Trespassing on Einstein's Lawn (26 page)

My dad was right about Hawking radiation. The usual story went something like this. Thanks to quantum uncertainty, pairs of virtual particles and antiparticles are constantly popping out of the vacuum. Fleeting ghosts, they surface for an instant, then meet and annihilate, disappearing back into the seething quantum sea. Should such a pair happen to emerge near a black hole, they can be divided by the horizon. Unable to partake in their mutual annihilation, the particle outside the horizon escapes into space while its antiparticle partner falls toward the singularity. Alone, separated from its partner, the escaped virtual particle becomes real. To an observer outside the black hole, it appears that the horizon is radiating. Meanwhile, the negative energy of the antiparticle shrinks the black hole ever so slightly, so it loses mass and appears to slowly evaporate.

Particles, however, are really excitations of fields—quantum fields that, even in their lowest energy states, fluctuate around a mean value, the zero-point energy. A positive frequency fluctuation corresponds to the presence of a virtual particle, while a negative frequency fluctuation corresponds to a virtual antiparticle. But things get interesting when there's an event horizon.

In an infinite, unbounded space, frequencies of every possible wavelength are equally represented, so they cancel one another out, leaving behind what appears to be calm, empty space. But when you stick an event horizon into that space, everything changes. The vacuum is totally different depending on which side of the horizon you're on. The space outside the horizon is now finite and bounded. Its energy changes and, with it, everything else. New vacuum, new fields, new particles.

Horizons create particles by restructuring the vacuum
, I jotted in my notebook. And on further thought I added,
Like the Casimir effect?
The whole situation sounded familiar. In the Casimir effect, two parallel, uncharged metal plates hovering just microns apart get pushed together by a mysterious force. The force is really just the vacuum. Outside the plates, the vacuum's zero-point energy extends infinitely throughout space, so every possible wavelength of the field modes is present and accounted for. But inside the tiny gap between the plates, only certain wavelengths can fit. Waves only come in whole numbers—you can't have half a wave—so only wavelengths that can fit entirely within the space between the plates count. The plates restructure the vacuum, leaving the vacuum outside the plates different from the vacuum inside. The difference creates a force: the stronger vacuum outside pushes on the plates, and the weaker vacuum inside can't handle the pressure. It's like a child trying to hold a door shut while an entire army pushes it open. Physicists have watched this battle of the nothings play out in the lab, and it's true—the plates snap together like magnets, only there are no magnetic forces to be found. There's just nothing.

I had always thought the Casimir effect was amazing, because it made the inner workings of the vacuum visible to the naked eye. When you're talking about vacuum modes it all sounds very esoteric and theoretical, but when you have two plates crashing together like cymbals it suddenly becomes very real.

In the case of the black hole, it was becoming clear that, just like the metal plate, the event horizon restructures the vacuum. Stick an event horizon in some region of spacetime and it restricts the wavelengths of zero-point energy that can fit there. This changes the energy
of the vacuum and, with it, its fluctuations, spawning particles that wouldn't otherwise be there. Those are Hawking particles, and they come in every variety, from photons to quarks. They are particles born as shifts in the nothingness, carved out by the boundaries of space and time.

But when I thought about it I realized that it was kind of an insane analogy. I mean, a horizon is different from a metal plate in a major way: you can't sail right through a metal plate. But you can fall right through an event horizon. How could a horizon be real enough to restructure the vacuum but ephemeral enough to allow stuff to pass through?

That question kept me up at night for weeks. That and the scurrying of quantum rats. During the days, seated proudly at my new editorial desk at the magazine, I plowed ahead with my research, itching to find an answer. And I quickly realized that the most powerful research tool at my disposal was my official
New Scientist
email address. That thing worked wonders. When I couldn't figure something out on my own, I would just fire off a casual email to a physicist.
Oh, hello there, superfamous physicist, I'm an editor at
New Scientist
and I'm looking into a possible story about this randomly specific aspect of black hole mechanics or quantum field theory or whatnot, and I'd love to get a better handle on it—could I bother you to explain a few things?
I'd click “Send” and within a day or two I'd have detailed answers to whatever questions had been bugging me. It was like magic. Being a “freelance science journalist” with an AOL address had worked pretty well, but being an “editor” at a major science magazine was on a whole other level. Some days I could hardly believe my luck.

Little by little, I pieced together the strange properties of event horizons. Einstein had shown that in terms of reference frames, there are two types of observers: those in uniform, inertial motion and those who are accelerating. When there's a black hole involved it's pretty crucial to know which type of observer you are. The accelerated observer is the guy who manages to safely skirt the black hole's pull and remain outside the horizon. He's accelerating because as gravity tugs at him, he has to run faster and faster just to get the hell out of there. In my mind I named him Safe. An inertial observer isn't quite so lucky. He
falls in, plunging through the horizon and down into the dark depths toward whatever lurks in there. If you don't accelerate, you'll never outrun gravity. The inertial observer's fate is sealed. I named him Screwed.

According to Safe, the horizon has some rather extreme physical properties. It comes equipped with all the strange effects of relativity—near the horizon, light is stretched to huge proportions and time slows down so much that it grinds to a halt at the horizon's edge. Not only does time stop at the horizon, but space does, too. The horizon marks the edge of reality. And since its area is entropy, the horizon is hot—hot enough to vaporize anything that gets too close, leaving nothing but ashes scattered among the Hawking radiation.

Screwed, however, doesn't see any of this. According to him, the horizon doesn't even exist. Assuming the black hole is big enough, he sails right through without noticing a thing. He doesn't see light stretch, time slow, or space stop. He doesn't feel heat. He doesn't see Hawking radiation. Screwed sees nothing other than ordinary, empty space.

Two guys look at the exact same patch of universe and one sees empty space while the other sees particles? It was so bizarre, I could barely wrap my head around it. Something had gone wrong with reality. And suddenly I understood exactly what it was.

I had never realized how deep Hawking's discovery had been, either. I had always harbored suspicions that his fame had been over-hyped thanks to his illness—there's just something about a man speaking with a robot voice that makes it seem extraordinarily profound. But as the implications of Hawking radiation sank in, I realized that if anything, Hawking is totally underrated. I mean, everyone knows who he is, but how many people know what he did or, more important, why it matters? Particles can be observer-dependent. Particles aren't invariant.
Particles aren't ultimately real.

The particles of Hawking radiation are literal embodiments of the observer-dependence of the vacuum. In a flat, boundless space, all observers agree on the lowest energy state, the state devoid of particles, the vacuum—which is basically to say that all observers agree on what nothing is. An event horizon undermines that agreement. The horizon places a boundary on the space; it restructures the vacuum. But only accelerated observers see the boundary; inertial observers see
flat, boundless space. What looks like something to one observer looks like nothing to another.

At first glance, a black hole's event horizon doesn't seem to be observer-dependent. After all, a black hole is a concrete, localized object; there's one sitting in the center of our galaxy right now. It seems like we'd all agree on the objective, observer-independent existence of its horizon, but that's only because every one of us is walking comfortably in Safe's shoes. If we bothered to think about Screwed plunging into that omphalic abyss, we'd realize that the horizon doesn't exist for everyone—it just exists for enough of us to trick us into thinking of it as an objective feature of the world. Once we realize that it's not—that it's observer-dependent—we suddenly see that the Hawking particles, whose existence is chained to the horizon's, are ultimately observer-dependent, too.

My father was right: I had found my thesis. The philosophical reverberations of this thing could bust the Richter scale. Ever since the atomists in ancient Greece, particles had been considered the basic units of material reality—solid, objective, indisputable. Relativity taught us that observers might disagree on the location of a particle in space or time, but they'd all agree that there was a particle
somewhere.
And sure, quantum mechanics made particles fuzzier, but again their very existence remained safe and sound. The idea that different observers can disagree on whether or not a particle exists
at all
is far weirder than anything relativity or quantum theory alone ever cooked up. Particles are the so-called building blocks of reality—so if their very existence depends on whom you ask, what happens to reality?

I set to work on my thesis. My life was all Hawking radiation all the time—all day every day at the office, and all night every night in my Planck-scale flat. At work, I found ways to turn what I was researching into articles so that I could continue reading about horizons and entropy and particle ontology without raising any suspicion. At night, the tenuous turning of pages, the soft clicks of the keyboard, and the occasional rustle of an invisible rat served as a satisfying soundtrack to my quiet pursuit.

There were times when I wished my father was there: when I discovered an amazing fact, when I was tortured by a question I couldn't answer, when I was convinced that I was really on to something big. I came to know the sound of one hand high-fiving. But through frequent emails I kept him abreast of everything I was learning, and, faintly echoing through the dark sky over the Atlantic and down a sparsely lit cul-de-sac in Notting Hill, I could hear him cheering me on.

I could also hear his question echoing annoyingly in my mind:
Isn't black hole radiation a rare situation? Does it really apply to matter in general?

It was a fair point. Even my mother, who worries more than anyone I know, doesn't worry about black holes. If black holes are so far removed from daily life, did it even matter if Hawking particles weren't real? Were they nothing more than theoretical curiosities?

In the throes of my research I quickly discovered that black holes aren't the only source of event horizons. In fact, there was a far more prosaic source: acceleration. If an observer is accelerating, light from certain far corners of the universe will never, no matter how much time lapses, be able to reach him, so long as he keeps accelerating. This sounded hard to believe until I thought back to those spacetime diagrams my dad had drawn years earlier on his yellow legal pad. The path of a light beam through spacetime is a straight line, but the path of an accelerated observer is a curve. Just as some light beams are about to collide with the observer, he swerves off along his curved path, successfully avoiding the light which has no choice but to continue along its rigid trajectory. Thus, there's a whole region of the universe from which light will never reach the accelerated observer. A whole region that's fundamentally out of touch. Dark. Like a black hole.