NEVER trust the textbooks, even the ones written by great scientists. In his celebrated 1972 tome Gravitation and Cosmology, Nobel prizewinning physicist Steven Weinberg called the existence of black holes “very hypothetical”, writing that “there is no [black hole] in the gravitational field of any known object of the universe”. He was dead wrong. Radio astronomers had already been detecting signals from matter falling into black holes for decades without realising. Today we have lots of evidence that the sky is teeming with them.