Despite 50 years of research on high-power microwaves, the US military has yet to produce a usable weapon. For some Pentagon officials, the demonstration in October 2007 must have seemed like a dream come true — an opportunity to blast reporters with a beam of energy that causes searing pain. The event in Quantico, Virginia, was to be a rare public showing for the US Air Force's Active Denial System: a prototype non-lethal crowd-control weapon that emits a beam of microwaves at 95 gigahertz. Radiation at that frequency penetrates less than half a millimetre into the skin, so the beam...