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.