Models of stress fluctuations in granular media

We investigate in detail two models describing how stresses propagate and fluctuate in granular media. The first one is a scalar model where only the vertical component of the stress tensor is considered. In the continuum limit, this model is equivalent to a diffusion equation (where the r\^ole of time is played by the vertical coordinate) plus a randomly varying convection term. We calculate the response and correlation function of this model, and discuss several properties, in particular related to the stress distribution function. We then turn to the tensorial model, where the basic starting point is a wave equation which, in the absence of disorder, leads to a ray-like propagation of stress. In the presence of disorder, the rays acquire a diffusive width and the angle of propagation is shifted. A striking feature is that the response function becomes negative, which suggests that the contact network is mechanically unstable to very weak perturbations. The stress correlation function reveals characteristic features related to the ray-like propagation, which are absent in the scalar description. Our analytical calculations are confirmed and extended by a numerical analysis of the stochastic wave equation.Comment: 32 pages, latex, 18 figures and 6 diagram

Switching dynamics between metastable ordered magnetic state and nonmagnetic ground state - A possible mechanism for photoinduced ferromagnetism -

By studying the dynamics of the metastable magnetization of a statistical mechanical model we propose a switching mechanism of photoinduced magnetization. The equilibrium and nonequilibrium properties of the Blume-Capel (BC) model, which is a typical model exhibiting metastability, are studied by mean field theory and Monte Carlo simulation. We demonstrate reversible changes of magnetization in a sequence of changes of system parameters, which would model the reversible photoinduced magnetization. Implications of the calculated results are discussed in relation to the recent experimental results for prussian blue analogs.Comment: 12 pages, 13 figure

Histogram Monte Carlo study of next-nearest-neighbor Ising antiferromagnet on a stacked triangular lattice

Critical properties of the Ising model on a stacked triangular lattice, with antiferromagnetic first and second-neighbor in-plane interactions, are studied by extensive histogram Monte Carlo simulations. The results, in conjunction with the recently determined phase diagram, strongly suggest that the transition from the period-3 ordered state to the paramagnetic phase remains in the xy universality class. This conclusion is in contrast with a previous suggestion of mean-field tricritical behavior.Comment: 13 pages (RevTex 3.0), 10 figures available upon request, CRPS-93-0

Collapse to Black Holes in Brans-Dicke Theory: II. Comparison with General Relativity

We discuss a number of long-standing theoretical questions about collapse to black holes in the Brans-Dicke theory of gravitation. Using a new numerical code, we show that Oppenheimer-Snyder collapse in this theory produces black holes that are identical to those of general relativity in final equilibrium, but are quite different from those of general relativity during dynamical evolution. We find that there are epochs during which the apparent horizon of such a black hole passes {\it outside\/} the event horizon, and that the surface area of the event horizon {\it decreases\/} with time. This behavior is possible because theorems which prove otherwise assume $R_{ab}l^al^b \ge 0$ for all null vectors $l^a$. We show that dynamical spacetimes in Brans-Dicke theory can violate this inequality, even in vacuum, for any value of $\omega$.Comment: 24 pages including figures, uuencoded gz-compressed postscript, Submitted to Phys Rev

Notes on the Cuban Revolution

I recall standing high on the tribune in the Plaza of the Revolution on January 2, 1961 just as I did twenty-eight years later and straining my neck unsuccessfully to see the end of the crowd. And Fidel. with five microphones, the number he still uses to ensure that his words will reach even the far end of the rally, denouncing the imperialists with the same fervour and some of the same language as he used on December 5, 1988. So much in Cuba has changed in thirty revolutionary years. Fidel at 62 and gray in hair and beard no longer shows that spontaneous grin of wonder. The casinos, beggars, sex shows and aura of free and dirty third world capitalism remain only in the film archives, captured on celluloid. So much remains the same, not just the same as before the revolution, but the same as the 19th Century, and even before. Cuba is Spain and Africa, the old world and the new. It is U.S. gangsters and gamblers, baseball players and novelists. A little of Cuba is the Soviet Union, much of it is Caribbean and Latin American. It is culturally diverse and provincial. It is also, in Fidel's mind, the last bastion of socialism left in the world. The visitor sees the superficial, members of the Cuban militia - more than a million strong throughout the nation - fastidiously dressed, occupying the front position of the crowd. the crack troops, including the Special Forces marching by with precision movements, as they do every five years. Under thousands of the neatly pressed military blouses militia members wear santerioz beads

The Cuba Conundrum

What should Progressives Think and Do

Borders: The New Berlin Walls

Current frontiers, the encumbrances to workers' travel, derive not only from historic settlements, wars and treaties, but also from ideologies and sentiments of racism and nationalism, constructs that distinguished a worker or petty merchant in one country from his counterpart in another. Ideologies that divided workers or prevented competition at one historical period, continue to function in the 'new global village', frustrating the creation of a global and mobile labour force, which would allow capital to reduce further the socially necessary cost of labour. Even as improved transportation allowed for increased mobility, social attitudes and laws obstructed people of darker skin or different customs from working inside national borders. Over centuries, in the age of empire, a virtual pigmentocracy was established and it set the standards and rules of residence and citizenship within the North American and West European nations - with implicit or explicit bias against people from the South and the East. Eurocentric axioms continue to provoke fear among members of the working and middle classes that hordes of 'uncivilized' immigrants will pollute, dilute or in some form pervert the direct line of western culture that runs from Greeks and Romans through Europe of the Reformation and Enlightenment and into the present; all held together by a White Christ and Moses that people of darker hues cannot truly share