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  • richardmitnick 5:09 pm on February 5, 2016 Permalink | Reply
    Tags: , , , String Theory,   

    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.

    Superconductivity
    Superconductivity

    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: , , , String Theory   

    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 .

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

     
  • richardmitnick 9:22 pm on December 30, 2015 Permalink | Reply
    Tags: , , , String Theory   

    From Ethan Siegel: “Why String Theory Is Not A Scientific Theory” 

    Starts with a bang
    Starts with a Bang

    12.30.15
    Ethan Siegel

    Temp 1
    Image credit: flickr user Trailfan, via https://www.flickr.com/photos/7725050@N06/631503428.

    Scientists work on it, it’s consistent with science, and it hopes to be the biggest scientific breakthrough of all. But it’s missing one key ingredient.

    “As of now, string theorists have no explanation of why there are three large dimensions as well as time, and the other dimensions are microscopic. Proposals about that have been all over the map.” -Edward Witten

    There are a lot of different ways to define science, but perhaps one that everyone can agree on is that it’s a process by which:

    knowledge about the natural world or a particular phenomenon is gathered,
    a testable hypothesis is put forth concerning a natural, physical explanation for that phenomenon,
    that hypothesis is then tested and either validated or falsified,
    and an overarching framework — or scientific theory — is constructed to explain the hypothesis and that makes predictions about other phenomena,
    which is then tested further, and either validated, in which case new phenomena to test are sought (back to step 3), or falsified, in which case a new testable hypothesis is put forth (back to step 2)…

    and so on. This scientific process always involves the continued gathering of more data, the continued refining or outright replacing of hypotheses when the realm of validity of the theory is exceeded, and testing that subjects that theory to either further validation or potential falsification.

    That’s how science has always progressed, whether we’ve recognized it or not. Heliocentrism replaced geocentrism because it explained phenomena that geocentrism couldn’t, including:

    Jupiter’s moons,
    the phases and relative sizes of Venus and Mars at different times of year,
    and the periodicity of cometary orbits.

    2
    Image credit: public domain work by Wikimedia Commons users Nichalp and Sagredo, of the phases (and angular size) of Venus in the heliocentric model.

    Newtonian gravity superseded Kepler’s laws because of its additional predictive power, combining terrestrial and celestial mechanics. Even Einstein’s relativity, both special and general, came about because of the failures of Newtonian mechanics to account for behavior close to the speed of light and in strong gravitational fields. It took observations well beyond what was capable of in Newton’s time, such as the measurements of the lifetimes of particles produced in radioactive decays and the orbit of Mercury around the Sun over the course of centuries. The continued gathering of data — in new regimes, at higher precision and over longer timescales — allowed us to see the cracks in the scientific theories du jour, as well as where the potential to expand beyond them were.

    Now, we come to the present day. [Albert] Einstein’s general relativity is still our leading theory of gravity, having passed every experimental and observational test tossed its way, from gravitational lensing to relativistic frame dragging to the decay of binary pulsar orbits, while three other fundamental forces — electromagnetism and the strong and weak nuclear forces — are described by quantum field theories. These two classes of theories are fundamentally incompatible and incomplete on their own, and indicate that there is more to the Universe than we currently understand, despite the success of the Standard Model and the need for a quantum theory of gravity.

    10
    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.

    3
    Image credit: NASA, of an artist’s concept of Gravity Probe B orbiting the Earth to measure space-time curvature.

    NASA Gravity Probe B
    NASA/Gravity Probe B

    One option for a solution to this conundrum is string theory., or the idea that everything we perceive as a particle or force is simply an excitation of a closed or open string, vibrating at specific but unique frequencies.

    It may seem that, by calling it “string theory” and presenting it as a possible solution to a scientific question, we’ve already answered in the affirmative: yes, string theory is a scientific theory. But it’s only a theory in the mathematical sense, which means it has its own set of axioms, postulates, elements, as well as the theorems and corollaries that can be derived from them. Set theory, group theory and number theory are all examples of mathematical theories, and string theory is another such example.

    4
    Image credit: Wikimedia Commons user Lunch, of a 2-D projection of a Calabi-Yau manifold, one popular method of compactifying the extra, unwanted dimensions of String Theory.

    But is it a physical theory?

    It makes physical predictions, such as:

    the existence of ten dimensions,
    that the fundamental constants are determined by the “vacuum” of string theory,
    the existence of supersymmetric particles,

    Supersymmetry standard model
    Standard Model of Supersymmetry

    and that there is a mathematically equivalent relation between a theory of quantum gravity in, say, five-dimensional space and a field theory without gravity on the boundary (and hence, in four dimensions) of that space.

    These are, no doubt, predictions about the physical Universe. But can we test any of these predictions?

    5
    Image credit: public domain work by Wikimedia Commons user Rogilbert.

    The answer, so far, is no. The first one is a huge problem: we need to get rid of six dimensions to get back the Universe we see, and there are more ways to do it than there are atoms in the Universe. What’s worse, is that each way you do it gives a different “vacuum” for string theory, with no clear way to get the fundamental constants that describe the Universe we inhabit, which is the second prediction. The third prediction has come up empty, but we would need to achieve energies that are ~1015 times higher than what the LHC can produce to rule out string theory entirely and falsify it.

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN, the most powerful particle accelerator ever built.

    Moreover, supersymmetric particles is not a unique prediction of string theory; finding them would only mean that string theory isn’t ruled out, not that it’s right. And the last prediction is only a mathematical one, not a physical one. It doesn’t give us anything specific to look for or test about our Universe.

    Although there was an entire conference on it earlier this month, spurred by a controversial opinion piece written a year ago by George Ellis and Joe Silk, the answer is very clear: no, string theory is not a scientific theory. The way people are trying to turn it into science is — as Sabine Hossenfelder and Davide Castelvecchi report — by redefining what “science” is.

    6
    Image credit: Gideon Pisanty, of Tulipa agenensis sharonensis (Dinsm.) Feinbrun, Dor-Habonim Beach, Israel, February 26, 2012.

    How absurd! If I showed you a tulip and said, “this is a rose,” you could show me all the roses in the world and say, “no, these are roses, that is a tulip.” If I then changed the definition of a rose to include tulips, would that cause a tulip to become a rose? Or would I merely be turning a useful definition and distinction into a less useful one?

    7
    Image credit: public domain, retrieved from https://pixabay.com/en/globe-earth-country-continents-73397/.

    If you want to rise to the level of a scientific theory, you have to make a testable — and hence, falsifiable or validatable — predictions. Even a physical state that arises as a consequence of an established theory, such as the multiverse, isn’t a scientific theory until we have a way to confirm or refute it; it’s only a hypothesis, even if it’s a good hypothesis. What’s interesting about string theory is that when it was first proposed, it was called the string hypothesis, as it was recognized this idea hadn’t yet risen to the status of a full-fledged theory. (Of course, at that time, it hypothesized that strings were the fundamental entity inside of atomic nuclei, rather than quarks and gluons.)

    8
    Image credit: G.S. Sharov (Tver State U.), 2013, via http://inspirehep.net/record/1233875.

    It’s still a physical hypothesis, and perhaps someday it will become a physically interesting scientific theory. When that day comes, we’ll all proudly welcome string theory into the fold as science. Until then, we can all agree that string theory is interesting for the possibilities it holds. Whether those possibilities are relevant or meaningful for our Universe, however, is a question science is unable to address today.

    See the full article here .

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 4:24 pm on December 24, 2015 Permalink | Reply
    Tags: , , , , , String Theory   

    From Ethan Siegel: “What Are Quantum Gravity’s Alternatives To String Theory?” 

    Starts with a bang
    Starts with a Bang

    12.24.15
    Ethan Siegel

    1
    Image credit: CPEP (Contemporary Physics Education Project), NSF/DOE/LBNL.

    If there is a quantum theory of gravity, is String Theory the only game in town?

    “I just think too many nice things have happened in string theory for it to be all wrong. Humans do not understand it very well, but I just don’t believe there is a big cosmic conspiracy that created this incredible thing that has nothing to do with the real world.” –Edward Witten

    The Universe we know and love — with [Albert] Einstein’s General Relativity as our theory of gravity and quantum field theories of the other three forces — has a problem that we don’t often talk about: it’s incomplete, and we know it. Einstein’s theory on its own is just fine, describing how matter-and-energy relate to the curvature of space-and-time. Quantum field theories on their own are fine as well, describing how particles interact and experience forces. Normally, the quantum field theory calculations are done in flat space, where spacetime isn’t curved. We can do them in the curved space described by Einstein’s theory of gravity as well (although they’re harder — but not impossible — to do), which is known as semi-classical gravity. This is how we calculate things like Hawking radiation and black hole decay.

    2
    Image credit: NASA, via http://www.nasa.gov/topics/universe/features/smallest_blackhole.html

    But even that semi-classical treatment is only valid near and outside the black hole’s event horizon, not at the location where gravity is truly at its strongest: at the singularities (or the mathematically nonsensical predictions) theorized to be at the center. There are multiple physical instances where we need a quantum theory of gravity, all having to do with strong gravitational physics on the smallest of scales: at tiny, quantum distances. Important questions, such as:

    What happens to the gravitational field of an electron when it passes through a double slit?
    What happens to the information of the particles that form a black hole, if the black hole’s eventual state is thermal radiation?
    And what is the behavior of a gravitational field/force at and around a singularity?

    3
    Image credit: Nature 496, 20–23 (04 April 2013) doi:10.1038/496020a, via http://www.nature.com/news/astrophysics-fire-in-the-hole-1.12726.

    In order to explain what happens at short distances in the presence of gravitational sources — or masses — we need a quantum, discrete, and hence particle-based theory of gravity. The known quantum forces are mediated by particles known as bosons, or particles with integer spin. The photon mediates the electromagnetic force, the W-and-Z bosons mediate the weak force, while the gluons mediate the strong force. All these types of particles have a spin of 1, which for massive (W-and-Z) particles mean they can take on spin values of -1, 0, or +1, while for massless ones (like gluons and photons), they can take on values of -1 or +1 only.

    The Higgs boson is also a boson, although it doesn’t mediate any forces, and has a spin of 0. Because of what we know about gravitation — General Relativity is a tensor theory of gravity — it must be mediated by a massless particle with a spin of 2, meaning it can take on a spin value of -2 or +2 only.

    This is fantastic! It means that we already know a few things about a quantum theory of gravity before we even try to formulate one! We know this because whatever the true quantum theory of gravity turns out to be, it must be consistent with General Relativity when we’re not at very small distances from a massive particle or object, just as — 100 years ago — we knew that General Relativity needed to reduce to Newtonian gravity in the weak-field regime.

    4
    Image credit: NASA, of an artist’s concept of Gravity Probe B orbiting the Earth to measure space-time curvature.

    NASA Gravity Probe B
    Gravity Probe B

    The big question, of course is how? How do you quantize gravity in a way that’s correct (at describing reality), consistent (with both GR and QFT), and hopefully leads to calculable predictions for new phenomena that might be observed, measured or somehow tested. The leading contender, of course, is something you’ve long heard of: String Theory.

    String Theory is an interesting framework — it can include all of the standard model fields and particles, both the fermions and the bosons.

    0
    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.

    It includes also a 10-dimensional Tensor-Scalar theory of gravity: with 9 space and 1 time dimensions, and a scalar field parameter. If we erase six of those spatial dimensions (through an incompletely defined process that people just call compactification) and let the parameter (ω) that defines the scalar interaction go to infinity, we can recover General Relativity.

    5
    Image credit: NASA/Goddard/Wade Sisler, of Brian Greene presenting on String Theory.

    But there are a whole host of phenomenological problems with String Theory. One is that it predicts a large number of new particles, including all the supersymmetric ones, none of which have been found.

    Supersymmetry standard model
    Standard Model of Supersymmetry

    It claims to not need to need “free parameters” like the standard model has (for the masses of the particles), but it replaces that problem with an even worse one. String theory refers to “10⁵⁰⁰ possible solutions,” where these solutions refer to the vacuum expectation values of the string fields, and there’s no mechanism to recover them; if you want String Theory to work, you need to give up on dynamics, and simply say, “well, it must’ve been anthropically selected.” There are frustrations, drawbacks, and problems with the very idea of String Theory. But the biggest problem with it may not be these mathematical ones. Instead, it may be that there are four other alternatives that may lead us to quantum gravity instead; approaches that are completely independent of String Theory.

    6
    Image credit: Wikimedia Commons user Linfoxman, of an illustration of a quantized “fabric of space.”

    1.) Loop Quantum Gravity [reader, please take the time to visit this link and read the article]. LQG is an interesting take on the problem: rather than trying to quantize particles, LQG has as one of its central features that space itself is discrete. Imagine a common analogy for gravity: a bedsheet pulled taut, with a bowling ball in the center. Rather than a continuous fabric, though, we know that the bedsheet itself is really quantized, in that it’s made up of molecules, which in turn are made of atoms, which in turn are made of nuclei (quarks and gluons) and electrons.

    Space might be the same way! Perhaps it acts like a fabric, but perhaps it’s made up of finite, quantized entities. And perhaps it’s woven out of “loops,” which is where the theory gets it name from. Weave these loops together and you get a spin network, which represents a quantum state of the gravitational field. In this picture, not just the matter itself but space itself is quantized. The way to go from this idea of a spin network to a perhaps realistic way of doing gravitational computations is an active area of research, one that saw a tremendous leap forward made in just 2007/8, so this is still actively advancing.

    7
    Image credit: Wikimedia Commons user & reasNink, generated with Wolfram Mathematica 8.0.

    2.) Asymptotically Safe Gravity. This is my personal favorite of the attempts at a quantum theory of gravity. Asymptotic freedom was developed in the 1970s to explain the unusual nature of the strong interaction: it was a very weak force at extremely short distances, then got stronger as (color) charged particles got farther and farther apart. Unlike electromagnetism, which had a very small coupling constant, the strong force has a large one. Due to some interesting properties of QCD, if you wound up with a (color) neutral system, the strength of the interaction fell off rapidly. This was able to account for properties like the physical sizes of baryons (protons and neutrons, for example) and mesons (pions, for example).

    Asymptotic safety, on the other hand, looks to solve a fundamental problem that’s related to this: you don’t need small couplings (or couplings that tend to zero), but rather for the couplings to simply be finite in the high-energy limit. All coupling constants change with energy, so what asymptotic safety does is pick a high-energy fixed point for the constant (technically, for the renormalization group, from which the coupling constant is derived), and then everything else can be calculated at lower energies.

    At least, that’s the idea! We’ve figured out how to do this in 1+1 dimensions (one space and one time), but not yet in 3+1 dimensions. Still, progress has been made, most notably by Christof Wetterich, who had two ground breaking papers in the 1990s. More recently, Wetterich used asymptotic safety — just six years ago — to calculate a prediction for the mass of the Higgs boson before the LHC found it. The result?

    9
    Image credit: Mikhail Shaposhnikov & Christof Wetterich.

    Amazingly, what it indicated was perfectly in line with what the LHC wound up finding.

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

    It’s such an amazing prediction that if asymptotic safety is correct, and — when the error bars are beaten down further — the masses of the top quark, the W-boson and the Higgs boson are finalized, there may not even be a need for any other fundamental particles (like SUSY particles) for physics to be stable all the way up to the Planck scale. It’s not only very promising, it has many of the same appealing properties of string theory: quantizes gravity successfully, reduces to GR in the low energy limit, and is UV-finite. In addition, it beats string theory on at least one account: it doesn’t need the addition of new particles or parameters that we have no evidence for! Of all the string theory alternatives, this one is my favorite.

    3.) Causal Dynamical Triangulations. This idea, CDT, is one of the new kids in town, first developed only in 2000 by Renate Loll and expanded on by others since. It’s similar to LQG in that space itself is discrete, but is primarily concerned with how that space itself evolves. One interesting property of this idea is that time must be discrete as well! As an interesting feature, it gives us a 4-dimensional spacetime (not even something put in a priori, but something that the theory gives us) at the present time, but at very, very high energies and small distances (like the Planck scale), it displays a 2-dimensional structure. It’s based on a mathematical structure called a simplex, which is a multi-dimensional analogue of a triangle.

    10
    Image credit: screenshot from the Wikipedia page for Simplex, via https://en.wikipedia.org/wiki/Simplex.

    A 2-simplex is a triangle, a 3-simplex is a tetrahedron, and so on. One of the “nice” features of this option is that causality — a notion held sacred by most human beings — is explicitly preserved in CDT. (Sabine has some words on CDT here, and its possible relation to asymptotically safe gravity.) It might be able to explain gravity, but it isn’t 100% certain that the standard model of elementary particles can fit suitably into this framework. It’s only major advances in computation that have enabled this to become a fairly well-studied alternative of late, and so work in this is both ongoing and relatively young.

    4.) Emergent gravity. And finally, we come to what’s probably the most speculative, recent of the quantum gravity possibilities. Emergent gravity only gained prominence in 2009, when Erik Verlinde proposed entropic gravity, a model where gravity was not a fundamental force, but rather emerged as a phenomenon linked to entropy. In fact, the seeds of emergent gravity go back to the discoverer of the conditions for generating a matter-antimatter asymmetry, Andrei Sakharov, who proposed the concept back in 1967. This research is still in its infancy, but as far as developments in the last 5–10 years go, it’s hard to ask for more than this.

    11
    Image credit: flickr gallery of J. Gabas Esteban.

    We’re sure we need a quantum theory of gravity to make the Universe work at a fundamental level, but we’re not sure what that theory looks like or whether any of these five avenues (string theory included) are going to prove fruitful or not. String Theory is the best studied of all the options, but Loop Quantum Gravity is a rising second, with the others being given serious consideration at long last. They say the answer’s always in the last place you look, and perhaps that’s motivation enough to start looking, seriously, in newer places.

    See the full article here .

    Please help promote STEM in your local schools.

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 2:12 pm on August 27, 2015 Permalink | Reply
    Tags: , , String Theory,   

    From Symmetry: “Looking for strings inside inflation” 

    Symmetry

    August 27, 2015
    Troy Rummler

    1

    Theorists from the Institute for Advanced Study have proposed a way forward in the quest to test string theory.

    Two theorists recently proposed a way to find evidence for an idea famous for being untestable: string theory. It involves looking for particles that were around 14 billion years ago, when a very tiny universe hit a growth spurt that used 15 billion times more energy than a collision in the Large Hadron Collider.

    Scientists can’t crank the LHC up that high, not even close. But they could possibly observe evidence of these particles through cosmological studies, with the right technological advances.
    Unknown particles

    During inflation—the flash of hyperexpansion that happened 10-33 seconds after the big bang— particles were colliding with astronomical power. We see remnants of that time in tiny fluctuations in the haze of leftover energy called the cosmic microwave background [CMB].

    Cosmic Background Radiation Planck
    CMB per Planck

    ESA Planck
    ESA/Planck

    Scientists might be able to find remnants of any prehistoric particles that were around during that time as well.

    “If new particles existed during inflation, they can imprint a signature on the primordial fluctuations, which can be seen through specific patterns,” says theorist Juan Maldacena of the Institute for Advanced Study at Princeton University.

    Maldacena and his IAS collaborator, theorist Nima Arkani-Hamed, have used quantum field theory calculations to figure out what these patterns might look like. The pair presented their findings at an annual string theory conference held this year in Bengaluru, India, in June.

    The probable, impossible string

    String theory is frequently summed up by its basic tenet: that the fundamental units of matter are not particles. They are one-dimensional, vibrating strings of energy.

    The theory’s purpose is to bridge a mathematic conflict between quantum mechanics and [Albert] Einstein’s theory of general relativity. Inside a black hole, for example, quantum mechanics dictates that gravity is impossible. Any attempt to adjust one theory to fit the other causes the whole delicate system to collapse. Instead of trying to do this, string theory creates a new mathematical framework in which both theories are natural results. Out of this framework emerges an astonishingly elegant way to unify the forces of nature, along with a correct qualitative description of all known elementary particles.

    As a system of mathematics, string theory makes a tremendous number of predictions. Testable predictions? None so far.

    Strings are thought to be the smallest objects in the universe, and computing their effects on the relatively enormous scales of particle physics experiments is no easy task. String theorists predict that new particles exist, but they cannot compute their masses.

    To exacerbate the problem, string theory can describe a variety of universes that differ by numbers of forces, particles or dimensions. Predictions at accessible energies depend on these unknown or very difficult details. No experiment can definitively prove a theory that offers so many alternative versions of reality.
    Putting string theory to the test

    But scientists are working out ways that experiments could at least begin to test parts of string theory. One prediction that string theory makes is the existence of particles with a unique property: a spin of greater than two.

    Spin is a property of fundamental particles. Particles that don’t spin decay in symmetric patterns. Particles that do spin decay in asymmetric patterns, and the greater the spin, the more complex those patterns get. Highly complex decay patterns from collisions between these particles would have left signature impressions on the universe as it expanded and cooled.

    Scientists could find the patterns of particles with greater than spin 2 in subtle variations in the distribution of galaxies or in the cosmic microwave background, according to Maldacena and Arkani-Hamed. Observational cosmologists would have to measure the primordial fluctuations over a wide range of length scales to be able to see these small deviations.

    The IAS theorists calculated what those measurements would theoretically be if these massive, high-spin particles existed. Such a particle would be much more massive than anything scientists could find at the LHC.

    A challenging proposition

    Cosmologists are already studying patterns in the cosmic microwave background. Experiments such as Planck, BICEP and POLAR BEAR are searching for polarization, which would be evidence that a nonrandom force acted on it.

    BICEP 2
    BICEP 2 interior
    BICEP

    POLARBEAR McGill Telescope
    PolarBear

    If they rewind the effects of time and mathematically undo all other forces that have interacted with this energy, they hope that what pattern remains will match the predicted twists imbued by inflation.

    The patterns proposed by Maldacena and Arkani-Hamed are much subtler and much more susceptible to interference. So any expectation of experimentally finding such signals is still a long way off.

    But this research could point us toward someday finding such signatures and illuminating our understanding of particles that have perhaps left their mark on the entire universe.
    The value of strings

    Whether or not anyone can prove that the world is made of strings, people have proven that the mathematics of string theory can be applied to other fields.

    In 2009, researchers discovered that string theory math could be applied to conventional problems in condensed matter physics. Since then researchers have been applying string theory to study superconductors.

    Fellow IAS theorist Edward Witten, who received the Fields Medal in 1990 for his mathematical contributions to quantum field theory and Supersymmetry, says Maldacena and Arkani-Hamed’s presentation was among the most innovative work he saw at the Strings ‘15 conference.

    Witten and others believe that such successes in other fields indicate that string theory actually underlies all other theories at some deeper level.

    “Physics—like history—does not precisely repeat itself,” Witten says. However, with similar structures appearing at different scales of lengths and energies, “it does rhyme.”

    See the full article here.

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 1:57 pm on January 28, 2015 Permalink | Reply
    Tags: , , , String Theory   

    From NPR: “The Most Dangerous Ideas In Science” 

    NPR

    National Public Radio (NPR)

    January 27, 2015
    Adam Frank, University of Rochester

    1
    Some physicists are pushing back against ideas like string theory and the multiverse. Here, we see a computer-generated image of a black hole, which might, ultimately, be explained by ideas like string theory.

    There’s a battle going on at the edge of the universe, but it’s getting fought right here on Earth. With roots stretching back as far as the ancient Greeks, in the eyes of champions on either side, this fight is a contest over nothing less than the future of science. It’s a conflict over the biggest cosmic questions humans can ask and the methods we use — or can use — to get answers for those questions.

    Cosmology is the study of the universe as a whole: its structure, its origins and its fate. Fundamental physics is the study of reality’s bedrock entities and their interactions. With these job descriptions it’s no surprise that cosmology and fundamental physics share a lot of territory. You can’t understand how the universe evolves after the Big Bang (a cosmology question) without understanding how matter, energy, space and time interact (a fundamental physics question). Recently, however, something remarkable has been happening in both these fields that’s raising hackles with some scientists. As physicists George Ellis and Joseph Silk recently put it in Nature:

    “This year, debates in physics circles took a worrying turn. Faced with difficulties in applying fundamental theories to the observed Universe, some researchers called for a change in how theoretical physics is done. They began to argue — explicitly — that if a theory is sufficiently elegant and explanatory, it need not be tested experimentally, breaking with centuries of philosophical tradition of defining scientific knowledge as empirical.”

    The root of the problem rests with two ideas/theories now central for some workers in cosmology (even if they remain problematic for physicists as a whole). The first is string theory, which posits that the world is made up not of point particles but of tiny vibrating strings. String theory only works if the universe has many “extra” dimensions of space other than the three we experience. The second idea is the so-called multiverse which, in its most popular form, claims more than one distinct universe emerged from the Big Bang. Instead, adherents claim, there may be an almost infinite (if not truly infinite) number of parallel “pocket universes,” each with their own version of physics.

    Both string theory and the multiverse are big, bold reformulations of what we mean when we say the words “physical reality.” That is reason enough for them to be contentious topics in scientific circles. But in the pursuit of these ideas, something else — something new — has emerged. Rather than focusing just on questions about the nature of the cosmos, the new developments raise critical questions about the basic rules of science [scientific method] when applied to something like the universe as a whole.

    Here is the problem: Both string theory and the multiverse posit entities that may, in principle or in practice, be unobservable. Evidence for the extra dimensions needed to make string theory work is likely to require a particle accelerator of astronomical proportions. And the other pocket universes making up the multiverse may lie permanently over our “horizon,” such that we will never get direct observations of their existence. It’s this specific aspect of the theories that has scientists like Ellis and Silk so concerned. As they put it:

    “These unprovable hypotheses are quite different from those that relate directly to the real world and that are testable through observations — such as the standard model of particle physics and the existence of dark matter and dark energy. As we see it, theoretical physics risks becoming a no-man’s-land between mathematics, physics and philosophy that does not truly meet the requirements of any.”

    What they, and others, find particularly worrisome is the claim that our attempts to push back frontiers in cosmology and fundamental physics have taken us into a new domain where new rules of science are needed. Some call this domain “post-empirical” science. Recently, for example, the philosopher of physics Richard Dawid has argued that in spite of the fact that no evidence for string theory exists (even after three decades of intense study), it must still be considered the best candidate for a path forward. As Dawid puts it, such arguments include “no-one has found a good alternative to string theory. Another [reason to accept string theory is] one uses the observation that theories without alternatives tended to be viable in the past.”

    Sean Carroll, a highly respected and philosophically astute physicist, takes a different approach from Dawid. For Carroll, it is the concept of falsifiability, which was central to Sir Karl Raimund Popper’s famous philosophy of science, that is too limited for the playing fields we now find ourselves working on. As Carroll writes:

    “Whether or not we can observe [extra dimensions or other universes] directly, the entities involved in these theories are either real or they are not. Refusing to contemplate their possible existence on the grounds of some a priori principle, even though they might play a crucial role in how the world works, is as non-scientific as it gets.”

    Thus, for Carroll, even if a theory predicts entities that can’t be directly observed, if there are indirect consequences of their existence we can confirm, then those theories (and those entities) must be included in our considerations.

    Other scientists, however, are not convinced. High-energy physicist Sabine Hossenfelder called Dawid’s kind of post-empirical science an “oxymoron.” More importantly, for scientists like Paul Steinhardt and collaborators, the new ideas are becoming “post-modern.” They use the term in the sense that without more definitive connections to data, the ideas will not be abandoned because a community exists that continues to support them.

    This is the possibility that troubles Ellis and Silk most of all:

    “In our view, the issue boils down to clarifying one question: What potential observational or experimental evidence is there that would persuade you that the theory is wrong and lead you to abandoning it? If there is none, it is not a scientific theory.”

    String theory and the multiverse are exciting ideas in and of themselves. If either one were true, it would have revolutionary consequences for our understanding of the cosmos. But, as debates about post-empirical science and falsifiability demonstrate, critics of these untested theories fear they may be leading the field down a difficult — and ultimately damaging — path. That’s why, one way or another, they may be science’s most dangerous ideas.

    See the full article here.

    My indebtedness to Don Lincoln of FNAL for pointing out this article using a Facebook post. Thanks, Dr Lincoln

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    • s7hummel 1:48 am on January 29, 2015 Permalink | Reply

      was a little empty without YOU! Welcome back!

      Like

    • richardmitnick 3:51 am on January 29, 2015 Permalink | Reply

      I don’t understand your comment. I approved it just to ask you what you mean. I have been posting constantly.

      Like

    • s7hummel 2:03 am on January 30, 2015 Permalink | Reply

      seemed to me that a few days there was no your entries. But maybe i missed something. Indeed, what can YOU expect from a stupid Pole. Sorry!

      Like

    • richardmitnick 4:34 am on January 30, 2015 Permalink | Reply

      Hey, no problem. I am glad to have you aboard. I hope that you continue to find articles interesting.

      Liked by 1 person

  • richardmitnick 9:31 am on January 27, 2015 Permalink | Reply
    Tags: , , , , String Theory,   

    From Huff Post: “The Future of Physics” 

    Huffington Post
    The Huffington Post

    01/26/2015

    Dr. Sten Odenwald, Astronomer, National Institute of Aerospace

    In another few months the Large Hadron Collider. will be powered up to explore its maximum energy range. Many physicists fervently hope we will see definite signs of “new physics,” especially a phenomenon called “supersymmetry.” In the simplest view, the Standard Model souped-up with supersymmetry will offer a massive new partner particle for every known particle (electron, quark, neutrino, etc). One of these, called the neutralino, may even explain dark matter itself!

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

    Supersymmetry standard model
    Standard Model of Supersymmetry

    But Wait, There’s More!

    Supersymmetry is the foundational cornerstone on which string theory rests. That’s why physicists call this “stringy” theory of matter “superstring theory.” If the LHC does not turn up any signs of supersymmetry during the next two or three years, not only will simple modifications to the current Standard Model be ruled out, but the most elegant forms of supersymmetry theory will fall too.

    As an astronomer I am not too worried. The verified Standard Model is now fully capable of accounting for everything we see in the universe since a trillion-trillion-trillionth of a second after the Big Bang to the present time, once you include gravity, and don’t worry too much about dark matter and dark energy.

    2
    The Standard Model of elementary particles, with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    But we need some explanation for dark energy and dark matter to complete our understanding of the cosmos, and for that we still need our physicist friends to show the way. Currently their only answers involve supersymmetry theory. If this idea falls, astronomers will be completely stumped to explain what governs our universe on the largest scales.

    Beyond supersymmetry, we also have the huge investment of talent that has pursued superstring theory since the early 1980s. Without supersymmetry, the “super” part of string theory also falls, and you end up with a non-super string theory that is clunky, inelegant and pretty dismal in accounting for the finer details of our physical world, often termed “quantum gravity.” A big part of this is the idea that our universe inhabits more than four dimensions — perhaps as many as 11!

    On April 26, 2006, I had the following exchanges with Stanford theoretical physicist Leonard Susskind, who is widely regarded as one of the fathers of string theory, along with other provocative and foundational ideas such as the “holographic universe.” His comments are still relevant seven years later.

    3

    Odenwald: Why is it so important for physicists to consider the universe having more than four dimensions, as the mathematics of superstring theory seems to require?

    Susskind: Almost all working high-energy theoretical physicists are convinced some sort of extra dimensions are needed to explain the complexity of elementary particles. That particles move in extra dimensions is another way of talking about the fact that they have more complex properties than just position and velocity. It is hard to find a serious paper about particle phenomenology that doesn’t in some way use the tools of superstring theory. Furthermore, we all agree that the origin of elementary particles is most likely at the planck scale and cannot be understood without a good theory of quantum gravity.

    Odenwald: So if superstring theory were found to be an incorrect model for our particular universe, is that like turning the clock back to circa 1978 in physics?

    Susskind: I agree that going back to the ’70s would be turning the clock back in the sense that we would be ignoring the vast amount of mathematical knowledge that has been gained over the subsequent years, mostly from string theory. That is just not going to happen. The changes in our theoretical understanding of quantum field theory, gravity, black holes, are completely irreversible. [String theory mathematics] has even worked its way into nuclear physics and heavy ion collisions as well as into condensed matter physics.

    Odenwald: Kind of a hard place for modern theoreticians to revisit, but for astronomy and cosmology the 1970s seem not such a bad place. Without superstring theory, we would still have cosmological inflation. Without superstring theory, what happened at the instant of the Big Bang would remain unknown, logically indescribable, and still a great puzzle… as it always has.

    Susskind: [Not quite.] Recent cosmology has been completely dominated by studying the cosmic microwave background [CMB] and inflationary theory.

    Cosmic Background Radiation Planck
    CMB per ESA/Planck

    ESA Planck
    ESA/Planck

    Gravitational Wave Background
    Gravitational waves from BICEP2 in support of Inflation theory not yet accepted

    BICEP 2
    BICEP2

    The CMB fluctuation spectrum is widely understood as a quantum effect. Inflation is a gravitational effect. Is there any question that quantum gravity (quantum plus gravity) will be the framework for understanding the early universe? No, there is not. Also I might add that the old inflation that you are referring to was a disaster. It didn’t work. Many inflationary cosmologists like Linde, Vilenkin, and Guth are looking to string theory for possible answers to the puzzles of inflation.

    Odenwald: Is it fair to say that superstring theory is “too big to fail”? I am reminded of the alledged quotation by Einstein as he was awaiting news about a major test of relativity in 1919. A reporter is said to have asked, “Well, what would it mean if your theory is wrong?” to which Einstein allegedly replied, “Then I would feel sorry for the good Lord; the theory is correct!” Is the physics community in the same predicament because superstring theory is a “beautiful” theory that seems to explain so much, and its mathematics is so impeccable that it is used in other theoretical settings in physics?

    Susskind: Exactly right. However, it is fair to say that while theorists were developing powerful tools, they mostly had wrong expectations for what the theory was indicating. Most theorists hoped that string theory would lead to an absolutely unique set of particles, coupling constants, with exactly vanishing cosmological constant. What we have learned from the theory itself is that it is a theory of tremendous diversity. Unexpectedly, string theory is most comfortable with a huge multiverse of tremendous variety instead of the small “knowable” and unique universe we once imagined.

    Odenwald: So what would our theoretical explanations for our universe look like without added dimensions, quantum gravity or string theory?

    Susskind: Without these things the world as we know it couldn’t exist. Giving up quantum gravity means giving up either the ideas of quantum [mechanics] or of gravity [and general relativity]. In a cosmological context quantum gravity is responsible for the primordial density fluctuations [we directly observe in the CMB] that ultimately condensed to form stars, galaxies, planets, etc. Without string theory we should not have the diversity of possibilites that allow pocket universes [Alan Guth’s term] with the ultra-fine tuning needed to insure conditions for our kind of life.

    Odenwald: If string theory loses its experimental support at the LHC, wouldn’t it be far worse than merely going back to cosmology circa 1975 or even 1965? We would have to question the very mathematical tools we have been using for the last 50 years!

    Susskind: I agree with your analysis, except that I would add: Expect the unexpected. Unforseen surprises are the rule in science, not the exception. Remember: Stuff happens.

    Odenwald: If superstring theory falls, are there any competing theories out there that could hold out some hope?

    Susskind: Not as far as I know.

    • * * * *

    So there you have it. The Large Hadron Collider absolutely has to find some clue about supersymmetry, or superstring theory is compromised, we will have no good idea about dark matter, and we will definitely be in a bad place until the super-LHC is built in the 2030s.

    Patience, however, is a still a necessary virtue. Physicists were in this same quandary before the Higgs boson was finally discovered after 50 years of increasingly panicked searching.

    See the full article here.

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  • richardmitnick 7:21 am on January 10, 2015 Permalink | Reply
    Tags: , , , , String Theory   

    From Byron Jennings at Quantum Diaries: “String Theory and the Scientific Method” 

    Jan 9, 2015

    b
    Byron Jennings, Triumf Lab

    It seems some disagreements are interminable: the Anabaptists versus the Calvinists, capitalism versus communism, the Hatfields versus the McCoys, or string theorists versus their detractors. It is the latter I will discuss here although the former may be more interesting. This essay is motivated by a comment in the December 16, 2014 issue of Nature by George Ellis and Joe Silk. The comment takes issue with attempts by some string theorists and cosmologists to redefine the scientific method by eliminating the need for experimental testing and relying on elegance or similar criteria instead. I have a lot of sympathy with Ellis and Silk’s point of view but believe that it is up to scientists to define what science is and that hoping for deliverance by outside people, like philosophers, is doomed to failure.

    To understand what science is and what science is not, we need a well-defined model for how science behaves. Providing that well-defined model is the motivation behind each of my essays. The scientific method is quite simple: build models of how the universe works based on observation and simplicity. Then test them by comparing their predictions against new observation. Simplicity is needed since observations underdetermine the models (see for example: Willard Quine’s (1908 –2000) essay: The Two Dogmas of Empiricism). Note also that what we do is build models: the standard model of particle physics, the nuclear shell model, string theory, etc. Quine refers to the internals of the models as myths and fictions. Henri Poincaré (1854 – 1912) talks of conventions and Hans Vaihinger (1852 –1933) of the philosophy of as if otherwise known as fictionalism. Thus it is important to remember that our models, even the so-called theory of everything, are only models and not reality.

    s
    The Standard Model of elementary particles, with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    v
    Partly filled valence orbitals for both neutrons and protons appear at energies over the filled inert core orbitals, in the shell model of the atomic nucleus

    It is the feedback loop of observation, model building and testing against new observation that define science and give it its successes. Let me repeat: The feedback loop is essential. To see why, consider example of astrology and why scientists reject it. Its practitioners consider it to be the very essence of elegance. Astrology uses careful measurements of current planetary locations and mathematics to predict their future locations, but it is based on an epistemology that places more reliance on the eloquence of ancient wisdom than on observation. Hence there is no attempt to test astrological predictions against observations. That would go against their fundamental principles of eloquence and the superiority of received knowledge to observation. Just as well, since astrological predictions routinely fail. Astrology’s failures provide a warning to those who wish to replace prediction and simplicity with other criteria. The testing of predictions against observation and simplicity are hard taskmasters and it would be nice to escape their tyranny but that path is fraught with danger, as astrology illustrates. The feedback loop from science has even been picked up by the business management community and has been built into the very structure of the management standards (see ISO Annex SL for example). It would be shame if management became more scientific than physics.

    But back to string theory. Gravity has always been a tough nut to crack. [Sir] Isaac Newton (1643 – 1727) proposed the decidedly inelegant idea of instantaneous action at a distance and it served well until 1905 and the development of special theory of relativity. Newton’s theory of gravity and special relativity are inconsistent since the latter rules out instantaneous action at a distance. In 1916, Albert Einstein (1879 – 1955) with an honorable mention to David Hilbert (1862 – 1943) proposed the general theory of relativity to solve the problem. In 1919, the prediction of the general theory of relativity for the bending of light by the sun was confirmed by an observation by [Sir] Arthur Eddington (1882 – 1944). Notice the progression: conflict between two models, proposed solution, confirmed prediction, and then acceptance.

    Like special relativity and Newtonian gravity, general relativity and quantum mechanics are incompatible with one another. This has led to attempts to generate a combined theory. Currently string theory is the most popular candidate, but it seems to be stuck at the stage general relativity was in 1917 or maybe even 1915: a complicated (some would say elegant, others messy) mathematical theory but unconfirmed by experiment. Although progress is definitely being made, string theory may stay where it is for a long time. The problem is that the natural scale of quantum gravity is the Planck mass and this scale is beyond what we can explore directly by experiment. However, there is one place that quantum gravity may have left observable traces and that is in its role in the early Universe. There are experimental hints that may indicate a signature in the cosmic microwave background radiation but we must await further experimental results. In the meantime, we must accept that current theories of quantum gravity are doubly uncertain. Uncertain, in the first instance, because, like all scientific models, they may be rendered obsolete by new a understanding and uncertain, in the second instance, because they have not been experimentally verified through testable predictions.

    Cosmic Background Radiation Planck
    Cosmic Microwave Background per ESA/Planck

    ESA Planck
    ESA/Planck

    Let’s now turn to the question of multiverses. This is an even worse dog’s breakfast than quantum gravity. The underlying problem is the fine tuning of the fundamental constants needed in order for life as we know it to exist. What is needed for life, as we do not know it, to exist is unknown. There are two popular ideas for why the Universe is fined tuned. One is that the constants were fine-tuned by an intelligent designer to allow for life as we know it. This explanation has the problem that by itself it can explain anything but predict nothing. An alternate is that there are many possible universes, all existing, and we are simply in the one where we can exist. This explanation has the problem that by itself it can explain anything but predict nothing. It is ironic that to avoid an intelligent designer, a solution based on an equally dubious just so story is proposed. Since we are into just so stories, perhaps we can compromise by having the intelligent designer choosing one of the multiverses as the one true Universe. This leaves the question of who the one true intelligent designer is. As an old farm boy, I find the idea that Audhumbla, the cow of the Norse creation myth, is the intelligent designer to be the most elegant. Besides the idea of elegance, as a deciding criterion in science, has a certain bovine aspect to it. Who decides what constitutes elegance? Everyone thinks their own creation is the most elegant. This is only possible in Lake Wobegon, where all the women are strong, all the men are good-looking, and all the children are above average (A PRAIRIE HOME COMPANION – Garrison Keillor (b. 1942)). Not being in Lake Wobegon, we need objective criteria for what constitutes elegance. Good luck with that one.

    Some may think the discussion in the last paragraph is frivolous, and quite by design it is. This is to illustrate the point that once we allow the quest for knowledge to escape from the rigors of the scientific method’s feedback loop all bets are off and we have no objective reason to rule out astrology or even the very elegant Audhumbla. However, the idea of an intelligent designer or multiverses can still be saved if they are an essential part of a model with a track record of successful predictions. For example, if that animal I see in my lane is Fenrir, the great gray wolf, and not just a passing coyote, then the odds swing in favor of Audhumbla as the intelligent designer and Ragnarok is not far off. More likely, evidence will eventually be found in the cosmic microwave background or elsewhere for some variant of quantum gravity. Until then, patience (on both sides) is a virtue.

    See the full article here.

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  • richardmitnick 11:54 am on January 10, 2014 Permalink | Reply
    Tags: , , , String Theory   

    From Fermilab: “Physics in a Nutshell – Superstrings” 


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

    Friday, Jan. 10, 2014
    Don Lincoln

    Fermilab Don Lincoln
    Dr. Don Lincoln

    One of the oldest scientific questions in history is “What are the ultimate building blocks of the universe?” Today’s article talks about a cool idea called “superstrings,” tiny subatomic strings that play a cosmic and subatomic symphony. Superstrings are a possible answer to the ancient question surrounding the identity of the universe’s smallest components. Understanding this answer requires some historical context.

    fuzzball
    Fuzzball of String Theory from Wikipedia

    The first recorded debate on the subject was written down about 2,500 years ago in Greece, with a philosopher named Democritus making the most accurate guess when he postulated discrete units he called atomos. The question was left to the realm of the philosophers for millennia until the 1700s, when modern chemistry began to shed light on the topic using empirical techniques. With the identification of the atoms of the chemical elements, central features of Democritus’ model were validated.

    Over the next two-plus centuries, scientists proposed and discovered increasingly smaller components that make up our universe. They discovered the familiar proton, neutron and electron that make up atoms. Then they learned that the proton and neutron contained even smaller components called quarks. They also discovered that the two types of quarks that make up protons and neutrons, called the up and down quarks, weren’t the only quarks out there.

    Today we know the building blocks of matter can be classified into two types called quarks and leptons, each of which consists of six examples. The six quarks are called up, down, charm, strange, top and bottom. The six leptons are called electron, electron neutrino, muon, muon neutrino, tau and tau neutrino. With so many fundamental particles, a new organizing principle is needed to make sense of what was understood to be the universe’s building blocks.

    So what the heck is the story with these particles of the Standard Model, the modern-day atomos of Democritus? What role do they play in our understanding of the subatomic world?

    sm
    Standard Model of Particle Physics

    Well, we don’t know the answer to that in detail. We do know of patterns. Up, charm and top quarks all have the same electrical charge but rather different masses, with up being the lightest and top the heaviest. Down, strange and bottom also have identical electric charge and increasing mass. The electron, muon and tau lepton exhibit the same behavior. Naturally, when we see recurring patterns like this, we go looking for an explanation. One such explanation is superstrings.

    Unlike what we’ve seen before, with each particle being composed of an even smaller one, superstrings break the pattern. Superstring theory postulates that the ultimate building block of matter consists of tiny, tiny “strings” that vibrate. Strings that vibrate the least are the quarks and leptons with the smallest mass, while the heavier particles have more energetic vibrations. Employing a musical metaphor, it’s as if the electron might be a B-flat, while the bottom quark might be an F-sharp above that.

    There are some bizarre consequences to this idea. Early calculations using superstring theory yielded nonsense. For example, when adding up all the things that could happen when two strings interacted, physicists found that there was more than a 100 percent chance something would happen. Since 100 percent is the maximum, that was a very bad outcome. These calculations were done in ordinary three dimensional space.

    Then a curious soul played around with the same calculation, but using more than three spatial dimensions. Scientists found that when four dimensions were invoked, the answer was more sensible, but still above 100 percent. By adding more and more dimensions, the answer became more and more reasonable. When 10 dimensions were invoked, the result gave the expected 100 percent. This is the reason people talk about 10 dimensions of space and time.

    Physicists put forward five different versions of superstring theory, but it turned out that by invoking one additional dimension, all the five versions turned out to be the same. This is the reason you might have heard that there are 11 dimensions of space and time.

    It is important to note that the idea of superstrings has not been proven empirically. It could well be wrong. However, the idea that the universe could have a single building block (the string) and that all the observed particles of nature are just different vibrations is a very attractive idea. Another benefit of superstring theory is that it incorporates gravity very easily. Other ideas unifying the various forces have a tough time with gravity. This isn’t a reason to believe in superstrings, but it is a reason to find the theory to be attractive.

    The story of superstrings is much bigger than I can describe here. I’ve only given the quickest possible description. If the idea of the entire universe being a cosmic melody played by tiny strings interests you, you might be interested in reading Brian Greene’s book The Elegant Universe.

    See the full article here.

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  • richardmitnick 2:54 pm on July 25, 2013 Permalink | Reply
    Tags: , , , , , , String Theory, Superfluids   

    From M.I.T.: “Superfluid turbulence through the lens of black holes” 

    Study finds behavior of the turbulent flow of superfluids is opposite that of ordinary fluids.

    July 25, 2013
    Jennifer Chu, MIT News Office

    “A superfluid moves like a completely frictionless liquid, seemingly able to propel itself without any hindrance from gravity or surface tension. The physics underlying these materials — which appear to defy the conventional laws of physics — has fascinated scientists for decades.

    fluid
    Black hole physics shows that superfluids in turbulence behave much like cigarette smoke. Image: Christine Daniloff

    Think of the assassin T-1000 in the movie “Terminator 2: Judgment Day” — a robotic shape-shifter made of liquid metal. Or better yet, consider a real-world example: liquid helium. When cooled to extremely low temperatures, helium exhibits behavior that is otherwise impossible in ordinary fluids. For instance, the superfluid can squeeze through pores as small as a molecule, and climb up and over the walls of a glass. It can even remain in motion years after a centrifuge containing it has stopped spinning.

    Now physicists at MIT have come up with a method to mathematically describe the behavior of superfluids — in particular, the turbulent flows within superfluids. They publish their results this week in the journal Science.

    ‘Turbulence provides a fascinating window into the dynamics of a superfluid,’ says Allan Adams, an associate professor of physics at MIT. ‘Imagine pouring milk into a cup of tea. As soon as the milk hits the tea, it flares out into whirls and eddies, which stretch and split into filigree. Understanding this complicated, roiling turbulent state is one of the great challenges of fluid dynamics. When it comes to superfluids, whose detailed dynamics depend on quantum mechanics, the problem of turbulence is an even tougher nut to crack.’

    To describe the underlying physics of a superfluid’s turbulence, Adams and his colleagues drew comparisons with the physics governing black holes. At first glance, black holes — extremely dense, gravitationally intense objects that pull in surrounding matter and light — may not appear to behave like a fluid. But the MIT researchers translated the physics of black holes to that of superfluid turbulence, using a technique called holographic duality.

    Consider, for example, a holographic image on a magazine cover. The data, or pixels, in the image exist on a flat surface, but can appear three-dimensional when viewed from certain angles. An engineer could conceivably build an actual 3-D replica based on the information, or dimensions, found in the 2-D hologram.

    ‘If you take that analogy one step further, in a certain sense you can regard various quantum theories as being a holographic image of a world with one extra dimension,’ says Paul Chesler, a postdoc in MIT’s Department of Physics.

    Taking this cosmic line of reasoning, Adams, Chesler and colleagues used holographic duality as a ‘dictionary’ to translate the very well-characterized physics of black holes to the physics of superfluid turbulence.

    To the researchers’ surprise, their calculations showed that turbulent flows of a class of superfluids on a flat surface behave not like those of ordinary fluids in 2-D, but more like 3-D fluids, which morph from relatively uniform, large structures to smaller and smaller structures. The result is much like cigarette smoke: From a burning tip, smoke unfurls in a single stream that quickly disperses into smaller and smaller eddies. Physicists refer to this phenomenon as an “energy cascade.”

    ‘For superfluids, whether such energy cascades exist is an open question,’ says Hong Liu, an associate professor of physics at MIT. ‘People have been making all kinds of claims, but there hasn’t been any smoking-gun type of evidence that such a cascade exists. In a class of superfluids, we produced very convincing evidence for the direction of this kind of flow, which would otherwise be very hard to obtain.’”

    See the full article here.


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