Triggered by observations in radio astronomy, scientists discovered extremely high-energy "quasi-stellar radio sources" - or quasars for short. How could this enormous amount of energy be explained, even though quasars are relatively small and far away? Einstein's general theory of relativity was a starting point with the possibility of exploding stars that collapse in a gravitational collapse and take surrounding matter and energy with them. The gravitational forces are so strong that even light can no longer escape. But how can we make the invisible measurable?