-hot chaos” 

    Perimeter Institute
    Perimeter Institute

    January 8, 2015
    Tenille Bonoguore

    Perimeter researcher Michal P. Heller has created a shortcut to understand primordial matter in its most extreme state.

    To recreate the birth of the universe, most physicists believe that you would first need a big bang. This would be followed almost instantly by the appearance of an ultra-hot blob of plasma, the primordial soup that formed the basis of everything in existence.


    Quark-gluon plasma

    It’s not exactly a process that is easily replicated, but in heavy-ion colliders, experimentalists are creating “little bangs” to produce tiny droplets of quark-gluon plasma (QGP).

    The experiments offer a glimpse of the kind of matter that filled the early universe, but there’s a problem. In its earliest stages – between “bang” and “goop” – the system is in extreme distortion. In fact, it’s not until the system cools down that it reaches equilibrium and can be assessed.

    It is this period of ultra-hot chaos that intrigues Perimeter Institute postdoctoral researcher Michal P. Heller. And in a new paper published recently in Physical Review Letters, he has brought together two seemingly different fields of study – hydrodynamics and string theory – to help describe it.

    In ordinary matter, quarks do not exist in isolation; they are always bound extremely tightly by gluons in atomic nuclei. But in extreme conditions – say, in temperatures a million times hotter than the sun – these bonds can “melt,” forming an ultra-hot, almost-frictionless plasma in which quarks and gluons move freely.

    QGP has recently been created by smashing atomic nuclei against each other at the Relativistic Heavy-Ion Collider at the Brookhaven National Laboratory in the United States, and at the Large Hadron Collider at CERN in Switzerland.

    BNL RHIC
    BNL RHIC Campus
    RHIC at BNL

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

    The tiny droplets produced by these “little bangs” exist for brief flashes of time, like super-hot fireballs that quickly expand and cool into ordinary matter. In the moment after the collision, the ultra-hot system is in extreme distortion, existing at the threshold of being called a fluid.

    Despite the fact that theorists know the microscopic rules that govern such ultra-energetic collisions, questions still abound. It is clear these systems only become plasma once they settle enough to reach equilibrium. Working out what happens in the chaotic interim, before the system equilibrates, requires extremely complex computations.

    The standard modelling for plasma experiments uses relativistic hydrodynamics, a theory similar to that describing the motion of water but which also incorporates [Albert] Einstein’s special relativity. (This is because QGP and its microscopic constituents move with large velocities, at which relativistic effects become important.)

    Some researchers, including Heller, have simplified the problem by equating the relaxation of the QGP “fireball” to a black hole reaching equilibrium in a hypothetical five-dimensional space. This approach takes methods derived from string theory, and applies them to the physics of the “little bang” experiments and the droplets of QGP they create.

    Now, Heller and his co-authors have put forward a computational technique that is something of a further short-cut for theorists: instead of doing the calculations using five-dimensional Einstein equations – which is very complicated – they have developed a way to incorporate part of those calculations into a four-dimensional description that is coupled to conventional hydrodynamics equations.

    “The paper shows something about the theories of relativistic hydrodynamics that was known, but not many people had thought about it seriously,” Heller says. “Our observation opens up a new possibility of describing transient relaxation effects governing the approach to the quark-gluon plasma phase.”

    This work is a return of sorts for Heller, a Polish scientist who came to Perimeter in 2014 from the University of Amsterdam. His research career began with a 2007 paper studying theories of second-order relativistic hydrodynamics, which factors causal evolution into standard fluid dynamics. In 2012 and 2013, his work in string theory and strong gravity brought him back to those theories, but with a new perspective.

    “What’s been fun is coming back to the project I started my research career with, and realizing that the things which I thought several years ago were simple are actually not so trivial and have far-reaching consequences,” he says.

    When he was younger, he thought the universe could be understood through one simple model. Now, he sees much more nuance: “At some point, you start appreciating that everything is complex and interconnected.”

    QGP is like that, too, he says. While researchers would probably like to create QGP in a simple state of equilibrium (so that they can introduce their own distortions and measure the effects), reality is much more complex. Current experimental and theoretical approaches aren’t sensitive enough to capture and analyze in detail the droplets’ initial, highly distorted state, but Heller’s paper is a step in this direction.

    “Is that a choice? It’s more of a necessity, at least given what we have available here on Earth,” he says.

    Should other researchers build on this work, combining it with complementary approaches of initial state physics to construct some sort of a hybrid, he says these generalized theories of hydrodynamics “will be a crucial ingredient of whatever comes next.”

    See the full article here.

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    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 3:12 pm on October 17, 2014 Permalink | Reply
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    From Perimeter: “The Last Gasp of a Black Hole” 

    Perimeter Institute
    Perimeter Institute

    October 17, 2014
    No Writer Credit

    New research from Perimeter shows that two of the strangest features of quantum mechanicsentanglement and negative energy – might be two faces of one coin.

    Quantum mechanics is, notoriously, weird. Take entanglement: when two or more particles are entangled, their states are linked together, no matter how far apart they go.

    If the idea makes your classical mind twitch, you’re in good company. At the heart of everything, according to quantum mechanics, nature has a certain amount of irreducible jitter. Even nothing – the vacuum of space – can jitter, or as physicists say, fluctuate. When it does, a particle and its anti-particle can pop into existence.

    For example, an electron and an anti-electron (these are called positrons) might pop into existence out of the vacuum. We know that they each have a spin of one half, which might be either up or down. We also know that these particles were created from nothing and so, to balance the books, the total spin must add up to zero. Finally, we know that the spin of either particle is not determined until it is measured.

    So suppose the electron and the positron fly apart a few metres or a few light years, and then a physicist comes by to measure the spin of, say, the electron. She discovers that the electron is spin up, and in that moment, the electron becomes spin up. Meanwhile, a few metres or a few light years away, the positron becomes spin down. Instantly. That is the strangeness of quantum entanglement.

    Negative energy is less well known than entanglement, but no less weird. It begins with the idea – perhaps already implied by the positron and electron popping out of nowhere – that empty space is not empty. It is filled with quantum fields, and the energy of those fields can fluctuate a little bit.

    In fact, the energy of these fields can dip under the zero mark, albeit briefly. When that happens, a small region of space can, for a short span of time, weigh less than nothing – or at least less than the vacuum. It’s a little bit like finding dry land below sea level.

    Despite their air of strangeness, entanglement and negative energy are both well-explored topics. But now, new research, published as a Rapid Communication in Physical Review D, is hinting that these two strange phenomena may be linked in a surprising way.

    The work was done by Perimeter postdoctoral fellow Matteo Smerlak and former postdoc Eugenio Bianchi (now on the faculty at Penn State and a Visiting Fellow at Perimeter). “Negative energy and entanglement are two of the most striking features of quantum mechanics,” says Smerlak. “Now, we think they might be two sides of the same coin.”

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    Perimeter Postdoctoral Researcher Matteo Smerlak

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    Perimeter Visiting Fellow Eugenio Bianchi

    Specifically, the researchers proved mathematically that any external influence that changes the entanglement of a system in its vacuum state must also produce some amount of negative energy. The reverse, they say, is also true: negative energy densities can never be produced without entanglement being directly affected.

    At the moment, the result only applies to certain quantum fields in two dimensions – to light pulses travelling up and down a thin cable, for instance. And it is with light that the Perimeter researchers hope that their new idea can be directly tested.

    “Some quantum states which have negative energy are known, and one of them is called a ‘squeezed state,’ and they can be produced in the lab, by optical devices called squeezers,” says Smerlak. The squeezers manipulate light to produce an observable pattern of negative energy.

    Remember that Smerlak and Bianchi’s basic argument is that if an external influence affects vacuum entanglement, it will also release some negative energy. In a quantum optics setup, the squeezers are the external influence.

    Experimentalists should be able to look for the correlation between the entanglement patterns and the negative energy densities which this new research predicts. If these results hold up – always a big if in brand new work – and if they can make the difficult leap from two dimensions to the real world, then there will be startling implications for black holes.

    Like optical squeezers, black holes also produce changes in entanglement and energy density. They do this by separating entangled pairs of particles and preferentially selecting the ones with negative energy.

    Remember that the vacuum is full of pairs of particles and antiparticles blinking into existence. Under normal circumstances, they blink out again just as quickly, as the particle and the antiparticle annihilate each other. But just at a black hole’s event horizon, it sometimes happens that one of the particles is sucked in, while the other escapes. The small stream of escaping particles is known as Hawking radiation.

    By emitting such radiation, black holes slowly give up their energy and mass, and eventually disappear. Black hole evaporation, as the process is known, is a hot topic in physics. This new research has the potential to change the way we think about it.

    “In the late stages of the evaporation of a black hole, the energy released from the black hole will turn negative,” says Smerlak. And if a black hole releases negative energy, then its total energy goes up, not down. “It means that the black hole will shrink and shrink and shrink – for zillions of years – but in the end, it will release its negative energy in a gasp before dying. Its mass will briefly go up.”

    Call it the last gasp of a black hole.

    See the full article here.

    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.

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  • richardmitnick 7:35 pm on August 7, 2014 Permalink | Reply
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    From Perimeter Institute: “The Black Hole at the Birth of the Universe” 

    Perimeter Institute
    Perimeter Institute

    August 7, 2014
    Colin Hunter

    The big bang poses a big question: if it was indeed the cataclysm that blasted our universe into existence 13.7 billion years ago, what sparked it?

    Three Perimeter Institute researchers have a new idea about what might have come before the big bang. It’s a bit perplexing, but it is grounded in sound mathematics, testable, and enticing enough to earn the cover story in Scientific American, called The Black Hole at the Beginning of Time.

    What we perceive as the big bang, they argue, could be the three-dimensional “mirage” of a collapsing star in a universe profoundly different than our own.

    “Cosmology’s greatest challenge is understanding the big bang itself,” write Perimeter Institute Associate Faculty member Niayesh Afshordi, Affiliate Faculty member and University of Waterloo professor Robert Mann, and PhD student Razieh Pourhasan.

    Conventional understanding holds that the big bang began with a singularity – an unfathomably hot and dense phenomenon of spacetime where the standard laws of physics break down. Singularities are bizarre, and our understanding of them is limited.

    “For all physicists know, dragons could have come flying out of the singularity,” Afshordi says in an interview with Nature.

    The problem, as the authors see it, is that the big bang hypothesis has our relatively comprehensible, uniform, and predictable universe arising from the physics-destroying insanity of a singularity. It seems unlikely.

    So perhaps something else happened. Perhaps our universe was never singular in the first place.

    Their suggestion: our known universe could be the three-dimensional “wrapping” around a four-dimensional black hole’s event horizon. In this scenario, our universe burst into being when a star in a four-dimensional universe collapsed into a black hole.

    In our three-dimensional universe, black holes have two-dimensional event horizons – that is, they are surrounded by a two-dimensional boundary that marks the “point of no return.” In the case of a four-dimensional universe, a black hole would have a three-dimensional event horizon.

    In their proposed scenario, our universe was never inside the singularity; rather, it came into being outside an event horizon, protected from the singularity. It originated as – and remains – just one feature in the imploded wreck of a four-dimensional star.

    The researchers emphasize that this idea, though it may sound “absurd,” is grounded firmly in the best modern mathematics describing space and time. Specifically, they’ve used the tools of holography to “turn the big bang into a cosmic mirage.” Along the way, their model appears to address long-standing cosmological puzzles and – crucially – produce testable predictions.

    Of course, our intuition tends to recoil at the idea that everything and everyone we know emerged from the event horizon of a single four-dimensional black hole. We have no concept of what a four-dimensional universe might look like. We don’t know how a four-dimensional “parent” universe itself came to be.

    But our fallible human intuitions, the researchers argue, evolved in a three-dimensional world that may only reveal shadows of reality.

    They draw a parallel to Plato’s allegory of the cave, in which prisoners spend their lives seeing only the flickering shadows cast by a fire on a cavern wall.

    “Their shackles have prevented them from perceiving the true world, a realm with one additional dimension,” they write. “Plato’s prisoners didn’t understand the powers behind the sun, just as we don’t understand the four-dimensional bulk universe. But at least they knew where to look for answers.”

    See the full article here.

    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.

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  • richardmitnick 7:01 am on July 4, 2014 Permalink | Reply
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    From The Perimeter Institute: “From Pencil Marks To Quantum Computers” 

    Perimeter Institute
    Perimeter Institute

    July 3, 2014
    Erin Bow

    Pick up a pencil. Make a mark on a piece of paper. Congratulations: you are doing cutting-edge condensed matter physics. You might even be making the first mark on the road to quantum computers, according to new Perimeter research.

    Introducing graphene

    One of the hottest materials in condensed matter research today is graphene.

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    Graphene is an atomic-scale honeycomb lattice made of carbon atoms.

    Graphene had an unlikely start: it began with researchers messing around with pencil marks on paper. Pencil “lead” is actually made of graphite, which is a soft crystal lattice made of nothing but carbon atoms. When pencils deposit that graphite on paper, the lattice is laid down in thin sheets. By pulling that lattice apart into thinner sheets – originally using Scotch tape – researchers discovered that they could make flakes of crystal just one atom thick.

    The name for this atom-scale chicken wire is graphene. Those folks with the Scotch tape, Andre Geim and Konstantin Novoselov, won the 2010 Nobel Prize for discovering it. “As a material, it is completely new – not only the thinnest ever but also the strongest,” wrote the Nobel committee. “As a conductor of electricity, it performs as well as copper. As a conductor of heat, it outperforms all other known materials. It is almost completely transparent, yet so dense that not even helium, the smallest gas atom, can pass through it.”

    Developing a theoretical model of graphene

    Graphene is not just a practical wonder – it’s also a wonderland for theorists. Confined to the two-dimensional surface of the graphene, the electrons behave strangely. All kinds of new phenomena can be seen, and new ideas can be tested. Testing new ideas in graphene is exactly what Perimeter researchers Zlatko Papić and Dmitry (Dima) Abanin set out to do.

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    Perimeter postdoctoral researcher Zlatko Papić

    abannin
    Perimeter Faculty member Dmitry Abanin

    “Dima and I started working on graphene a very long time ago,” says Papić. “We first met in 2009 at a conference in Sweden. I was a grad student and Dima was in the first year of his postdoc, I think.”

    The two young scientists got to talking about what new physics they might be able to observe in the strange new material when it is exposed to a strong magnetic field.

    “We decided we wanted to model the material,” says Papić. They’ve been working on their theoretical model of graphene, on and off, ever since. The two are now both at Perimeter Institute, where Papić is a postdoctoral researcher and Abanin is a faculty member. They are both cross-appointed with the Institute for Quantum Computing (IQC) at the University of Waterloo.

    In January 2014, they published a paper in Physical Review Letters presenting new ideas about how to induce a strange but interesting state in graphene – one where it appears as if particles inside it have a fraction of an electron’s charge.

    It’s called the fractional quantum Hall effect (FQHE), and it’s head turning. Like the speed of light or Planck’s constant, the charge of the electron is a fixed point in the disorienting quantum universe.

    Every system in the universe carries whole multiples of a single electron’s charge. When the FQHE was first discovered in the 1980s, condensed matter physicists quickly worked out that the fractionally charged “particles” inside their semiconductors were actually quasiparticles – that is, emergent collective behaviours of the system that imitate particles.

    Graphene is an ideal material in which to study the FQHE. “Because it’s just one atom thick, you have direct access to the surface,” says Papić. “In semiconductors, where FQHE was first observed, the gas of electrons that create this effect are buried deep inside the material. They’re hard to access and manipulate. But with graphene you can imagine manipulating these states much more easily.”

    In the January paper, Abanin and Papić reported novel types of FQHE states that could arise in bilayer graphene – that is, in two sheets of graphene laid one on top of another – when it is placed in a strong perpendicular magnetic field. In an earlier work from 2012, they argued that applying an electric field across the surface of bilayer graphene could offer a unique experimental knob to induce transitions between FQHE states. Combining the two effects, they argued, would be an ideal way to look at special FQHE states and the transitions between them.

    Experimental tests

    Two experimental groups – one in Geneva, involving Abanin, and one at Columbia, involving both Abanin and Papić – have since put the electric field + magnetic field method to good use. The paper by the Columbia group appears in the July 4 issue of Science. A third group, led by Amir Yacoby of Harvard, is doing closely related work.

    “We often work hand-in-hand with experimentalists,” says Papić. “One of the reasons I like condensed matter is that often even the most sophisticated, cutting-edge theory stands a good chance of being quickly checked with experiment.”

    Inside both the magnetic and electric field, the electrical resistance of the graphene demonstrates the strange behaviour characteristic of the FQHE. Instead of resistance that varies in a smooth curve with voltage, resistance jumps suddenly from one level to another, and then plateaus – a kind of staircase of resistance. Each stair step is a different state of matter, defined by the complex quantum tangle of charges, spins, and other properties inside the graphene.

    “The number of states is quite rich,” says Papić. “We’re very interested in bilayer graphene because of the number of states we are detecting and because we have these mechanisms – like tuning the electric field – to study how these states are interrelated, and what happens when the material changes from one state to another.”

    For the moment, researchers are particularly interested in the stair steps whose “height” is described by a fraction with an even denominator. That’s because the quasiparticles in that state are expected to have an unusual property.

    There are two kinds of particles in our three-dimensional world: fermions (such as electrons), where two identical particles can’t occupy one state, and bosons (such as photons), where two identical particles actually want to occupy one state. In three dimensions, fermions are fermions and bosons are bosons, and never the twain shall meet.

    But a sheet of graphene doesn’t have three dimensions – it has two. It’s effectively a tiny two-dimensional universe, and in that universe, new phenomena can occur. For one thing, fermions and bosons can meet halfway – becoming anyons, which can be anywhere in between fermions and bosons. The quasiparticles in these special stair-step states are expected to be anyons.

    In particular, the researchers are hoping these quasiparticles will be non-Abelian anyons, as their theory indicates they should be. That would be exciting because non-Abelian anyons can be used in the making of qubits.

    Graphene qubits?

    Qubits are to quantum computers what bits are to ordinary computers: both a basic unit of information and the basic piece of equipment that stores that information. Because of their quantum complexity, qubits are more powerful than ordinary bits and their power grows exponentially as more of them are added. A quantum computer of only a hundred qubits can tackle certain problems beyond the reach of even the best non-quantum supercomputers. Or, it could, if someone could find a way to build stable qubits.

    The drive to make qubits is part of the reason why graphene is a hot research area in general, and why even-denominator FQHE states – with their special anyons – are sought after in particular.

    “A state with some number of these anyons can be used to represent a qubit,” says Papić. “Our theory says they should be there and the experiments seem to bear that out – certainly the even-denominator FQHE states seem to be there, at least according to the Geneva experiments.”

    That’s still a step away from experimental proof that those even-denominator stair-step states actually contain non-Abelian anyons. More work remains, but Papić is optimistic: “It might be easier to prove in graphene than it would be in semiconductors. Everything is happening right at the surface.”

    It’s still early, but it looks as if bilayer graphene may be the magic material that allows this kind of qubit to be built. That would be a major mark on the unlikely line between pencil lead and quantum computers.

    See the full article here.

    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.

     
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