1,836 research outputs found
Experiments on Internet Response
This paper suggests a generalized distribution of response times to new
information for human populations in the absence of deadlines.
This has important implications for psychological and social studies as well
the study of dynamical networks such as the WWW.Comment: 4 figures 6 page
Characterization of large price variations in financial markets
Statistics of drawdowns (loss from the last local maximum to the next local
minimum) plays an important role in risk assessment of investment strategies.
As they incorporate higher ( two) order correlations, they offer a better
measure of real market risks than the variance or other cumulants of daily (or
some other fixed time scale) of returns. Previous results have shown that the
vast majority of drawdowns occurring on the major financial markets have a
distribution which is well-represented by a stretched exponential, while the
largest drawdowns are occurring with a significantly larger rate than predicted
by the bulk of the distribution and should thus be characterized as outliers.
In the present analysis, the definition of drawdowns is generalised to
coarse-grained drawdowns or so-called epsilon-drawdowns and a link between such
epsilon-outliers and preceding log-periodic power law bubbles previously
identified is established.Comment: 7 pages and 5 figures. To be published in Physica A, Proceedings of
International Econophysics Conference, Bali, August 28-31, 200
Large financial crashes
We propose that large stock market crashes are analogous to critical points
studied in statistical physics with log-periodic correction to scaling. We
extend our previous renormalization group model of stock market prices prior to
and after crashes [D. Sornette et al., J.Phys.I France 6, 167, 1996] by
including the first non-linear correction. This predicts the existence of a
log-frequency shift over time in the log-periodic oscillations prior to a
crash. This is tested on the two largest historical crashes of the century, the
october 1929 and october 1987 crashes, by fitting the stock market index over
an interval of 8 years prior to the crashes. The good quality of the fits, as
well as the consistency of the parameter values obtained from the two crashes,
promote the theory that crashes have their origin in the collective ``crowd''
behavior of many interacting agents.Comment: 14 pages and 4 figures. To be published in Physica
Influence of the water content in protoplanetary discs on planet migration and formation
The temperature and density profiles of protoplanetary discs depend crucially
on the mass fraction of micrometre-sized dust grains and on their chemical
composition. A larger abundance of micrometre-sized grains leads to an overall
heating of the disc, so that the water ice line moves further away from the
star. An increase in the water fraction inside the disc, maintaining a fixed
dust abundance, increases the temperature in the icy regions of the disc and
lowers the temperature in the inner regions. Discs with a larger silicate
fraction have the opposite effect. Here we explore the consequence of the dust
composition and abundance for the formation and migration of planets. We find
that discs with low water content can only sustain outwards migration for
planets up to 4 Earth masses, while outwards migration in discs with a larger
water content persists up to 8 Earth masses in the late stages of the disc
evolution. Icy planetary cores that do not reach run-away gas accretion can
thus migrate to orbits close to the host star if the water abundance is low.
Our results imply that hot and warm super-Earths found in exoplanet surveys
could have formed beyond the ice line and thus contain a significant fraction
in water. These water-rich super-Earths should orbit primarily around stars
with a low oxygen abundance, where a low oxygen abundance is caused by either a
low water-to-silicate ratio or by overall low metallicity.Comment: 15 pages, 12 figures, accepted by A&
The Nasdaq crash of April 2000: Yet another example of log-periodicity in a speculative bubble ending in a crash
The Nasdaq Composite fell another on Friday the 14'th of April
2000 signaling the end of a remarkable speculative high-tech bubble starting in
spring 1997. The closing of the Nasdaq Composite at 3321 corresponds to a total
loss of over 35% since its all-time high of 5133 on the 10'th of March 2000.
Similarities to the speculative bubble preceding the infamous crash of October
1929 are quite striking: the belief in what was coined a ``New Economy'' both
in 1929 and presently made share-prices of companies with three digits
price-earning ratios soar. Furthermore, we show that the largest draw downs of
the Nasdaq are outliers with a confidence level better than 99% and that these
two speculative bubbles, as well as others, both nicely fit into the
quantitative framework proposed by the authors in a series of recent papers.Comment: 15 pages including 7 figures. Accepted in Eur. Phys. J. The revised
version contains significant parametric and non-parametric statistical tests
which establishes the outlier nature of the largest market events and
provides an objective definition of a cras
Critical Crashes
We argue that the word ``critical'' in the title is not purely literary.
Based on our and other previous work on nonlinear complex dynamical systems, we
summarize present evidence, on the Oct. 1929, Oct. 1987, Oct. 1987 Hong-Kong,
Aug. 1998 global market events and on the 1985 Forex event, for the hypothesis
advanced four years ago that stock market crashes are caused by the slow
buildup of long-range correlations between traders leading to a collapse of the
stock market in one critical instant. We qualify the log-periodic oscillations
using a novel non-parametric method that does not rely on any fit: the
corresponding log-periodogram exhibits a strong statistically significant peak
for all six crashes examined, pointing at approximately the same prefered
scaling ratio around 2.Comment: 7 pages, 5 figure
Critical Ruptures
The fracture of materials is a catastrophic phenomenon of considerable
technological and scientific importance. Here, we analysed experiments designed
for industrial applications in order to test the concept that, in heterogeneous
materials such as fiber composites, rocks, concrete under compression and
materials with large distributed residual stresses, rupture is a genuine
critical point, i.e. the culmination of a self-organization of damage and
cracking characterized by power law signatures. Specifically, we analyse the
acoustic emissions recorded during the pressurisation of spherical tanks of
kevlar or carbon fibers pre-impregnated in a resin matrix wrapped up around a
thin metallic liner (steel or titanium) fabricated and instrumented by
A\'erospatiale-Matra Inc. These experiments are performed as part of a routine
industrial procedure which tests the quality of the tanks prior to shipment and
varies in nature. We find that the seven acoustic emission recordings of seven
pressure tanks which was brought to rupture exhibit clear acceleration in
agreement with a power law ``divergence'' expected from the critical point
theory. In addition, we find strong evidence of log-periodic corrections that
quantify the intermittent succession of accelerating bursts and quiescent
phases of the acoustic emissions on the approach to rupture. An improved model
accounting for the cross-over from the non-critical to the critical region
close to the rupture point exhibits interesting predictive potential.Comment: 24 pages including 50 figure
Modeling the Stock Market prior to large crashes
We propose that the minimal requirements for a model of stock market price
fluctuations should comprise time asymmetry, robustness with respect to
connectivity between agents, ``bounded rationality'' and a probabilistic
description. We also compare extensively two previously proposed models of
log-periodic behavior of the stock market index prior to a large crash. We find
that the model which follows the above requirements outperforms the other with
a high statistical significance.Comment: 18 pages with 4 figures. Submitted to Eur.Phys.
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