For most of the 20th century, scientists assumed black holes were eternal. Once something fell in, it was gone forever, and the black hole just kept growing as more matter dropped into it. In 1974, the physicist Stephen Hawking turned that assumption upside down.
Using a combination of quantum physics and general relativity, Hawking showed that black holes aren’t quite as inescapable as they look. Pairs of particles can pop into existence near the event horizon. Sometimes one of the pair falls in, while the other escapes - taking a tiny amount of energy with it. Over time, the black hole slowly loses mass through this process, now called Hawking radiation.
The catch is that this evaporation is extremely slow. A black hole the mass of the Sun would take about 10^67 years to evaporate completely - almost unimaginably longer than the current age of the universe. Bigger ones take even longer. Smaller ones evaporate faster. In a far enough future, though, every black hole will be gone.
