Electromagnetic radiation is everywhere. But nobody knew it until 1864, when a British scientist called James Maxwell saw the light - the only visible kind - and proposed the existence of invisible varieties.
Fourteen years later, Heinrich Hertz showed Maxwell's hunch to be correct, and gave his name to the system we still use for measuring their frequency. Gamma rays, X-rays, infrared and ultra-violet light - even microwaves - have sparked all sorts of technological innovations. But perhaps none has changed our lives quite so profoundly as radio waves.
Distant stars, pulsars and quasars give off radio waves, and modern radio telescopes have enabled us to map the universe. But 100 years ago, a much simpler invention was making waves. When Guglielmo Marconi sent a signal across the Atlantic in 1901 ("serenely ignoring the curvature of the Earth", as he put it), he proved that radio waves didn't travel in straight lines, and started a communications revolution.
Unlike telegraphs, radio transmitters were, of course, wire-less, which meant Morse code signals could be sent and received at sea, providing a lifeline for shipping - the Titanic's 700 survivors presented Marconi with a golden medal in thanks. The first radio sttion - KDKA in Pittsburgh, Pennsylvania - was set up soon afterwards in 1920. Two years later, the BBC was born, but early radio announcers, clearly in awe of the new invention, were warned: "Do not sneeze into the microphone. If you cough, you will deafen thousands."
Until television arrived, radio was our number one means of mass communication. Families would huddle round sets during the Second World War, listening to news from the front and Churchill's stirring speeches. Hitler's spin doctor, Goebbels, used it as a propaganda machine, sending out the infamous Lord Haw-Haw broadcasts. And Orson Welles's deadpan announcement that Earth was being invaded by Martians, as part of a production of H G Wells's War of the Worlds, prompted panic evacuations across the United States.
Cheap and portable, simple to build and easy to operate, the radio has been loved by truckers on the citizens' band, hospitals and amateur hams. Pirate stations have sailed its airwaves and revolutionaries have seized its stations. It's the home of the afternoon play, rolling news and ball-by-ball commentary. Music lovers couldn't live without it - think of all the songs about it. And remember where you heard them first.