Dynamic exponent in Extremal models of Pinning

The depinning transition of a front moving in a time-independent random potential is studied. The temporal development of the overall roughness w(L,t) of an initially flat front, $w(t)\propto t^\beta$, is the classical means to have access to the dynamic exponent. However, in the case of front propagation in quenched disorder via extremal dynamics, we show that the initial increase in front roughness implies an extra dependence over the system size which comes from the fact that the activity is essentially localized in a narrow region of space. We propose an analytic expression for the $\beta$ exponent and confirm this for different models (crack front propagation, Edwards-Wilkinson model in a quenched noise, ...).Comment: RevTex, 3 figures .ep

Musical chairs: a comment on the credit crisis.

Uncertainty –that is, a rise in unknown and immeasurable risk rather than the measurable risk that the financial sector specializes in managing– is at the heart of the recent liquidity crisis. The financial instruments and derivative structures underpinning the recent growth in credit markets are complex. Because of the rapid proliferation of these instruments, market participants cannot refer to a historical record to measure how these financial structures will behave during a time of stress. These two factors, complexity and lack of history, are the preconditions for rampant uncertainty. We explain how a rise in uncertainty can cause a liquidity crisis and discuss central bank policies in this context.

Strong light fields coax intramolecular reactions on femtosecond time scales

Energetic H$_2^+$ ions are formed as a result of intra-molecular rearrangement during fragmentation of linear alcohols (methanol, ethanol, propanol, hexanol, and dodecanol) induced by intense optical fields produced by 100 fs long, infrared, laser pulses of peak intensity 8$\times10^{15}$ W cm$^{-2}$. Polarization dependent measurements show, counterintuitively, that rearrangement is induced by the strong optical field within a single laser pulse, and that it occurs before Coulomb explosion of the field-ionized multiply charged alcohols

Psychoacoustic Test to Determine Sound Quality Metric Indicators of Rotorcraft Noise Annoyance

Noise certification metrics such as Effective Perceived Noise Level and Sound Exposure Level are used to ensure that helicopters meet regulations, but these metrics may not be good indicators of annoyance since noise complaints against helicopters persist. Sound quality (SQ) metrics, specifically fluctuation strength, tonality, impulsiveness, roughness, and sharpness, are explored to determine their relationship with annoyance. A psychoacoustic test was conducted at the NASA Langley Research Center Exterior Effects Room to assess annoyance to helicopter-like sounds over a range of SQ metric values. The amplitude, phase, and frequency of the AS350 helicopter main and tail rotor blade passage signal harmonics were manipulated to produce 105 unique helicopter-like sounds with prescribed values of SQ metrics. All sounds were set to roughly the same loudness level. These sounds were played to 40 subjects who rated each sound for annoyance. Analyses given in this paper point to which SQ metrics are important to the helicopter noise annoyance response

A Stochastic Description for Extremal Dynamics

We show that extremal dynamics is very well modelled by the "Linear Fractional Stable Motion" (LFSM), a stochastic process entirely defined by two exponents that take into account spatio-temporal correlations in the distribution of active sites. We demonstrate this numerically and analytically using well-known properties of the LFSM. Further, we use this correspondence to write an exact expressions for an n-point correlation function as well as an equation of fractional order for interface growth in extremal dynamics.Comment: 4 pages LaTex, 3 figures .ep