Hot Topics in General Relativity and Gravitation

In this centennial anniversary of General Relativity, we will hold a conference on Hot Topics in General Relativity and Gravitation with the motivation to emphasize the tremendous progresses that have been made in Astrophysics and in Cosmology since Einstein’s discovery of General Relativity (GR) in 1915. This international conference will be held at ICISE as part of the Rencontres du Vietnam.

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European Physical Society Conference on High Energy Physics

The European Physical Society Conference on High Energy Physics (EPS-HEP) is one of the major international conferences that reviews the field every second year since 1971 organized by the High Energy and Particle Physics Divison of the European Physical Society. The latest conferences in this series were held in Stockholm, Grenoble, Krakow, Manchester, Lisbon and Aachen.

AdS/CFT, self-adjoint extension and the resolution of cosmological singularities

In the context of Einstein gravity coupled to matter satisfying fairly general energy conditions, cosmological singularities are unavoidable, as was proven by Hawking and Penrose many year ago. One of the hopes for superstring theory is that a solution to this problem may be found. Clearly, however, a non-perturbative formulation of string theory must be used. The AdS/CFT correspondence is a proposed definition of non-perturbative string theory (including gravity) in terms of a conformal field theory which does not involve gravity. It has been conjectured that the AdS/CFT correspondence can be used to resolve cosmological singularities. Initial work in this direction was done by different groups, including Craps-Hertog-Turok, Khoury-Ovrut-Steinhardt-Turok, Skenderis-McFadden, Silverstein-Horowitz and Das-Tridevi.

Strings 2015

Strings 2015 conference will be held from 22 – 26 June 2015 at the International Centre for Theoretical Sciences, Bengaluru. This conference is the latest in a sequence of annual conferences, that bring the entire string theory community together.

String Theory & Cosmology: New Ideas Meet New Experimental Data

This is the golden age of cosmology. Once a philosophical subject, cosmology has burgeoned into a precision science as ground and space-based astronomical observations supply a wealth of unprecedently precise cosmological measurements. Questions that were recently the stuff of speculation can now be analyzed in the context of rigorous, predictive theoretical frameworks whose viability is determined by observational data. To address key questions about our universe, especially at the energy scales characteristic of its earliest moments, one must invoke a theory of quantum gravity, such as string theory. Conversely, observational cosmology is our most promising window for testing fundamental theories at ultra-high energies.

International Conference on Quantum Gravity, Cosmology and String Theory

The Center for Fundamental Physics (CFP) at Zewail City of Science and Technology is organizing its first international conference on Quantum Gravity, Cosmology and String Theory (QGCS15) in Giza, Egypt from 11 to 15 January 2016. 

The potential of string theory as an elegant unified description of physics

Nell’Agosto del 1984 due fisici arrivarono ad elaborare una formula che aprì una nuova finestra verso la comprensione della teoria delle stringhe. Lo scorso mese di Dicembre, Michael Green dell’Università di Cambridge e John Schwarz del California Institute of Technology sono stati insigniti del Fundamental Physics Prize 2014, uno dei premi della serie “Breakthrough Prizes” che riguarda le scienze fisiche e biologiche. La citazione del premio, che ammonta a 3 milioni di dollari, dice “per aver introdotto nuove prospettive sulla gravità quantistica e l’unificazione delle forze“.

Green and Schwarz are known for their pioneering work in string theory, postulated as a way of explaining the fundamental constituents of the Universe as tiny vibrating strings. Different types of elementary particles arise in this theory as different vibrational harmonics (or ‘notes’). The scope of string theory has broadened over the past few years and is currently being applied to a far wider field than that for which it was first devised, which has taken those who research into it in unexpected directions. Although the term ‘string theory’ was not coined till 1971, it had its genesis in a paper by the Italian physicist Gabriele Veneziano in 1968, published when Green was a research student in Cambridge. Green was rapidly impressed by its potential and began working seriously on it in the early 1970s. As he explains in the accompanying film, he stuck with string theory during a period when it was overshadowed by other developments in elementary particle physics. As a result of a chance meeting at the CERN accelerator laboratory in Switzerland in the summer of 1979, Green (then a researcher at Queen Mary, London) began to work on string theory with Schwarz. Green says that the relative absence of interest in string theory during the 1970s and early 1980s was actually helpful: it allowed him and a small number of colleagues to focus on their research well away from the limelight. “Initially we were not sure that the theory would be consistent, but as we understood it better we became more and more convinced that the theory had something valuable to say about the fundamental particles and their forces”, he says. In August 1984 the two researchers, while working at the Aspen Center for Physics in Colorado, famously understood how string theory avoids certain inconsistencies (known as ‘anomalies’) that plague more conventional theories in which the fundamental particles are points rather than strings. This convinced other researchers of the potential of string theory as an elegant unified description of fundamental physics. “Suddenly our world changed – and we were called on to give lectures and attend meetings and workshops”, remembers Green. String theory was back on track as a construct that offered a compelling explanation for the fundamental building blocks of the Universe: many researchers shifted the focus of their work into this newly-promising field and, as a result of this upturn in interest, developments in string theory began to take new and unexpected directions. Ideas formulated in the past few years, indicate that string theory has an overarching mathematical structure that may be useful for understanding a much wider variety of problems in theoretical physics that the theory was originally supposed to explain, this includes problems in condensed matter, superconductivity, plasma physics and the physics of fluids. Green is a passionate believer in the exchange of ideas and he values immensely his interaction with the latest generation of researchers to be tackling some of the knottiest problems in particle physics and associated fields. “The best ideas come from the young people entering the field and we need to make sure we continue to attract them into research. It is particularly evident that at present we fail to encourage sufficient numbers of young women to think about careers in physics”, he says. “Scientific research is by its nature competitive and there are, of course, professional jealousies – but there’s also a strong tradition of collaboration in theoretical physics and advances in the subject feel like a communal activity.” In 2009 Green was appointed Lucasian Professor of Mathematics at Cambridge. It comes with a legacy that Green describes as daunting: his immediate predecessor was Professor Stephen Hawking and in its 350-year history the chair has been held by a series of formidable names in the history of mathematical sciences.

The challenges of pushing forward the boundaries in a field that demands thinking in not three dimensions but as many as 11 are tremendous. The explanation of the basic building blocks of nature as different harmonics of a string is only a small part of string theory, and is the feature that is easiest to put across to the general public as it is relatively straightforward to visualise.

“Far harder to articulate in words are concepts to do with explaining how time and space might emerge from the theory”, says Green. “Sometimes you hit a problem that you just can’t get out of your head and carry round with you wherever you are. It’s almost a cliché that it’s often when you’re relaxing that a solution will spontaneously present itself”. Like his colleagues Green is motivated by wonderment at the world and the excitement of being part of a close community grappling with fundamental questions. He is often asked to justify the cost of research that can seem so remote from everyday life, and that cannot be tested in any conventional sense. In response he gives the example of the way in which quantum mechanics has revolutionised the way in which many of us live. In terms of developments that may come from advances in string theory, he says: “We can’t predict what the eventual outcomes of our research will be. But, if we are successful, they will certainly be huge and in the meantime, string theory provides a constant stream of unexpected surprises.”

Michael Green will be giving a lecture, ‘The pointless Universe’, as part of Cambridge Science Festival on Thursday 13 March, 5pm-6pm, at Lady Mitchell Hall, Sidgwick Site, Cambridge. The event is free but requires pre-booking.

University of Cambridge: Strings that surprise: how a theory scaled up

Heavy quarks as ideal probes for quark-gluon plasma studies

Subito dopo il Big Bang, lo spazio era caratterizzato da una sorta di “zuppa primordiale” composta di quark e gluoni, ossia particelle di materia e di interazioni fondamentali. Questo plasma super denso si raffreddò quasi istantaneamente e la sua, seppure breve, esistenza contribuì sostanzialmente a creare le condizioni iniziali da cui si è successivamente evoluto il nostro Universo. Ma per capire meglio queste fasi iniziali della storia cosmica, gli scienziati devono ricreare nei grandi acceleratori di particelle quel plasma primordiale: è il caso del Relativistic Heavy Ion Collider (RHIC) presso il Brookhaven National Laboratory (BNL) dove si stanno analizzando i dati degli ultimi anni grazie ad un esperimento noto come STAR (Solenoidal Tracker at RHIC). A tale complesso è stato aggiunto di recente un nuovo rivelatore, denominato Heavy Flavor Tracker, il più avanzato nel suo genere e che servirà per studiare i processi di decadimento degli adroni costituiti da quark charm e bottom.

Scientists and engineers at the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab), have played a major role in the development of the STAR Heavy Flavor Tracker. The STAR HFT is actually the collective name for three separate silicon-based detector systems that make it possible for the first time to directly track the decay products of hadrons comprised of flavors (types) of quarks, “charm” and “bottom,” with heavy mass. Heavy quarks are considered ideal probes for quark-gluon plasma studies; however, their low production yield and short life-span (a fraction of a microsecond) make them difficult to study in heavy ion collisions that also produce huge quantities of light flavor particles. The HFT was first conceived nearly 15 years ago by Berkeley Lab’s Howard Wieman, a physicist with the Lab’s Nuclear Sciences Division who also played a prominent role in the creation of STAR. The HFT construction project, which began a few years later, was initially led at Berkeley Lab by Hans Georg Ritter, a physicist who served as head of the Nuclear Science Division’s Relativistic Nuclear Collisions program (RNC) for many years. “The HFT enables precision tracking measurements of heavy quarks at low momentum where the particle production is most sensitive to the bulk medium created in heavy ion collisions”, says Nu Xu, a physicist also with Berkeley Lab’s Nuclear Science Division who is the current spokesperson for the STAR experiment. “This allows us to distinguish the decay vertices of heavy flavor particles from primary vertices and significantly reduces combinational background, which yields cleaner measurements with a higher level of significance”. The importance of the HFT’s precision measurements at low momentum to quark-gluon plasma studies is explained by Peter Jacobs, a Berkeley Lab physicist who now heads the Nuclear Science Division’s RNC program. “Theorists claim they can calculate the dynamical behavior of heavy quarks in matter more accurately than that of light quarks or gluons. Some even think they can calculate the dynamical behavior of heavy quarks in the quark-gluon plasma using models inspired by string theory”, Jacobs says. “One of the things we will be testing with the HFT is the different predictions of the behavior of heavy flavors in the quark-gluon plasma made by string-inspired models versus more conventional physics”.

Berkeley Lab scientists and engineers are now developing a new, larger version of the HFT which they propose to be fabricated for the ALICE detector at CERN’s Large Hadron Collider. “If approved, this will be an upgrade to the Inner Tracking System of the ALICE experiment at the LHC that is a direct follow-on to the STAR HFT, utilizing a number of HFT developments”, says Jacobs. “It is proposed to be installed during the next long LHC shutdown in 2018 and will essentially be a 25 giga-pixel camera made up of 11 square meters of silicon, about 30 times larger than the HFT at STAR”.

LBL: Heavy Flavor Tracker for STAR

arXiv: Heavy Flavor Tracker (HFT) : A new inner tracking device at STAR

Testing the string theory with astronomy

Il “Sacro Graal” della fisica potrebbe venire alla luce. E’ oggi quello che un gruppo di fisici del Department of Physics, Astronomy and Geosciences presso la Towson University (TU) sperano di aver trovato dopo quasi mezzo secolo di ricerche: verificare sperimentalmente una delle teorie più elusive e più complicate da capire, la teoria delle stringhe, osservando il moto dei pianeti, della Luna e degli asteroidi in una sorta di reminiscenza di uno dei più famosi test realizzati da Galileo sulla caduta dei gravi dalla Torre di Pisa.

“Scientists have joked about how string theory is promising…and always will be promising, for lack of being able to test it”, says James Overduin, professor in Towson’s Department of Physics, Astronomy and Geosciences and lead author on a paper about the test TU scientists are developing. The paper was presented at the 223rd Meeting of the American Astronomical Society in Washington.

String theory posits an explanation for the connection between all the forces in the Universe. If it sounds overly broad, it is; string theory is nicknamed “the theory of everything.”

Scientific theories need tests in order to be truly valid, and string theory hasn’t been testable because the energy level and size to see its effects are just too big. “What we have identified is a straightforward method to detect cracks in general relativity that could be explained by string theory, with almost no strings attached”, Overduin explains. For most people, the understanding of string theory goes about as far as CBS’s “The Big Bang Theory” can convey it. The very basic explanation of the complex concept is that all matter and energy in the Universe is made of one-dimensional strings, a quintillion times smaller than the extremely tiny hydrogen atom. That means the strings are too small to detect indirectly, and finding signs of them in an instrument like a particle accelerator would require millions of times more energy than what was used to uncover, for example, the Higgs boson, a particle pivotal to the explanation and further proof of particle theory. The Higgs boson was posited in the 1960s, around the same time as string theory’s introduction; the boson’s identification was announced in 2012. The TU team’s string theory test borrows from Galilean and Newtonian laws of gravity. History holds that Galileo tested rates of acceleration by simultaneously dropping balls of two different weights off the Tower of Pisa to demonstrate that, despite the weight difference, they would hit the ground at the same time. Newton later found that Jupiter and its moons, in their orbits, “fall” at the same rate of acceleration toward the Sun. Much later, Einstein developed the theory of relativity when he recognized that gravity pulls all masses with precisely the same amount of strength, regardless of size.

Overduin and his team use those understandings for their test because string theory posits violations of Einstein’s relativity. It asserts that there are other fields that couple with objects differently, depending on the objects’ composition. That makes them accelerate differently, even within the same gravitational field.

But why does it matter? According to Overduin, the answer is nothing short of revolution. “Every time physicists have succeeded in unifying two different branches of physics, society has been transformed”, Overduin says. The Scientific Revolution was born of Newton’s unification of physics and astronomy. The Industrial Revolution, steam engines leading to train and boat transportation, began after physicists unified mechanics and heat. Electrification came when James Clerk Maxwell unified electricity and magnetism. Einstein’s relativity ushered in the Atomic Age, and then the Information Age, when relativity entered the quantum mechanics. That leaves two parts of physics still unconnected: gravitation and everything else. Physicists believe unifying them, as a test of string theory could do, would spark yet another revolution. But for all this time, they couldn’t do it. Towson University scientists might.

TU: TU scientists may spark revolution with string theory test

arXiv: Expanded solar-system limits on violations of the equivalence principle

Is our Universe a hologram?

E’ circolata di recente nei media la notizia pubblicata da Nature secondo la quale un gruppo di fisici giapponesi avrebbero formulato una teoria che “potrebbe essere considerata l’evidenza più chiara sul fatto che il nostro Universo sarebbe una gigantesca proiezione“. Nei loro articoli, Yoshifumi Hyakutake e colleghi della Ibaraki University in Giappone spiegano  come la loro idea suggerisca che la realtà fisica, così come noi la concepiamo, potrebbe essere in definitiva un ologramma appartenente ad un altro spazio bidimensionale.

In 1997, theoretical physicist Juan Maldacena proposed that an audacious model of the Universe in which gravity arises from infinitesimally thin, vibrating strings could be reinterpreted in terms of well-established physics. The mathematically intricate world of strings, which exist in nine dimensions of space plus one of time, would be merely a hologram: the real action would play out in a simpler, flatter cosmos where there is no gravity. Maldacena’s idea thrilled physicists because it offered a way to put the popular but still unproven theory of strings on solid footing, and because it solved apparent inconsistencies between quantum physics and Einstein’s theory of gravity. It provided physicists with a mathematical “Rosetta stone”, a ‘duality’, that allowed them to translate back and forth between the two languages, and solve problems in one model that seemed intractable in the other and vice versa (see ‘Collaborative physics: String theory finds a bench mate‘). But although the validity of Maldacena’s ideas has pretty much been taken for granted ever since, a rigorous proof has been elusive.

In two papers posted on the arXiv repository, Yoshifumi Hyakutake of Ibaraki University in Japan and his colleagues now provide, if not an actual proof, at least compelling evidence that Maldacena’s conjecture is true.

In one paper, Hyakutake computes the internal energy of a black hole, the position of its event horizon (the boundary between the black hole and the rest of the Universe), its entropy and other properties based on the predictions of string theory as well as the effects of so-called virtual particles that continuously pop into and out of existence (see ‘Astrophysics: Fire in the Hole!‘). In the other, he and his collaborators calculate the internal energy of the corresponding lower-dimensional cosmos with no gravity. The two computer calculations match. “It seems to be a correct computation”, says Maldacena, who is now at the Institute for Advanced Study in Princeton, New Jersey and who did not contribute to the team’s work.

The findings “are an interesting way to test many ideas in quantum gravity and string theory”, Maldacena adds.

The two papers, he notes, are the culmination of a series of articles contributed by the Japanese team over the past few years. “The whole sequence of papers is very nice because it tests the dual [nature of the universes] in regimes where there are no analytic tests. They have numerically confirmed, perhaps for the first time, something we were fairly sure had to be true, but was still a conjecture, namely that the thermodynamics of certain black holes can be reproduced from a lower-dimensional Universe”, says Leonard Susskind, a theoretical physicist at Stanford University in California who was among the first theoreticians to explore the idea of holographic universes. Neither of the model universes explored by the Japanese team resembles our own, Maldacena notes. The cosmos with a black hole has ten dimensions, with eight of them forming an eight-dimensional sphere. The lower-dimensional, gravity-free one has but a single dimension, and its menagerie of quantum particles resembles a group of idealized springs, or harmonic oscillators, attached to one another. Nevertheless, says Maldacena, the numerical proof that these two seemingly disparate worlds are actually identical gives hope that the gravitational properties of our Universe can one day be explained by a simpler cosmos purely in terms of quantum theory.

Nature: Simulations back up theory that Universe is a hologram

arXiv: Quantum Near Horizon Geometry of Black 0-Brane

arXiv: Holographic description of quantum black hole on a computer

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