Gravitational waves, black holes, and astronomy without light
On 25 September 2015 scientists at the LIGO experiment detected something that no human had ever seen before: a gravitational wave. This wave was emitted by two black holes that lived and died more than a billion years ago. Each of the black holes was around thirty times as massive as our own sun, and when they merged they gave out so much energy that they temporarily outshone every star in the observable Universe put together.