Mike Hutchings / Reuters

I have a confession to make. For a long time—years, really—I thought Stephen Hawking was overrated.

He was just so famous, an icon, and I found it hard to imagine that his contributions to physics were really proportional to his fame. There’s just something about a guy who speaks in a computer voice that automatically makes him sound like a genius. Like someone who knows things no mortal human could ever know.

Not that I was immune to his celebrity. I was fortunate to have met Hawking on a few occasions, at physics conferences I was attending as a journalist. Once, during a lecture, I found myself sitting directly behind him. I tried my best to pay attention to the speaker, but I was mesmerized by the words flashing across the computer screen mounted to the arm of Hawking’s wheelchair. Paralyzed by a motor neuron disease, Hawking had one last functioning muscle in his cheek, and by twitching it he could control the cursor on his monitor. The cursor constantly scrolled though a catalog of his most commonly used words, and with a properly timed twitch he could select one from the list, slowly and arduously building up sentences for that oracular voice to deliver. In his otherworldly presence, I couldn’t help thinking those sentences contained the answers to the universe.

Later I learned about Hawking’s work. I learned that in the 1970s he had performed a remarkable calculation, in an attempt to disprove the work of another physicist who had annoyed him. The result ended up proving three things. First, that revenge is an excellent fuel for genius. Second, that I was a moron, because if anything, Hawking is underrated. His physics was brilliant and the only thing that was disproportionate was the fact that everyone has heard of Stephen Hawking but few people know what he really did that was so great. Something about black holes? The universe? Time? The profound meaning of his work is all too often overlooked. The meaning, that is, of the third thing he proved: that particles are not ultimately real.

Physicists define something as “real”—truly, ultimately, fundamentally real—if it remains invariant across reference frames. If that sounds abstract, it’s not—in fact, it’s how normal people define reality, too. Say you suddenly see a purple elephant in the corner of the room. You might wonder whether the elephant is really there or if you’re having some kind of breakdown. Instinctively, you know there are two ways to find out. The first is to get up, walk over to the elephant and tread a careful circle around it, viewing it from every angle, eyeing it suspiciously. If at some angle it disappears, you’ll know it was more likely a mirage than a mammal. The other strategy is to turn to the guy next to you and ask, “Do you see an elephant?” If he says no (or stares at you blankly), you’ll probably want to call a neurologist. Because you know, intuitively, that something is only real if it persists in every point of view.

Just because something’s not ultimately real doesn’t mean it’s a hallucination. Take a rainbow. Is it real? Not really. It’s not a hallucination, but it’s also not a physical object hanging in the sky. You can’t go touch it because it’s a product of your reference frame, a lucky confluence of circumstance, your standing in the right place at the right time with the sun streaming in from behind you and the light being refracted by the moisture in the air. Ask the guy next to you, “Do you see that rainbow?” and he’ll probably say yes, but run the test of walking around it, and you’ll see it disappear. Its existence is dependent on your reference frame. It’s a product of physics, but it’s not invariant. If you want to find the fundamental ingredients of ultimate reality, you have to find the invariants.

Particles always seemed like good candidates. After all, they comprise all the stuff in the universe. They give things heft and solidity and object-hood. They’re the reason there are things at all.

But Hawking’s calculation suggested otherwise.

To appreciate what Hawking did, there’s one more thing you need to know. According to quantum mechanics, empty space isn’t really empty. The so-called uncertainty principle tells us that there’s a trade-off between time and energy—the more defined the one, the vaguer the other. That means that on very short timescales—fractions of fractions of fractions of seconds—large amounts of energy can (and do) bubble up out of empty space. To ensure it’s all paid back in full, the energy manifests as pairs of particles and antiparticles, which, in the blink of an eye, will collide and annihilate, the existence of one canceling out the existence of the other, returning the energy back to the emptiness from whence it came. This cycle of creation and destruction is happening all the time, right now, all around us, particles emerging in pairs and disappearing, but it all happens so quickly that we call them “virtual particles”—not because they’re fundamentally different from ordinary particles, but because they don’t stick around long enough to count.

Hawking realized that something different—something profound—happens when virtual particle pairs arise in the presence of an event horizon. An event horizon marks an edge beyond which light can’t reach an observer, rendering the far side of the horizon fatefully dark. A black hole, for instance, is surrounded by an event horizon, the edge beyond which light can’t escape gravity’s clutches to reach an external observer.

When a pair of virtual particles bubble up out of empty space near a horizon, something extraordinary happens. The horizon can separate the pairs, so that while one particle travels out into the universe, its partner falls behind the horizon into the black hole. Now they can’t annihilate, so instead of disappearing, they just stick around. The virtual particles are no longer virtual. They’re real—as real as any other particle. You could collect a bunch of them and build a chair.

But there would be something really weird about that chair. It would owe its very existence to the horizon—but the horizon is not like a brick wall sitting in space, blocking the light. A horizon is like a rainbow. It’s a feature of certain reference frames, namely the reference frame of the observer who is lucky enough not to fall into the black hole. Most textbooks call him Bob, but I like to call him Safe. There’s another kind of an observer who is not so lucky. An observer in inertial free fall cannot escape the black hole’s gravity; he falls straight through the horizon into dark. I like to call him Screwed.

From Safe’s point of view, particles that can no longer be annihilated are streaming out from the horizon, as if the black hole is radiating. But for Screwed, the horizon doesn’t exist. He falls straight through it. And without a horizon to separate them, the virtual particles and antiparticles, from Screwed’s point of view, continue to annihilate as usual, so that where Safe sees a stream of particles, Screwed sees nothing but empty space.

That difference in what these two observers see means everything. It means that the particles are not invariant. Hawking’s discovery was as radical as it was monumental: Particles aren’t ultimately real. He showed that the very meaning of a particle, its existence, depends on your reference frame, not only in the vicinity of a black hole but everywhere, because every one of us is surrounded by a horizon.

We live in a universe that’s expanding at an accelerated rate, faraway galaxies being pushed out of our field of vision at speeds proportional to their distance. Their light tries to reach us, but space-time just keeps growing, preventing the light from covering any ground. Far from here, the space-time grows faster than the light, trapping it there, a light beam on a treadmill; it will never reach us, no matter how long we wait. The boundary separating the light that will reach us from the light that won’t is an event horizon, precisely of the kind you’d find around a black hole. Because each of us occupies a unique point in space, we each have our own unique horizon. Technically speaking, every one of us is Screwed. Not to be morbid, but all the light in the universe will eventually be swept away by the expansion of space-time, and we’ll be left here in the dark, our Milky Way a lonely beacon in a swelling, spreading nothing. Of course, if we were out there, in one of those distant galaxies falling off the edge of the universe, everything would seem just fine. From their perspective, we’d be the ones exiting the observable universe at light speed, and they’d be left alone in the void.

The point is, every one of us lives in a region of space-time delineated by a cosmic horizon, and that horizon defines what we mean by a particle, and whether or not one exists, not only out there at the horizon’s edge, but here right in front of us, too.

When Hawking set out to do his calculation, his disease had already made it impossible for him to write out long equations by hand, and he was forced to do it all in his head. That inspired him to think about things in a totally new way—to think not in numbers but in shapes. In geometry. He saw, in his mind’s eye, how event horizons affect the entire space-time they bind—he saw how they determine the symmetries of the whole space-time, which in turn determine what we mean by a vacuum and what we mean by a particle. He saw this grand, global picture, which changed everything, and he attributed it, in part, to his illness, which he once said was the best thing that ever happened to him.

It’s no exaggeration to say that Hawking’s discovery has driven, and continues to drive, theoretical physics forward for the last four decades. It’s because the three great pillars of physics—general relativity, quantum mechanics and thermodynamics—all collide in his brilliant calculation, pointing the way toward some deeper unified theory. And it’s because it’s rife with paradoxes, and there’s nothing better for physics than a paradox forcing you to question your most basic assumptions about the world. Finally, it’s because Hawking’s discovery that the very building blocks of that world, of everything we see around us, of us, are not ultimately real has forced us to ask, well, what is? The truth is, no one knows. The search for reality continues, but it was Hawking who blazed the trail. He—and his physics—deserve to be better known.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.