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TOPIC: Superstrings


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Posts: 131433
Date:
Professor Lisa Randall
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Professor Randall is a theoretical particle physicist who sees past the rest of us to a world of extra dimensions and parallel universes. Hers is a world of warped geometry, sink-holes and branes a world that fills glaring gaps in current thinking, and can finally explain why gravity is so weak!
Now while this might sound like so much Greek just wait. Randalls latest book, written for the layman, is called Warped Passages: Unravelling the Mysteries of the Universes Hidden Dimensions so shes had plenty of practice explaining these high-flying ideas to English majors.

Click here to listen to Lisa Randalls lecture at IDEAS Boston on the WGBH Forum Network.

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Date:
Testing String Theory
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Title: Testing String Theory with CMB
Authors: Renata Kallosh, Andrei Linde

Future detection/non-detection of tensor modes from inflation in CMB observations presents a unique way to test certain features of string theory. Current limit on the ratio of tensor to scalar perturbations, r=T/S, is r < 0.3, future detection may take place for r > 10^{-2}-10^{-3}. At present all known string theory inflation models predict tensor modes well below the level of detection. Therefore a possible experimental discovery of tensor modes may present a challenge to string cosmology.
The strongest bound on r in string inflation follows from the observation that in most of the models based on the KKLT construction, the value of the Hubble constant H during inflation must be smaller than the gravitino mass. For the gravitino mass in the usual range, m_{3/2} < O(1) TeV, this leads to an extremely strong bound r < 10^-24. A discovery of tensor perturbations with r > 10^-3 would imply that the gravitinos in this class of models are superheavy, m_{3/2} > 10^13 GeV. This would have important implications for particle phenomenology based on string theory.

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-- Edited by Blobrana at 09:54, 2007-04-09

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Michio Kaku
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Dr. Michio Kaku. Henry Semat Professor of Theoretical Physics - City University of New York. Author: "PARALLEL WORLDS - A JOURNEY THROUGH CREATION, HIGHER DIMENSIONS, AND THE FUTURE OF THE COSMOS"



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RE: Superstrings
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If Michael Turner had known what he was in for, he might have stayed home. As the moderator of a debate held here last night at the National Museum of Natural History, the University of Chicago cosmologist had the unenviable task of trying to crown a winner in a match-up between Brian Greene and Lawrence Krauss, two physics heavyweights duking it out over the merits--or lack thereof--of the so-called Theory of Everything.
String theory assumes that elementary particles are tiny vibrating strings that exist in multiple dimensions. In trying to unite Einstein's theory of gravity with quantum mechanics, it hopes to answer mysteries about the beginning of the universe and the very nature of matter, energy, and time. The claims are deep, and opponents of the theory say the findings so far have been shallow, even nonexistent. Last night's debate did little to settle the argument, but a packed house of academics, physics geeks, and just-curious laypeople seemed to enjoy themselves nonetheless.

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Date:
String Theory
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String Theory


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Date:
Cosmic string loops
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Title: Cosmic string loops in the expanding universe
Authors: Ken D. Olum, Vitaly Vanchurin
(revised v2)

We study the production of loops in the cosmic string network in the expanding background by means of a numerical simulation exact in the flat-spacetime limit and first-order in the expansion rate. We find an initial regime characterised by production of small loops at the scale of the initial correlation length, but later we see the emergence of a scaling regime of loop production. This qualitatively agrees with earlier expectations derived from the results of flat-spacetime simulations. In the final scaling regime we find that the characteristic length of loops scales as about 0.1 t in both radiation and matter eras.

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-- Edited by Blobrana at 15:48, 2007-03-12

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Posts: 131433
Date:
String theory
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String theory argues that all matter is composed of string-like shreds in a 10-dimensional hyperspace assembled in various forms. It has won acclaim from many who appreciate the theory's elegant mathematics and ambition to unite quantum mechanics and general relativity, and skepticism from others who cite the theory's lack of a practical track record. String theory, the doubters say, makes no testable predictions.
But this isn't exactly true. Indeed, the theory has not yet been experimentally vindicated in the realm of quantum gravity, but has been put into play in the realm of high-energy ion collisions, the kind carried out at Brookhaven's Relativistic Heavy Ion Collider (RHIC). A few years ago string practitioners attempted to establish a relationship between the 10-dimensional string world and the 4-dimensional (3 spatial dimensions plus time) world in which we observe interactions among quark-filled particles like protons.
This duality between string theory and the theory of the strong nuclear force, quantum chromodynamics (QCD), was recently used to interpret puzzling early results from RHIC, namely the suppression of energetic quark jets that should have emerged from the fireball formed when two heavy nuclei (such as gold) collide head on. The thinking was that perhaps the plasma of quarks and gluons (quarks bursting free from their customary proton and meson groupings) wasn't a gas of weakly interacting particles (as was originally thought) but a gas of strongly interacting particles, so strong that any energetic quarks that might have escaped the fireball (initiating a secondary avalanche, or jet, or quarks) would quickly be slowed and stripped of energy on its way through the tumultuous quark-gluon plasma (QGP) environment.
Two new papers by Hong Liu and Krishna Rajagopal of (MIT) and Urs Wiedemann (CERN) address this problem. The first paper calculates a specific quark-suppression parameter (namely, how much the quarks, each attached to a string dangling "downward" into a fifth dimension, are pushed around as they traverse the quark-gluon plasma) that agrees closely with the experimentally observed value.
Rajagopal says that in the second paper, the same authors make a specific testable prediction using string theory that bears not just on missing jets of energetic light quarks (up, down, and strange quarks), but on the melting or dissociation temperatures of bound states of heavy quarks (charm-anticharm or bottom-antibottom pairs) moving through the quark-gluon plasma with sufficiently high velocity, as will be produced in future experiments at RHIC and the Large Hadron Collider (LHC) under construction at CERN.

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Date:
Extra dimensions
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A satellite to be launched next year could see signs of extra dimensions in the afterglow of the big bang, a new study says.
Some theories such as string theory that attempt to unify all known forces into a single "theory of everything" posit the existence of extra spatial dimensions beyond the three familiar ones.
But string theory has proven stubbornly resistant to experimental tests (although some physicists say it could be tested in the Large Hadron Collider scheduled to open by the end of 2007).

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RE: Superstrings
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Peering backward in time to an instant after the big bang, physicists at the University of Wisconsin-Madison have devised an approach that may help unlock the hidden shapes of alternate dimensions of the universe.
A new study demonstrates that the shapes of extra dimensions can be "seen" by deciphering their influence on cosmic energy released by the violent birth of the universe 13 billion years ago. The method, published today (Feb. 2) in Physical Review Letters, provides evidence that physicists can use experimental data to discern the nature of these elusive dimensions the existence of which is a critical but as yet unproven element of string theory, the leading contender for a unified "theory of everything."

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-- Edited by Blobrana at 17:49, 2007-02-02

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Posts: 131433
Date:
String theory
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String theory could be ruled out by experiments at the Large Hadron Collider (LHC), a particle accelerator scheduled to open by the end of 2007, a new study says. The finding offers a new approach for testing this potential "theory of everything", a goal that has so far proven elusive.
According to string theory, particles like electrons and photons are actually tiny, vibrating strings. The beauty of the theory is that it accounts for all of the known forces including gravity, which the standard model of physics does not. But its critics have complained that there is essentially no way to test it

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