Can a Turing Player identify itself?

We show that the problem of whether two Turing Machines are functionally equivalent is undecidable and explain why this is significant for the theory of repeated play and evolution

Tortuosity of lightning return stroke channels

Data obtained from photographs of lightning are presented on the tortuosity of return stroke channels. The data were obtained by making piecewise linear fits to the channels, and recording the cartesian coordinates of the ends of each linear segment. The mean change between ends of the segments was nearly zero in the horizontal direction and was about eight meters in the vertical direction. Histograms of these changes are presented. These data were used to create model lightning channels and to predict the electric fields radiated during return strokes. This was done using a computer generated random walk in which linear segments were placed end-to-end to form a piecewise linear representation of the channel. The computer selected random numbers for the ends of the segments assuming a normal distribution with the measured statistics. Once the channels were simulated, the electric fields radiated during a return stroke were predicted using a transmission line model on each segment. It was found that realistic channels are obtained with this procedure, but only if the model includes two scales of tortuosity: fine scale irregularities corresponding to the local channel tortuosity which are superimposed on large scale horizontal drifts. The two scales of tortuosity are also necessary to obtain agreement between the electric fields computed mathematically from the simulated channels and the electric fields radiated from real return strokes. Without large scale drifts, the computed electric fields do not have the undulations characteristics of the data

An empirical analysis of the welfare magnet debate using the NLSY

This paper examines the extent to which differences in welfare generosity across states lead to interstate migration. Using microdata from the National Longitudinal Survey of Youth between 1979 and 1992, we employ a quasi-experimental design that utilizes the categorical eligibility of the welfare system. The "treatment" group consists of all those in the survey who appear eligible to participate in Aid to Families with Dependent Children. The "control" group contains those who are poor but ineligible for other reasons. The pattern of cross-state moves among poor single women with children who are likely to be eligible for benefits (treatment-group members) is compared to the pattern among other poor households. We find little evidence indicating that welfare-induced migration is a widespread phenomenon.

The intergenerational correlation in AFDC participation: Welfare trap or poverty trap?

Several recent studies have shown that daughters whose mothers have participated in the welfare program Aid to Families with Dependent Children (AFDC), are themselves more likely to participate in AFDC when they head their own household. Other studies have shown that the earnings of parents and their children are highly correlated across generations. This suggests that any variable correlated with income such as AFDC participation will also be correlated across generations. This paper uses data from the original and youth cohorts of the National Longitudinal Surveys to investigate the question of whether the link in mother-daughter welfare participation is a causal relationship, or whether it can be explained by the expected intergenerational correlation in earnings. Several reduced-form probit equations are estimated, and attention is directed to the potential endogeneity of key explanatory variables. The empirical findings suggest that much of the observed correlation in AFDC participation across generations can be explained by the intergenerational correlation of income and other family characteristics.

Non-equilibrium mechanics and dynamics of motor activated gels

The mechanics of cells is strongly affected by molecular motors that generate forces in the cellular cytoskeleton. We develop a model for cytoskeletal networks driven out of equilibrium by molecular motors exerting transient contractile stresses. Using this model we show how motor activity can dramatically increase the network's bulk elastic moduli. We also show how motor binding kinetics naturally leads to enhanced low-frequency stress fluctuations that result in non-equilibrium diffusive motion within an elastic network, as seen in recent \emph{in vitro} and \emph{in vivo} experiments.Comment: 21 pages, 8 figure

Estimation of Kalman filter model parameters from an ensemble of tests

A methodology for estimating initial mean and covariance parameters in a Kalman filter model from an ensemble of nonidentical tests is presented. In addition, the problem of estimating time constants and process noise levels is addressed. Practical problems such as developing and validating inertial instrument error models from laboratory test data or developing error models of individual phases of a test are generally considered

Detecting many-body entanglements in noninteracting ultracold atomic fermi gases

We explore the possibility of detecting many-body entanglement using time-of-flight (TOF) momentum correlations in ultracold atomic fermi gases. In analogy to the vacuum correlations responsible for Bekenstein-Hawking black hole entropy, a partitioned atomic gas will exhibit particle-hole correlations responsible for entanglement entropy. The signature of these momentum correlations might be detected by a sensitive TOF type experiment.Comment: 5 pages, 5 figures, fixed axes labels on figs. 3 and 5, added reference

The structure of lightning flashes HF-UHF: 12 September 1975, Atlanta, Georgia

Simultaneous measurement of sferics at 3, 30, 139, and 295 MHz were made during thunderstorms. Wideband electronics and an analogue tape recorder continuously recorded the radiation from lightning with about 300 kHz of bandwidth. The data were obtained during the passage of a cold front. Flashing rate, burst rate and the structure of individual flashes were recorded. The record of a typical flash begins with a sudden burst of closely spaced pulses whose temporal structure is typical of the stepped leader, and ends in a large pulse suggestive of a first return stroke. The remainder of the flash consists of a sequence of pulses of varying amplitude separated by quiet periods of the order of milliseconds. The shape of these pulses and the temporal structure suggest that the first few large pulses are return strokes. Other discharges begin with widely spaced discrete pulses and resemble the preceding discharge less the leader and return stroke phase. The radiation exhibits a similar structure, at each of the frequencies monitored

Unfolding cross-linkers as rheology regulators in F-actin networks

We report on the nonlinear mechanical properties of a statistically homogeneous, isotropic semiflexible network cross-linked by polymers containing numerous small unfolding domains, such as the ubiquitous F-actin cross-linker Filamin. We show that the inclusion of such proteins has a dramatic effect on the large strain behavior of the network. Beyond a strain threshold, which depends on network density, the unfolding of protein domains leads to bulk shear softening. Past this critical strain, the network spontaneously organizes itself so that an appreciable fraction of the Filamin cross-linkers are at the threshold of domain unfolding. We discuss via a simple mean-field model the cause of this network organization and suggest that it may be the source of power-law relaxation observed in in vitro and in intracellular microrheology experiments. We present data which fully justifies our model for a simplified network architecture.Comment: 11 pages, 4 figures. to appear in Physical Review