Chapter 1
 
Entrance
 
Black holes are nothing.
 
Black holes are special because there's nothing there. There is no thing there.
 
I probably accepted black holes whole, as complete conceptual entities, before I was able to doubt, before I had intuition to combat. They were fodder for fantasy. I received the fact of their existence without resistance. Gullible and without prejudice, I could see the plausibility, appreciate the curiosity, their peculiarity, accept the universe as it was presented. Maybe it was the same for you. It's very unlikely that this is your first encounter with the astrophysical oddity that is a black hole, the warp in spacetime so strong that not even light can escape.
 
I don't know what it was like where you were, but I had a pretty unspectacular view, astronomically speaking, out of my childhood bedroom. I would edge to the foot of the bed and strain to look at the sky through the window frame, a portal to a green yard below, the seams between neighboring plots demarcated by shrubs or trees, patchwork real estate scooped together under a rounded cap of atmosphere. The ground would darken first, then the trees, but the dome would hang on to the vaguest splattering of light for the longest. Ours was not an urban burnished sky, but still light pollution diluted the view that I stared into night after night. I never expected the vista to improve or expected anything new, only the usual indistinct few bright speckles, like rain stains on a windshield.
 
I don't remember when the feeling started, the first time that longing tugged at me, but before I even recognized the desire, I pined to know what else was out there, like a restless dog pacing at the door. I wanted to be free of ordinary confines. To fly there and explore. Frustrated by the fact of the heaviness of my feet on the Earth, striding at the base of the sky, restless to be let in.
I wonder how many of us inherited this longing, millennium after millennium, generation after generation, child after child bound to the crust, rapt at the illusion of a ceiling, compelled to crack through and defy our puniness and our limitations.
 
As a kid, I never imagined I would become a scientist. If you had told me I would become a physicist, I'm sure I would have been offended. Scientists build bombs and memorize equations. I'm not sure if I acquired that cliché or if the stereotype was one of my own invention--that scientists could not be creative, or just weren't out of stubbornness. I didn't understand the profound freedom of constrained creativity, visionary originality that explodes in the face of fundamental limits.
 
Limits have incited revolutions. The limit of the speed of light hinted at relativity. Einstein fantasized about riding a light beam and imagined that time stood still. He gave up the absolutism of an inflexible space and time in favor of the absolutism of the speed of light. That concession forced us to reconceive of our universe as an impermanent, living process with an origin in a big bang, still expanding with the energy of creation, and home to extravagances like black holes.
 
The quantum revolution was incited in parallel by the limit imposed by Heisenberg's uncertainty principle, which asserts that particles as we thought we knew them do not exist. We are pressured to reimagine the fundamental nature of reality as an uncertain fog of possibilities, of particles that are there and not there. The pressure to yield to this seemingly terrible constraint guided us toward revelations that would otherwise have been unattainable. We rewrote reality in an astonishing new language. We have discovered quarks, photons, neutrinos, condensed matter, dead neutron stars, the Higgs boson, superconductors, and quantum computers in the most precisely tested paradigm in all of physics.
 
The computer revolution too was incited by a limit, an insurmountable limit to mathematical knowledge. The incompleteness theorems, which proved that there were mathematical facts that could never be proven, led Alan Turing to dream of artificial intelligence and biological machines. Turing proved that most facts about numbers are unknowable: there is an infinite list of irrational numbers with an infinite list of unpredictable digits. His musings led him to imagine a machine that one day will think and to the epiphany that, indeed, we are machines who think.
 
The severity of the constraints imposed by physical law and mathematical precision does not squash creativity. The limits are the scaffolding enabling creativity. Limits can be worthy adversaries that galvanize our best, most inventive, most agile natures. Before I succumbed to the seduction of the elegance and transcendence of limits, I did not understand the thrill of imagination crashing into truth.
 
As a student, not yet on a trajectory to become a scientist, I was nostalgic for that longing to leap into the atmospheric pool of blue midnight high out my window. I realized I missed that sensation, the urgent desire to leave the planet. Wherever I was by then, I was still under the same sky. From a slightly different perspective I trod at the foot of that blue dome, that overly lit blanched blue sky, and wanted in. Deterred by reality (we've never gone farther than the Moon), I redirected my focus down to the page, following mathematics to places our bodies couldn't go. Mathematics alone cannot tell us what specifically is out there in our universe. The mathematics can speculate only about what is possible. And sometimes mathematics allows us to explore pure potential before any physical manifestations of that potential are discovered.
 
Black holes were like that, a purely mathematical construct on the page, benign in virtual form, in typescript on paper, unverified for decades, unaccepted for decades, absurd, maligned and denied by some great geniuses of the twentieth century, until physical evidence of real black holes in the galaxy was discovered. Find one just a few thousand light-years away--a light-year being the distance light can travel in a year, nearly 10 trillion kilometers, the distance you would travel driving at the average highway speed limit for 10 million years. Take a left at that yellow star and veer toward that star cluster. Wandering at the base of the sky, we are under them. We are above them. Black holes in their abiding darkness are scattered plentifully among the stars, which themselves are scattered plentifully, like somber glitter infiltrating the void. We are in orbit around one in the center of our Milky Way galaxy. We are pulled toward another in the Andromeda galaxy.
 
I want to influence your perception of black holes, to shuck away the husk a bit, get closer to their darkest selves, to marvel at their peculiarity and their prodigious character. We can take a road less traveled, follow a series of simple observations that culminate in an intuitive impression of the objects of our attention, which are not objects at all, not things in the conventional sense.
 
A friend takes me out in New York to discuss the essentials to include in a black hole survival guide. An accomplished science writer, he asks me to clarify: "Don't I already know everything about black holes?"
 
"Do you know they are nothing?"
 
He looks at me unblinking for a long time, considering. He pauses to toss some salted peanuts into his mouth and while chewing he says, "I guess I don't know anything about black holes." Then slightly bemused at this realization, we eat the rest of the bar snacks and drink our wine while we talk about things more familiar.

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Excerpted from Black Hole Survival Guide by Janna Levin
All rights reserved by the original copyright owners. Excerpts are provided for display purposes only and may not be reproduced, reprinted or distributed without the written permission of the publisher.