Late in 2018, the gravitational wave observatory, LIGO, announced that they had detected the most distant and massive source of ripples of spacetime ever monitored: waves triggered by pairs of black holes colliding in deep space.
Only since 2015 have we been able to observe these invisible astronomical bodies, which can be detected only by their gravitational attraction. The history of our hunt for these enigmatic objects traces back to the 18th century, but the crucial phase took place in a suitably dark period of human history – World War II.
The concept of a body that would trap light, thereby becoming invisible to the rest of the universe, had first been considered by the natural philosophers John Michell and later Pierre-Simon Laplace in the 18th century. They used Newton’s gravitational laws to calculate the escape velocity of a light particle from a body, predicting the existence of stars so dense that light could not escape from them. Michell called them “dark stars”.