In the first of the series of Making Science, Tom Whipple, Science Editor at the Times, explores the strange history of a 'death ray’ that supposedly promised to change modern warfare forever. In 1924, engineer and inventor Harry Grindell Matthews claimed to have created a beam that could stop an engine, ignite gunpowder, and incapacitate enemy soldiers from up to four miles away. Harry Grindell Matthews never revealed how his technology worked and few had seen the ray in action. So was it true? Perhaps the science of electromagnetic spectrum holds the answer.