Issues

Strong shock waves can create very hot plasma. What temperatures can shock waves produce? What physical phenomena become important with increasing shock speed? What is the present state of our understanding of shock waves, where do they occur in nature, and how strong a shock wave can now be produced in laboratory devices? These are the questions discussed below.

The fundamental constants of nature are so interrelated that a measurement affecting one affects them all. The author became interested when Millikan's oil‐drop value of the electron charge was different from the value given by x‐ray determination of crystal spacings. To assist in finding the true values, he invented a method for plotting various functions of the constants in a space of as many coordinates as there are constants. If all measurements are consistent, the plotted functions intersect in a point. When they do not intersect, one examines standard deviations, which correspond to thicknesses of surfaces, in an effort to find out what is wrong. In three decades, searches of this kind have reduced uncertainties in the constants from a fraction of a percent to, at most, tens of parts per million.

Theorists from East and West recently spent two months at the IAEA Trieste Center discussing particles and high energies. The author, a reader in theoretical physics at the University of Cambridge and a fellow of Trinity College, recalls here what was said of SU(6), relativity, “nearly conserved” parity, and broken symmetries.

In the two decades during which the US Army has conducted its annual frequency‐control symposia, electromagnetic oscillations from atoms in transition have joined mechanical vibrations of crystals as frequency standards. Masers, in which hydrogen, ammonia, or rubidium is the active medium, and cesium beams function alongside quartz oscillators. Current developments are turning both crystal and atomic standards into more accurate and easier‐to‐use devices.