How Stephen Hawking transformed our understanding of black holes

An artist’s illustration of a supermassive black hole ever discovered. (Robin Dienel, courtesy of the Carnegie Institution for Science)

There's a lot we still don't know about black holes, but these light-gobbling behemoths would be even more mysterious if Stephen Hawking hadn't plumbed their inky depths.

For starters, the famed cosmologist, who died yesterday (March 14) at the age of 76, helped give more solid mathematical backing to the concept of black holes, the existence of which was predicted by Albert Einstein's 1915 theory of general relativity.

"Hawking actually proved some rigorous mathematical theorems about Einstein's equations for gravity that showed that, under quite general circumstances, there were places where the equations broke down — what are called singularities," said Tom Banks, a professor of physics and astronomy at Rutgers University-New Brunswick in New Jersey. "And, in particular, the region inside of a black hole is such a singularity." [Stephen Hawking: A Physics Icon Remembered in Photos]

But it was Hawking's investigation of black holes' nature that would prove revolutionary. Initially, his work suggested that a black hole could never get smaller — specifically, that the surface area of its spherical event horizon, the point beyond which nothing can escape, could never decrease.

Similarly, the second law of thermodynamics holds that the "entropy," or disorder, of a closed system can never go down. And, in the early 1970s, physicist Jacob Bekenstein explicitly connected the concepts, proposing that a black hole's entropy is linked to the area of its event horizon.

Hawking was originally skeptical of this idea, Banks said. After all, entropy and black holes didn't seem to go together: Black holes were supposed to radiate no energy of any sort — hence the name — and you can't have entropy without radiation.

But then Hawking crunched the numbers, in a way that nobody had ever done before.

"He then showed that, if you added quantum mechanics to the game, you could show that, in fact, black holes were not really black," Banks told Space.com. "They actually emitted radiation."

This radiation comes from "virtual particles," which are constantly popping into and out of existence in the bizarre quantum realm. They do so in matter-antimatter pairs, one of which has positive energy and the other negative energy.

Ordinarily, these pairs immediately annihilate each other. But if this pair popping occurred at the boundary of a black hole's event horizon, one particle could, theoretically, get gobbled up while the other rocketed off into space. If the negative-energy particle were eaten, the black hole's mass would shrink by a tiny amount, and the object would emit a minuscule bit of radiation.

Hawking worked out this idea in 1974, which is why this hypothesized black-hole light is known as Hawking radiation, or Hawking-Bekenstein radiation. Nobody has spotted such emissions yet, but most physicists believe the emissions exist. Therefore, they posit, all black holes will shrink away to nothingness eventually, after there's no matter left for them to scarf down. (This will happen on almost unimaginably long timescales for big black holes; some calculations suggest that the last of the supermassive monsters at the cores of galaxies won't die for another 10^100 years or so.)

Though unquestionably a genius, Hawking wasn't always right, and one of his high-profile mistakes concerned black holes. The cosmologist famously posited that the information carried by every particle — data characterizing its spin and mass, for example — hoovered up by a black hole would be lost when the black hole evaporated. [Stephen Hawking's Most Far-Out Ideas About Black Holes]

Most other physicists disagreed, and for good reason, Banks said.

"It leads to equations that are in massive contradiction with known experimental facts," he said. "There are certain types of idealized black holes that you can construct in string-theory models, and there, it's quite clear that there's no loss of information."

Instead, this information must seep back out into the universe via Hawking radiation before the black hole dissipates, most physicists think. Hawking eventually came around to this position, Banks said.

Hawking's black-hole work has also spurred physicists to rethink their understanding of the universe on a more general level, Banks said. Previously, physicists had assumed that entropy scales with a system's volume, so the entropy-area link that Hawking and Bekenstein established came as a big surprise.

"In a way, Hawking's observation led to a potential revolution in the way we model nature, period," Banks said. "Part of that hasn't been realized yet. We don't actually have such a theory that everybody agrees is correct, but it's sort of the big challenge that Hawking's work made."

Hawking inspired deep thought and reflection in more than just his fellow physicists and cosmologists, of course. For decades, laypeople around the world have marveled at the way Hawking fought through his debilitating motor neuron disease to make breakthrough discoveries and bring exciting research to the masses in his best-selling books.

"It was remarkable how resilient he was, and how determined he was," said Banks, who knew Hawking personally. "That was one of the most awe-inspiring parts of being around him."

Originally published on Space.com.