6,020 research outputs found
An Infinite Dimensional Symmetry Algebra in String Theory
Symmetry transformations of the space-time fields of string theory are
generated by certain similarity transformations of the stress-tensor of the
associated conformal field theories. This observation is complicated by the
fact that, as we explain, many of the operators we habitually use in string
theory (such as vertices and currents) have ill-defined commutators. However,
we identify an infinite-dimensional subalgebra whose commutators are not
singular, and explicitly calculate its structure constants. This constitutes a
subalgebra of the gauge symmetry of string theory, although it may act on
auxiliary as well as propagating fields. We term this object a {\it weighted
tensor algebra}, and, while it appears to be a distant cousin of the
-algebras, it has not, to our knowledge, appeared in the literature before.Comment: 14 pages, Plain TeX, report RU93-8, CTP-TAMU-2/94, CERN-TH.7022/9
Fluctuation Theorems for Entropy Production and Heat Dissipation in Periodically Driven Markov Chains
Asymptotic fluctuation theorems are statements of a Gallavotti-Cohen symmetry
in the rate function of either the time-averaged entropy production or heat
dissipation of a process. Such theorems have been proved for various general
classes of continuous-time deterministic and stochastic processes, but always
under the assumption that the forces driving the system are time independent,
and often relying on the existence of a limiting ergodic distribution. In this
paper we extend the asymptotic fluctuation theorem for the first time to
inhomogeneous continuous-time processes without a stationary distribution,
considering specifically a finite state Markov chain driven by periodic
transition rates. We find that for both entropy production and heat
dissipation, the usual Gallavotti-Cohen symmetry of the rate function is
generalized to an analogous relation between the rate functions of the original
process and its corresponding backward process, in which the trajectory and the
driving protocol have been time-reversed. The effect is that spontaneous
positive fluctuations in the long time average of each quantity in the forward
process are exponentially more likely than spontaneous negative fluctuations in
the backward process, and vice-versa, revealing that the distributions of
fluctuations in universes in which time moves forward and backward are related.
As an additional result, the asymptotic time-averaged entropy production is
obtained as the integral of a periodic entropy production rate that generalizes
the constant rate pertaining to homogeneous dynamics
(G)hosting television: Ghostwatch and its medium
This article’s subject is Ghostwatch (BBC, 1992), a drama broadcast on Halloween night of 1992 which adopted the rhetoric of live non-fiction programming, and attracted controversy and ultimately censure from the Broadcasting Standards Council. In what follows, we argue that Ghostwatch must be understood as a televisually-specific artwork and artefact. We discuss the programme’s ludic relationship with some key features of television during what Ellis (2000) has termed its era of ‘availability’, principally liveness, mass simultaneous viewing, and the flow of the television super-text. We trace the programme’s television-specific historicity whilst acknowledging its allusions and debts to other media (most notably film and radio). We explore the sophisticated ways in which Ghostwatch’s visual grammar and vocabulary and deployment of ‘broadcast talk’ (Scannell 1991) variously ape, comment upon and subvert the rhetoric of factual programming, and the ends to which these strategies are put. We hope that these arguments collectively demonstrate the aesthetic and historical significance of Ghostwatch and identify its relationship to its medium and that medium’s history. We offer the programme as an historically-reflexive artefact, and as an exemplary instance of the work of art in television’s age of broadcasting, liveness and co-presence
As time goes by
A rather simple and non-technical exposition of our new approach to {\em
Time, Quantum Physics, Black-Hole dynamics}, and {\em Cosmology}, based on
non-critical string theory, is provided. A new fundamental principle, the {\em
Procrustean Principle}, that catches the essence of our approach is postulated:
the low-energy world is {\em unavoidably} an ``open" system due to the
spontaneous truncation of the {\em delocalized, topological} string modes in
continuous interaction with the low-lying-{\em localized} string modes. The
origin of space-time, the expansion of the Universe, the entropy increase and
accompanied irreversibility of time, as well as the collapse of the
wavefunction are all very neatly tied together. Possible observable
consequences include: quantum relaxation with time of the Universal,
fundamental constants, like the velocity of light and the Planck constant
decreasing towards their asymptotic values, and the cosmological
constant diminishing towards zero; possible violation of {\em CPT}
invariance in the system, possible apparent non-conservation of
angular momentum, and possible loss of quantum coherence in SQUID-type
experiments.Comment: CERN-TH.7260/94, 84 pages Latex (no figures
All-optical switching in granular ferromagnets caused by magnetic circular dichroism
Magnetic recording using circularly polarised femto-second laser pulses is an emerging technology that would allow write speeds much faster than existing field driven methods. However, the mechanism that drives the magnetisation switching in ferromagnets is unclear. Recent theories suggest that the interaction of the light with the magnetised media induces an opto-magnetic field within the media, known as the inverse Faraday effect. Here we show that an alternative mechanism, driven by thermal excitation over the anisotropy energy barrier and a difference in the energy absorption depending on polarisation, can create a net magnetisation over a series of laser pulses in an ensemble of single domain grains. Only a small difference in the absorption is required to reach magnetisation levels observed experimentally and the model does not preclude the role of the inverse Faraday effect but removes the necessity that the opto-magnetic field is 10 s of Tesla in strength
On the dynamical behavior of the ABC model
We consider the ABC dynamics, with equal density of the three species, on the
discrete ring with sites. In this case, the process is reversible with
respect to a Gibbs measure with a mean field interaction that undergoes a
second order phase transition. We analyze the relaxation time of the dynamics
and show that at high temperature it grows at most as while it grows at
least as at low temperature
Physical, chemical and biological processes in Lake Vostok and other Antarctic subglacial lakes
Over 70 lakes have now been identified beneath the Antarctic ice sheet. Although water from none of the lakes has been sampled directly, analysis of lake ice frozen (accreted) to the underside of the ice sheet above Lake Vostok, the largest of these lakes, has allowed inferences to be made on lake water chemistry and has revealed small quantities of microbes. These findings suggest that Lake Vostok is an extreme, yet viable, environment for life. All subglacial lakes are subject to high pressure (∼350 atmospheres), low temperatures (about -3 °C) and permanent darkness. Any microbes present must therefore use chemical sources to power biological processes. Importantly, dissolved oxygen is available at least at the lake surface, from equilibration with air hydrates released from melting basal glacier ice. Microbes found in Lake Vostok's accreted ice are relatively modern, but the probability of ancient lake-floor sediments leads to a possibility of a very old biota at the base of subglacial lakes
Effective Lagrangians for Orientifold Theories
We construct effective Lagrangians of the Veneziano-Yankielowicz (VY) type
for two non-supersymmetric theories which are orientifold daughters of
supersymmetric gluodynamics (containing one Dirac fermion in the two-index
antisymmetric or symmetric representation of the gauge group). Since the parent
and daughter theories are planar equivalent, at N\to\infty the effective
Lagrangians in the orientifold theories basically coincide with the bosonic
part of the VY Lagrangian.
We depart from the supersymmetric limit in two ways. First, we consider
finite (albeit large) values of N. Then 1/N effects break supersymmetry. We
suggest seemingly the simplest modification of the VY Lagrangian which
incorporates these 1/N effects, leading to a non-vanishing vacuum energy
density. We analyze the spectrum of the finite-N non-supersymmetric daughters.
For N=3 the two-index antisymmetric representation (one flavor) is equivalent
to one-flavor QCD. We show that in this case the scalar quark-antiquark state
is heavier than the corresponding pseudoscalar state, `` eta' ''. Second, we
add a small fermion mass term. The fermion mass term breaks supersymmetry
explicitly. The vacuum degeneracy is lifted. The parity doublets split. We
evaluate the splitting. Finally, we include the theta-angle and study its
implications.Comment: LaTeX, 21 page
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