black hole

A black hole is a region of space where matter has collapsed so tightly that the curvature of spacetime becomes overwhelming. Inside a boundary called the event horizon, every possible path forward in time leads deeper in, so nothing can climb back out, not even light. From the outside it looks utterly black, because no signal it swallows is ever returned.

Think of spacetime as a stretched rubber sheet and mass as a weight pressing on it. A modest weight makes a gentle dip; a black hole is a weight so heavy the sheet folds into a bottomless funnel, and once you slide past the rim there is no slope steep enough to climb. The 'escape speed' at the horizon equals the speed of light, and since nothing local outruns light, escape is impossible.

Black holes are not science fiction. They form when massive stars run out of fuel and collapse, and giant ones millions to billions of times the Sun's mass sit at the centres of galaxies. We see them indirectly: by stars whipping around an invisible companion, by gas glowing fiercely as it spirals in, and in 2019 and 2022 by direct images of the dark shadow cast on surrounding light.