The Story of Black Holes — Stephen Hawking and the Star That Swallows Light

A place not even light can escape

A black hole is an object where an enormous mass is compressed into a tiny space, making its gravity unimaginably strong. So powerful is that gravity that even light, the fastest thing in the universe, can never return once it is drawn in. The boundary beyond which light can no longer escape is called the "event horizon," and whatever happens inside it can send no signal at all to the outside universe.

How black holes are born

When a star far more massive than the Sun explodes as a supernova at the end of its life, its leftover core cannot withstand its own gravity and collapses endlessly to a single point, becoming a "stellar-mass black hole." At the center of a galaxy, meanwhile, sits a "supermassive black hole" millions to billions of times the mass of the Sun — and at the center of our own Milky Way hides "Sagittarius A*," about 4 million times the Sun's mass.

A black hole itself is dark and invisible, but gas spiraling inward circles the event horizon and heats to millions of degrees, giving off intense light and X-rays. The light of this "accretion disk," and the motion of stars orbiting seemingly empty space, are how we confirm a black hole's existence. In 2019 the Event Horizon Telescope (EHT) captured the first-ever image of the dark shadow of a black hole ringed by such a disk.

Stephen Hawking and the "shining black hole"

The British physicist Stephen Hawking (1942–2018) was diagnosed at age 21 with ALS (Lou Gehrig's disease), which gradually stiffens all the body's muscles, and was told he had only a few years to live. Yet, relying on a wheelchair and a voice synthesizer, he lived half a century more and became the foremost theoretical physicist leading black hole research in the late 20th century.

Hawking's most famous discovery was that "black holes are not entirely black after all." In 1974, applying quantum mechanics, he showed theoretically that when particles and antiparticles are ceaselessly created just outside the event horizon and one of them escapes, a black hole gives off a very faint light and heat. Today this phenomenon is called "Hawking radiation."

Black holes evaporate, too

The startling conclusion of Hawking radiation is that black holes are not eternal. It means that, losing energy little by little, a black hole "evaporates" and, given infinite time, eventually disappears. The rate, however, is unbelievably slow: for a single large black hole to evaporate takes far longer than the current age of the universe.

This discovery became an important clue toward uniting gravity (general relativity) and quantum mechanics — two branches of physics that had never fit together. Through his popular book "A Brief History of Time," Hawking brought the mysteries of the universe to a wide audience, and he left us the words: "We are just an advanced breed of monkeys on a minor planet of a very average star. But we can understand the universe. That makes us something very special."