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  • richardmitnick 7:17 am on August 13, 2016 Permalink | Reply
    Tags: , , , , , , Quanta Magazine   

    From Quanta: “What No New Particles Means for Physics” 

    Quanta Magazine
    Quanta Magazine

    August 9, 2016
    Natalie Wolchover

    Olena Shmahalo/Quanta Magazine

    Physicists at the Large Hadron Collider (LHC) in Europe have explored the properties of nature at higher energies than ever before, and they have found something profound: nothing new.

    It’s perhaps the one thing that no one predicted 30 years ago when the project was first conceived.

    The infamous “diphoton bump” that arose in data plots in December has disappeared, indicating that it was a fleeting statistical fluctuation rather than a revolutionary new fundamental particle. And in fact, the machine’s collisions have so far conjured up no particles at all beyond those catalogued in the long-reigning but incomplete “Standard Model” of particle physics.

    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.
    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.

    In the collision debris, physicists have found no particles that could comprise dark matter, no siblings or cousins of the Higgs boson, no sign of extra dimensions, no leptoquarks — and above all, none of the desperately sought supersymmetry particles that would round out equations and satisfy “naturalness,” a deep principle about how the laws of nature ought to work.

    CERN ATLAS Higgs Event
    CERN ATLAS Higgs Event

    CERN CMS Higgs Event
    CERN CMS Higgs Event

    “It’s striking that we’ve thought about these things for 30 years and we have not made one correct prediction that they have seen,” said Nima Arkani-Hamed, a professor of physics at the Institute for Advanced Study in Princeton, N.J.

    The news has emerged at the International Conference on High Energy Physics in Chicago over the past few days in presentations by the ATLAS and CMS experiments, whose cathedral-like detectors sit at 6 and 12 o’clock on the LHC’s 17-mile ring.

    CERN/ATLAS detector
    CERN/ATLAS detector

    CERN/CMS Detector
    CERN/CMS Detector

    Both teams, each with over 3,000 members, have been working feverishly for the past three months analyzing a glut of data from a machine that is finally running at full throttle after being upgraded to nearly double its previous operating energy. It now collides protons with 13 trillion electron volts (TeV) of energy — more than 13,000 times the protons’ individual masses — providing enough raw material to beget gargantuan elementary particles, should any exist.

    Lucy Reading-Ikkanda for Quanta Magazine

    So far, none have materialized. Especially heartbreaking for many is the loss of the diphoton bump, an excess of pairs of photons that cropped up in last year’s teaser batch of 13-TeV data, and whose origin has been the speculation of some 500 papers by theorists. Rumors about the bump’s disappearance in this year’s data began leaking in June, triggering a community-wide “diphoton hangover.”

    “It would have single-handedly pointed to a very exciting future for particle experiments,” said Raman Sundrum, a theoretical physicist at the University of Maryland. “Its absence puts us back to where we were.”

    The lack of new physics deepens a crisis that started in 2012 during the LHC’s first run, when it became clear that its 8-TeV collisions would not generate any new physics beyond the Standard Model. (The Higgs boson, discovered that year, was the Standard Model’s final puzzle piece, rather than an extension of it.) A white-knight particle could still show up later this year or next year, or, as statistics accrue over a longer time scale, subtle surprises in the behavior of the known particles could indirectly hint at new physics. But theorists are increasingly bracing themselves for their “nightmare scenario,” in which the LHC offers no path at all toward a more complete theory of nature.

    Some theorists argue that the time has already come for the whole field to start reckoning with the message of the null results. The absence of new particles almost certainly means that the laws of physics are not natural in the way physicists long assumed they are. “Naturalness is so well-motivated,” Sundrum said, “that its actual absence is a major discovery.”

    Missing Pieces

    The main reason physicists felt sure that the Standard Model could not be the whole story is that its linchpin, the Higgs boson, has a highly unnatural-seeming mass. In the equations of the Standard Model, the Higgs is coupled to many other particles. This coupling endows those particles with mass, allowing them in turn to drive the value of the Higgs mass to and fro, like competitors in a tug-of-war. Some of the competitors are extremely strong — hypothetical particles associated with gravity might contribute (or deduct) as much as 10 million billion TeV to the Higgs mass — yet somehow its mass ends up as 0.125 TeV, as if the competitors in the tug-of-war finish in a near-perfect tie. This seems absurd — unless there is some reasonable explanation for why the competing teams are so evenly matched.

    Maria Spiropulu of the California Institute of Technology, pictured in the LHC’s CMS control room, brushed aside talk of a nightmare scenario, saying, “Experimentalists have no religion.” Courtesy of Maria Spiropulu

    Supersymmetry, as theorists realized in the early 1980s, does the trick. It says that for every “fermion” that exists in nature — a particle of matter, such as an electron or quark, that adds to the Higgs mass — there is a supersymmetric “boson,” or force-carrying particle, that subtracts from the Higgs mass. This way, every participant in the tug-of-war game has a rival of equal strength, and the Higgs is naturally stabilized. Theorists devised alternative proposals for how naturalness might be achieved, but supersymmetry had additional arguments in its favor: It caused the strengths of the three quantum forces to exactly converge at high energies, suggesting they were unified at the beginning of the universe. And it supplied an inert, stable particle of just the right mass to be dark matter.

    “We had figured it all out,” said Maria Spiropulu, a particle physicist at the California Institute of Technology and a member of CMS. “If you ask people of my generation, we were almost taught that supersymmetry is there even if we haven’t discovered it. We believed it.”

    Standard model of Supersymmetry DESY
    Standard model of Supersymmetry DESY

    Hence the surprise when the supersymmetric partners of the known particles didn’t show up — first at the Large Electron-Positron Collider in the 1990s, then at the Tevatron in the 1990s and early 2000s, and now at the LHC. As the colliders have searched ever-higher energies, the gap has widened between the known particles and their hypothetical superpartners, which must be much heavier in order to have avoided detection. Ultimately, supersymmetry becomes so “broken” that the effects of the particles and their superpartners on the Higgs mass no longer cancel out, and supersymmetry fails as a solution to the naturalness problem. Some experts argue that we’ve passed that point already. Others, allowing for more freedom in how certain factors are arranged, say it is happening right now, with ATLAS and CMS excluding the stop quark — the hypothetical superpartner of the 0.173-TeV top quark — up to a mass of 1 TeV. That’s already a nearly sixfold imbalance between the top and the stop in the Higgs tug-of-war. Even if a stop heavier than 1 TeV exists, it would be pulling too hard on the Higgs to solve the problem it was invented to address.

    “I think 1 TeV is a psychological limit,” said Albert de Roeck, a senior research scientist at CERN, the laboratory that houses the LHC, and a professor at the University of Antwerp in Belgium.

    Some will say that enough is enough, but for others there are still loopholes to cling to. Among the myriad supersymmetric extensions of the Standard Model, there are more complicated versions in which stop quarks heavier than 1 TeV conspire with additional supersymmetric particles to counterbalance the top quark, tuning the Higgs mass. The theory has so many variants, or individual “models,” that killing it outright is almost impossible. Joe Incandela, a physicist at the University of California, Santa Barbara, who announced the discovery of the Higgs boson on behalf of the CMS collaboration in 2012, and who now leads one of the stop-quark searches, said, “If you see something, you can make a model-independent statement that you see something. Seeing nothing is a little more complicated.”

    Particles can hide in nooks and crannies. If, for example, the stop quark and the lightest neutralino (supersymmetry’s candidate for dark matter) happen to have nearly the same mass, they might have stayed hidden so far. The reason for this is that, when a stop quark is created in a collision and decays, producing a neutralino, very little energy will be freed up to take the form of motion. “When the stop decays, there’s a dark-matter particle just kind of sitting there,” explained Kyle Cranmer of New York University, a member of ATLAS. “You don’t see it. So in those regions it’s very difficult to look for.” In that case, a stop quark with a mass as low as 0.6 TeV could still be hiding in the data.

    Experimentalists will strive to close these loopholes in the coming years, or to dig out the hidden particles. Meanwhile, theorists who are ready to move on face the fact that they have no signposts from nature about which way to go. “It’s a very muddled and uncertain situation,” Arkani-Hamed said.

    New Hope

    Many particle theorists now acknowledge a long-looming possibility: that the mass of the Higgs boson is simply unnatural — its small value resulting from an accidental, fine-tuned cancellation in a cosmic game of tug-of-war — and that we observe such a peculiar property because our lives depend on it. In this scenario, there are many, many universes, each shaped by different chance combinations of effects. Out of all these universes, only the ones with accidentally lightweight Higgs bosons will allow atoms to form and thus give rise to living beings. But this “anthropic” argument is widely disliked for being seemingly untestable.

    In the past two years, some theoretical physicists have started to devise totally new natural explanations for the Higgs mass that avoid the fatalism of anthropic reasoning and do not rely on new particles showing up at the LHC. Last week at CERN, while their experimental colleagues elsewhere in the building busily crunched data in search of such particles, theorists held a workshop to discuss nascent ideas such as the relaxion hypothesis — which supposes that the Higgs mass, rather than being shaped by symmetry, was sculpted dynamically by the birth of the cosmos — and possible ways to test these ideas. Nathaniel Craig of the University of California, Santa Barbara, who works on an idea called neutral naturalness, said in a phone call from the CERN workshop, “Now that everyone is past their diphoton hangover, we’re going back to these questions that are really aimed at coping with the lack of apparent new physics at the LHC.”

    Arkani-Hamed, who, along with several colleagues, recently proposed another new approach called Nnaturalness, said, “There are many theorists, myself included, who feel that we’re in a totally unique time, where the questions on the table are the really huge, structural ones, not the details of the next particle. We’re very lucky to get to live in a period like this — even if there may not be major, verified progress in our lifetimes.”

    As theorists return to their blackboards, the 6,000 experimentalists with CMS and ATLAS are reveling in their exploration of a previously uncharted realm. “Nightmare, what does it mean?” said Spiropulu, referring to theorists’ angst about the nightmare scenario. “We are exploring nature. Maybe we don’t have time to think about nightmares like that, because we are being flooded in data and we are extremely excited.”

    There’s still hope that new physics will show up. But discovering nothing, in Spiropulu’s view, is a discovery all the same — especially when it heralds the death of cherished ideas. “Experimentalists have no religion,” she said.

    Some theorists agree. Talk of disappointment is “crazy talk,” Arkani-Hamed said. “It’s actually nature! We’re learning the answer! These 6,000 people are busting their butts and you’re pouting like a little kid because you didn’t get the lollipop you wanted?”

    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 3:21 pm on August 11, 2016 Permalink | Reply
    Tags: , Deuteron, New Measurement Deepens Proton Puzzle, , Quanta Magazine   

    From Quanta: “New Measurement Deepens Proton Puzzle” 

    Quanta Magazine
    Quanta Magazine

    August 11, 2016
    Natalie Wolchover

    Researchers fired a laser at a gas of muonic deuterium in order to measure the size of its nucleus. Courtesy of Randolf Pohl

    The same group that discovered a curious discrepancy in measurements of the size of the proton, giving rise to the “proton radius puzzle,” has now found a matching discrepancy in measurements of a nuclear particle called the deuteron. The new finding, to appear on August 12 in Science, increases the slim chance that something is truly amiss, rather than simply mismeasured, in the heart of atoms.

    The puzzle is that the proton — the positively charged particle found in atomic nuclei, which is actually a fuzzy ball of quarks and gluons — is measured to be ever so slightly larger when it is orbited by an electron than when it is orbited by a muon, a sibling of the electron that’s 207 times as heavy but otherwise identical. It’s as if the proton tightens its belt in the muon’s presence. And yet, according to the reigning theory of particle physics, the proton should interact with the muon and the electron in exactly the same way. As hundreds of papers have pointed out since the proton radius puzzle was born in 2010, a shrinking of the proton in the presence of a muon would most likely signify the existence of a previously unknown fundamental force — one that acts between protons and muons, but not between protons and electrons. (Interestingly, this new physics could also explain a long-standing discrepancy in the measurement of the muon’s anomalous magnetic moment.)

    This “would, of course, be fantastic,” said Randolf Pohl of the Max Planck Institute of Quantum Optics in Garching, Germany, who led both the 2010 experiment and the new study. “But the most realistic thing is that it’s not new physics.”

    The harsh reality is that the proton radius is extremely hard to measure, making such a measurement error-prone. It’s especially tough in the typical case where a proton is orbited by an electron, as in a regular hydrogen atom. Numerous groups have attempted this measurement over many decades; their average value for the proton radius is just shy of 0.88 femtometers. But Pohl’s group, seeking greater precision, set out in 1998 to measure the proton radius in “muonic hydrogen,” since the muon’s heft makes the proton’s size easier to probe. Twelve years later, the scientists reported in Nature a value for the proton radius that was far more precise than any single previous measurement using regular hydrogen, but which, at 0.84 femtometers, fell stunningly short of the average.

    The question is: Were all the measurements using regular hydrogen simply off — all accidentally too large? When I first corresponded with Pohl in 2013, the year he and his colleagues reported an updated muonic hydrogen measurement in Science, he emailed me a plot showing how, historically, measurements of physical constants have often drifted dramatically as techniques change and improve before converging on their correct values. “Quite instructive, no?” Pohl wrote. He was keeping things in perspective.

    Examples of how the measured values of constants can vary dramatically before converging on their correct values. Particle Data Group

    But he and his group were also keeping at it. Already, they had begun the study that is finally being published this week.

    This time, they measured the radius of the deuteron, the nucleus of a deuterium atom (an isotope of hydrogen) that is comprised of a proton and a neutron. They measured it in muonic deuterium, in which a muon orbits a deuteron. The scientists then compared their measurement to the deuteron radius as measured in regular, electron-orbited deuterium, and asked: Is there a deuteron radius puzzle to match the proton’s?

    Their experiment probes the deuteron radius as follows: When electrons or muons orbit the deuteron in a certain energy level, they actually spend much of their time inside the deuteron, which, like a solar system, has a lot of empty space. Being inside the deuteron reduces the attraction that the electron or muon feels to it, since the deuteron’s charge pulls in different directions, partly canceling out. And so, paradoxically, the more time an electron or muon spends inside the deuteron, the less strongly bound it is, and the more easily it can jump away. The muon, because it’s so much heavier, orbits the deuteron much more tightly than the electron does, and so it is far more likely to be found inside. This means it experiences a much more greatly reduced deuteron charge; this larger reduction due to the deuteron’s structure is why the muon is a more precise probe of its radius.

    To actually measure that radius, the researchers fire a laser at a gas of muonic deuterium, causing muons to jump to a higher energy level that does not overlap with the nucleus. The team can pinpoint the energy required for the muon to undergo the transition, revealing how weakly bound the muon was when residing partly inside the deuteron. From this they can figure out where “inside the deuteron” begins — that is, its radius.

    When they did this, Pohl and company found that the deuteron radius is smaller when measured in muonic deuterium compared to the average value using electronic deuterium, just as with the proton radius discrepancy. The size difference scales from proton to deuteron exactly as they would expect if both effects come from a new force. “So now there are two discrepancies, and they are completely independent,” aside from being measured by the same group, Pohl said.

    Still, Pohl is highly skeptical that the puzzle is evidence of new fundamental physics.

    His personal guess is that physicists have misgauged the Rydberg constant, a factor that goes into calculating the expected differences between atomic energy levels. While it is considered one of the most accurately measured constants, a small error could account for the proton and deuteron radius puzzles.

    To test this possibility, physicists in Toronto are attempting to measure the proton radius in a way that sidesteps the Rydberg constant. Other experiments are under way to test alternative hypotheses, mundane and exciting alike. Pohl’s group is diving into muonic helium, a system in which the effects of a new force, if it exists, should be enhanced, since there are two protons. We’ll keep you posted.

    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 1:37 pm on August 4, 2016 Permalink | Reply
    Tags: , Miranda Cheng, Monstrous moonshine, Quanta Magazine,   

    From Quanta: “Moonshine Master Toys With String Theory” 

    Quanta Magazine
    Quanta Magazine

    August 4, 2016
    Natalie Wolchover

    The physicist-mathematician Miranda Cheng is working to harness a mysterious connection between string theory, algebra and number theory.

    Ilvy Njiokiktjien for Quanta Magazine

    After the Eyjafjallajökull volcano erupted in Iceland in 2010, flight cancellations left Miranda Cheng stranded in Paris. While waiting for the ash to clear, Cheng, then a postdoctoral researcher at Harvard University studying string theory, got to thinking about a paper that had recently been posted online. Its three coauthors had pointed out a numerical coincidence connecting far-flung mathematical objects. “That smells like another moonshine,” Cheng recalled thinking. “Could it be another moonshine?”

    She happened to have read a book about the “monstrous moonshine,” a mathematical structure that unfolded out of a similar bit of numerology: In the late 1970s, the mathematician John McKay noticed that 196,884, the first important coefficient of an object called the j-function, was the sum of one and 196,883, the first two dimensions in which a giant collection of symmetries called the monster group could be represented. By 1992, researchers had traced this farfetched (hence “moonshine”) correspondence to its unlikely source: string theory, a candidate for the fundamental theory of physics that casts elementary particles as tiny oscillating strings. The j-function describes the strings’ oscillations in a particular string theory model, and the monster group captures the symmetries of the space-time fabric that these strings inhabit.

    By the time of Eyjafjallajökull’s eruption, “this was ancient stuff,” Cheng said — a mathematical volcano that, as far as physicists were concerned, had gone dormant. The string theory model underlying monstrous moonshine was nothing like the particles or space-time geometry of the real world. But Cheng sensed that the new moonshine, if it was one, might be different. It involved K3 surfaces — the geometric objects that she and many other string theorists study as possible toy models of real space-time.

    By the time she flew home from Paris, Cheng had uncovered more evidence that the new moonshine existed. She and collaborators John Duncan and Jeff Harvey gradually teased out evidence of not one but 23 new moonshines: mathematical structures that connect symmetry groups on the one hand and fundamental objects in number theory called mock modular forms (a class that includes the j-function) on the other. The existence of these 23 moonshines, posited in their Umbral Moonshine Conjecture in 2012, was proved by Duncan and coworkers late last year.

    Meanwhile, Cheng, 37, is on the trail of the K3 string theory underlying the 23 moonshines — a particular version of the theory in which space-time has the geometry of a K3 surface. She and other string theorists hope to be able to use the mathematical ideas of umbral moonshine to study the properties of the K3 model in detail. This in turn could be a powerful means for understanding the physics of the real world where it can’t be probed directly — such as inside black holes. An assistant professor at the University of Amsterdam on leave from France’s National Center for Scientific Research, Cheng spoke with Quanta Magazine about the mysteries of moonshines, her hopes for string theory, and her improbable path from punk-rock high school dropout to a researcher who explores some of the most abstruse ideas in math and physics. An edited and condensed version of the conversation follows.

    Ilvy Njiokiktjien for Quanta Magazine

    QUANTA MAGAZINE: You do string theory on so-called K3 surfaces. What are they, and why are they important?

    MIRANDA CHENG: String theory says there are 10 space-time dimensions. Since we only perceive four, the other six must be curled up or “compactified” too small to see, like the circumference of a very thin wire. There’s a plethora of possibilities — something like 10^500 — for how the extra dimensions might be compactified, and it’s almost impossible to say which compactification is more likely to describe reality than the rest. We can’t possibly study the physical properties of all of them. So you look for a toy model. And if you like having exact results instead of approximated results, which I like, then you often end up with a K3 compactification, which is a middle ground for compactifications between too simple and too complicated. It also captures the key properties of Calabi-Yau manifolds [the most highly studied class of compactifications] and how string theory behaves when it’s compactified on them. K3 also has the feature that you can often do direct and exact computations with it.

    What does K3 actually look like?

    You can think of a flat torus, then you fold it so that there’s a line or corner of sharp edges. Mathematicians have a way to smooth it, and the result of smoothing a folded flat torus is a K3 surface.

    So you can figure out what the physics is in this setup, with strings moving through this space-time geometry?

    Yes. In the context of my Ph.D., I explored how black holes behave in this theory. Once you have the curled-up dimensions being K3-related Calabi-Yaus, black holes can form. How do these black holes behave — especially their quantum properties?

    So you could try to solve the information paradox—the long-standing puzzle of what happens to quantum information when it falls inside a black hole.

    Absolutely. You can ask about the information paradox or properties of various types of black holes, like realistic astrophysical black holes or supersymmetric black holes that come out of string theory. Studying the second type can shed light on your realistic problems because they share the same paradox. That’s why trying to understand string theory in K3 and the black holes that arise in that compactification should also shed light on other problems. At least, that’s the hope, and I think it’s a reasonable hope.

    Do you think string theory definitely describes reality? Or is it something you study purely for its own sake?

    I personally always have the real world at the back of my mind — but really, really, really back. I use it as sort of an inspiration for determining roughly the big directions I’m going in. But my day-to-day research is not aimed at solving the real world. I see it as differences in taste and style and personal capabilities. New ideas are needed in fundamental high-energy physics, and it’s hard to say where those new ideas will come from. Understanding the basic, fundamental structures of string theory is needed and helpful. You’ve got to start somewhere where you can compute things, and that leads, often, to very mathematical corners. The payoff to understanding the real world might be really long term, but that’s necessary at this stage.

    Have you always had a knack for physics and math?

    As a child in Taiwan I was more into literature — that was my big thing. And then I got into music when I was 12 or so — pop music, rock, punk. I was always very good at math and physics, but I wasn’t really interested in it. And I always found school insufferable and was always trying to find a way around it. I tried to make a deal with the teacher that I wouldn’t need to go into the class. Or I had months of sick leave while I wasn’t sick at all. Or I skipped a year here and there. I just don’t know how to deal with authority, I guess.

    And the material was probably too easy. I skipped two years, but that didn’t help. So then they moved me to a special class and that made it even worse, because everybody was very competitive, and I just couldn’t deal with the competition at all. Eventually I was super depressed, and I decided either I would kill myself or not go to school. So I stopped going to school when I was 16, and I also left home because I was convinced that my parents would ask me to go back to school and I really didn’t want to do that. So I started working in a record shop, and by that time I also played in a band, and I loved it.

    How did you get from there to string theory?

    Long story short, I got a little bit discouraged or bored. I wanted to do something else aside from music. So I tried to go back to university, but then I had the problem that I hadn’t graduated from high school. But before I quit school I was in a special class for kids who are really good in science. I could get in the university with this. So I thought, OK, great, I’ll just get into university first by majoring in physics or math, and then I can switch to literature. So I enrolled in the physics department, having a very on- and off-again relationship to it, going to class every now and then, and then trying to study literature, while still playing in the band. Then I realized I’m not good enough in literature. And also there was a very good teacher teaching quantum mechanics. Just once I went to his class and thought, that’s actually pretty cool. I started paying a bit more attention to my studies of math and physics, and I started to find peace in it. That’s what started to attract me about math and physics, because my other life in the band playing music was more chaotic somehow. It sucks a lot of emotions out of you. You’re always working with people, and the music is too much about life, about emotions — you have to give a lot of yourself to it. Math and physics seems to have this peaceful quiet beauty. This space of serenity.

    Then at the end of university I thought, well, let me just have one more year to study physics, then I’m really done with it and can move on with my life. So I decided to go to Holland to see the world and study some physics, and I got really into it there.

    You got your master’s at Utrecht under Nobel Prize-winning physicist Gerard ’t Hooft, and then you did your Ph.D. in Amsterdam. What drew you in?

    Working with [’t Hooft] was a big factor. But just learning more is also a big factor — to realize that there are so many interesting questions. That’s the big-picture part. But for me the day-to-day part is also important. The learning process, the thinking process, really the beauty of it. Every day you encounter some equations or some way of thinking, or this fact leads to that fact — I thought, well, this is pretty. Gerard is not a string theorist — he’s very open-minded about what the correct area of quantum gravity should be — so I got exposed to a few different options. I got attracted by string theory because it’s mathematically rigorous, and pretty.

    With the work you’re doing now, aside from the beauty, are you also drawn to the mystery of these connections between seemingly different parts of math and physics?

    The mystery part connects to the bad side of my character, which is the obsessive side. That’s one of the driving forces that I would call slightly negative from the human point of view, though not the scientist point of view. But there’s also the positive driving force, which is that I really enjoy learning different stuff and feeling how ignorant I am. I enjoy that frustration, like, “I know nothing about this subject; I really want to learn!” So that’s one motivation — to be at this boundary place between math and physics. Moonshine is a puzzle that might require inspirations from everywhere and knowledge from everywhere. And the beauty, certainly — it’s a beautiful story. It’s kind of hard to say why it is beautiful. It’s beautiful not the same way as a song is beautiful or a picture is beautiful.

    What’s the difference?

    Typically a song is beautiful because it triggers certain emotions. It resonates with part of your life. Mathematical beauty is not that. It’s something much more structured. It gives you a feeling of something much more permanent, and independent of you. It makes me feel small, and I like that.

    What is a moonshine, exactly?

    A moonshine relates representations of a finite symmetry group to a function with special symmetries [ways that you can transform the function without affecting its output]. Underlying this relationship, at least in the case of monstrous moonshine, is a string theory. String theory has two geometries. One is the “worldsheet” geometry. If you have a string — essentially a circle — moving in time, then you get a cylinder. That’s what we call the worldsheet geometry; it’s the geometry of the string itself. If you roll the cylinder and connect the two ends, you get a torus. The torus gives you the symmetry of the j-function. The other geometry in string theory is space-time itself, and its symmetry gives you the monster group.

    We don’t know yet, but these are educated guesses: To have a moonshine tells you that this theory has to have an algebraic structure [you have to be able to do algebra with its elements]. If you look at a theory and you ask what kind of particles you have at a certain energy level, this question is infinite, because you can go to higher and higher energies, and then this question goes on and on. In monstrous moonshine, this is manifested in the fact that if you look at the j-function, there are infinitely many terms that basically capture the energy of the particles. But we know there’s an algebraic structure underlying it — there’s a mechanism for how the lower energy states can be related to higher energy states. So this infinite question has a structure; it’s not just random.

    As you can imagine, having an algebraic structure helps you understand what the structure is that captures a theory — how, if you look at the lower energy states, they will tell you something about the higher energy states. And then it also gives you more tools to do computations. If you want to understand something at a high-energy level [such as inside black holes], then I have more information about it. I can compute what I want to compute for high-energy states using this low-energy data I already have in hand. That’s the hope.

    Umbral moonshine tells you that there should be a structure like this that we don’t understand yet. Understanding it more generally will force us to understand this algebraic structure. And that will lead to a much deeper understanding of the theory. That’s the hope.

    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 8:41 am on July 29, 2016 Permalink | Reply
    Tags: , , Quanta Magazine   

    From Quanta: “Neutrinos Hint of Matter-Antimatter Rift” 

    Quanta Magazine
    Quanta Magazine

    July 28, 2016
    Natalie Wolchover

    As neutrinos and antineutrinos change flavors they may illuminate the differences between matter and antimatter. Olena Shmahalo/Quanta Magazine

    An early sign that neutrinos behave differently than antineutrinos suggests an answer to one of the biggest questions in physics.

    In the same underground observatory in Japan where, 18 years ago, neutrinos were first seen oscillating from one “flavor” to another — a landmark discovery that earned two physicists the 2015 Nobel Prize — a tiny anomaly has begun to surface in the neutrinos’ oscillations that could herald an answer to one of the biggest mysteries in physics: why matter dominates over antimatter in the universe.

    The anomaly, detected by the T2K experiment, is not yet pronounced enough to be sure of, but it and the findings of two related experiments “are all pointing in the same direction,” said Hirohisa Tanaka of the University of Toronto, a member of the T2K team who presented the result to a packed audience in London earlier this month.

    “A full proof will take more time,” said Werner Rodejohann, a neutrino specialist at the Max Planck Institute for Nuclear Physics in Heidelberg who was not involved in the experiments, “but my and many others’ feeling is that there is something real here.”

    The long-standing puzzle to be solved is why we and everything we see is matter-made. More to the point, why does anything — matter or antimatter — exist at all? The reigning laws of particle physics, known as the Standard Model, treat matter and antimatter nearly equivalently, respecting (with one known exception) so-called charge-parity, or “CP,” symmetry: For every particle decay that produces, say, a negatively charged electron, the mirror-image decay yielding a positively charged antielectron occurs at the same rate.

    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.
    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.

    But this cannot be the whole story. If equal amounts of matter and antimatter were produced during the Big Bang, equal amounts should have existed shortly thereafter. And since matter and antimatter annihilate upon contact, such a situation would have led to the wholesale destruction of both, resulting in an empty cosmos.

    Somehow, significantly more matter than antimatter must have been created, such that a matter surplus survived the annihilation and now holds sway. The question is, what CP-violating process beyond the Standard Model favored the production of matter over antimatter?

    Many physicists suspect that the answer lies with neutrinos — ultra-elusive, omnipresent particles that pass unfelt through your body by the trillions each second.

    To that end, starting in 2010, scientists with the T2K experiment generated beams of neutrinos or antineutrinos in Tokai, Japan, and aimed them toward the Super-Kamiokande neutrino observatory, a sensor-lined tank of 50,000 tons of pure water located nearly 200 miles away in Kamioka. Occasionally, these ghostly particles interacted with atoms inside the water tank, generating detectable flashes of radiation. Detecting a difference in the behavior of the neutrinos and antineutrinos would provide an important clue about the preponderance of matter over antimatter, perhaps opening up a route beyond the Standard Model to a more complete theory of nature. Already, the strange properties of neutrinos provide a possible outline of that fuller story.

    At the Super-Kamiokande observatory in Kamioka, Japan — shown here when it was being filled with water in 2006 — neutrinos interact with atoms inside the water, generating flashes of radiation that are picked up by the surrounding sensors. Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo

    Primordial Neutrinos

    The 1998 discovery that neutrinos switch flavors on the fly “may change our most fundamental theories,” President Bill Clinton said at the time, “from the nature of the smallest subatomic particles to how the universe itself works.”

    Neutrino oscillations defied the Standard Model’s prediction that the particles are massless, like photons. In order for neutrinos to oscillate, each of their three possible flavors (electron, muon and tau) must be a quantum-mechanical mixture, or “superposition,” of three possible masses. Quantum superpositions evolve over time. So a neutrino might start out with its three mass components giving it pure muon flavor, but as the components evolve at different rates, electron flavor gradually enters the mixture, and the neutrino will have some probability of being measured as an electron neutrino.

    There’s no mechanism within the Standard Model by which neutrinos might acquire their tiny, nonzero masses. Also unknown is why all neutrinos are observed to be “left-handed,” spinning clockwise with respect to their direction of motion, while all antineutrinos are right-handed, spinning counterclockwise.

    Experts overwhelmingly favor a double-duty explanation of neutrino mass and single-handedness called the “seesaw mechanism,” whereby the known, lightweight, left-handed neutrinos have much heavier right-handed counterparts, and the known antineutrinos likewise have superheavy left-handed counterparts (the light and heavy masses are inversely related, like two sides of a seesaw). For this seesaw explanation to work, the neutrinos and antineutrinos on each side of the seesaw must actually be the same particle, except for their opposite handedness. Numerous experiments are now hunting for an extremely rare radioactive decay that would confirm this “Majorana” nature of neutrinos, thereby shoring up the logic of the seesaw mechanism.

    If the theory is correct, then the heavy neutrinos and antineutrinos would have populated the hot young universe, when there was enough energy to beget beastly particles. They would have since decayed. Physicists wonder: Might their decays have produced the matter-antimatter asymmetry? This is the question to which an answer may be emerging — and much sooner than expected.

    Tilted Seesaw

    There’s good reason to think that neutrinos violate CP symmetry. The one established instance of CP violation in the laws of physics arises among the quarks — the building blocks of protons and neutrons — whose flavor mixing is described by a mathematical matrix similar to the one for neutrino mixing. In the quark case, though, the value of a numerical factor in the matrix that creates a disparity between quarks and antiquarks is very small. Quarks and antiquarks behave far too symmetrically to account for the universe’s matter-antimatter imbalance.

    Lucy Reading-Ikkanda for Quanta Magazine

    But the neutrino mixing matrix comes equipped with its own factor by which neutrinos and antineutrinos can violate CP symmetry. (Paradoxically, they can behave differently from one another even if they are Majorana particles, identical except for their opposite handedness.) If the lightweight neutrinos and antineutrinos violate CP symmetry, then the hypothetical heavy primordial neutrinos and antineutrinos must as well, and their asymmetric decays could easily have produced the universe’s glut of matter. Discovering CP violation among the lightweight neutrinos “will boost that general framework,” said Neal Weiner, a theoretical physicist at New York University.

    The question is, how large will the CP-violation factor be? “The fear was that it would be small,” said Patricia Vahle, a physicist at the College of William & Mary — so small that the current generation of experiments wouldn’t detect any difference between neutrinos’ and antineutrinos’ behavior. “But it is starting to look like maybe we will be lucky,” she said.

    To search for CP violation, the T2K scientists looked for evidence that neutrinos and antineutrinos oscillated between muon and electron flavors with unequal probabilities as they traveled between Tokai and Kamioka. The amount of CP violation once again works like a seesaw, with the rate of muon-to-electron neutrino conversions on one side, and corresponding antineutrino conversions on the other. The larger the value of the factor in the matrix, the greater the seesaw’s tilt.

    If the seesaw is balanced, signifying perfect CP symmetry, then (accounting for differences in the production and detection rates of neutrinos and antineutrinos), the T2K scientists would have expected to detect roughly 23 electron neutrino candidates and seven electron antineutrino candidates in Kamioka, Tanaka said. Meanwhile, if CP symmetry is “maximally” violated — the seesaw tilted fully toward more neutrino oscillations and fewer antineutrino oscillations — then 27 electron neutrinos and six electron antineutrinos should have been detected. The actual numbers were even more skewed. “What we observed are 32 electron neutrino candidates and four electron antineutrino candidates,” Tanaka said.

    With so few total events, it’s too soon to know whether the apparent tilt of the seesaw, signifying a large amount of CP violation, is real or a statistical aberration. Two other new hints of CP violation, however, strengthen the case. First, the newly running NOvA experiment, which generates a beam of muon neutrinos in Illinois and measures electron neutrinos in Minnesota, found a large number of these oscillations, again suggesting that the seesaw may be tilted in favor of neutrino oscillations and away from antineutrino oscillations.

    FNAL/NOvA experiment
    FNAL/NOvA experiment

    Second, researchers at the Super-Kamiokande observatory detected a similar enhancement of electron neutrinos coming from Earth’s atmosphere. (T2K and NOvA both plan to submit their findings for publication later this year.)

    Vahle, who presented NOvA’s new results this month in London, urged caution; even when the T2K and NOvA results are combined, their statistical significance remains at a low level known as “2 sigma,” where there’s still a 5 percent chance the apparent deviation from CP symmetry is a random fluke. The results “do give me hope that finding CP violation in neutrino oscillations won’t be as hard as many feared it would be,” she said, “but we aren’t there yet.”

    If CP violation among neutrinos is real and as large as it currently seems, then the evidence will slowly strengthen in the coming years. T2K’s signal could reach 3-sigma significance by the mid-2020s. “It’s a very exciting time as we look forward to a lot more data from both experiments,” said Peter Shanahan, a NOvA co-spokesperson.

    It isn’t yet known exactly how CP violation in the light neutrino oscillations would translate into CP-violating decays of the heavy set. But discovering the former would point physicists in the latter’s general direction. “If we are starting to see [CP violation] in the neutrino sector, it is certainly a critical result,” Weiner said.

    See the full article here .

    Please help promote STEM in your local schools.

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

  • richardmitnick 7:45 am on July 23, 2016 Permalink | Reply
    Tags: Asimina Arvanitaki, , , , , Quanta Magazine,   

    From Quanta: Women in Science – “Mining Black Hole Collisions for New Physics” Asimina Arvanitaki 

    Quanta Magazine
    Quanta Magazine

    July 21, 2016
    Joshua Sokol

    Asimina Arvanitaki during a July visit to the CERN particle physics laboratory in Geneva, Switzerland. Samuel Rubio for Quanta Magazine

    When physicists announced in February that they had detected gravitational waves firsthand, the foundations of physics scarcely rattled. The signal exactly matched the expectations physicists had arrived at after a century of tinkering with Einstein’s theory of general relativity. “There is a question: Can you do fundamental physics with it? Can you do things beyond the standard model with it?” said Savas Dimopoulos, a theoretical physicist at Stanford University. “And most people think the answer to that is no.”

    Asimina Arvanitaki is not one of those people. A theoretical physicist at Ontario’s Perimeter Institute of Theoretical Physics, Arvanitaki has been dreaming up ways to use black holes to explore nature’s fundamental particles and forces since 2010, when she published a paper with Dimopoulos, her mentor from graduate school, and others. Together, they sketched out a “string axiverse,” a pantheon of as yet undiscovered, weakly interacting particles. Axions such as these have long been a favored candidate to explain dark matter and other mysteries.

    In the intervening years, Arvanitaki and her colleagues have developed the idea through successive papers. But February’s announcement marked a turning point, where it all started to seem possible to test these ideas. Studying gravitational waves from the newfound population of merging black holes would allow physicists to search for those axions, since the axions would bind to black holes in what Arvanitaki describes as a “black hole atom.”

    “When it came up, we were like, ‘Oh my god, we’re going to do it now, we’re going to look for this,’” she said. “It’s a whole different ball game if you actually have data.”

    That’s Arvanitaki’s knack: matching what she calls “well-motivated,” field-hopping theoretical ideas with the precise experiment that could probe them. “By thinking away from what people are used to thinking about, you see that there is low-hanging fruit that lie in the interfaces,” she said. At the end of April, she was named the Stavros Niarchos Foundation’s Aristarchus Chair at the Perimeter Institute, the first woman to hold a research chair there.

    It’s a long way to come for someone raised in the small Grecian village of Koklas, where the graduating class at her high school — at which both of her parents taught — consisted of nine students. Quanta Magazine spoke with Arvanitaki about her plan to use black holes as particle detectors. An edited and condensed version of those discussions follows.

    QUANTA MAGZINE: When did you start to think that black holes might be good places to look for axions?

    ASIMINA ARVANITAKI: When we were writing the axiverse paper, Nemanja Kaloper, a physicist who is very good in general relativity, came and told us, “Hey, did you know there is this effect in general relativity called superradiance?” And we’re like, “No, this cannot be, I don’t think this happens. This cannot happen for a realistic system. You must be wrong.” And then he eventually convinced us that this could be possible, and then we spent like a year figuring out the dynamics.

    What is superradiance, and how does it work?

    An astrophysical black hole can rotate. There is a region around it called the “ergo region” where even light has to rotate. Imagine I take a piece of matter and throw it in a trajectory that goes through the ergo region. Now imagine you have some explosives in the matter, and it breaks apart into pieces. Part of it falls into the black hole and part escapes into infinity. The piece that is coming out has more total energy than the piece that went in the black hole.

    You can perform the same experiment by scattering radiation from a black hole. Take an electromagnetic wave pulse, scatter it from the black hole, and you see that the pulse you got back has a higher amplitude.

    So you can send a pulse of light near a black hole in such a way that it would take some energy and angular momentum from the black hole’s spin?

    This is old news, by the way, this is very old news. In ’72 Press and Teukolsky wrote a Nature paper that suggested the following cute thing. Let’s imagine you performed the same experiment as the light, but now imagine that you have the black hole surrounded by a giant mirror. What will happen in that case is the light will bounce on the mirror many times, the amplitude [of the light] grows exponentially, and the mirror eventually explodes due to radiation pressure. They called it the black hole bomb.

    The property that allows light to do this is that light is made of photons, and photons are bosons — particles that can sit in the same space at the same time with the same wave function. Now imagine that you have another boson that has a mass. It can [orbit] the black hole. The particle’s mass acts like a mirror, because it confines the particle in the vicinity of the black hole.

    In this way, axions might get stuck around a black hole?

    This process requires that the size of the particle is comparable to the black hole size. Turns out that [axion] mass can be anywhere from Hubble scale — with a quantum wavelength as big as the universe — or you could have a particle that’s tiny in size.

    So if they exist, axions can bind to black holes with a similar size and mass. What’s next?

    What happens is the number of particles in this bound orbit starts growing exponentially. At the same time the black hole spins down. If you solve for the wave functions of the bound orbits, what you find is that they look like hydrogen wave functions. Instead of electromagnetism binding your atom, what’s binding it is gravity. There are three quantum numbers you can describe, just the same. You can use the exact terminology that you can use in the hydrogen atom.

    How could we check to see if any of the black holes LIGO finds have axion clouds orbiting around black hole nuclei?

    This is a process that extracts energy and angular momentum from the black hole. If you were to measure spin versus mass of black holes, you should see that in a certain mass range for black holes you see no quickly rotating black holes.

    This is where Advanced LIGO comes in.

    LSC LIGO Scientific Collaboration
    VIRGO Collaboration bloc

    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

    You saw the event they saw. [Their measurements] allowed them to measure the masses of the merging objects, the mass of the final object, the spin of the final object, and to have some information about the spins of the initial objects.

    If I were to take the spins of the black holes before they merged, they could have been affected by superradiance. Now imagine a graph of black hole spin versus mass. Advanced LIGO could maybe get, if the things that we hear are correct, a thousand events per year. Now you have a thousand data points on this plot. So you may trace out the region that is affected by this particle just by those measurements.

    That would be supercool.

    That’s of course indirect. So the other cool thing is that it turns out there are signatures that have to do with the cloud of particles themselves. And essentially what they do is turn the black hole into a gravitational wave laser.

    Awesome. OK, what does that mean?

    Yeah, what that means is important. Just like you have transitions of electrons in an excited atom, you can have transitions of particles in the gravitational wave atom. The rate of emission of gravitational waves from these transitions is enhanced by the 1080 particles that you have. It would look like a very monochromatic line. It wouldn’t look like a transient. Imagine something now that emits a signal at a very fixed frequency.

    Where could LIGO expect to see signals like this?

    In Advanced LIGO, you actually see the birth of a black hole. You know when and where a black hole was born with a certain mass and a certain spin. So if you know the particle masses that you’re looking for, you can predict when the black hole will start growing the [axion] cloud around it. It could be that you see a merger in that day, and one or 10 years down the line, they go back to the same position and they see this laser turning on, they see this monochromatic line coming out from the cloud.

    You can also do a blind search. Because you have black holes that are roaming the universe by themselves, and they could still have some leftover cloud around them, you can do a blind search for monochromatic gravitational waves.

    Were you surprised to find out that axions and black holes could combine to produce such a dramatic effect?

    Oh my god yes. What are you talking about? We had panic attacks. You know how many panic attacks we had saying that this effect, no, this cannot be true, this is too good to be true? So yes, it was a surprise.

    The experiments you suggest draw from a lot of different theoretical ideas — like how we could look for high-frequency gravitational waves with tabletop sensors, or test whether dark matter oscillates using atomic clocks. When you’re thinking about making risky bets on physics beyond the standard model, what sorts of theories seem worth the effort?

    What is well motivated? Things that are not: “What if you had this?” People imagine: “What if dark matter was this thing? What if dark matter was the other thing?” For example, supersymmetry makes predictions about what types of dark matter should be there. String theory makes predictions about what types of particles you should have. There is always an underlying reason why these particles are there; it’s not just the endless theoretical possibilities that we have.

    And axions fit that definition?

    This is a particle that was proposed 30 years ago to explain the smallness of the observed electric dipole moment of the neutron. There are several experiments around the world looking for it already, at different wavelengths. So this particle, we’ve been looking for it for 30 years. This can be the dark matter. That particle solves an outstanding problem of the standard model, so that makes it a good particle to look for.

    Now, whether or not the particle is there I cannot answer for nature. Nature will have to answer.

    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 9:44 am on June 10, 2016 Permalink | Reply
    Tags: , , Quanta Magazine,   

    From Quanta: Women in Science “An Explorer of Quantum Borderlands” Suchitra Sebastian 

    Quanta Magazine
    Quanta Magazine

    June 9, 2016
    Maggie McKee

    Suchitra Sebastian is building a new lab to study exotic quantum behavior at Cambridge University. Philipp Ammon for Quanta Magazine

    Suchitra Sebastian is a fringe physicist. Not a crackpot — she lectures at the University of Cambridge and has published a string of papers in Science and Nature. But she likes to venture into the borderlands between forms of matter that other physicists have already explored. There, in the liminal space where the particles in a material begin to change from one configuration to another, new quantum effects appear. “A lot of it is really exciting phenomena that emerges before it’s theoretically predicted,” Sebastian said with delight.

    Last year, she and her colleagues discovered what appeared to be electrons looping their way through an insulator, a type of material that by definition prevents such movement. The observation, in a substance called samarium hexaboride, is still not understood. But Sebastian says one possibility is that what was looping was not electrons but an entirely new kind of subatomic building block.

    Interactions between electrons create wavelike disturbances — known as quasiparticles — that serve as the basic components of almost every complex material. The known quasiparticles tend to act like heavier versions of electrons, but not so in this case. “In samarium hexaboride, the possibility is that the electron itself has broken apart,” said Sebastian. “So instead of thinking of the electron as the building block, we would need to think of fractional parts of the electron as building blocks.” These fractional quasiparticles would create an entirely new way to understand the universe of materials.

    Sebastian herself moves between very different worlds. Before delving into science, she worked as a management consultant, and now she performs in experimental theater pieces when she’s not in the lab. “I kind of intensely do different things,” she says. “If I spend too much time doing the analytical physics side, I’m, like, gasping for oxygen.” Research, she said, “is not about drawing within the lines. It’s about discovery and creativity.”

    Quanta Magazine spoke with Sebastian about her research and her unconventional path to science. An edited and condensed version of the interview follows.

    QUANTA MAGAZINE: You search for quantum effects that are entirely new to science. How do you go about looking for them?

    SUCHITRA SEBASTIAN: One option is to look at many, many, many, materials. You can say, “I’m trying to find the one that does something very different.” You may never find it. What I realized is you can use external conditions — pressures or temperatures or magnetic fields — to manipulate a material and move it into a region where it does something really interesting and where new quantum properties emerge.

    Where is that magical region?

    Water can be water, or it can be ice, or it can be steam. These are the same material but in different phases. In the quantum world you can also have different phases. You can have the same material and the same electrons, but the interactions can result either in the substance organizing into one kind of material — so under certain conditions, you can have a magnet — or you exert pressure on the material — you press it — and then it quantum configures in a slightly different way and the magnet transforms into a superconductor. The region I’m excited about is the region between these phases, which is a quantum critical region. Between one phase and another phase you get this intermediate region, where it’s unknown what might happen, and you can have completely new forms of matter emerging.

    What gives rise to the quantum effects?

    Simple materials with weak electron interactions can be modeled just in terms of the electrons’ propensity to hop around, which can be averaged over the entire material. But in more strongly interacting materials, the repulsive force due to interactions between each of the trillions and trillions of electrons is stronger than their propensity to hop around. In this case, the resulting collective effects are almost impossible to predict and can be dramatically different from individual electron behavior.

    Last year, you found an unexpected quantum effect in a material called samarium hexaboride. What was so surprising?

    We think of metals as carrying electricity, and insulators as not. Electrons in metals are traveling long distances, and they’re carrying charge — it’s how electricity flows. In insulators, electrons are largely stuck in one position. That’s why electricity is not being transported in insulators. Samarium hexaboride was predicted to be in a class of materials known as topological insulators, where current flows only on the surface and not in the bulk [the material’s interior].

    We were really, really shocked when the magnetization we measured in the sample started showing wiggles that are characteristic of an electron executing orbits. We see these electrons that travel really long orbits, and they’re coming from the bulk. But the bulk is insulating; those electrons are barely moving. How is it conceivable they’re traveling in these large orbits?

    What might be going on?

    We assume it’s the electrons that are traveling in these orbits, but at the same time, the electrons can’t possibly be moving because there’s no charge moving.

    Maybe what’s happening is that when the electrons come together in this quantum ensemble, we cannot describe the physics in terms of the individual electrons anymore. The possibility is that the electron itself has broken apart. So instead of thinking of the electron as the building block, we would need to think of fractional parts of the electron as building blocks.

    One new finding in support of this is that we do have heat transport through this material, but there are no charges being transported. So maybe you need to think about particles that carry [the quantum property of] spin — which carry heat — and not charge. One possibility is a type of neutral quasiparticle known as a spinon, which carries spin and not charge.

    If it turned out that the electron was no longer the basic building block, that would be very shocking. You can count on the fingers of one hand how many such cases there are.

    Why would it be so shocking?

    It’s always shocking when we discover the fundamental building block of matter isn’t what we thought it was. Originally we thought the fundamental building block was the atom. Then subatomic particles — neutrons, protons and electrons — were discovered.

    When we describe complex materials, we think of the fundamental building block as the electron. One of the overarching quests in condensed matter physics is to find a material that behaves radically differently, as though the electron has broken down, which means we need a new description — not in terms of electronlike quasiparticles but in terms of quasiparticles based on fractional components of an electron.

    One of the clearest examples is the fractional quantum Hall effect, in which instead of an electron that carries charge as the building block, one observes the fundamental building block to be fragments of an electron that carry fractional charges.

    Video: Sebastian talks about how extreme conditions can create unexpected quantum behavior. Philipp Ammon for Quanta Magazine
    Access mp4 video here .

    You have also created a superconductor — a material that transmits electricity without any resistance — by squeezing a magnet with a diamond anvil, a process you call “quantum alchemy.” What strategies do you use to search for new types of superconductors?

    Thus far, if you look at existing superconductors, they’re often close to being magnetic, and they’re often close to being insulating. Another promising feature is layered materials, so that rather than being cubic in their crystal structure, the materials are quite two-dimensional — their crystal structure is stretched out.

    What’s important is to take materials with these promising properties, then apply pressure and go from a non-superconductor into a superconductor. You’re probably not going to start with an optimal high-temperature superconductor by doing this. But if we can make several superconductors from different families of materials, then you’re going to be able to start making a road map: “Now I know these properties of this material gave me a superconductor, but it wasn’t as good as this other one where I started with a material with these other properties.” When you start finding patterns, that will give you a good bridge to then pursue the ultimate optimization of superconductivity.

    What would this optimization lead to, practically?

    One of the big implications of a room-temperature superconductor would be that you could transport energy over very long distances — from anywhere in the world to anywhere else — without any loss. Renewables will be a big part of the future for sustainable energy. What’s not always recognized is that we also need to transport the renewable energy. Solar energy will be abundant in the Sahara, but the most populous cities are, for instance, in New York. How are you going to transport energy over thousands of miles? If you wave a magic wand and get to ideal superconductors, a really long cable would take energy from where it’s created to where it’s most needed. We could start thinking of renewable energy in a more holistic way. You could start thinking more of a worldwide grid.

    Is science doing enough to find more superconductors?

    I don’t think there is a very big effort to search for new superconductors. There are these two questions: How do we understand copper oxide superconductors, which are the one known family of materials that superconduct above liquid-nitrogen temperatures at ambient conditions? And how do we create new superconductors? My impression is about 90 percent of the field’s effort is focused on how to understand copper oxide superconductors. That is a really interesting and important area, but I think there is way too little effort and interest in how to make new superconductors. Personally I feel that scientists should be partnering with industry, or partnering with organizations that are familiar with the idea of assembly-line processes, to accelerate this process of discovery. It’s such an important problem.

    You have some experience working with industry. After studying physics in college, you went on to do an MBA and work as a management consultant for several years before returning to physics. Why did you leave science for a while?

    As much as I love physics and I’m passionate about it, I think the field itself is quite insular, and it does tend to be not very diverse. I didn’t identify with the kind of people I saw doing physics. They didn’t seem very fun or interesting. It seemed like they were really locked into these little worlds, and they knew a lot about what they did, but I felt like they were totally out of touch with the rest of the world. I really need to be engaged with everything around me in different ways. How does the world work, how does economics work, how do governments run? I’m interested in the social implications of what we do. I actually applied to an MBA, I applied to physics, I applied to engineering, and I applied to literature. I interviewed for all of them, and the MBA interview — it was quite an interesting interview. This is how I made my decision.

    But you didn’t enjoy consulting in the end?

    They sell it to you as out-of-the-box thinking and meeting all these interesting people in different fields. But doing it I realized it is not that out-of-the-box thinking. The longer you stay in it, the less it becomes about discovering solutions and the more it becomes about trying to network and market the same old solutions.

    But I will say that I think that is why I recognize how important communication is and how important it is to get your audience on board. I think scientists are sometimes quite complacent in thinking they know a lot about something very specific, and really, if someone doesn’t understand, that’s their problem. But if you’re making a pitch and your client doesn’t buy it, that is your problem. It’s up to you to learn to make a compelling case so you bring people on board. The difference really was, in my consulting work, I wasn’t passionate enough about the solution I was pitching to want to do it the rest of my life. It’s nice, but does it change the world? No. But with physics, I’m passionate about it. It has the potential to revolutionize the world.

    So did you find fulfillment with physics?

    I think it took time to find my own way. I think I needed all those experiences. When I came back to physics, now I’m able to keep all the different parts of me and develop them. Now I can be a more balanced person and recognize that I can do physics without being that stereotypical physicist.

    Physics isn’t just about writing equations on a board and sitting in front of a computer. Science is about exploring new worlds. There are lots of people who would approach the world in that way, but they don’t recognize that physics is about that. We need to attract more of these people to physics.

    You have said, “Who I am is at the heart of the science I do.” Can you explain that?

    I think often people have this impression that the science you do is removed from the person you are. It’s almost like people are swappable. I feel like in physics they don’t really recognize that the person you are is integral to the science you do. The way different people do physics is completely different. I choose to do extremely exploratory physics where I deliberately choose problems where I don’t know what the answer is going to be. But I recognize that for other people, they need to do the kind of physics where you have to be incredibly careful and take years setting something up because you’re looking for the 10th digit to prove a hypothesis. In order to learn more about the world around us, you need to come at it in different ways.

    How have you tried to draw more people to science?

    I’ve done things called soapbox talks before. We go into public spaces — it’s like science busking. It’s literally upturned wooden boxes, and a few people engage with pedestrians walking by, saying, “Do you want to learn about some cool science?” People are really excited about it.

    In your free time, you do theater, and you just spearheaded an art exhibit to celebrate the opening of a new science building at the University of Cambridge. Do you plan to do more art-science events?

    Engaging through the arts is really new for me. But I’m really excited about how well it worked the first time. We are having conversations about taking it forward. Creativity happens when you bring together disparate worlds. If you just do the one thing, and you meet people very similar to you, I think this often reinforces certain ways of thinking and deepens ruts. But where does creativity come from? It comes when you have different approaches intersecting. I think physical spaces are really, really important for this — where people come together, where you have chance encounters. I just really think it’s important to have these interfaces and porous boundaries to break down any kind of siloing.

    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 3:10 pm on May 10, 2016 Permalink | Reply
    Tags: , , Quanta Magazine, Tiny Tests Seek the Universe’s Big Mysteries   

    From Quanta: “Tiny Tests Seek the Universe’s Big Mysteries” 

    Quanta Magazine
    Quanta Magazine

    May 3, 2016
    Joshua Sokol

    Huge supercolliders aren’t the only way to search for new physical phenomena. A new generation of experiments that can fit on a tabletop are probing the nature of dark matter and dark energy and searching for evidence of extra dimensions.

    Access mp4 video here .
    Video: David Moore of Stanford University describes how, inside this chamber, silica spheres probe for distortions of gravity. Peter DaSilva for Quanta Magazine

    To answer some of the biggest unsolved questions in the cosmos, you might not need a supercollider. For decades, theorists have been dreaming up a Wild West of exotic physics that could be visible at scales just below the thickness of a dollar bill — provided you build a clever-enough experiment, one small enough to fit on a tabletop. Over distances of a few dozen microns — a little thinner than that dollar — known forces like gravity could get weird, or, even more exciting, previously unknown forces could pop up. Now a new generation of tabletop experiments is coming online to look into these phenomena.

    One such experiment uses levitated spheres of silica — “basically a glass bead that we hold up using light,” according to Andrew Geraci, the lead investigator — to search for hidden forces far weaker than anything we can imagine. In a paper* uploaded to the scientific preprint site arxiv.org in early March, his team announced that they had detected sensitivities of a few zeptonewtons — a level of force 21 orders of magnitude below a newton, which is about what is needed to depress a computer key.

    “A bathroom scale might be able to tell your weight to maybe 0.1 newtons if it was very accurate,” said Geraci, a physicist at the University of Nevada, Reno. “If you had a single virus land on you, that would be about 10^–19 newtons, so we’re about two orders of magnitude below that.”

    The targets of these searches feature in some of the most compelling questions in physics, including those that center on the nature of gravity, dark matter and dark energy. “There’s a whole panoply of things these experiments could look for,” said Nima Arkani-Hamed, a physicist at the Institute for Advanced Study in Princeton, N.J. For example, dark matter, the massive stuff whose existence has been inferred only on astronomical scales, might leave faint electric charges behind when it interacts with ordinary particles. Dark energy, the pressure powering the accelerating expansion of the universe, might make itself felt through so-called “chameleon” particles that a tabletop experiment could theoretically be able to spot. And certain theories predict that gravity will be much weaker than expected at short range, while others predict that it will be stronger. If the extra dimensions posited by string theory exist, the tug of gravity between objects separated by a micron might exceed what Isaac Newton’s law predicts by a factor of 10 billion.

    Janet Conrad, a physicist at the Massachusetts Institute of Technology who is not directly involved with any of these small-scale searches, thinks that they complement the work done at massive accelerators such as the Large Hadron Collider.

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

    “We are like dinosaurs. We have gotten bigger, and bigger, and bigger,” she said. But experiments like these offer the chance for a more agile kind of fundamental physics, in which individual researchers with small devices can make a big impact. “I really do believe that this is a new field,” she said.

    For theorists like Arkani-Hamed, what happens just beyond the limits of our vision is interesting because of a curious numerical connection. The Planck scale, the infinitesimal size scale in which quantum gravity is thought to rule, is 16 orders of magnitude smaller than the weak scale, the neighborhood of particle physics explored in the Large Hadron Collider.

    Theories that blend these length scales often compare the two. (Physicists will take the length of the weak scale, square it, then divide this number by the length of the Planck scale.) The result of the comparison yields a range of distances matching what may be another fundamental scale: one that runs between a micron and a millimeter. Here, Arkani-Hamed suspects, new forces and particles may arise.

    Similar sizes arise when physicists consider the dark energy that fills empty space throughout the universe. When that energy density is associated with a length scale on which particles may be acting, it turns out to be about 100 microns — again suggesting this neighborhood would be an auspicious place to look for signs of new physics.

    One such search started in the late 1990s, after Arkani-Hamed and two colleagues suggested that gravity may be leaking into extra dimensions of space, a process that would explain why gravity is far weaker than the other forces known to physics. At scales smaller than the extra dimensions, before gravity had a chance to leak away, its attraction would be stronger than expected. The researchers calculated that these dimensions could be as big as a millimeter in size.

    This inspired Eric Adelberger and his colleagues to search for those dimensions. They already had the device to do it. In the 1980s, Adelberger and the so-called Eöt-Wash group at the University of Washington had built a device called a “torsion balance” that would twist in response to small forces. At first the group used the balance to search for a “fifth force” that had been proposed based on century-old experimental results. They failed to find it. “We built an apparatus, and we found that this thing wasn’t true,” Adelberger said. “It was so much fun, and it was much easier than we thought it would be.”

    Now they set out to work on Arkani-Hamed’s prediction that gravity would be much stronger at small distances — before it has a chance to leak into extra dimensions — than when objects are farther away.

    Since 2001, the team has published results from four torsion balances, each more sensitive than the last. So far, any diminutive dimensions haven’t revealed themselves. The team first reported that gravity acts normally at a distance of 218 microns. Then they reduced this number to 197 microns, then 56, and finally 42, as reported in a 2013 study. Today, their data come from two different instruments with pendulums. One pendulum twists at a rate determined by the strength of gravity; the other should stay still unless gravity behaves unexpectedly.

    But they haven’t been able to shrink their measurements much beyond 42 microns. Currently, they’re tweaking the 2013 analysis, and they hope to publish updated numbers soon. While Adelberger is hesitant to cite the new limit they’re pushing for, he said it’s unlikely to be under 20 microns. “When you first do something, the bar is relatively low,” he said. “It gets so much harder when you make the distances shorter.”

    Techniques borrowed from atomic physics may indicate another way down the ladder, even to nanoscopic scales.

    In 2010, Geraci, then a physicist at the National Institute of Standards and Technology in Boulder, Colo., suggested a scheme****to probe hidden forces at tiny scales. Instead of using the pendulums at Washington, small-force hunters could use spheres of silica levitated by lasers. By measuring how nearby objects change the position of a floating bead, this kind of experiment can look at the forces spanning just a few microns.

    The experiment is able to probe scales of smaller lengths, but there’s a catch. Gravity is most easily measured using massive objects. Geraci’s design, now built, uses spheres just 0.3 microns in size. David Moore, a physicist at Stanford University who works in the lab of Giorgio Gratta, has his own working version that uses larger silica spheres about five microns in diameter. Compared to the Eöt-Wash team, which uses torsion balances that are a few centimeters wide, both experiments trade away the larger gravitational signals for more precision at close range.

    Geraci’s and Moore’s masses are so light that the teams are not yet able to directly measure the gravitational pull of nearby objects; they can only see it if it turns out stronger than predicted by Newton’s law. That may make it harder to determine if gravity or something else is behind anything strange they might see. “One thing we always like to point out about gravity is that having the force sensitivity to see gravity is basically table stakes to play the game,” said Charlie Hagedorn, a postdoc at Washington. Adelberger adds, “If you want to know what gravity does, you’ve got to be able to see it.

    But to Geraci and Moore, the levitated beads are a general platform they can use to investigate small physics beyond just gravity. “The vision here is that once you’re able to measure these tiny forces, there’s a lot you can do,” Moore said. At the end of 2014, Moore conducted a search for particles with electric charges much smaller than one electron. Some models of dark matter suggest these “millicharged” particles could have formed in the early universe, and could still be lurking in ordinary matter.

    To try to find these particles, Moore held positively charged spheres between a pair of electrodes. He then zapped the entire apparatus with flashes of ultraviolet light to knock electrons off the electrodes. These electrons then attached to the positively charged spheres, turning them neutral. Then he applied an electric field. If any millicharged particles were still stuck on the spheres, they would impart a small force. Moore didn’t see any effects, which means that any millicharged particles must have an exceedingly small charge, or the particles themselves must be rare, or both.

    In a more recent test published** in April, Moore, working with his colleagues Alex Rider and Charles Blakemore, also used the microspheres to look for so-called “chameleon” particles that may explain dark energy. They didn’t find any, a result that echoed one published*** last year in the journal Science by a team at the University of California, Berkeley.

    “These small-scale experiments are — I don’t know what it’s called in English — ‘wild goose chase’?” said Savas Dimopoulos, a physicist at Stanford who was a co-author of the paper with Arkani-Hamed that proposed the search for millimeter-size extra dimensions. “You don’t really know where to look, but you look wherever you can.”

    For Dimopoulos, these tabletop searches are an appealing cottage industry. They offer a cheap alternative way to study provocative theories. “These ideas have been proposed over the last 40 years, but they’ve been staying on the back burner, because the main focus of fundamental physics has been accelerators,” he said.

    It’s a pitch Dimopoulos has been honing in talks over the last three years. Several experiments like those aimed at short-range forces are in the works, but they’re underfunded and underappreciated. “The field doesn’t even have a proper name,” he said.

    What might help is what Dimopoulos calls a “super lab” — a facility that would bring many such tabletop experiments together under one roof, like the research communities that have built up around high-energy projects like the Large Hadron Collider. Conrad, for her part, would like these endeavors to be better supported while still remaining at universities.

    Either way, both argue that more effort is warranted in the search for lower-energy particles, especially those predicted to lurk at scales only a little smaller than the width of a human hair. “There is a whole zoo of these things,” Dimopoulos said. “High energy is not the only frontier that exists.”

    *Science paper:
    Zeptonewton force sensing with nanospheres in an optical lattice

    **Science paper:
    Search for Screened Interactions Below the Dark Energy Length Scale Using Optically Levitated Microspheres

    ***Science paper
    Atom-interferometry constraints on dark energy

    ****Science paper:
    Short-range force detection using optically-cooled levitated microspheres

    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 5:02 pm on April 19, 2016 Permalink | Reply
    Tags: , , , , Quanta Magazine   

    From Quanta: “Physicists Hunt for the Big Bang’s Triangles” 

    Quanta Magazine
    Quanta Magazine

    The story of the universe’s birth — and evidence for string theory — could be found in triangles and myriad other shapes in the sky.

    Hannes Hummel and Olena Shmahalo/Quanta Magazine

    April 19, 2016
    Natalie Wolchover

    Once upon a time, about 13.8 billion years ago, our universe sprang from a quantum speck, ballooning to one million trillion trillion trillion trillion trillion trillion times its initial volume (by some estimates) in less than a billionth of a trillionth of a trillionth of a second. It then continued to expand at a mellower rate, in accordance with the known laws of physics.

    So goes the story of cosmic inflation, the modern version of the Big Bang theory.

    Inflationary Universe. NASA/WMAP
    Inflationary Universe. NASA/WMAP

    Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey
    Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey

    SDSS Telescope at Apache Point, NM, USA
    SDSS Telescope at Apache Point, NM, USA

    That single short, outrageous growth spurt fits all existing cosmological data well and accounts for the universe’s largeness, smoothness, flatness and lack of preferred direction.

    Cosmic Microwave Background per ESA/Planck
    Cosmic Microwave Background per ESA/Planck


    But as an explanation of how and why the universe began, inflation falls short. The questions it raises — why the growth spurt happened, how it happened, what (if anything) occurred beforehand — have confounded cosmologists since the theory emerged in the 1980s. “We have very strong evidence that there was this period of inflation,” said Matthew Kleban, a cosmologist at New York University. “But we have no idea — or we have many, many ideas — too many ideas — what inflation was, fundamentally.”

    To understand the origin of the universe, today’s cosmologists seek to identify the unknown driver of inflation, dubbed the “inflaton.” Often envisioned as a field of energy permeating space and driving it apart, the inflaton worked, experts say, like a clock. With each tick, it doubled the size of the universe, keeping nearly perfect time — until it stopped. Theorists like Kleban, then, are the clocksmiths, devising altogether hundreds of different models that might replicate the clockwork of the Big Bang.

    Like many cosmological clocksmiths, Kleban is an expert in string theory — the dominant candidate for a “theory of everything” that attempts to describe nature across all distances, times and energies. The known equations of physics falter when applied to the tiny, fleeting and frenzied environment of the Big Bang, in which they struggle to cram an enormous amount of energy into infinitesimal space and time. But string theory flourishes in this milieu, positing extra spatial dimensions that diffuse the energy. Familiar point particles become, at this highest energy and zoom level, one-dimensional “strings” and higher-dimensional, membranous “branes,” all of which traverse a 10-dimensional landscape. These vibrating, undulating gears may have powered the Big Bang’s clock.

    At his office on a recent afternoon, Kleban sketched his latest inflaton design on the blackboard. First, he drew a skinny cylinder to depict the string landscape. Its length represented the three spatial dimensions of macroscopic reality, and its circumference signified the six other spatial dimensions that string theory says exist, but which are too small to see. On the side of the cylinder, he drew a circle. This is Kleban’s timepiece: a membrane that bubbles into being and naturally expands. As its inflating interior forms a new universe, its energy incrementally ticks down in clocklike fashion each time the expanding circle winds around the cylinder’s circumference and overlaps itself. When the energy of the “brane” dilutes, the clock stops ticking, and inflation ends. It’s a scheme that some string cosmologists have hailed for its economy. “I think it’s pretty plausible that some version of this happens,” he said.

    A sketch by the string theorist and cosmologist Matthew Kleban of his Big Bang model known as unwinding inflation. Olena Shmahalo/Quanta Magazine

    Though Kleban acknowledges that it’s too soon to tell whether he or anyone else is on to something, plans are under way to find out.

    The record of the inflaton’s breakneck ticking can be read in the distribution of galaxies, galaxy clusters and superclusters that span the cosmos. These structures (and everything in them, including you) are artifacts of “mistakes in the clock,” as Matias Zaldarriaga, a cosmologist at the Institute for Advanced Study (IAS) in Princeton, N.J., put it. That is, time is intrinsically uncertain, and so the universe inflated at slightly different rates in different places and moments, producing density variations throughout. The jitter in time can also be thought of as a jitter in energy that occurred as pairs of particles spontaneously surfaced all over an “inflaton field” and stretched apart like two points on an inflating balloon. These particles were the seeds that gravity grew into galactic structures over the course of eons. The pairs of structures spanning the largest distances in the sky today came from the earliest quantum fluctuations during inflation, while structures that are closer together were produced later. This nested distribution across all cosmic distance scales “is telling you in detail that the clock was ticking,” said Nima Arkani-Hamed, a theoretical physicist at IAS. “But it doesn’t tell you anything about what it was made of.”

    To reverse-engineer the clockwork, cosmologists are seeking a new kind of data. Their calculations indicate that galaxies and other structures are not merely randomly spread out in pairs across the sky; instead, they have a slight tendency to be arranged in more complex configurations: triangles, rectangles, pentagons and all manner of other shapes, which trace back not just to quantum jitter in the Big Bang’s clock, but to a much more meaningful turning of the gears.

    Teasing out the cosmological triangles and other shapes — which have been named “non-Gaussianities” to contrast them with the Gaussian bell curve of randomly distributed pairs of structures — will require more precise observations of the cosmos than have been made to date. And so plans are being laid for a timeline of increasingly sensitive experiments. “We’re going to have far more information than we have now, and sensitivity to far subtler effects than we can probe now,” said Marc Kamionkowski, a cosmologist at Johns Hopkins University. In the meantime, theorists are making significant progress in determining what shapes to look for and how to look for them. “There’s been a great renaissance of understanding,” said Eva Silverstein, a string cosmologist at Stanford University who devised the dimensional-winding mechanism used by Kleban, as well as many clock designs of her own.

    The rigorous study of non-Gaussianities took off in 2002, when Juan Maldacena, a revered, monklike theorist at IAS, calculated what’s known as the “gravitational floor”: the minimum number of triangles and other shapes that are guaranteed to exist in the sky, due to the unavoidable effect of gravity during cosmic inflation. Cosmologists had been struggling to calculate the gravitational floor for more than a decade, since it would provide a concrete goal for experimenters. If the floor is reached, and still no triangles are detected, Maldacena explained, “then inflation is wrong.”

    When Maldacena first calculated the gravitational floor, actually detecting it seemed a distant goal indeed. At the time, all precise knowledge of the universe’s birth came from observations of the “cosmic microwave background” — the oldest light in the sky, which illuminates a two-dimensional slice of the infant universe as it appeared 380,000 years after the Big Bang. Based on the limited number of nascent structures that appear in this 2-D snapshot, it seemed impossible that their slight propensity to be configured in triangles and other shapes could ever be detected with statistical certainty. But Maldacena’s work gave theorists the tools to calculate other, more pronounced forms of non-Gaussianity that might exist in the sky, due to stronger effects than gravity. And it motivated researchers to devise better ways to search for the signals.

    A year after Maldacena made his calculation, Zaldarriaga and collaborators showed that measuring the distribution of galaxies and groupings of galaxies that make up the universe’s “large-scale structure” would yield many more shapes than observing the cosmic microwave background. “It’s a 3-D versus 2-D argument,” said Olivier Doré, a cosmologist at NASA’s Jet Propulsion Laboratory who is working on a proposed search for non-Gaussianities in the large-scale structure. “If you start counting triangles in 3-D like you can do with galaxy surveys, there are really many more you can count.”

    The notion that counting more shapes in the sky will reveal more details of the Big Bang is implied in a central principle of quantum physics known as “unitarity.” Unitarity dictates that the probabilities of all possible quantum states of the universe must add up to one, now and forever; thus, information, which is stored in quantum states, can never be lost — only scrambled. This means that all information about the birth of the cosmos remains encoded in its present state, and the more precisely cosmologists know the latter, the more they can learn about the former.

    But how did details of the Big Bang get encoded in triangles and other shapes? According to Zaldarriaga, Maldacena’s calculation “opened up the understanding of how it comes about.” In a universe governed by quantum mechanics, all of nature’s constituents are cross-wired, morphing into and interacting with one another with varying degrees of probability. This includes the inflaton field, the gravitational field, and whatever else existed in the primordial universe: Particles arising in these fields would have morphed into and scattered with each other to produce triangles and other geometric configurations, like billiard balls scattering on a table.

    Lucy Reading-Ikkanda for Quanta Magazine

    These dynamical events would be mixed in with the more mundane quantum jitter from those particle pairs that popped up in the inflaton field and engendered so-called “two-point correlations” throughout the sky. A pair of particles might, for instance, have surfaced in some other primordial field, and one member of this pair might then have decayed into two inflaton particles while the other decayed into just a single inflaton particle, yielding a three-point correlation, or triangle, in the sky. Or, two mystery particles might have collided and split into four inflaton particles, producing a four-point correlation. Rarer events would have yielded five-point, six-point and even higher-point correlations, with their numbers, sizes and interior angles encoding the types and relationships of the particles that produced them. The unitarity principle promises that by tallying the shapes ever more precisely, cosmologists will achieve an increasingly detailed account of the primordial universe, just as physicists at the Large Hadron Collider in Europe hone their theory of the known particles and look for evidence of new ones by collecting statistics on how particles morph and scatter during collisions.

    Following Maldacena’s calculation of the gravitational floor, other researchers demonstrated that even many simple inflationary models generate much more pronounced non-Gaussianity than the bare minimum. Clocksmiths like Silverstein and Kleban have since been busy working out the distinct set of triangles that their models would produce — predictions that will become increasingly testable in the coming years. Progress accelerated in 2014, when a small experiment based at the South Pole appeared to make a momentous discovery about the universe’s birth. The announcement drummed up interest in cosmological triangles, even though the supposed discovery ultimately proved a grave disappointment.

    As news began to spread on March 17, 2014, that the “smoking gun” of cosmic inflation had been detected, Stanford University’s press office posted a celebratory video on YouTube. In the footage, the cosmologist Andrei Linde, one of the decorated pioneers of inflationary cosmology, and his wife, the string and supergravity theorist and cosmologist Renata Kallosh, answer their door to find their Stanford colleague Chao-Lin Kuo on the doorstep, accompanied by a camera crew.

    “It’s five sigma, at point two,” Kuo says in the video.

    “Discovery?” Kallosh asks, after a beat. She hugs Kuo, almost melting, as Linde exclaims, “What?”

    Viewers learn that BICEP2, an experiment co-led by Kuo, has detected a swirl pattern in the cosmic microwave background that would have been imprinted by ripples in space-time known as “primordial gravitational waves.”

    BICEP 2
    BICEP 2

    And these could only have arisen during cosmic inflation, as corkscrew-like particles popped up in the gravitational field and then became stretched and permanently frozen into the shape of the universe.

    In the next scene, Linde sips champagne with his wife and their guest. In the early 1980s, Linde, Alexei Starobinsky, Alan Guth and other young cosmologists devised the theory of cosmic inflation as a patch for the broken 1930s-era Big Bang theory, which described the universe as expanding outward from a “singularity” — a nonsensical point of infinite density — and couldn’t explain why the universe hadn’t become mottled and contorted as it grew. Cosmic inflation provided a clever fix for these problems, and BICEP2’s finding suggested that the theory was conclusively proved.

    Gravitational Wave Background
    Gravitational Wave Background [?] from BICEP2

    “If this is true,” Linde says to the camera, “this is a moment of understanding of nature of such a magnitude that it just overwhelms. Let’s see. Let’s just hope that this is not a trick.”

    To many researchers, the most exciting thing about the alleged discovery was the strength of the swirl signal, measured as r = 0.2. The measurement indicated that inflation occurred at an extremely high energy scale and at the earliest moments in time, near the time-energy domain where gravity, as well as the effects of strings, branes or other exotica, would have been strong. The higher the energy scale of inflation, the more cross-wiring there would be between the inflaton and these other primordial ingredients. The result would be pronounced triangles and other non-Gaussianities in the sky.

    “After BICEP, we all stopped what we were doing and started thinking about inflation,” Arkani-Hamed said. “Inflation is like having a gigantic particle accelerator at much higher energy scales than you can get to on Earth.” The question became how such an accelerator would operate, he said, “and if there really was exotic stuff up there [near the inflation scale], how you could go about looking for it.”

    As these investigations took off, more details of BICEP2’s analysis emerged. It became clear that the discovery was indeed a trick of nature: The team’s telescope at the South Pole had picked up the swirly glow of galactic dust rather than the effect of primordial gravitational waves. A mix of anguish and anger swept through the field. Two years on, primordial gravitational waves still haven’t been detected. In January, BICEP2’s predecessor, the BICEP/Keck Array, reported that the value of r can be no more than 0.07, which lowers the ceiling on the energy scale of inflation and moves it further below the scale of strings or other exotic physics.

    Keck Array
    Keck Array

    Nonetheless, many researchers were now aware of the potential gold mine of information contained in triangles and other non-Gaussianities. It had become apparent that these fossils from inflation were worth digging for, even if they were buried deeper than BICEP2 had briefly promised. “Yeah, r went down a little bit,” Maldacena said. But it’s not so bad, in his opinion: A relatively high scale is still possible.

    In a paper last spring that drew on previous work by other researchers, Maldacena and Arkani-Hamed used symmetry arguments to show that a key feature of string theory could manifest itself in triangles. String theory predicts an infinite tower of “higher-spin states” — essentially, strings vibrating at an infinitely rising sequence of pitches. So far, no fundamental particles with a “spin” value greater than two have been discovered. Maldacena and Arkani-Hamed showed that the existence of such a higher-spin state would result in alternating peaks and troughs in the strength of the signal produced by triangles in the sky as they grow more elongated. For string theorists, this is exciting. “You can’t build a consistent interacting theory of such a particle except if you have an infinite tower of them” like the tower in string theory, explained Daniel Baumann, a theoretical cosmologist at the University of Amsterdam. Finding the oscillatory pattern in the triangles in the sky would confirm that this tower exists. “Just seeing one particle of spin greater than two would be indicative of string theory being present.”

    Other researchers are pursuing similarly general predictions. In February, Kamionkowski and collaborators reported detailed information about primordial particles that is encoded in the geometry of four-point correlations, which “get interesting,” he said, because four points can lie flat or sweep into the third dimension. Observing the signals predicted by Arkani-Hamed, Maldacena and Kamionkowski would be like striking gold, but the gold is buried deep: Their strength is probably near the gravitational floor and will require at least 1,000 times the sensitivity of current equipment to detect. Other researchers prefer to tinker with bespoke string models that predict more pronounced triangles and other shapes. “So far we’ve explored only, I think, a very small fraction of the possibilities for non-Gaussianity,” Kamionkowski said.

    Meanwhile, Linde and Kallosh are pushing in a totally different direction. Over the past three years, they’ve become enamored with a class of models called “cosmological alpha-attractors” that do not predict any non-Gaussianities above the gravitational floor at all. According to these models, cosmic inflation was completely pure, driven by a solitary inflaton field. The field is described by a Kähler manifold, which maps onto the geometric disk seen in Escher’s drawing of angels and devils. The Escherian geometry provides a continuum of possible values for the energy scale of inflation, including values so low that the inflaton’s cross-wiring to the gravitational field and other primordial fields would be extremely weak. If such a model does describe the universe, then swirls, triangles and other shapes might never be detected.

    Linde isn’t bothered by this. In supporting the alpha-attractor models, he and Kallosh are staking a position in favor of simplicity and theoretical beauty, at the expense of ever knowing for sure whether their cosmological origin story is correct. An alpha-attractor universe, Linde said, is like one of the happy families in the famous opening line of Anna Karenina. As he paraphrased Tolstoy: “Any happy family, well, they look in a sense alike. But all unhappy families — they’re unhappy for different reasons.”

    Will our universe turn out to be “happy” and completely free of distinguishing features? Baumann, who co-authored a book last year on string cosmology, argues that models like Linde’s and Kallosh’s are too simple to be plausible. “They are building these models from the bottom up,” he said. “Introducing a single field, trying to be very minimal — it would have been a beautiful model of the world.” But, he said, when you try to embed inflation into a fundamental theory of nature, it’s very hard to engineer a single field acting by itself, immune to the effects of everything else. “String theory has many of these effects; you can’t ignore them.”

    And so the search for triangles and other non-Gaussianities is under way. Between 2009 and 2013, the Planck space telescope mapped the cosmic microwave background at the highest resolution yet, and scientists have since been scouring the map for statistical excesses of triangles and other shapes. As of their most recent analysis, they haven’t found any; given the sensitivity of their instruments and their 2-D searching ground, they only ever had an outside chance of doing so. But the scientists are continuing to parse the data in new ways, with another non-Gaussianity analysis expected this year.

    Hiranya Peiris, an astrophysicist at University College London who searches for non-Gaussianities in the Planck data, said that she and her collaborators are taking cues from string cosmologists in determining which signals to look for. Peiris is keen to test a string-inflationary mechanism called axion monodromy, including variants recently developed by Silverstein and collaborators Raphael Flauger, Mehrdad Mibabayi, and Leonardo Senatore that generate an oscillatory pattern in triangles as a function of their size that can be much more pronounced than the pattern studied by Arkani-Hamed and Maldacena. To find such a signal, Peiris and her team must construct templates of the pattern and match them with the data “in a very numerically intensive and demanding analysis,” she said. “Then we have to do careful statistical tests to make sure we are not being fooled by random fluctuations in the data.”

    Some string models have already been ruled out by this data analysis. Regarding the public debate about whether string theory is too divorced from empirical testing to count as science, Silverstein said, “I find it surreal, because we are currently doing some traditional science with string theory.”

    LSST/Camera, built at SLAC
    LSST Interior
    LSST telescope, currently under construction in Chile
    LSST camera, built at SLAC, LSST telescope, currently under construction in Chile.

    Moving forward, cosmologists plan to scour ever larger volumes of the universe’s large-scale structure. Starting in 2020, the proposed SPHEREx mission could measure non-Gaussianity sensitively enough in the distribution of 300 million galaxies to determine whether inflation was driven by one clock or two cross-wired clocks (according to models of the theory known as single- and multi-field inflation, respectively).


    “Just to reach this level would dramatically reduce the number of possible inflation theories,” said Doré, who is working on the SPHEREx project. A few years further out, the Large Synoptic Survey Telescope will map 20 billion cosmological structures. If the statistical presence of triangles is not detected in the universe’s large-scale structure, there is yet another, perhaps final, approach. By mapping an ultra-faint radio signal called the 21-centimeter line, which is emitted by hydrogen atoms and traces back to the creation of the first stars, cosmologists would be able to measure even more “modes,” or arrangements of structures. “It’s going to have information about the whole volume of the universe,” Maldacena said.

    If or when triangles show up, they will, one by one, reveal the nature of the inflaton clock and why it ticked. But will enough clues be gathered before we run out of sky in which to gather them?

    The promise of unitarity — that information can be scrambled but never lost — comes with a caveat.

    “If we assume we can make perfect measurements and we have an infinite sky and so on,” Maldacena said, “then in principle all the interactions and information about particles during inflation is contained in these correlators” — that is, three-point correlations, four-point correlations and so on. But perfect measurements are impossible. And worse, the sky is finite. There is a cosmic horizon: the farthest distance from which light has had time to reach us, and thus, beyond which we cannot see. During inflation, and over the entire history of the accelerating expansion of the universe since then, swirls, triangles, quadrilaterals and other shapes have been flying past this horizon and out of sight. And with them, the subtlest of signals, associated with the rarest, highest-energy processes during inflation, are lost: Cosmologists will never be able to gather enough statistics in our finite patch of sky to tease them out, precluding a complete accounting of nature’s fundamental constituents.

    In his paper with Maldacena, Arkani-Hamed initially included a discussion of this issue, but he removed most of it. He finds the possibility of a limit to knowledge “tremendously disturbing” and sees it as evidence that quantum mechanics must be extended. One possible way to do this is suggested by his work on the amplituhedron, which casts quantum mechanical probabilities (and with them, unitarity) as emergent consequences of an underlying geometry. He plans to discuss this possibility in a forthcoming paper that will relate an analogue of the amplituhedron to non-Gaussianities in the sky.

    People vary in the extent to which they are bothered by a limit to knowledge. “I’m more practical,” Zaldarriaga said. “There are, like, tens or many tens or orders of magnitude more modes that in principle we could see, that we have not been able to measure just because of technological or theoretical inability. So, these ‘in principle’ questions are interesting, but we are way before this point.”

    Kleban also feels hopeful. “Yeah, it’s a finite amount of information,” he said. “But you could say the same thing about evolution, right? There’s a limited number of fossils, and yet we have a pretty good idea of what happened, and it’s getting better and better.”

    If all goes well, enough fossils will turn up in the sky to tell a more complete story. A vast searching ground awaits.

    See the full article here .

    Please help promote STEM in your local schools.

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

  • richardmitnick 5:09 pm on February 5, 2016 Permalink | Reply
    Tags: , Quanta Magazine, , ,   

    From SA: “Taming Superconductors with String Theory” 

    Scientific American

    Scientific American

    February 4, 2016
    Kevin Hartnett, Quanta Magazine

    The physicist Subir Sachdev borrows tools from string theory to understand the puzzling behavior of superconductors.

    String theory was devised as a way to unite the laws of quantum mechanics with those of gravity [General Relativity], with the goal of creating the vaunted theory of everything.

    Subir Sachdev is taking the “everything” literally. He’s applying the mathematics of string theory to a major problem at the other end of physics — the behavior of a potentially revolutionary class of materials known as high-temperature superconductors.


    These materials are among the most promising and the most perplexing. Unlike regular superconductors, which need to be cooled almost to absolute zero (–273.15 degrees Celsius) to pass a frictionless current of electricity, high-temperature superconductors yield the same remarkable performance under more accommodating conditions. Since the first high-temperature superconductor was discovered in 1986, physicists have found other materials that exhibit superconductivity at successively higher temperatures, with the current record standing at –70 degrees Celsius.

    This progress has occurred despite the fact that physicists don’t understand how these superconductors work. Broadly speaking, many condensed-matter physicists study how electrons — the carriers of electrical current — move through a given material. In an ordinary conductor like copper or gold, the electrons flow through a lattice formed by the copper or gold atoms. In an insulator like diamond, electrons tend to stay put. In superconductors, electrons move through the underlying atomic lattice with no energy loss at all. For three decades, physicists have been unable to develop a comprehensive theory that explains how electrons in high-temperature superconductors behave.

    A particularly interesting question is how the behavior of the material changes with temperature — in particular, how conductors transition from ordinary to super as the temperature drops. Scientists call this a “quantum phase change,” with the two phases being the property of the material on either side of the transition temperature.

    Sachdev, a condensed-matter physicist at Harvard University, explains that the challenge is one of scale. A typical chunk of material has trillions upon trillions of electrons. When those electrons interact with one another — as they do in superconductors — they become impossible to keep track of. In some phases of matter, physicists have been able to overcome this scale issue by modeling swarms of electrons as quasiparticles, quantum excitations that behave a lot like individual particles. But the quasiparticle strategy doesn’t work in high-temperature superconductors, forcing physicists to look for another way to impose collective order on the behavior of electrons in these materials.

    In 2007 Sachdev had a startling insight: He realized that certain features of string theory correspond to the electron soup found in high-temperature superconductors. In the years since, Sachdev has developed models in string theory that offer ways to think about the electron behavior in high-temperature superconductors. He’s used these ideas to design real-world experiments with materials like graphene — a flat sheet of carbon atoms — which have properties in common with the materials that interest him.

    In a forthcoming paper in Science, he and his collaborators use methods borrowed from string theory to correctly predict experimental results related to the flow of heat and electrical charge in graphene. Now he hopes to apply his insights to high-temperature superconductors themselves.

    Quanta Magazine spoke with Sachdev about how the electrons in high-temperature superconductors are related to black holes, his recent success with graphene, and why the biggest name in condensed-matter physics is skeptical that the string-theory approach works at all. An edited and condensed version of the interview follows.

    QUANTA MAGAZINE: What’s going on inside a high-temperature superconductor?
    SUBIR SACHDEV: The difference between old materials and the new materials is that in older materials, electrons conduct electricity independent of one another. They obey the exclusion principle, which says electrons can’t occupy the same quantum state at the same time and that they move independently of one another. In the new materials that I, and many others, have been studying, it’s clear that this independent-electron model fails. The general picture is that they move cooperatively and, in particular, they’re entangled — their quantum properties are linked.

    This entanglement makes high-temperature superconductors much more complicated to model than regular superconductors. How have you been looking at the problem?
    Generally I approach this through the classification of the quantum phases of matter. Examples of simple quantum phases are simple metals like silver and gold, or simple insulators like diamonds. Many of these phases are well-understood and appear everywhere in our daily lives. Since we discovered high-temperature superconductors, and many other new materials, we’ve been trying to understand the other physical properties that can emerge when you have trillions of electrons obeying quantum principles and also interacting with each other. At the back of my mind is the hope that this broad attack on classifying quantum phases of matter will lead to a deeper understanding of high-temperature superconductors.

    How far have you gotten?
    There has been great progress in understanding the theory of quantum phase transitions, which involves taking two phases of quantum matter that are very different from each other and adjusting some parameter — say, pressure on a crystal — and asking what happens when the material goes from one phase to the other. There has been a huge amount of progress for a wide class of quantum phase transitions. We now understand many different kinds of phases we didn’t know existed before.

    But a full theory of how electrons behave in high-temperature superconductors has been difficult to develop. Why?
    If you have a single electron moving through a lattice, then you really only need to worry about the different positions that electron can occupy. Even though the number of positions is large, that pretty much is something you can handle on a computer.

    But once you start talking about many electrons, you have to think about it very differently. One way to think about it is to imagine that each site on the lattice can be either empty or full. With N sites it’s 2N, so the possibilities are unimaginably vast. In this vast set of possibilities, you have to classify what are reasonable things an electron would tend to do. That in a nutshell is why it’s a difficult problem.

    Returning to phase transitions, you’ve spent a lot of time studying what happens to a high-temperature superconductor when it grows too warm. At this point, it becomes a so-called “strange metal.” Why would understanding strange metals help you to understand high-temperature superconductors?
    If you start with a superconductor and raise the temperature, there’s a critical temperature at which the superconductivity disappears. Right above this temperature you get a type of metal that we call a strange metal because many of its properties are very different from ordinary metals. Now imagine reversing the path, so that the phase of a system is changing from a strange-metal state to a superconducting state as it goes below the critical temperature. If we’re going to determine the temperature at which this happens, we need to compare the energies of the quantum states on either side of the critical temperature. But strange metals look strange in every respect, and we have only the simplest models for their physical properties.

    What makes strange metals so different from other unique quantum phases?
    In certain phases, [quantum] excitations generally behave like new emergent particles. They are quasiparticles. Their inner structure is very complicated, but from the outside they look like ordinary particles. The quasiparticle theory of many-body states pretty much applies to all states we’ve discovered in the older materials.

    Strange metals are one of the most prominent cases we know where quasiparticle theory fails. That’s why it’s so much harder to study them, because this basic tool of many-body theory doesn’t apply.

    You had the idea that string theory might be useful for understanding quantum phases that lack quasiparticles, like strange metals. How is string theory useful in this setting?
    From my point of view, string theory was another powerful mathematical tool for understanding large numbers of quantum-entangled particles. In particular, there are certain phases of string theory in which you can imagine that the ends of strings are sticking to a surface. If you are an ant moving on the surface, you only see the ends of the string. To you, these ends look like particles, but really the particles are connected by a string that goes to an extra dimension. To you, these particles sitting on the surface will appear entangled, and it is the string in the extra dimension which is entangling the particles. It’s a different way of describing entanglement.

    Now you could imagine continuing that process, not just with two electrons, but with four, six, infinitely many electrons, looking at the different entangled states the electrons can form. This is closely connected to the classification of phases of matter. It’s a hierarchical description of entanglement, where each electron finds a partner, and then the pairs entangle with other pairs, and so on. You can build this hierarchical structure using the stringy description. So it is one approach to talking about the entanglement of trillions of electrons.

    This application of string theory to strange metals has some interesting implications. For instance, it’s led you to draw connections between strange metals and the properties of black holes. How do you get from one to the other?
    In the string-theory picture, [changing the density of electrons] corresponds to putting a charge on a black hole. Many people have been studying this in the last five years or so — trying to understand things about strange metals from the properties of charged black holes. I have a recent paper in which I actually found a certain artificial model of electrons moving on a lattice where many properties precisely match the properties of charged black holes.

    I’ve read that Philip Anderson, considered by many people to be the most-influential living condensed-matter physicist, is skeptical that string theory is really useful for understanding strange metals. Do you know if that’s true?
    I think that’s correct. He’s told me himself that he doesn’t believe any of this, but, you know, what can I say, he’s a brilliant man with his own point of view. I would say that when we first proposed the idea in 2007, it certainly sounded crazy. A lot of progress has been made since then. I have a new paper with Philip Kim and others where it turns out that with graphene, which is a slightly less-strange metal, many of the methods inspired by string theory have led to quantitative predictions that have been verified by experiments.

    I think that’s been one of the best successes of the string-theory methods so far. It literally works; you can get the numbers right. But graphene is a simple system, and whether these methods are going to work for high-temperature superconductors hasn’t yet been proven.

    Could you say more about why Anderson might be skeptical of the approach you’ve taken?
    If you go back and actually look at string-theory models, on the surface they look very different from the kinds of models you need for high-temperature superconductors. You look at the stringy models and their constituents, and it appears absurd that these are connected to the constituents of the high-temperature superconductors. But if you take the point of view that, OK, I’m not literally saying this model is going to be found in [high-temperature superconductors], this is just a model that helps me make progress on difficult issues, like how do materials without quasiparticles behave, string theory gives you examples of one of these materials that’s reliably solvable.

    How literally are you using string theory? Is it a direct application, or are you drawing inspiration from it?
    It’s closer to the inspiration side of things. Once you’ve solved the model, it gives you a lot of insight into other models that you may not be able to solve. After six or seven years of work closer to the string-theory side, we think we’ve learned a lot. For us the next step appears to be working in more realistic systems using inspiration we got from more solvable models.

    How might the string-theory models, plus the work on graphene, put you in a position to understand the properties of high-temperature superconductors?
    As you change the density of electrons in high-temperature superconductors, there’s a much more dramatic change in which the electrons go from a regime where it seems only a few electrons are mobile to one where all electrons are mobile. We’re understanding that there’s a special point called the optimal density where there seems to be a dramatic change in the quantum state of electrons. And right near this point is where the strange metal is also observed. We’re trying to work out microscopic theories of this special point where the quantum state changes, and stringy models can teach us a lot about such quantum-critical points. Once we have the full framework, we’re hopeful and optimistic that we can take many of the insights from graphene and apply them to this more complicated model. That’s where we are.

    See the full article here .

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  • richardmitnick 4:31 pm on January 13, 2016 Permalink | Reply
    Tags: , , Quanta Magazine,   

    From Quanta: “String Theory Meets Loop Quantum Gravity” 

    Quanta Magazine
    Quanta Magazine

    January 12, 2016
    Sabine Hossenfelder

    Temp 1

    Eight decades have passed since physicists realized that the theories of quantum mechanics and gravity [Albert Einstein’s Theory of General Relativity] don’t fit together, and the puzzle of how to combine the two remains unsolved. In the last few decades, researchers have pursued the problem in two separate programs — string theory and loop quantum gravity — that are widely considered incompatible by their practitioners. But now some scientists argue that joining forces is the way forward.

    Among the attempts to unify quantum theory and gravity, string theory has attracted the most attention. Its premise is simple: Everything is made of tiny strings. The strings may be closed unto themselves or have loose ends; they can vibrate, stretch, join or split. And in these manifold appearances lie the explanations for all phenomena we observe, both matter and space-time included.

    Loop quantum gravity, by contrast, is concerned less with the matter that inhabits space-time than with the quantum properties of space-time itself. In loop quantum gravity, or LQG, space-time is a network. The smooth background of Einstein’s theory of gravity is replaced by nodes and links to which quantum properties are assigned. In this way, space is built up of discrete chunks. LQG is in large part a study of these chunks.

    This approach has long been thought incompatible with string theory. Indeed, the conceptual differences are obvious and profound. For starters, LQG studies bits of space-time, whereas string theory investigates the behavior of objects within space-time. Specific technical problems separate the fields. String theory requires that space-time have 10 dimensions; LQG doesn’t work in higher dimensions. String theory also implies the existence of supersymmetry, in which all known particles have yet-undiscovered partners.

    Supersymmetry standard model
    Standard Model of Supersymmetry

    Supersymmetry isn’t a feature of LQG.

    These and other differences have split the theoretical physics community into deeply divergent camps. “Conferences have segregated,” said Jorge Pullin, a physicist at Louisiana State University and co-author of an LQG textbook. “Loopy people go to loopy conferences. Stringy people go to stringy conferences. They don’t even go to ‘physics’ conferences anymore. I think it’s unfortunate that it developed this way.”

    But a number of factors may be pushing the camps closer together. New theoretical findings have revealed potential similarities between LQG and string theory. A young generation of string theorists has begun to look outside string theory for methods and tools that might be useful in the quest to understand how to create a “theory of everything.” And a still-raw paradox involving black holes and information loss has given everyone a fresh dose of humility.

    Moreover, in the absence of experimental evidence for either string theory or LQG, mathematical proof that the two are in fact opposite sides of the same coin would bolster the argument that physicists are progressing toward the correct theory of everything. Combining LQG and string theory would truly make it the only game in town.

    An Unexpected Link

    An effort to solve some of LQG’s own internal problems has led to the first surprising link with string theory. Physicists who study LQG lack a clear understanding of how to zoom out from their network of space-time chunks and arrive at a large-scale description of space-time that dovetails with Einstein’s general theory of relativity — our best theory of gravity. More worrying still, their theory can’t reconcile the special case in which gravity can be neglected. It’s a malaise that befalls any approach reliant on chunking-up space-time: In Einstein’s theory of special relativity, an object will appear to contract depending on how fast an observer is moving relative to it. This contraction also affects the size of space-time chunks, which are then perceived differently by observers with different velocities. The discrepancy leads to problems with the central tenet of Einstein’s theory — that the laws of physics should be the same no matter what the observer’s velocity.

    “It’s difficult to introduce discrete structures without running into difficulties with special relativity,” said Pullin. In a brief paper he wrote in 2014 with frequent collaborator Rodolfo Gambini, a physicist at the University of the Republic in Montevideo, Uruguay, Pullin argued that making LQG compatible with special relativity necessitates interactions that are similar to those found in string theory.

    That the two approaches have something in common seemed likely to Pullin since a seminal discovery in the late 1990s by Juan Maldacena, a physicist at the Institute for Advanced Study in Princeton, N.J. Maldacena matched up a gravitational theory in a so-called anti-de Sitter (AdS) space-time with a field theory (CFT — the “C” is for “conformal”) on the boundary of the space-time. By using this AdS/CFT identification, the gravitational theory can be described by the better-understood field theory.

    The full version of the duality is a conjecture, but it has a well-understood limiting case that string theory plays no role in. Because strings don’t matter in this limiting case, it should be shared by any theory of quantum gravity. Pullin sees this as a contact point.

    Herman Verlinde, a theoretical physicist at Princeton University who frequently works on string theory, finds it plausible that methods from LQG can help illuminate the gravity side of the duality. In a recent paper, Verlinde looked at AdS/CFT in a simplified model with only two dimensions of space and one of time, or “2+1” as physicists say. He found that the AdS space can be described by a network like those used in LQG. Even though the construction presently only works in 2+1, it offers a new way to think about gravity. Verlinde hopes to generalize the model to higher dimensions. “Loop quantum gravity has been seen too narrowly. My approach is to be inclusive. It’s much more intellectually forward-looking,” he said.

    But even having successfully combined LQG methods with string theory to make headway in anti-de Sitter space, the question remains: How useful is that combination? Anti-de Sitter space-times have a negative cosmological constant (a number that describes the large-scale geometry of the universe); our universe has a positive one. We just don’t inhabit the mathematical construct that is AdS space.

    Verlinde is pragmatic. “One idea is that [for a positive cosmological constant] one needs a totally new theory,” he said. “Then the question is how different that theory is going to look. AdS is at the moment the best hint for the structure we are looking for, and then we have to find the twist to get a positive cosmological constant.” He thinks it’s time well spent: “Though [AdS] doesn’t describe our world, it will teach us some lessons that will guide us where to go.”

    Coming Together in a Black Hole

    Verlinde and Pullin both point to another chance for the string theory and loop quantum gravity communities to come together: the mysterious fate of information that falls into a black hole. In 2012, four researchers based at the University of California, Santa Barbara, highlighted an internal contradiction in the prevailing theory. They argued that requiring a black hole to let information escape would destroy the delicate structure of empty space around the black hole’s horizon, thereby creating a highly energetic barrier — a black hole “firewall.” This firewall, however, is incompatible with the equivalence principle that underlies general relativity, which holds that observers can’t tell whether they’ve crossed the horizon. The incompatibility roiled string theorists, who thought they understood black hole information and now must revisit their notebooks.

    But this isn’t a conundrum only for string theorists. “This whole discussion about the black hole firewalls took place mostly within the string theory community, which I don’t understand,” Verlinde said. “These questions about quantum information, and entanglement, and how to construct a [mathematical] Hilbert space – that’s exactly what people in loop quantum gravity have been working on for a long time.”

    Meanwhile, in a development that went unnoted by much of the string community, the barrier once posed by supersymmetry and extra dimensions has fallen as well. A group around Thomas Thiemann at Friedrich-Alexander University in Erlangen, Germany, has extended LQG to higher dimensions and included supersymmetry, both of which were formerly the territory of string theory.

    More recently, Norbert Bodendorfer, a former student of Thiemann’s who is now at the University of Warsaw, has applied methods of LQG’s loop quantization to anti-de Sitter space. He argues that LQG can be useful for the AdS/CFT duality in situations where string theorists don’t know how to perform gravitational computations. Bodendorfer feels that the former chasm between string theory and LQG is fading away. “On some occasions I’ve had the impression that string theorists knew very little about LQG and didn’t want to talk about it,” he said. “But [the] younger people in string theory, they are very open-minded. They are very interested what is going on at the interface.”

    “The biggest difference is in how we define our questions,” said Verlinde. “It’s more sociological than scientific, unfortunately.” He doesn’t think the two approaches are in conflict: “I’ve always viewed [string theory and loop quantum gravity] as parts of the same description. LQG is a method, it’s not a theory. It’s a method to think of quantum mechanics and geometry. It’s a method that string theorists can use and are actually using. These things are not incompatible.”

    Not everyone is so convinced. Moshe Rozali, a string theorist at the University of British Columbia, remains skeptical of LQG: “The reason why I personally don’t work on LQG is the issue with special relativity,” he said. “If your approach does not respect the symmetries of special relativity from the outset, then you basically need a miracle to happen at one of your intermediate steps.” Still, Rozali said, some of the mathematical tools developed in LQG might come in handy. “I don’t think that there is any likelihood that string theory and LQG are going to converge to some middle ground,” he said. “But the methods are what people normally care about, and these are similar enough; the mathematical methods could have some overlap.”

    Not everyone on the LQG side expects the two will merge either. Carlo Rovelli, a physicist at the University of Marseille and a founding father of LQG, believes his field ascendant. “The string planet is infinitely less arrogant than ten years ago, especially after the bitter disappointment of the non-appearance of supersymmetric particles,” he said. “It is possible that the two theories could be parts of a common solution … but I myself think it is unlikely. String theory seems to me to have failed to deliver what it had promised in the ’80s, and is one of the many ‘nice-idea-but-nature-is-not-like-that’ that dot the history of science. I do not really understand how can people still have hope in it.”

    For Pullin, declaring victory seems premature: “There are LQG people now saying, ‘We are the only game in town.’ I don’t subscribe to this way of arguing. I think both theories are vastly incomplete.”

    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.

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