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  • richardmitnick 10:52 am on June 21, 2021 Permalink | Reply
    Tags: "Klarman postdoc seeks ‘theory of everything’ by approximation", Attempting to unify gravity with other fundamental forces of physics., , , , , , , String Theory   

    From Cornell Chronicle (US) : “Klarman postdoc seeks ‘theory of everything’ by approximation” 

    From Cornell Chronicle (US)

    June 21, 2021
    Kate Blackwood
    cunews@cornell.edu

    1
    Francesco Sgarlata.

    Two pillar theories in physics – general relativity and quantum mechanics – stand up well on their own, but are incompatible with each other.

    “These two theories describe two different regimes of phenomena,” said Francesco Sgarlata, a Klarman Postdoctoral Fellow in physics in the College of Arts and Sciences (A&S).

    2

    Quantum mechanics, he said, describes physical phenomena at atomic or sub-atomic scales; general relativity describes very large phenomena.

    “The two theories are both correct in that they both predict very well, and we don’t have any violation of these theories. However, the two theories are inconsistent with each other,” Sgarlata said, adding that the inconsistencies show up in processes at extremely small scales.

    A member of the first cohort of six Klarman Fellows, Sgarlata is using his three-year fellowship to join theoretical physicists at Cornell and around the world in trying to solve this inconsistency.

    Physicists have long sought a “theory of everything,” or theory of quantum gravity, that would unify quantum mechanics and general relativity. In recent decades, researchers have tried a top-down approach, trying to come up with a unifying theory, such as string theory.

    Sgarlata, in contrast, is taking a bottom-up approach to finding a theory of quantum gravity, which attempts to unify gravity with other fundamental forces of physics.

    “We seek an approximation,” he said. “We don’t know what this theory of everything is. [Instead,] we are trying to write down some theory which can be seen as an approximation of quantum gravity, and we study what conditions this theory will have in order to be a good approximation of quantum gravity.”

    Sgarlata is working with Cornell’s theoretical physics community, including his faculty host, Csaba Csaki, professor of physics (A&S), and Thomas Hartman, associate professor of physics (A&S), to “identify some hidden properties of quantum gravity,” one at a time – and then build from there.

    “Francesco’s research is on the fundamental properties of particles and forces,” Hartman said. “His goal is to understand what particles are consistent with basic principles of relativity and quantum mechanics, and how these particles can interact.”

    Sgarlata’s background is in particle physics, Hartman said, while his own background is in black hole physics and string theory.

    “There is a lot of overlap, but these are two different perspectives,” Hartman said, “so this is a great opportunity for us to collaborate on new ideas. We are working on joining forces and combining our approaches.”

    To find conditions necessary to support a theory of quantum gravity, Sgarlata and collaborators focus on “first principles” – those we experience in everyday life but are difficult to prove mathematically. One example is causality – the link between cause and effect.

    “If I punch you, you will start feeling pain after I punch you, not before,” Sgarlata said. “We assume that this theory of everything respects causality.”

    Other first principles the researchers consider are unitarity (probabilities must add up to 1); and locality (particles only interact with neighboring particles.)

    From a “swampland” of possible theories arise islands of probable theories, Sgarlata said, narrowing the scope. “We get some constraints on the parameters of the theory,” he said.

    Hartman said that Sgarlata uses methods from particle physics to develop and interpret theories of physics at high energies.

    “In some cases, his methods can even be used to understand some corners of the more mysterious theory of quantum gravity at ultrashort distances,” Hartman said. “Over the next couple years, I think Francesco’s research at Cornell will lead to better insight into fundamental particles and new connections between particles, gravity and black holes.”

    The Klarman Fellowship, Sgarlata said, offers independence to pursue research collaborations toward solving the biggest problems in physics.

    “We have the tools to understand features of quantum gravity,” he said. “Today we are reinterpreting these concepts in a more modern way, and we are discovering new concepts of physics just by our interpretations.”

    See the full article here .


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    Stem Education Coalition

    Once called “the first American university” by educational historian Frederick Rudolph, Cornell University represents a distinctive mix of eminent scholarship and democratic ideals. Adding practical subjects to the classics and admitting qualified students regardless of nationality, race, social circumstance, gender, or religion was quite a departure when Cornell was founded in 1865.

    Today’s Cornell reflects this heritage of egalitarian excellence. It is home to the nation’s first colleges devoted to hotel administration, industrial and labor relations, and veterinary medicine. Both a private university and the land-grant institution of New York State, Cornell University is the most educationally diverse member of the Ivy League.

    On the Ithaca campus alone nearly 20,000 students representing every state and 120 countries choose from among 4,000 courses in 11 undergraduate, graduate, and professional schools. Many undergraduates participate in a wide range of interdisciplinary programs, play meaningful roles in original research, and study in Cornell programs in Washington, New York City, and the world over.

    Cornell University (US) is a private, statutory, Ivy League and land-grant research university in Ithaca, New York. Founded in 1865 by Ezra Cornell and Andrew Dickson White, the university was intended to teach and make contributions in all fields of knowledge—from the classics to the sciences, and from the theoretical to the applied. These ideals, unconventional for the time, are captured in Cornell’s founding principle, a popular 1868 quotation from founder Ezra Cornell: “I would found an institution where any person can find instruction in any study.”

    The university is broadly organized into seven undergraduate colleges and seven graduate divisions at its main Ithaca campus, with each college and division defining its specific admission standards and academic programs in near autonomy. The university also administers two satellite medical campuses, one in New York City and one in Education City, Qatar, and Jacobs Technion-Cornell Institute(US) in New York City, a graduate program that incorporates technology, business, and creative thinking. The program moved from Google’s Chelsea Building in New York City to its permanent campus on Roosevelt Island in September 2017.

    Cornell is one of the few private land grant universities in the United States. Of its seven undergraduate colleges, three are state-supported statutory or contract colleges through the SUNY – The State University of New York (US) system, including its Agricultural and Human Ecology colleges as well as its Industrial Labor Relations school. Of Cornell’s graduate schools, only the veterinary college is state-supported. As a land grant college, Cornell operates a cooperative extension outreach program in every county of New York and receives annual funding from the State of New York for certain educational missions. The Cornell University Ithaca Campus comprises 745 acres, but is much larger when the Cornell Botanic Gardens (more than 4,300 acres) and the numerous university-owned lands in New York City are considered.

    Alumni and affiliates of Cornell have reached many notable and influential positions in politics, media, and science. As of January 2021, 61 Nobel laureates, four Turing Award winners and one Fields Medalist have been affiliated with Cornell. Cornell counts more than 250,000 living alumni, and its former and present faculty and alumni include 34 Marshall Scholars, 33 Rhodes Scholars, 29 Truman Scholars, 7 Gates Scholars, 55 Olympic Medalists, 10 current Fortune 500 CEOs, and 35 billionaire alumni. Since its founding, Cornell has been a co-educational, non-sectarian institution where admission has not been restricted by religion or race. The student body consists of more than 15,000 undergraduate and 9,000 graduate students from all 50 American states and 119 countries.

    History

    Cornell University was founded on April 27, 1865; the New York State (NYS) Senate authorized the university as the state’s land grant institution. Senator Ezra Cornell offered his farm in Ithaca, New York, as a site and $500,000 of his personal fortune as an initial endowment. Fellow senator and educator Andrew Dickson White agreed to be the first president. During the next three years, White oversaw the construction of the first two buildings and traveled to attract students and faculty. The university was inaugurated on October 7, 1868, and 412 men were enrolled the next day.

    Cornell developed as a technologically innovative institution, applying its research to its own campus and to outreach efforts. For example, in 1883 it was one of the first university campuses to use electricity from a water-powered dynamo to light the grounds. Since 1894, Cornell has included colleges that are state funded and fulfill statutory requirements; it has also administered research and extension activities that have been jointly funded by state and federal matching programs.

    Cornell has had active alumni since its earliest classes. It was one of the first universities to include alumni-elected representatives on its Board of Trustees. Cornell was also among the Ivies that had heightened student activism during the 1960s related to cultural issues; civil rights; and opposition to the Vietnam War, with protests and occupations resulting in the resignation of Cornell’s president and the restructuring of university governance. Today the university has more than 4,000 courses. Cornell is also known for the Residential Club Fire of 1967, a fire in the Residential Club building that killed eight students and one professor.

    Since 2000, Cornell has been expanding its international programs. In 2004, the university opened the Weill Cornell Medical College in Qatar. It has partnerships with institutions in India, Singapore, and the People’s Republic of China. Former president Jeffrey S. Lehman described the university, with its high international profile, a “transnational university”. On March 9, 2004, Cornell and Stanford University(US) laid the cornerstone for a new ‘Bridging the Rift Center’ to be built and jointly operated for education on the Israel–Jordan border.

    Research

    Cornell, a research university, is ranked fourth in the world in producing the largest number of graduates who go on to pursue PhDs in engineering or the natural sciences at American institutions, and fifth in the world in producing graduates who pursue PhDs at American institutions in any field. Research is a central element of the university’s mission; in 2009 Cornell spent $671 million on science and engineering research and development, the 16th highest in the United States. Cornell is classified among “R1: Doctoral Universities – Very high research activity”.

    For the 2016–17 fiscal year, the university spent $984.5 million on research. Federal sources constitute the largest source of research funding, with total federal investment of $438.2 million. The agencies contributing the largest share of that investment are the Department of Health and Human Services and the National Science Foundation(US), accounting for 49.6% and 24.4% of all federal investment, respectively. Cornell was on the top-ten list of U.S. universities receiving the most patents in 2003, and was one of the nation’s top five institutions in forming start-up companies. In 2004–05, Cornell received 200 invention disclosures; filed 203 U.S. patent applications; completed 77 commercial license agreements; and distributed royalties of more than $4.1 million to Cornell units and inventors.

    Since 1962, Cornell has been involved in unmanned missions to Mars. In the 21st century, Cornell had a hand in the Mars Exploration Rover Mission. Cornell’s Steve Squyres, Principal Investigator for the Athena Science Payload, led the selection of the landing zones and requested data collection features for the Spirit and Opportunity rovers. NASA-JPL/Caltech(US) engineers took those requests and designed the rovers to meet them. The rovers, both of which have operated long past their original life expectancies, are responsible for the discoveries that were awarded 2004 Breakthrough of the Year honors by Science. Control of the Mars rovers has shifted between National Aeronautics and Space Administration(US)’s JPL-Caltech (US) and Cornell’s Space Sciences Building.

    Further, Cornell researchers discovered the rings around the planet Uranus, and Cornell built and operated the telescope at Arecibo Observatory located in Arecibo, Puerto Rico(US) until 2011, when they transferred the operations to SRI International, the Universities Space Research Association (US) and the Metropolitan University of Puerto Rico [Universidad Metropolitana de Puerto Rico](US).

    The Automotive Crash Injury Research Project was begun in 1952. It pioneered the use of crash testing, originally using corpses rather than dummies. The project discovered that improved door locks; energy-absorbing steering wheels; padded dashboards; and seat belts could prevent an extraordinary percentage of injuries.

    In the early 1980s, Cornell deployed the first IBM 3090-400VF and coupled two IBM 3090-600E systems to investigate coarse-grained parallel computing. In 1984, the National Science Foundation began work on establishing five new supercomputer centers, including the Cornell Center for Advanced Computing, to provide high-speed computing resources for research within the United States. As an National Science Foundation (US) center, Cornell deployed the first IBM Scalable Parallel supercomputer.

    In the 1990s, Cornell developed scheduling software and deployed the first supercomputer built by Dell. Most recently, Cornell deployed Red Cloud, one of the first cloud computing services designed specifically for research. Today, the center is a partner on the National Science Foundation XSEDE-Extreme Science Engineering Discovery Environment supercomputing program, providing coordination for XSEDE architecture and design, systems reliability testing, and online training using the Cornell Virtual Workshop learning platform.

    Cornell scientists have researched the fundamental particles of nature for more than 70 years. Cornell physicists, such as Hans Bethe, contributed not only to the foundations of nuclear physics but also participated in the Manhattan Project. In the 1930s, Cornell built the second cyclotron in the United States. In the 1950s, Cornell physicists became the first to study synchrotron radiation.

    During the 1990s, the Cornell Electron Storage Ring, located beneath Alumni Field, was the world’s highest-luminosity electron-positron collider. After building the synchrotron at Cornell, Robert R. Wilson took a leave of absence to become the founding director of DOE’s Fermi National Accelerator Laboratory(US), which involved designing and building the largest accelerator in the United States.

    Cornell’s accelerator and high-energy physics groups are involved in the design of the proposed ILC-International Linear Collider(JP) and plan to participate in its construction and operation. The International Linear Collider(JP), to be completed in the late 2010s, will complement the CERN Large Hadron Collider(CH) and shed light on questions such as the identity of dark matter and the existence of extra dimensions.

    As part of its research work, Cornell has established several research collaborations with universities around the globe. For example, a partnership with the University of Sussex(UK) (including the Institute of Development Studies at Sussex) allows research and teaching collaboration between the two institutions.

     
  • richardmitnick 9:58 am on May 25, 2021 Permalink | Reply
    Tags: "Was Einstein wrong? Why some astrophysicists are questioning the theory of space-time", As in history revolutions are the lifeblood of science., , , Erwin Schrödinger's cat, General Theory of Relativity., , Loop Quantum Gravity (LQG), Modular space-time theory, , , , String Theory, The Planck scale length-around a trillionth of a trillionth of a trillionth of a meter.   

    From Live Science : “Was Einstein wrong? Why some astrophysicists are questioning the theory of space-time” 

    From Live Science

    5.24.21
    Colin Stuart

    1
    Do we have to kill off the theory of space and time to make sense of the universe? (Image credit: Tobias Roetsch.)

    As in history revolutions are the lifeblood of science. Bubbling undercurrents of disquiet boil over until a new regime emerges to seize power. Then everyone’s attention turns to toppling their new ruler. The king is dead, long live the king.

    This has happened many times in the history of physics and astronomy. First, we thought Earth was at the center of the solar system — an idea that stood for over 1,000 years. Then Copernicus stuck his neck out to say that the whole system would be a lot simpler if we are just another planet orbiting the sun. Despite much initial opposition, the old geocentric picture eventually buckled under the weight of evidence from the newly invented telescope.

    Then Newton came along to explain that gravity is why the planets orbit the sun. He said all objects with mass have a gravitational attraction towards each other. According to his ideas we orbit the sun because it is pulling on us, the moon orbits Earth because we are pulling on it. Newton ruled for two-and-a-half centuries before Albert Einstein turned up in 1915 to usurp him with his General Theory of Relativity. This new picture neatly explained inconsistencies in Mercury’s orbit, and was famously confirmed by observations of a solar eclipse off the coast of Africa in 1919.

    Instead of a pull, Einstein saw gravity as the result of curved space. He said that all objects in the universe sit in a smooth, four-dimensional fabric called space-time. Massive objects such as the sun warp the space-time around them, and so Earth’s orbit is simply the result of our planet following this curvature. To us that looks like a Newtonian gravitational pull. This space-time picture has now been on the throne for over 100 years, and has so far vanquished all pretenders to its crown. The discovery of gravitational waves in 2015 was a decisive victory, but, like its predecessors, it too might be about to fall. That’s because it is fundamentally incompatible with the other big beast in the physics zoo: Quantum theory.

    The quantum world is notoriously weird. Single particles can be in two places at once, for example. Only by making an observation do we force it to ‘choose’. Before an observation we can only assign probabilities to the likely outcomes. In the 1930s, Erwin Schrödinger devised a famous way to expose how perverse this idea is. He imagined a cat in a sealed box accompanied by a vial of poison attached to a hammer. The hammer is hooked up to a device that measures the quantum state of a particle. Whether or not the hammer smashes the vial and kills the cat hinges on that measurement, but quantum physics says that until such a measurement is made, the particle is simultaneously in both states, which means the vial is both broken and unbroken and the cat is alive and dead.

    Such a picture cannot be reconciled with a smooth, continuous fabric of space-time. “A gravitational field cannot be in two places at once,” said Sabine Hossenfelder, a theoretical physicist at the Frankfurt Institute for Advanced Studies [Frankfurter Institut für fortgeschrittene Studien] (DE). According to Einstein, space-time is warped by matter and energy, but quantum physics says matter and energy exist in multiple states simultaneously — they can be both here and over there. “So where is the gravitational field?” asks Hossenfelder. “Nobody has an answer to that question. It’s kind of embarrassing,” she said.

    1
    Massive bodies warp the fabric of space and time around them, leading to nearby objects following a curved path. (Image credit: Take 27 Ltd.)

    Try and use general relativity and quantum theory together, and it doesn’t work. “Above a certain energy, you get probabilities that are larger than one,” said Hossenfelder. One is the highest probability possible — it means an outcome is certain. You can’t be more certain than certain. Equally, calculations sometimes give you the answer infinity, which has no real physical meaning. The two theories are therefore mathematically inconsistent. So, like many monarchs throughout history, physicists are seeking a marriage between rival factions to secure peace. They’re searching for a theory of quantum gravity— the ultimate diplomatic exercise in getting these two rivals to share the throne. This has seen theorists turn to some outlandish possibilities.

    Arguably the most famous is string theory. It’s the idea that sub-atomic particles such as electrons and quarks are made from tiny vibrating strings. Just as you can play strings on a musical instrument to create different notes, string theorists argue that different combinations of strings create different particles. The attraction of the theory is that it can reconcile general relativity and quantum physics, at least on paper. However, to pull that particular rabbit out of the hat, the strings have to vibrate across eleven dimensions — seven more than the four in Einstein’s space-time fabric. As yet there is no experimental evidence that these extra dimensions really exist. “It might be interesting mathematics, but whether it describes the space-time in which we live, we don’t really know until there is an experiment,” said Jorma Louko from the University of Nottingham (UK).

    2
    One way to reconcile general relativity and quantum theory says reality is made of vibrating strings. (Image credit: Science Photo Library.)

    Partly inspired by string theory’s perceived failings, other physicists have turned to an alternative called Loop Quantum Gravity (LQG). They can get the two theories to play nicely if they do away with one of the central tenets of general relativity: That space-time is a smooth, continuous fabric. Instead, they argue, space-time is made up of a series of interwoven loops — that it has structure at the smallest size scales. This is a bit like a length of cloth. At first glance it looks like one smooth fabric. Look closely, however, and you’ll see it is really made of a network of stitches. Alternatively, think of it like a photograph on a computer screen: Zoom in, and you’ll see it is really made of individual pixels.

    The trouble is that when LQG physicists say small, they mean really small. These defects in space-time would only be apparent on the level of the Planck scale —around a trillionth of a trillionth of a trillionth of a meter. That’s so tiny that there would be more loops in a cubic centimeter of space than cubic centimeters in the entire observable universe. “If space-time only differs on the Planck scale then this would be difficult to test in any particle accelerator,” says Louko. You’d need an atom smasher a 1,000-trillion-times more powerful than the Large Hadron Collider (LHC) at CERN. How, then, can you detect space-time defects that small? The answer is to look across a large area of space.

    Light arriving here from the furthest reaches of the universe has traveled through billions of light years of space-time along the way. While the effect of each space-time defect would be tiny, over those distances interactions with multiple defects might well add up to a potentially observable effect. For the last decade, astronomers have been using light from far-off Gamma Ray Bursts to look for evidence in support of LQG. These cosmic flashes are the result of massive stars collapsing at the ends of their lives, and there is something about these distant detonations we currently cannot explain. “Their spectrum has a systematic distortion to it,” said Hossenfelder, but no one knows if that is something that happens on the way here or if it’s something to do with the source of the bursts themselves. The jury is still out.

    3
    An alternate picture says space and time is not smooth, but instead made of a series of tiny loops. (Image credit: Science Photo Library.)

    To make progress, we might have to go a step further than saying space-time isn’t the smooth, continuous fabric Einstein suggested. According to Einstein, space-time is like a stage that remains in place whether actors are treading its boards or not —even if there were no stars or planets dancing around, space-time would still be there. However, physicists Laurent Freidel, Robert Leigh, and Djordje Minic think that this picture is holding us back. They believe space-time doesn’t exist independently of the objects in it. Space-time is defined by the way objects interact. That would make space-time an artifact of the quantum world itself, not something to be combined with it. “It may sound kooky,” said Minic, “but it is a very precise way of approaching the problem.”

    The attraction of this theory — called modular space-time — is that it might help solve another long-standing problem in theoretical physics regarding something called locality, and a notorious phenomenon in quantum physics called entanglement. Physicists can set up a situation whereby they bring two particles together and link their quantum properties. They then separate them by a large distance and find they are still linked. Change the properties of one and the other will change instantly, as if information has traveled from one to the other faster than the speed of light in direct violation of relativity. Einstein was so perturbed by this phenomenon that he called it ‘spooky action at a distance’.

    Modular space-time theory can accommodate such behavior by redefining what it means to be separated. If space-time emerges from the quantum world, then being closer in a quantum sense is more fundamental than being close in a physical sense. “Different observers would have different notions of locality,” said Minic, “it depends on the context.” It’s a bit like our relationships with other people. We can feel closer to a loved one far away than the stranger who lives down the street. “You can have these non-local connections as long as they are fairly small,” said Hossenfelder.

    Freidel, Leigh, and Minic have been working on their idea for the last five years, and they believe they are slowly making progress. “We want to be conservative and take things step-by-step,” said Minic, “but it is tantalizing and exciting”. It’s certainly a novel approach, one that looks to “gravitationalize” the quantum world rather than quantizing gravity as in LQG. Yet as with any scientific theory, it needs to be tested. At the moment the trio are working on how to fit time into their model.

    This may all sound incredibly esoteric, something only academics should care about, but it could have a more profound effect on our everyday lives. “We sit in space, we travel through time, and if something changes in our understanding of space-time this will impact not only on our understanding of gravity, but of quantum theory in general,” said Hossenfelder. “All our present devices only work because of quantum theory. If we understand the quantum structure of space-time better that will have an impact on future technologies — maybe not in 50 or 100 years, but maybe in 200,” she said.

    The current monarch is getting long in tooth, and a new pretender is long overdue, but we can’t decide which of the many options is the most likely to succeed. When we do, the resulting revolution could bear fruit not just for theoretical physics, but for all.

    See the full article here .

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  • richardmitnick 1:32 pm on April 26, 2019 Permalink | Reply
    Tags: , , , String Theory   

    From Penn Today: “Making sense of string theory” 


    From Penn Today

    April 25, 2019
    Erica K. Brockmeier

    1

    A Q&A with theoretical physicists Mirjam Cvetic, Ling Lin, and Muyang Liu about what string theory is and how their recent discovery of a “quadrillion solutions” might change the course of the field.

    Albert Einstein’s general theory of relativity provided physicists with both an improved understanding of gravity as well as new, unanswered questions. While it was groundbreaking, it wasn’t able to describe gravity as a consistent quantum theory, or one that successfully describes all of the forces of nature. To this day, Einstein’s dream of linking gravity with electromagnetism and the strong and weak nuclear forces into a single framework has yet to be realized.

    Two scientists later proposed an idea where gravity and electromagnetism could emerge from the same theoretical approach, but only with additional dimensions in the equations. While their theory was too simple to completely describe the universe, their idea of the “compactification” of dimensions eventually became the foundation of string theory research.

    Physicists at Penn have published a paper with a “quadrillion” string theory solutions that each describes a hypothetical universe with the same particles and fundamental forces as our own.

    Penn Today sat down with co-authors Mirjam Cvetic, Ling Lin, and Muyang Liu to learn more about what these solutions mean, how physicists use tiny strings to explain physical phenomena, and how the field of theoretical physics will progress in the future.

    What, broadly speaking, is string theory, and how did this theory come about?

    Cvetic: In understanding how nature works, we want to understand the origin of fundamental forces of nature. And in this context we explain particle physics in terms of quantum mechanical phenomena. Elementary particle physics is consistent with quantum mechanics, but we also have gravity theory that we want to describe in terms of quantum phenomena, and that’s where things get hard.

    Lin: It’s like the people who invented gravity had a different language than people who invented quantum mechanics.

    Cvetic: That’s the main motivation of string theory: Originally intended as a description of the strong nuclear force, people realized that it allows for a quantum description of gravity. The way we identify quantum particles in string theory, including quantum particles of gravity, is by vibrations, excitations of tiny strings. String theory as a consistent quantum theory does not live in three spatial/one time dimensions, but in 10 dimensions. So we are dealing with the idea of compactifying six extra dimensions, namely, shrinking them to small sizes. While unobservable to us, these dimensions can still be probed by the microscopic strings and affect how they behave.

    But there is a byproduct here: The shrinking of extra dimensions allows us to start describing particle physics. We observe not only the quantum particle of gravity but also the quantum particle of, say, electromagnetic interactions, which we call a photon.

    In some ways you say, “Oh gosh, extra dimensions, that’s trouble,” but these extra dimensions also naturally produce types of interactions in four dimensions other than gravity, which we did not ask for in the beginning. Depending on the geometric shapes of the extra dimensions, we may identify these interactions with other forces of nature, like electromagnetism and nuclear forces.

    In our current understanding, these forces are described by the so-called standard model of particle physics, but this does not include gravity. And that’s where string theory becomes an interesting field of research.

    What are the challenges of finally realizing Einstein’s dream of unifying the other forces with gravity?

    Lin: If you think about music, it’s like someone invented the notation, but what we actually observe in an experiment is a particular piece. The problem is that we don’t have a good system that allows us to write down what we observe in experiments, or, to use that same analogy, what we listen to in a concert hall, using the system we have.

    It’s like our sheet music can distinguish between half-tone steps, but there is other music that has finer intonational increments. So our current sheet music will never be able to capture that, and, if there’s a particular piece that has these kinds of changes, how do we capture these things?

    String theory is trying to propose a new system of writing down music, a new system of writing down theories of quantum gravity. But it’s not just a system to write down what we know for our world because we don’t even know all the features that are worth writing down.

    We have a few hints what specific features our system needs to provide, and what we are trying to do is explore more technical things, like do these kinds of mathematical tools actually help us in capturing features of the standard model.

    Your paper relied on methods from the F-theory branch of string theory. What are the benefits of this approach, and what does having a quadrillion solutions really mean?

    Cvetic: The beautiful thing about this regime of string theory is that we can describe its properties in terms of geometry: The shape of this additional compact space, how singular it is, how it determines properties of the particles in three space/one time dimension. So for certain properties, in particular to get the standard model particles out, the power of geometry helped us uncover examples where we can match it to the music of the standard model.

    Lin: The quadrillion solutions are related to the question of how special is our universe, the standard model and the particle physics phenomena that we observe, in what we call the string landscape. From a particle physics perspective, people think that, if I change certain parameters of the standard model, our world would be very drastically different, so it is special in some sense.

    In string theory we have this nice feature that everything comes in discrete numbers, so we can count how many solutions there are. What we show is that, yes, the standard model is special, but within string theory it has the potential to be realized in many different ways.

    What are the challenges of your work, and where do you go next?

    Cvetic: For consistency, the constructions from string theory rely on something called supersymmetry. We include supersymmetry because it’s a technical tool we need for deriving these properties, but it can be broken at large energies. This is an important issue because people would like to match, in all details, our constructions to experimental constraints where we don’t observe supersymmetry at low energies, so we would be required to address those things in more details.

    Lin: That’s one of the conceptual problems of string theory. If someone builds a new detector and finds these additional particles, associated with supersymmetry, at some higher energies than what we are currently reaching in experiments, that would be an advance on the experimental side which could help us a lot. On the other hand, not observing supersymmetry in the near future does not mean that string theory is wrong. It just means that we need to develop new frameworks and methods to improve our toolkit.

    In terms of what to do with these quadrillion examples, these are not just something to be put in a museum, but you can actually use these examples to test new conceptual frameworks and computational methods in string theory. Somebody else will maybe have some ideas, for example, how to break supersymmetry, and now that we have this huge ensemble to explore these ideas, and it’s so large that you could even think about using big data techniques.

    It’s like you produce a bunch of cars, and, even if you just smash them into a wall to test if your airbags are working, they are still providing some usefulness.

    What continues to excite you and inspire you about this area of research?

    Cvetic: I think one of the strengths of the Penn effort is that we ask questions from theory that are relevant to our colleagues in experimental high energy physics. So on one side, the questions we are asking are questions related to things that high energy experimentalists are testing in colliders, and on the other hand we are using techniques of formal string theory that tie us closely to our math department colleagues.

    Lin: What I find interesting about what we do, and more broadly what string theory provides, is the idea of dual descriptions for the same phenomena that suddenly makes certain aspects much easier to grasp. There have been these sorts of ideas floating around in theoretical physics, but it’s string theory that has made this notion of dualities much more present. These ideas have, for example, influenced works in condensed matter which have no immediate connection to string theory.

    And if one thinks from the mathematician’s perspective, what’s also very intriguing is that suddenly, after centuries where mathematicians provided tools for physicists, we’re now at a stage where we can use our intuition to tell mathematicians what to do. That’s unprecedented throughout the history of science, that physics is now guiding math.

    Liu: This interplay between physics and math is particularly fascinating to me in F-theory. The powerful dictionary between concepts in fundamental theoretical physics and beautiful abstract math allows us to translate many demanding questions that intrigue physicists into solvable questions in geometry. Conversely, our physical intuition can uncover novel theorems which are tough to prove under pure mathematical circumstances.

    Cvetic: I think F-theory is amazing. But to understand on a deeper level it’s like uncovering something beyond quantum gravity or beyond string theory. I think that, specifically, the important role of geometry in string theory and more generally in theoretical physics, has led to tremendous conceptual progress, and we may be just scratching the tip of the iceberg of some of these fundamental ideas.

    See the full article here .

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  • richardmitnick 12:10 pm on April 23, 2019 Permalink | Reply
    Tags: "Falsifiability and physics", , , , , , , , Karl Popper (1902-1994) "The Logic of Scientific Discovery", String Theory, ,   

    From Symmetry: “Falsifiability and physics” 

    Symmetry Mag
    From Symmetry

    04/23/19
    Matthew R. Francis

    1
    Illustration by Sandbox Studio, Chicago with Corinne Mucha

    Can a theory that isn’t completely testable still be useful to physics?

    What determines if an idea is legitimately scientific or not? This question has been debated by philosophers and historians of science, working scientists, and lawyers in courts of law. That’s because it’s not merely an abstract notion: What makes something scientific or not determines if it should be taught in classrooms or supported by government grant money.

    The answer is relatively straightforward in many cases: Despite conspiracy theories to the contrary, the Earth is not flat. Literally all evidence is in favor of a round and rotating Earth, so statements based on a flat-Earth hypothesis are not scientific.

    In other cases, though, people actively debate where and how the demarcation line should be drawn. One such criterion was proposed by philosopher of science Karl Popper (1902-1994), who argued that scientific ideas must be subject to “falsification.”

    Popper wrote in his classic book The Logic of Scientific Discovery that a theory that cannot be proven false—that is, a theory flexible enough to encompass every possible experimental outcome—is scientifically useless. He wrote that a scientific idea must contain the key to its own downfall: It must make predictions that can be tested and, if those predictions are proven false, the theory must be jettisoned.

    When writing this, Popper was less concerned with physics than he was with theories like Freudian psychology and Stalinist history. These, he argued, were not falsifiable because they were vague or flexible enough to incorporate all the available evidence and therefore immune to testing.

    But where does this falsifiability requirement leave certain areas of theoretical physics? String theory, for example, involves physics on extremely small length scales unreachable by any foreseeable experiment.

    String Theory depiction. Cross section of the quintic Calabi–Yau manifold Calabi yau.jpg. Jbourjai (using Mathematica output)

    Cosmic inflation, a theory that explains much about the properties of the observable universe, may itself be untestable through direct observations.

    Some critics believe these theories are unfalsifiable and, for that reason, are of dubious scientific value.

    At the same time, many physicists align with philosophers of science who identified flaws in Popper’s model, saying falsification is most useful in identifying blatant pseudoscience (the flat-Earth hypothesis, again) but relatively unimportant for judging theories growing out of established paradigms in science.

    “I think we should be worried about being arrogant,” says Chanda Prescod-Weinstein of the University of New Hampshire. “Falsifiability is important, but so is remembering that nature does what it wants.”

    Prescod-Weinstein is both a particle cosmologist and researcher in science, technology, and society studies, interested in analyzing the priorities scientists have as a group. “Any particular generation deciding that they’ve worked out all that can be worked out seems like the height of arrogance to me,” she says.

    Tracy Slatyer of MIT agrees, and argues that stringently worrying about falsification can prevent new ideas from germinating, stifling creativity. “In theoretical physics, the vast majority of all the ideas you ever work on are going to be wrong,” she says. “They may be interesting ideas, they may be beautiful ideas, they may be gorgeous structures that are simply not realized in our universe.”

    Particles and practical philosophy

    Take, for example, supersymmetry. SUSY is an extension of the Standard Model in which each known particle is paired with a supersymmetric partner.

    Standard Model of Supersymmetry via DESY

    The theory is a natural outgrowth of a mathematical symmetry of spacetime, in ways similar to the Standard Model itself. It’s well established within particle physics, even though supersymmetric particles, if they exist, may be out of scientists’ experimental reach.

    SUSY could potentially resolve some major mysteries in modern physics. For one, all of those supersymmetric particles could be the reason the mass of the Higgs boson is smaller than quantum mechanics says it should be.

    CERN CMS Higgs Event


    CERN ATLAS Higgs Event

    “Quantum mechanics says that [the Higgs boson] mass should blow up to the largest mass scale possible,” says Howard Baer of the University of Oklahoma. That’s because masses in quantum theory are the result of contributions from many different particles involved in interactions—and the Higgs field, which gives other particles mass, racks up a lot of these interactions. But the Higgs mass isn’t huge, which requires an explanation.

    “Something else would have to be tuned to a huge negative [value] in order to cancel [the huge positive value of those interactions] and give you the observed value,” Baer says. That level of coincidence, known as a “fine-tuning problem,” makes physicists itchy. “It’s like trying to play the lottery. It’s possible you might win, but really you’re almost certain to lose.”

    If SUSY particles turn up in a certain mass range, their contributions to the Higgs mass “naturally” solve this problem, which has been an argument in favor of the theory of supersymmetry. So far, the Large Hadron Collider has not turned up any SUSY particles in the range of “naturalness.”

    LHC

    CERN map


    CERN LHC Tunnel

    CERN LHC particles

    However, the broad framework of supersymmetry can accommodate even more massive SUSY particles, which may or may not be detectable using the LHC. In fact, if naturalness is abandoned, SUSY doesn’t provide an obvious mass scale at all, meaning SUSY particles might be out of range for discovery with any earthly particle collider. That point has made some critics queasy: If there’s no obvious mass scale at which colliders can hunt for SUSY, is the theory falsifiable?

    A related problem confronts dark matter researchers: Despite strong indirect evidence for a large amount of mass invisible to all forms of light, particle experiments have yet to find any dark matter particles. It could be that dark matter particles are just impossible to directly detect. A small but vocal group of researchers has argued that we need to consider alternative theories of gravity instead.

    Fritz Zwicky, the Father of Dark Matter research.No image credit after long search

    Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)

    U Washington ADMX Axion Dark Matter Experiment

    DEAP Dark Matter detector, The DEAP-3600, suspended in the SNOLAB deep in Sudbury’s Creighton Mine

    Dark Side-50 Dark Matter Experiment at Gran Sasso

    Slatyer, whose research involves looking for dark matter, considers the criticism partly as a problem of language. “When you say ‘dark matter,’ [you need] to distinguish dark matter from specific scenarios for what dark matter could be,” she says. “The community has not always done that well.”

    In other words, specific models for dark matter can stand or fall, but the dark matter paradigm as a whole has withstood all tests so far. But as Slatyer points out, no alternative theory of gravity can explain all the phenomena that a simple dark matter model can, from the behavior of galaxies to the structure of the cosmic microwave background.

    Prescod-Weinstein argues that we’re a long way from ruling out all dark matter possibilities. “How will we prove that the dark matter, if it exists, definitively doesn’t interact with the Standard Model?” she says. “Astrophysics is always a bit of a detective game. Without laboratory [detection of] dark matter, it’s hard to make definitive statements about its properties. But we can construct likely narratives based on what we know about its behavior.”

    Similarly, Baer thinks that we haven’t exhausted all the SUSY possibilities yet. “People say, ‘you’ve been promising supersymmetry for 20 or 30 years,’ but it was based on overly optimistic naturalness calculations,” he says. “I think if one evaluates the naturalness properly, then you find that supersymmetry is still even now very natural. But you’re going to need either an energy upgrade of LHC or an ILC [International Linear Collider] in order to discover it.”

    ILC schematic, being planned for the Kitakami highland, in the Iwate prefecture of northern Japan

    Beyond falsifiability of dark matter or SUSY, physicists are motivated by more mundane concerns. “Even if these individual scenarios are in principle falsifiable, how much money would [it] take and how much time would it take?” Slatyer says. In other words, rather than try to demonstrate or rule out SUSY as a whole, physicists focus on particle experiments that can be performed within a certain number of budgetary cycles. It’s not romantic, but it’s true nevertheless.

    2
    Illustration by Sandbox Studio, Chicago with Corinne Mucha

    Is it science? Who decides?

    Historically, sometimes theories that seem untestable turn out to just need more time. For example, 19th century physicist Ludwig Boltzmann and colleagues showed they could explain many results in thermal physics and chemistry if everything were made up of “atoms”—what we call particles, atoms, and molecules today—governed by Newtonian physics.

    Since atoms were out of reach of experiments of the day, prominent philosophers of science argued that the atomic hypothesis was untestable in principle, and therefore unscientific.

    However, the atomists eventually won the day: J. J. Thompson demonstrated the existence of electrons, while Albert Einstein showed that water molecules could make grains of pollen dance on a pond’s surface.

    Atoms provide a case study for how falsifiability proved to be the wrong criterion. Many other cases are trickier.

    For instance, Einstein’s theory of general relativity is one of the best-tested theories in all of science. At the same time, it allows for physically unrealistic “universes,” such as a “rotating” cosmos where movement back and forth in time is possible, which are contradicted by all observations of the reality we inhabit.

    General relativity also makes predictions about things that are untestable by definition, like how particles move inside the event horizon of a black hole: No information about these trajectories can be determined by experiment.

    The first image of a black hole, Messier 87 Credit Event Horizon Telescope Collaboration, via NSF 4.10.19

    Yet no knowledgeable physicist or philosopher of science would argue that general relativity is unscientific. The success of the theory is due to enough of its predictions being testable.

    Eddington/Einstein exibition of gravitational lensing solar eclipse of 29 May 1919

    Another type of theory may be mostly untestable, but have important consequences. One such theory is cosmic inflation, which (among other things) explains why we don’t see isolated magnetic monopoles and why the universe is a nearly uniform temperature everywhere we look.

    The key property of inflation—the extremely rapid expansion of spacetime during a tiny split second after the Big Bang—cannot be tested directly. Cosmologists look for indirect evidence for inflation, but in the end it may be difficult or impossible to distinguish between different inflationary models, simply because scientists can’t get the data. Does that mean it isn’t scientific?

    Inflation

    4
    Alan Guth, from Highland Park High School and M.I.T., who first proposed cosmic inflation

    HPHS Owls

    Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe) Date 2010 Credit: Alex MittelmannColdcreation

    Alan Guth’s notes:
    5

    “A lot of people have personal feelings about inflation and the aesthetics of physical theories,” Prescod-Weinstein says. She’s willing to entertain alternative ideas which have testable consequences, but inflation works well enough for now to keep it around. “It’s also the case that the majority of the cosmology community continues to take inflation seriously as a model, so I have to shrug a little when someone says it’s not science.”

    On that note, Caltech cosmologist Sean M. Carroll argues that many very useful theories have both falsifiable and unfalsifiable predictions. Some aspects may be testable in principle, but not by any experiment or observation we can perform with existing technology. Many particle physics models fall into that category, but that doesn’t stop physicists from finding them useful. SUSY as a concept may not be falsifiable, but many specific models within the broad framework certainly are. All the evidence we have for the existence of dark matter is indirect, which won’t go away even if laboratory experiments never find dark matter particles. Physicists accept the concept of dark matter because it works.

    Slatyer is a practical dark matter hunter. “The questions I’m most interested asking are not even just questions that are in principle falsifiable, but questions that in principle can be tested by data on the timescale of less than my lifetime,” she says. “But it’s not only problems that can be tested by data on a timescale of ‘less than Tracy’s lifetime’ are good scientific questions!”

    Prescod-Weinstein agrees, and argues for keeping an open mind. “There’s a lot we don’t know about the universe, including what’s knowable about it. We are a curious species, and I think we should remain curious.”

    See the full article here .


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  • richardmitnick 8:17 am on April 5, 2019 Permalink | Reply
    Tags: , In string theory a “solution” implies a vacuum of spacetime that is governed by Einstein’s theory of gravity coupled to a quantum field theory., In the past two decades a new branch of string theory called F-theory has allowed physicists to work with strongly interacting or strongly coupled strings, , , String theorists can use algebraic geometry to analyze the various ways of compactifying extra dimensions in F-theory and to find solutions., String Theory,   

    From Scientific American: “Found: A Quadrillion Ways for String Theory to Make Our Universe” 

    Scientific American

    From Scientific American

    Mar 29, 2019
    Anil Ananthaswamy

    Stemming from the “F-theory” branch of string theory, each solution replicates key features of the standard model of particle physics.

    1
    Photo: dianaarturovna/Getty Images

    Physicists who have been roaming the “landscape” of string theory — the space of zillions and zillions of mathematical solutions of the theory, where each solution provides the kinds of equations physicists need to describe reality — have stumbled upon a subset of such equations that have the same set of matter particles as exists in our universe.

    String Theory depiction. Cross section of the quintic Calabi–Yau manifold Calabi yau.jpg. Jbourjai (using Mathematica output)

    Standard Model of Supersymmetry via DESY

    But this is no small subset: there are at least a quadrillion such solutions, making it the largest such set ever found in string theory.

    According to string theory, all particles and fundamental forces arise from the vibrational states of tiny strings. For mathematical consistency, these strings vibrate in 10-dimensional spacetime. And for consistency with our familiar everyday experience of the universe, with three spatial dimensions and the dimension of time, the additional six dimensions are “compactified” so as to be undetectable.

    Different compactifications lead to different solutions. In string theory, a “solution” implies a vacuum of spacetime that is governed by Einstein’s theory of gravity coupled to a quantum field theory. Each solution describes a unique universe, with its own set of particles, fundamental forces and other such defining properties.

    Some string theorists have focused their efforts on trying to find ways to connect string theory to properties of our known, observable universe — particularly the standard model of particle physics, which describes all known particles and all their mutual forces except gravity.

    Much of this effort has involved a version of string theory in which the strings interact weakly. However, in the past two decades, a new branch of string theory called F-theory has allowed physicists to work with strongly interacting, or strongly coupled, strings.

    ____________________________________________________
    F-theory is a branch of string theory developed by Cumrun Vafa. The new vacua described by F-theory were discovered by Vafa and allowed string theorists to construct new realistic vacua — in the form of F-theory compactified on elliptically fibered Calabi–Yau four-folds. The letter “F” supposedly stands for “Father”.

    F-theory is formally a 12-dimensional theory, but the only way to obtain an acceptable background is to compactify this theory on a two-torus. By doing so, one obtains type IIB superstring theory in 10 dimensions. The SL(2,Z) S-duality symmetry of the resulting type IIB string theory is manifest because it arises as the group of large diffeomorphisms of the two-dimensional torus.

    More generally, one can compactify F-theory on an elliptically fibered manifold (elliptic fibration), i.e. a fiber bundle whose fiber is a two-dimensional torus (also called an elliptic curve). For example, a subclass of the K3 manifolds is elliptically fibered, and F-theory on a K3 manifold is dual to heterotic string theory on a two-torus. Also, the moduli spaces of those theories should be isomorphic.

    The large number of semirealistic solutions to string theory referred to as the string theory landscape, with 10 272 , 000 {\displaystyle 10^{272,000}} {\displaystyle 10^{272,000}} elements or so, is dominated by F-theory compactifications on Calabi–Yau four-folds.[3] There are about 10 15 {\displaystyle 10^{15}} 10^{15} of those solutions consistent with the Standard Model of particle physics.

    -Wikipedia

    ____________________________________________________

    “An intriguing, surprising result is that when the coupling is large, we can start describing the theory very geometrically,” says Mirjam Cvetic of the University of Pennsylvania in Philadelphia.

    This means that string theorists can use algebraic geometry — which uses algebraic techniques to tackle geometric problems — to analyze the various ways of compactifying extra dimensions in F-theory and to find solutions. Mathematicians have been independently studying some of the geometric forms that appear in F-theory. “They provide us physicists a vast toolkit”, says Ling Lin, also of the University of Pennsylvania. “The geometry is really the key… it is the ‘language’ that makes F-theory such a powerful framework.”

    Now, Cvetic, Lin, James Halverson of Northeastern University in Boston, and their colleagues have used such techniques to identify a class of solutions with string vibrational modes that lead to a similar spectrum of fermions (or, particles of matter) as is described by the standard model — including the property that all fermions come in three generations (for example, the electron, muon and tau are the three generations of one type of fermion).

    Standard Model of Particle Physics (LATHAM BOYLE AND MARDUS OF WIKIMEDIA COMMONS)

    The F-theory solutions found by Cvetic and colleagues have particles that also exhibit the handedness, or chirality, of the standard model particles. In particle physics lingo, the solutions reproduce the exact “chiral spectrum” of standard model particles. For example, the quarks and leptons in these solutions come in left and right-handed versions, as they do in our universe.

    The new work shows that there are at least a quadrillion solutions in which particles have the same chiral spectrum as the standard model, which is 10 orders of magnitude more solutions than had been found within string theory until now. “This is by far the largest domain of standard model solutions,” Cvetic says. “It’s somehow surprising and actually also rewarding that it turns out to be in the strongly coupled string theory regime, where geometry helped us.”

    A quadrillion — while it’s much, much smaller than the size of the landscape of solutions in F-theory (which at last count was shown to be of the order of 10272,000) — is a tremendously large number. “And because it’s a tremendously large number, and it gets something nontrivial in real world particle physics correct, we should take it seriously and study it further,” Halverson says.

    Further study would involve uncovering stronger connections with the particle physics of the real world. The researchers still have to work out the couplings or interactions between particles in the F-theory solutions — which again depend on the geometric details of the compactifications of the extra dimensions.

    It could be that within the space of the quadrillion solutions, there are some with couplings that could cause the proton to decay within observable timescales. This would clearly be at odds with the real world, as experiments have yet to see any sign of protons decaying. Alternatively, physicists could search for solutions that realize the spectrum of standard model particles that preserve a mathematical symmetry called R-parity. “This symmetry forbids certain proton decay processes and would be very attractive from a particle physics point of view, but is missing in our current models,” Lin says.

    Also, the work assumes supersymmetry, which means that all the standard model particles have partner particles. String theory needs this symmetry in order to ensure the mathematical consistency of solutions.

    But in order for any supersymmetric theory to tally with the observable universe, the symmetry has to be broken (much like how a diner’s selection of cutlery and drinking glass on her left or right side will “break” the symmetry of the table setting at a round dinner table). Else, the partner particles would have the same mass as standard model particles — and that is clearly not the case, since we don’t observe any such partner particles in our experiments.

    Crucially, experiments at the Large Hadron Collider (LHC) have also shown that supersymmetry — if it is the correct description of nature — is not broken even at the energy scales probed by the LHC, given that the LHC has yet to find any supersymmetric particles.

    String theorists think that supersymmetry might be broken only at extremely high energies that are not within experimental reach anytime soon. “The expectation in string theory is that high-scale [supersymmetry] breaking, which is fully consistent with LHC data, is completely possible,” Halverson says. “It requires further analysis to determine whether or not it happens in our case.”

    Despite these caveats, other string theorists are approving of the new work. “This is definitely a step forward in demonstrating that string theory gives rise to many solutions with features of the standard model,” says string theorist Washington Taylor of MIT.

    “It’s very nice work,” says Cumrun Vafa, one of the developers of F-theory, at Harvard University. “The fact you can arrange the geometry and topology to fit with not only Einstein’s equations, but also with the [particle] spectrum that we want, is not trivial. It works out nicely here.”

    But Vafa and Taylor both caution that these solutions are far from matching perfectly with the standard model. Getting solutions to match exactly with the particle physics of our world is one of the ultimate goals of string theory. Vafa is among those who think that, despite the immensity of the landscape of solutions, there exists a unique solution that matches our universe. “I bet there is exactly one,” he says. But, “to pinpoint this is not going to be easy.”

    See the full article here .


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  • richardmitnick 12:52 pm on December 28, 2018 Permalink | Reply
    Tags: , , , Scientists propose a new model with dark energy and our universe riding on an expanding bubble in an extra dimension, String Theory,   

    From phys.org: “Our universe: An expanding bubble in an extra dimension” 

    physdotorg
    From phys.org

    December 28, 2018
    Uppsala University

    1
    In their article, the scientists propose a new model with dark energy and our universe riding on an expanding bubble in an extra dimension. Credit: Suvendu Giri

    Uppsala University researchers have devised a new model for the universe – one that may solve the enigma of dark energy. Their new article, published in Physical Review Letters, proposes a new structural concept, including dark energy, for a universe that rides on an expanding bubble in an additional dimension.

    We have known for the past 20 years that the universe is expanding at an ever accelerating rate. The explanation is the “dark energy” that permeates it throughout, pushing it to expand. Understanding the nature of this dark energy is one of the paramount enigmas of fundamental physics.

    It has long been hoped that string theory will provide the answer. According to string theory, all matter consists of tiny, vibrating “stringlike” entities. The theory also requires there to be more spatial dimensions than the three that are already part of everyday knowledge. For 15 years, there have been models in string theory that have been thought to give rise to dark energy. However, these have come in for increasingly harsh criticism, and several researchers are now asserting that none of the models proposed to date are workable.

    In their article, the scientists propose a new model with dark energy and our universe riding on an expanding bubble in an extra dimension. The whole universe is accommodated on the edge of this expanding bubble. All existing matter in the universe corresponds to the ends of strings that extend out into the extra dimension. The researchers also show that expanding bubbles of this kind can come into existence within the framework of string theory. It is conceivable that there are more bubbles than ours, corresponding to other universes.

    The Uppsala scientists’ model provides a new, different picture of the creation and future fate of the universe, while it may also pave the way for methods of testing string theory.

    See the full article here .

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    About Phys.org in 100 Words

    Phys.org™ (formerly Physorg.com) is a leading web-based science, research and technology news service which covers a full range of topics. These include physics, earth science, medicine, nanotechnology, electronics, space, biology, chemistry, computer sciences, engineering, mathematics and other sciences and technologies. Launched in 2004, Phys.org’s readership has grown steadily to include 1.75 million scientists, researchers, and engineers every month. Phys.org publishes approximately 100 quality articles every day, offering some of the most comprehensive coverage of sci-tech developments world-wide. Quancast 2009 includes Phys.org in its list of the Global Top 2,000 Websites. Phys.org community members enjoy access to many personalized features such as social networking, a personal home page set-up, RSS/XML feeds, article comments and ranking, the ability to save favorite articles, a daily newsletter, and other options.

     
  • richardmitnick 10:15 am on September 12, 2018 Permalink | Reply
    Tags: "The fractal universe" Part 3, , Andrei Linde-Professor of Physics, , , , , , , String Theory   

    From Stanford University: “The fractal universe” Part 3 

    Stanford University Name
    From Stanford University

    1
    The concept of a multiverse, created in a fiery bloom of matter and radiation, is a central part of the String Theory Landscape. (Image credit: Eric Nyquist)

    September 12, 2018
    Ker Than

    Late one summer night nearly 40 years ago, Andrei Linde was seized by a sudden conviction that he knew how the universe was born. His nocturnal eureka moment would lead to the concept of a multiverse, a central part of the String Theory Landscape. This story is part 3 of a five-part series.

    Late one summer night in 1981, while still a junior research fellow at Lebedev Physical Institute in Moscow, Andrei Linde was struck by a revelation. Unable to contain his excitement, he shook awake his wife, Renata Kallosh, and whispered to her in their native Russian, “I think I know how the universe was born.”

    Kallosh, a theoretical physicist herself, muttered some encouraging words and fell back asleep. “It wasn’t until the next morning that I realized the full impact of what Andrei had told me,” recalled Kallosh, now a professor of physics at the Stanford Institute for Theoretical Physics.

    Linde’s nocturnal eureka moment had to do with a problem in cosmology that he and other theorists, including Stephen Hawking, had struggled with for months.

    A year earlier, a 32-year-old postdoc at SLAC National Accelerator Laboratory named Alan Guth shocked the physics community by proposing a bold modification to the Big Bang theory. According to Guth’s idea, which he called “inflation,” our universe erupted from a vacuum-like state and underwent a brief period of faster-than-light expansion.

    Inflation

    4
    Alan Guth, from Highland Park High School and M.I.T., who first proposed cosmic inflation

    HPHS Owls

    Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe) Date 2010 Credit: Alex Mittelmann Coldcreation

    Alan Guth’s notes:
    5

    In less than a billionth of a trillionth of a trillionth of a second, space-time doubled more than 60 times from a subatomic speck to a volume many times larger than the observable universe.

    Guth envisioned the powerful repulsive force fueling the universe’s exponential growth as a field of energy flooding space. As the universe unfurled, this “inflaton field” decayed, and its shed energy was transfigured into a fiery bloom of matter and radiation. This pivot, from nothing to something and timelessness to time, marked the beginning of the Big Bang. It also prompted Guth to famously quip that the inflationary universe was the “ultimate free lunch.”

    As theories go, inflation was a beauty. It explained in one fell swoop why the universe is so large, why it was born hot, and why its structure appears to be so flat and uniform over vast distances. There was just one problem – it didn’t work.

    Tunneling

    To conclude the unpacking of space-time, Guth borrowed a trick from quantum mechanics called “tunneling” to allow his inflaton field to randomly and instantly skip from a higher, less stable energy state to a lower one, thus bypassing a barrier that could not be scaled by classical physics.

    3
    Andrei Linde and Renata Kallosh, both professors of physics. (Image credit: L.A. Cicero)

    But closer inspection revealed that quantum tunneling caused the inflaton field to decay quickly and unevenly, resulting in a universe that was neither flat nor uniform. Aware of the fatal flaw in his theory, Guth wrote at the end of his paper on inflation: “I am publishing this paper in the hope that it will … encourage others to find some way to avoid the undesirable features of the inflationary scenario.”

    Guth’s plea was answered by Linde, who on that fateful summer night realized that inflation didn’t require quantum tunneling to work. Instead, the inflaton field could be modeled as a ball rolling down a hill of potential energy that had a very shallow, nearly flat slope. While the ball rolls lazily downhill, the universe is inflating, and as it nears the bottom, inflation slows further and eventually ends. This provided a “graceful exit” to the inflationary state that was lacking in Guth’s model and produced a cosmos like the one we observe. To distinguish it from Guth’s original model while still paying homage to it, Linde dubbed his model “new inflation.”

    Quantum birth of galaxies

    By the time Linde and Kallosh moved to Stanford in 1990, experiments had begun to catch up with the theory. Space missions were finding temperature variations in the energetic afterglow of the Big Bang – called the cosmic microwave background radiation – that confirmed a startling prediction made by the latest inflationary models. These updated models went by various names – “chaotic inflation,” “eternal inflation,” “eternal chaotic inflation” and many more – but they all shared in common the graceful exit that Linde pioneered.

    According to these models, galaxies like the Milky Way grew from faint wrinkles in the fabric of space-time. The density of matter in these wrinkles was slightly greater compared to surrounding areas and this difference was magnified during inflation, allowing them to attract even more matter. From these dense primordial seeds grew the cosmic structures we see today. “Galaxies are children of random quantum fluctuations produced during the first 10-35 seconds after the birth of the universe,” Linde said.

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

    Inflation predicted that these quantum fluctuations would leave imprints on the universe’s background radiation in the form of hotter and colder regions, and this is precisely what two experiments – dubbed COBE and WMAP – found. “After the COBE and WMAP experiments, inflation started to become part of the standard model of cosmology,” Shamit Kachru said.

    COBE/CMB


    NASA/COBE 1989 to 1993.

    CMB per NASA/WMAP


    NASA/WMAP 2001 to 2010

    CMB per ESA/Planck


    ESA/Planck 2009 to 2013

    5
    Shamit Kachru, Professor of Physics and Director, Stanford Institute for Theoretical Physics (Image credit: L.A. Cicero)

    The multiverse

    Linde and others later realized that the same quantum fluctuations that produced galaxies can give rise to new inflating regions in the universe. Even though inflation ended in our local cosmic neighborhood 14 billion years ago, it can still continue at the outermost fringes of the universe. The consequence is an ever-expanding sea of inflating space-time dotted with “island universes” or “pocket universes” like our own where inflation has ceased.

    Multiverse. Image credit: public domain, retrieved from https://pixabay.com/

    “As a result, the universe becomes a multiverse, an eternally growing fractal consisting of exponentially many exponentially large parts,” Linde wrote. “These parts are so large that for all practical purposes they look like separate universes.”

    Linde took the multiverse idea even further by proposing that each pocket universe could have differing properties, a conclusion that some string theorists were also reaching independently. “It’s not that the laws of physics are different in each universe, but their realizations,” Linde said. “An analogy is the relationship between liquid water and ice. They’re both H2O but realized differently.”

    Linde’s multiverse is like a cosmic funhouse filled with reality-distorting mirrors. Some pocket universes are resplendent with life, while others were stillborn because they were cursed with too few (or too many) dimensions, or with physics incompatible with the formation of stars and galaxies. An infinite number are exact replicas of ours, but infinitely more are only near-replicas. Right now, there could be countless versions of you inhabiting worlds with histories divergent from ours in ways large and small. In an infinitely expanding multiverse, anything that can happen will happen.

    “The inflationary universe is not just the ultimate free lunch, it’s the only lunch where all possible dishes are served,” Linde said.

    While disturbing to some, this eternal aspect of inflation was just what a small group of string theorists were looking for to help explain a surprise discovery that was upending the physics world – dark energy.

    Dark Energy Survey


    Dark Energy Camera [DECam], built at FNAL


    NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam at an altitude of 7200 feet

    See the full article here .


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    Stanford University campus. No image credit

    Leland and Jane Stanford founded the University to “promote the public welfare by exercising an influence on behalf of humanity and civilization.” Stanford opened its doors in 1891, and more than a century later, it remains dedicated to finding solutions to the great challenges of the day and to preparing our students for leadership in today’s complex world. Stanford, is an American private research university located in Stanford, California on an 8,180-acre (3,310 ha) campus near Palo Alto. Since 1952, more than 54 Stanford faculty, staff, and alumni have won the Nobel Prize, including 19 current faculty members

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  • richardmitnick 6:37 am on August 30, 2018 Permalink | Reply
    Tags: , , , Black Hole Firewalls Could Be Too Tepid to Burn, , , , , String Theory   

    From Nautilus: “Black Hole Firewalls Could Be Too Tepid to Burn” 

    Nautilus

    From Nautilus

    Aug 29, 2018
    Charlie Wood

    Artist’s conception of two merging black holes similar to those detected by LIGO Credit LIGO-Caltech/MIT/Sonoma State /Aurore Simonnet

    1
    String theorists elide a paradox about black holes by extinguishing the walls of fire feared to surround them. NASA

    Despite its ability to bend both minds and space, an Einsteinian black hole looks so simple a child could draw it. There’s a point in the center, a perfectly spherical boundary a bit farther out, and that’s it

    The point is the singularity, an infinitely dense, unimaginably small dot contorting space so radically that anything nearby falls straight in, leaving behind a vacuum. The spherical boundary marks the event horizon, the point of no return between the vacuum and the rest of the universe. But according to Einstein’s theory of gravity, the event horizon isn’t anything that an unlucky astronaut would immediately notice if she were to cross it. “It’s like the horizon outside your window,” said Samir Mathur, a physicist at Ohio State University. “If you actually walked over there, there’s nothing.”

    In 2012, however, this placid picture went up in flames. A team of four physicists took a puzzle first put forward by Stephen Hawking about what happens to all the information that falls into the black hole, and turned it on its head. Rather than insisting that an astronaut (often named Alice) pass smoothly over the event horizon, they prioritized a key postulate of quantum mechanics: Information, like matter and energy, must never be destroyed. That change ended up promoting the event horizon from mathematical boundary to physical object, one they colorfully named the wall of fire.

    “It can’t be empty, and it turns out it has to be full of a lot of stuff, a lot of hot stuff,” said Donald Marolf, a physicist at the University of California, Santa Barbara, and one of the four co-authors [no cited paper]. The argument caused an uproar in the theoretical physics community, much as if cartographers suggested that instead of an imaginary line on their maps, Earth’s equator was actually a wall of bright red bricks.

    The news of a structure at the boundary didn’t shock Mathur, however. For more than a decade he had been arguing that black holes are really balls of strings (from string theory) with hot, fuzzy surfaces. “As you come closer and closer it gets hotter and hotter, and that’s what causes the burning,” he explained.

    In recent years, Mathur has been refining his “fuzzball” description, and his most recent calculations bring marginally good news for Alice. While she wouldn’t live a long and healthy life, the horizon’s heat might not be what does her in.

    Fuzzballs are what you get when you apply string theory, a description of nature that replaces particles with strings, to extremely dense objects. Energize a particle and it can only speed up, but strings stretch and swell as well. That ability to expand, combined with additional flexibility from postulated extra dimensions, makes strings fluff up when enough of them are packed into a small space. They form a fuzzy ball that looks from afar like an ordinary black hole—it has the same size (for a given mass) and emits the same kind of “Hawking radiation” that all black holes emit. As a bonus, the slightly bumpy surface changes the way it emits particles and declaws Hawking’s information puzzle, according to Mathur. “It’s more like a planet,” he said, “and it radiates from that surface just like anything else.”

    33
    Olena Shmahalo / Quanta Magazine

    His new work extends arguments from 2014, which asked what would happen to Alice if she were to fall onto a supermassive fuzzball akin to the one at the heart of our galaxy—one with the mass of millions of suns. In such situations, the force of gravity dominates all others. Assuming this constraint, Mathur and his collaborator found that an incoming Alice particle had almost no chance of smashing into an outgoing particle of Hawking radiation. The surface might be hot, he said, but the way the fuzzball expands to swallow new material prevents anything from getting close enough to burn, so Alice should make it to the surface.


    In response, Marolf suggested that a medium-size fuzzball might still be able to barbecue Alice in other ways. It wouldn’t drag her in as fast, and in a collision at lower energies, forces other than gravity could singe her, too.

    Mathur’s team recently took a more detailed look at Alice’s experience with new calculations published in the Journal of High Energy Physics. They concluded that for a modest fuzzball—one as massive as our sun—the overall chance of an Alice particle hitting a radiation particle was slightly higher than they had found before, but still very close to zero. Their work suggested that you’d have to shrink a fuzzball down to a thousand times smaller than the nanoscale before burning would become likely.

    By allowing Alice to reach the surface more or less intact (she would still undergo an uncontroversial and likely fatal stretching), the theory might even end up restoring the Einsteinian picture of smooth passage across the boundary, albeit in a twisted form. There might be a scenario in which Alice went splat on the surface while simultaneously feeling as if she were falling through open space, whatever that might mean.

    “If you jump onto [fuzzballs] in one description, you break up into little strings. That’s the splat picture,” Mathur said. We typically assume that once her particles start breaking up, Alice ceases to be Alice. A bizarre duality in string theory, however, allows her strings to spread out across the fuzzball in an orderly way that preserves their connections, and, perhaps, her sense of self. “If you look carefully at what [the strings] are doing,” Mathur continued, “they’re actually spreading in a very coherent ball.”

    The details of Mathur’s picture remain rough. And the model rests entirely on the machinery of string theory, a mathematical framework with no experimental evidence. What’s more, not even string theory can handle the messiness of realistic fuzzballs. Instead, physicists focus on contrived examples such as highly organized, extra-frigid bodies with extreme features, said Marika Taylor, a string theorist at the University of Southampton in the U.K.

    Mathur’s calculations are exploratory, she said, approximate generalizations from the common features of the simple models. The next step is a theory that can describe the fuzzball’s surface at the quantum level, from the point of view of the string. Nevertheless, she agreed that the hot firewall idea has always smelled fishy from a string-theory perspective. “You suddenly transition from ‘I’m falling perfectly happily’ to ‘Oh my God, I’m completely destroyed’? That’s unsatisfactory,” she said.

    Marolf refrained from commenting on the latest results until he finished discussing them with Mathur, but said that he was interested in learning more about how the other forces had been accounted for and how the fuzzball surface would react to Alice’s visit. He also pointed out that Mathur’s black hole model was just one of many tactics for resolving Hawking’s puzzle, and there was no guarantee that anyone had hit on the right one. “Maybe the real world is crazier than even the things we’ve thought of yet,” he said, “and we’re just not being clever enough.”

    See the full article here .

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    Welcome to Nautilus. We are delighted you joined us. We are here to tell you about science and its endless connections to our lives. Each month we choose a single topic. And each Thursday we publish a new chapter on that topic online. Each issue combines the sciences, culture and philosophy into a single story told by the world’s leading thinkers and writers. We follow the story wherever it leads us. Read our essays, investigative reports, and blogs. Fiction, too. Take in our games, videos, and graphic stories. Stop in for a minute, or an hour. Nautilus lets science spill over its usual borders. We are science, connected.

     
  • richardmitnick 4:33 pm on August 20, 2018 Permalink | Reply
    Tags: , Anomalies, , Branes, , , , , , Parity violation, , , , String Theory, , , The second superstring revolution, Theorist John Schwarz   

    From Caltech: “Long and Winding Road: A Conversation with String Theory Pioneer” John Schwarz 

    Caltech Logo

    From Caltech

    08/20/2018

    Whitney Clavin
    (626) 395-1856
    wclavin@caltech.edu

    John Schwarz discusses the history and evolution of superstring theory.

    1
    John Schwarz. Credit: Seth Hansen for Caltech

    The decades-long quest for a theory that would unify all the known forces—from the microscopic quantum realm to the macroscopic world where gravity dominates—has had many twists and turns. The current leading theory, known as superstring theory and more informally as string theory, grew out of an approach to theoretical particle physics, called S-matrix theory, which was popular in the 1960s. Caltech’s John H. Schwarz, the Harold Brown Professor of Theoretical Physics, Emeritus, began working on the problem in 1971, while a junior faculty member at Princeton University. He moved to Caltech in 1972, where he continued his research with various collaborators from other universities. Their studies in the 1970s and 1980s would dramatically shift the evolution of the theory and, in 1984, usher in what’s known as the first superstring revolution.

    Essentially, string theory postulates that our universe is made up, at its most fundamental level, of infinitesimal tiny vibrating strings and contains 10 dimensions—three for space, one for time, and six other spatial dimensions curled up in such a way that we don’t perceive them in everyday life or even with the most sensitive experimental searches to date. One of the many states of a string is thought to correspond to the particle that carries the gravitational force, the graviton, thereby linking the two pillars of fundamental physics—quantum mechanics and the general theory of relativity, which includes gravity.

    We sat down with Schwarz to discuss the history and evolution of string theory and how the theory itself might have moved past strings.

    What are the earliest origins of string theory?

    The first study often regarded as the beginning of string theory came from an Italian physicist named Gabriele Veneziano in 1968. He discovered a mathematical formula that had many of the properties that people were trying to incorporate in a fundamental theory of the strong nuclear force [a fundamental force that holds nuclei together]. This formula was kind of pulled out of the blue, and ultimately Veneziano and others realized, within a couple years, that it was actually describing a quantum theory of a string—a one-dimensional extended object.

    How did the field grow after this paper?

    In the early ’70s, there were several hundred people worldwide working on string theory. But then everything changed when quantum chromodynamics, or QCD—which was developed by Caltech’s Murray Gell-Mann [Nobel Laureate, 1969] and others—became the favored theory of the strong nuclear force. Almost everyone was convinced QCD was the right way to go and stopped working on string theory. The field shrank down to just a handful of people in the course of a year or two. I was one of the ones who remained.

    How did Gell-Mann become interested in your work?

    Gell-Mann is the one who brought me to Caltech and was very supportive of my work. He took an interest in studies I had done with a French physicist, André Neveu, when we were at Princeton. Neveu and I introduced a second string theory. The initial Veneziano version had many problems. There are two kinds of fundamental particles called bosons and fermions, and the Veneziano theory only described bosons. The one I developed with Neveu included fermions. And not only did it include fermions but it led to the discovery of a new kind of symmetry that relates bosons and fermions, which is called supersymmetry. Because of that discovery, this version of string theory is called superstring theory.

    When did the field take off again?

    A pivotal change happened after work I did with another French physicist, Joël Scherk, whom Gell-Mann and I had brought to Caltech as a visitor in 1974. During that period, we realized that many of the problems we were having with string theory could be turned into advantages if we changed the purpose. Instead of insisting on constructing a theory of the strong nuclear force, we took this beautiful theory and asked what it was good for. And it turned out it was good for gravity. Neither of us had worked on gravity. It wasn’t something we were especially interested in but we realized that this theory, which was having trouble describing the strong nuclear force, gives rise to gravity. Once we realized this, I knew what I would be doing for the rest of my career. And I believe Joël felt the same way. Unfortunately, he died six years later. He made several important discoveries during those six years, including a supergravity theory in 11 dimensions.

    Surprisingly, the community didn’t respond very much to our papers and lectures. We were generally respected and never had a problem getting our papers published, but there wasn’t much interest in the idea. We were proposing a quantum theory of gravity, but in that era physicists who worked on quantum theory weren’t interested in gravity, and physicists who worked on gravity weren’t interested in quantum theory.

    That changed after I met Michael Green [a theoretical physicist then at the University of London and now at the University of Cambridge], at the CERN cafeteria in Switzerland in the summer of 1979. Our collaboration was very successful, and Michael visited Caltech for several extended visits over the next few years. We published a number of papers during that period, which are much cited, but our most famous work was something we did in 1984, which had to do with a problem known as anomalies.

    What are anomalies in string theory?

    One of the facts of nature is that there is what’s called parity violation, which means that the fundamental laws are not invariant under mirror reflection. For example, a neutrino always spins clockwise and not counterclockwise, so it would look wrong viewed in a mirror. When you try to write down a fundamental theory with parity violation, mathematical inconsistencies often arise when you take account of quantum effects. This is referred to as the anomaly problem. It appeared that one couldn’t make a theory based on strings without encountering these anomalies, which, if that were the case, would mean strings couldn’t give a realistic theory. Green and I discovered that these anomalies cancel one another in very special situations.

    When we released our results in 1984, the field exploded. That’s when Edward Witten [a theoretical physicist at the Institute for Advanced Study in Princeton], probably the most influential theoretical physicist in the world, got interested. Witten and three collaborators wrote a paper early in 1985 making a particular proposal for what to do with the six extra dimensions, the ones other than the four for space and time. That proposal looked, at the time, as if it could give a theory that is quite realistic. These developments, together with the discovery of another version of superstring theory, constituted the first superstring revolution.

    Richard Feynman was here at Caltech during that time, before he passed away in 1988. What did he think about string theory?

    After the 1984 to 1985 breakthroughs in our understanding of superstring theory, the subject no longer could be ignored. At that time it acquired some prominent critics, including Richard Feynman and Stephen Hawking. Feynman’s skepticism of superstring theory was based mostly on the concern that it could not be tested experimentally. This was a valid concern, which my collaborators and I shared. However, Feynman did want to learn more, so I spent several hours explaining the essential ideas to him. Thirty years later, it is still true that there is no smoking-gun experimental confirmation of superstring theory, though it has proved its value in other ways. The most likely possibility for experimental support in the foreseeable future would be the discovery of supersymmetry particles. So far, they have not shown up.

    What was the second superstring revolution about?

    The second superstring revolution occurred 10 years later in the mid ’90s. What happened then is that string theorists discovered what happens when particle interactions become strong. Before, we had been studying weakly interacting systems. But as you crank up the strength of the interaction, a 10th dimension of space can emerge. New objects called branes also emerge. Strings are one dimensional; branes have all sorts of dimensions ranging from zero to nine. An important class of these branes, called D-branes, was discovered by the late Joseph Polchinski [BS ’75]. Strings do have a special role, but when the system is strongly interacting, then the strings become less fundamental. It’s possible that in the future the subject will get a new name but until we understand better what the theory is, which we’re still struggling with, it’s premature to invent a new name.

    What can we say now about the future of string theory?

    It’s now over 30 years since a large community of scientists began pooling their talents, and there’s been enormous progress in those 30 years. But the more big problems we solve, the more new questions arise. So, you don’t even know the right questions to ask until you solve the previous questions. Interestingly, some of the biggest spin-offs of our efforts to find the most fundamental theory of nature are in pure mathematics.

    Do you think string theory will ultimately unify the forces of nature?

    Yes, but I don’t think we’ll have a final answer in my lifetime. The journey has been worth it, even if it did take some unusual twists and turns. I’m convinced that, in other intelligent civilizations throughout the galaxy, similar discoveries will occur, or already have occurred, in a different sequence than ours. We’ll find the same result and reach the same conclusions as other civilizations, but we’ll get there by a very different route.

    See the full article here .

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    The California Institute of Technology (commonly referred to as Caltech) is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering. Its 124-acre (50 ha) primary campus is located approximately 11 mi (18 km) northeast of downtown Los Angeles. “The mission of the California Institute of Technology is to expand human knowledge and benefit society through research integrated with education. We investigate the most challenging, fundamental problems in science and technology in a singularly collegial, interdisciplinary atmosphere, while educating outstanding students to become creative members of society.”

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  • richardmitnick 3:39 am on August 15, 2018 Permalink | Reply
    Tags: , , , , Dark Energy May Be Incompatible With String Theory, , , , String Theory   

    From Quanta Magazine: “Dark Energy May Be Incompatible With String Theory” 

    Quanta Magazine
    From Quanta Magazine

    August 9, 2018
    Natalie Wolchover

    1
    String theory permits a “landscape” of possible universes, surrounded by a “swampland” of logically inconsistent universes. In all of the simple, viable stringy universes physicists have studied, the density of dark energy is either diminishing or has a stable negative value, unlike our universe, which appears to have a stable positive value. Maciej Rebisz for Quanta Magazine

    On June 25, Timm Wrase awoke in Vienna and groggily scrolled through an online repository of newly posted physics papers. One title startled him into full consciousness.

    The paper, by the prominent string theorist Cumrun Vafa of Harvard University and collaborators, conjectured a simple formula dictating which kinds of universes are allowed to exist and which are forbidden, according to string theory. The leading candidate for a “theory of everything” weaving the force of gravity together with quantum physics, string theory defines all matter and forces as vibrations of tiny strands of energy. The theory permits some 10500 different solutions: a vast, varied “landscape” of possible universes. String theorists like Wrase and Vafa have strived for years to place our particular universe somewhere in this landscape of possibilities.

    But now, Vafa and his colleagues were conjecturing that in the string landscape, universes like ours — or what ours is thought to be like — don’t exist. If the conjecture is correct, Wrase and other string theorists immediately realized, the cosmos must either be profoundly different than previously supposed or string theory must be wrong.

    After dropping his kindergartner off that morning, Wrase went to work at the Vienna University of Technology, where his colleagues were also buzzing about the paper. That same day, in Okinawa, Japan, Vafa presented the conjecture at the Strings 2018 conference, which was streamed by physicists worldwide. Debate broke out on- and off-site. “There were people who immediately said, ‘This has to be wrong,’ other people who said, ‘Oh, I’ve been saying this for years,’ and everything in the middle,” Wrase said. There was confusion, he added, but “also, of course, huge excitement. Because if this conjecture was right, then it has a lot of tremendous implications for cosmology.”

    Researchers have set to work trying to test the conjecture and explore its implications. Wrase has already written two papers, including one that may lead to a refinement of the conjecture, and both mostly while on vacation with his family. He recalled thinking, “This is so exciting. I have to work and study that further.”

    The conjectured formula — posed in the June 25 paper by Vafa, Georges Obied, Hirosi Ooguri and Lev Spodyneiko and further explored in a second paper released two days later by Vafa, Obied, Prateek Agrawal and Paul Steinhardt — says, simply, that as the universe expands, the density of energy in the vacuum of empty space must decrease faster than a certain rate. The rule appears to be true in all simple string theory-based models of universes. But it violates two widespread beliefs about the actual universe: It deems impossible both the accepted picture of the universe’s present-day expansion and the leading model of its explosive birth.

    Dark Energy in Question

    Since 1998, telescope observations have indicated that the cosmos is expanding ever-so-slightly faster all the time, implying that the vacuum of empty space must be infused with a dose of gravitationally repulsive “dark energy.”

    In addition, it looks like the amount of dark energy infused in empty space stays constant over time (as best anyone can tell).

    But the new conjecture asserts that the vacuum energy of the universe must be decreasing.

    Vafa and colleagues contend that universes with stable, constant, positive amounts of vacuum energy, known as “de Sitter universes,” aren’t possible. String theorists have struggled mightily since dark energy’s 1998 discovery to construct convincing stringy models of stable de Sitter universes. But if Vafa is right, such efforts are bound to sink in logical inconsistency; de Sitter universes lie not in the landscape, but in the “swampland.” “The things that look consistent but ultimately are not consistent, I call them swampland,” he explained recently. “They almost look like landscape; you can be fooled by them. You think you should be able to construct them, but you cannot.”

    According to this “de Sitter swampland conjecture,” in all possible, logical universes, the vacuum energy must either be dropping, its value like a ball rolling down a hill, or it must have obtained a stable negative value. (So-called “anti-de Sitter” universes, with stable, negative doses of vacuum energy, are easily constructed in string theory.)

    The conjecture, if true, would mean the density of dark energy in our universe cannot be constant, but must instead take a form called “quintessence” — an energy source that will gradually diminish over tens of billions of years. Several telescope experiments are underway now to more precisely probe whether the universe is expanding with a constant rate of acceleration, which would mean that as new space is created, a proportionate amount of new dark energy arises with it, or whether the cosmic acceleration is gradually changing, as in quintessence models. A discovery of quintessence would revolutionize fundamental physics and cosmology, including rewriting the cosmos’s history and future. Instead of tearing apart in a Big Rip, a quintessent universe would gradually decelerate, and in most models, would eventually stop expanding and contract in either a Big Crunch or Big Bounce.

    Paul Steinhardt, a cosmologist at Princeton University and one of Vafa’s co-authors, said that over the next few years, “all eyes should be on” measurements by the Dark Energy Survey, WFIRST and Euclid telescopes of whether the density of dark energy is changing.

    Dark Energy Survey


    Dark Energy Camera [DECam], built at FNAL


    NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam at an altitude of 7200 feet

    NASA/WFIRST

    ESA/Euclid spacecraft

    “If you find it’s not consistent with quintessence,” Steinhardt said, “it means either the swampland idea is wrong, or string theory is wrong, or both are wrong or — something’s wrong.”

    Inflation Under Siege

    No less dramatically, the new swampland conjecture also casts doubt on the widely believed story of the universe’s birth: the Big Bang theory known as cosmic inflation.

    Inflation

    4
    Alan Guth, from Highland Park High School and M.I.T., who first proposed cosmic inflation

    HPHS Owls

    Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe) Date 2010 Credit: Alex MittelmannColdcreation

    Alan Guth’s notes:
    5

    According to this theory, a minuscule, energy-infused speck of space-time rapidly inflated to form the macroscopic universe we inhabit. The theory was devised to explain, in part, how the universe got so huge, smooth and flat.

    But the hypothetical “inflaton field” of energy that supposedly drove cosmic inflation doesn’t sit well with Vafa’s formula. To abide by the formula, the inflaton field’s energy would probably have needed to diminish too quickly to form a smooth- and flat-enough universe, he and other researchers explained. Thus, the conjecture disfavors many popular models of cosmic inflation. In the coming years, telescopes such as the Simons Observatory will look for definitive signatures of cosmic inflation, testing it against rival ideas.

    In the meantime, string theorists, who normally form a united front, will disagree about the conjecture. Eva Silverstein, a physics professor at Stanford University and a leader in the effort to construct string-theoretic models of inflation, thinks it is very likely to be false. So does her husband, the Stanford professor Shamit Kachru; he is the first “K” in KKLT, a famous 2003 paper (known by its authors’ initials) that suggested a set of stringy ingredients that might be used to construct de Sitter universes. Vafa’s formula says both Silverstein’s and Kachru’s constructions won’t work. “We’re besieged by these conjectures in our family,” Silverstein joked. But in her view, accelerating-expansion models are no more disfavored now, in light of the new papers, than before. “They essentially just speculate that those things don’t exist, citing very limited and in some cases highly dubious analyses,” she said.

    Matthew Kleban, a string theorist and cosmologist at New York University, also works on stringy models of inflation. He stresses that the new swampland conjecture is highly speculative and an example of “lamppost reasoning,” since much of the string landscape has yet to be explored. And yet he acknowledges that, based on existing evidence, the conjecture could well be true. “It could be true about string theory, and then maybe string theory doesn’t describe the world,” Kleban said. “[Maybe] dark energy has falsified it. That obviously would be very interesting.”

    Mapping the Swampland

    Whether the de Sitter swampland conjecture and future experiments really have the power to falsify string theory remains to be seen. The discovery in the early 2000s that string theory has something like 10^500 solutions killed the dream that it might uniquely and inevitably predict the properties of our one universe. The theory seemed like it could support almost any observations and became very difficult to experimentally test or disprove.

    In 2005, Vafa and a network of collaborators began to think about how to pare the possibilities down by mapping out fundamental features of nature that absolutely have to be true. For example, their “weak gravity conjecture” asserts that gravity must always be the weakest force in any logical universe. Imagined universes that don’t satisfy such requirements get tossed from the landscape into the swampland. Many of these swampland conjectures have held up famously against attack, and some are now “on a very solid theoretical footing,” said Hirosi Ooguri, a theoretical physicist at the California Institute of Technology and one of Vafa’s first swampland collaborators. The weak gravity conjecture, for instance, has accumulated so much evidence that it’s now suspected to hold generally, independent of whether string theory is the correct theory of quantum gravity.

    The intuition about where landscape ends and swampland begins derives from decades of effort to construct stringy models of universes. The chief challenge of that project has been that string theory predicts the existence of 10 space-time dimensions — far more than are apparent in our 4-D universe. String theorists posit that the six extra spatial dimensions must be small — curled up tightly at every point. The landscape springs from all the different ways of configuring these extra dimensions. But although the possibilities are enormous, researchers like Vafa have found that general principles emerge. For instance, the curled-up dimensions typically want to gravitationally contract inward, whereas fields like electromagnetic fields tend to push everything apart. And in simple, stable configurations, these effects balance out by having negative vacuum energy, producing anti-de Sitter universes. Turning the vacuum energy positive is hard. “Usually in physics, we have simple examples of general phenomena,” Vafa said. “De Sitter is not such a thing.”

    The KKLT paper, by Kachru, Renata Kallosh, Andrei Linde and Sandip Trivedi, suggested stringy trappings like “fluxes,” “instantons” and “anti-D-branes” that could potentially serve as tools for configuring a positive, constant vacuum energy. However, these constructions are complicated, and over the years possible instabilities have been identified. Though Kachru said he does not have “any serious doubts,” many researchers have come to suspect the KKLT scenario does not produce stable de Sitter universes after all.

    Vafa thinks a concerted search for definitely stable de Sitter universe models is long overdue. His conjecture is, above all, intended to press the issue. In his view, string theorists have not felt sufficiently motivated to figure out whether string theory really is capable of describing our world, instead taking the attitude that because the string landscape is huge, there must be a place in it for us, even if no one knows where. “The bulk of the community in string theory still sides on the side of de Sitter constructions [existing],” he said, “because the belief is, ‘Look, we live in a de Sitter universe with positive energy; therefore we better have examples of that type.’”

    His conjecture has roused the community to action, with researchers like Wrase looking for stable de Sitter counterexamples, while others toy with little-explored stringy models of quintessent universes. “I would be equally interested to know if the conjecture is true or false,” Vafa said. “Raising the question is what we should be doing. And finding evidence for or against it — that’s how we make progress.”

    See the full article here .


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    Please help promote STEM in your local schools.

    Stem Education Coalition

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

     
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