Hawking radiation was a big step in bringing together quantum mechanics and relativity
Stephen Hawking’s biggest accomplishment shone through both his physics work and his outreach to the public, bridging gaps between things that once seemed incompatible. Whether he was demonstrating that black holes did indeed radiate material or that popular culture could embrace the mysteries of the Universe, he had a way of making seemingly impossible things connect.
Before Hawking, black holes were considered the Universe’s most mysterious garbage collectors. It was once believed that nothing could escape the immense gravitational pull of one of these objects; they’re so dense that they even pull in light. But Hawking found that, in fact, something does escape a black hole: radiation. Thanks to his work, we now know that black holes aren’t even totally black. (They actually have a faint glow about them from the small amount of energy they radiate.) The equation Hawking came up with to explain how this phenomenon works became his most notable achievement, one that’s named for him: Hawking radiation. “He came up with the idea that black holes have a temperature,” Jonathan McDowell, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, tells The Verge.
Though his work upended what was thought to be a fundamental truth of black holes, Hawking radiation actually did some reconciliation. His work connected two conflicting concepts in theoretical physics: quantum mechanics and the theory of relativity. “Those are the two pillars on which physics now rests, but they’re really quite incompatible to one another,” Raphael Bousso, a theoretical physicist at UC Berkely who was once a student of Hawking’s, tells The Verge.
Quantum mechanics is all about how the Universe works at the smallest level — how teeny particles like electrons and positrons move and connect. If you want to know how atoms stick together, for instance, quantum mechanics has your back. On the other side of the scale is general relativity, which explains how gravity works. It’s the idea that large objects of the Universe — like planets, stars, and galaxies — actually bend the space and time around them. And that dictates how these objects interact with one another in space.
The two theories are both strong, backed up by lots of science and observation. But they seem to be in conflict, never truly fitting no matter how hard people try. And that’s a problem for physicists because they’re all about simplicity. “We want to be able to describe more and more phenomena with fewer fundamental ingredients,” Bousso says.
But Hawking found a way to bring big and small together. He looked at what happens around a really massive object — one with lots of gravity — on a very small scale. Specifically, he analyzed how particles are interacting at the edge of a black hole, known as the event horizon. This boundary is often referred to as the “point of no return.” Once you cross this line, you’re going into the black hole no matter what — unless you’ve figured out a way to travel faster than the speed of light. (Spoiler: you can’t.)
Here’s where we get small: according to quantum mechanics, the vacuum of space isn’t really empty. Instead, virtual particles are popping in and out of existence in the vacuum all the time. These particles pop up tangled together as opposite pairs: one particle with positive energy and one negative, a yin and a yang. What happens to one affects the other. Typically, the pair will come together quickly and cancel each other out. But if they form on the event horizon, that’s not what happens. Things start to get weird.
A particle pair that straddles the event horizon will be wrenched apart. The black hole sucks in the one particle with negative energy while the positive particle is flung away from the black hole. That escaping particle becomes the Hawking radiation, heated up by its escape. The doomed particle becomes part of the black hole. But since it has negative energy, it actually makes the black hole slightly smaller.
If you left a black hole alone, this process would go on for billions and billions of years. Eventually, the black hole would waste away — and then, because black holes are weird, explode. How big is the explosion? “Fairly small by astronomical standards,” Hawking wrote. But it’s still pretty damn big: about the size of 1 million one-megaton hydrogen bombs.
Of course, many black holes are usually surrounded by material that is constantly falling into them. But Hawking showed it was theoretically possible for a black hole to disappear over time in the right conditions. “Black holes won’t last forever,” says McDowell. “Long after all the other stars have died out, the black holes will be glowing and eventually blow up.”
This idea upended physics when it was published in 1974. But it also solved a huge puzzle: if nothing ever escapes from a black hole, that means they’re the Universe’s clean-up crew, eating material that never comes back. But that just didn’t make sense with other physics. There’s a law of thermodynamics that says that the randomness and chaos of a system — known as entropy — cannot decrease over time; our messy Universe can’t get cleaner. So how was it possible that black holes were vacuuming up the trash? Hawking’s discovery demonstrated that black holes don’t violate that law of thermodynamics: by emitting radiation, they are also keeping things chaotic. “[He] wasn’t trying to address this puzzle with thermodynamics. It just turned out to be exactly what was needed,” says Ted Jacobson, a theoretical physicist at the University of Maryland.
Hawking radiation didn’t completely solve everything, though. (What does?) It provided an important first step in bridging quantum mechanics and gravity. There are still a lot of things about big and small physics that have yet to be reconciled. Hawking radiation was just one way the ideas could work together.
Hawking radiation opened up some major questions, too. In quantum physics, a particle recipe — the orientation, mass, spin, and other traits of particles — is called information. That information sticks around. When you burn a piece of paper, for instance, the information of what was in that paper is contained in fire, smoke, and ash. If you wanted, you could put the paper back together because you had all the information from it. But Hawking radiation introduced a new conundrum: if black holes are losing mass, where does all their information go?
A black hole’s information is slowly disappearing when it wastes away — and that’s just not supposed to happen! The radiation that the black hole emits doesn’t actually contain information from the black hole, so it seems like all the details are disappearing along the way. “The information should not be completely lost, but in this process, it would be,” Katie Mack, a theoretical astrophysicist at North Carolina State University who is working on a book about the end of the Universe, tells The Verge. It’s called the black hole information paradox, and people have come up with tentative solutions for it, including Hawking himself. But it’s still not completely solved.
Still, what Hawking did was to take a huge step toward a unified theory of physics — a theory of everything, as the recent movie based on his life is titled. We still aren’t totally to that point, but Hawking started building the bridge. And he continued to work on that bridge up until he died. “He gave us problems to work on and directions to go with them… If we solve those problems, we’ll have a better understanding of which fundamental laws truly govern reality,” says Mack.
Hawking’s knack for connection extended beyond just physics. Black holes seem abstract, but he found ways to bring them to everyone. He made people excited about what’s out there. He was media savvy, appearing on The Simpsons, Star Trek, and more. He also wrote popular books about his work. So, in the same way that he connected the big and the small, he also brought people into the cosmological fold, inspiring new generations of scientists to continue learning more about the weirdness of space.