A ballistic gravimeter with dropping holographic grating

The principle of operation of a ballistic laser gravimeter based on a dropping holographic diffraction grating is described. The free-fall acceleration of the grating is determined from a change in the frequency of beats that arise during the interference of light beams diffracted on the hologram in the zeroth and first orders of diffraction. An experiment demonstrating this principle of measurement is described. The main distinctive features of the proposed gravimeter are simple design, compact size, and the possibility of using this device for analysis of high-frequency fluctuations in the gravitational-field strength.     

Differentiating information transfer and causal effect

The concepts of information transfer and causal effect have received much recent attention, yet often the two are not appropriately distinguished and certain measures have been suggested to be suitable for both. We discuss two existing measures, transfer entropy and information flow, which can be used separately to quantify information transfer and causal information flow respectively. We apply these measures to cellular automata on a local scale in space and time, in order to explicitly contrast them and emphasize the differences between information transfer and causality. We also describe the manner in which the measures are complementary, including the conditions under which they in fact converge. We show that causal information flow is a primary tool to describe the causal structure of a system, while information transfer can then be used to describe the emergent computation on that causal structure.