They are all spin-offs from technology developed for military use during the Second World War. It's been said that one effect of World War II was to condense twenty five years of technological progress into just five. In other words, our technology advanced in huge leaps and bounds, directly because of all the extra effort that people had to put in to fight the war. The radar of the Second World War used microwaves. It's these same microwaves that we use nowadays to heat our food up.
Nobody knows for sure.