American Scientist

In her new book, Black Hole Survival Guide, as a way of making the science of black holes more comprehensible, astrophysicist Janna Levin uses them as a “fantasy scape” for thought experiments, encouraging the reader to imagine viscerally the experience of a black hole encounter.

Black holes were unverified for decades, unaccepted for decades, absurd, maligned and denied by some great geniuses of the 20th 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. . . .

Weightlessness and Free Fall

Black holes are much maligned, depicted unfairly as behemoths when they are often benign and actually by nature quite small. Still, before you travel, you should do your research and consider the hazards. The perils, in fairness, are exceptional and unmatched if you are not careful. As with nature untamed here on Earth, black holes demand respect for safe navigation. After all, you are a trespasser on their territory.

Black holes are a gift, both physically and theoretically. They anchor galaxies, and they provide a laboratory for the exploration of the farthest reaches of the mind.

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Black holes were the unwanted product of the plasticity of space and time, grotesque and extreme deformations, grim instabilities. Honestly, they’re not such an anathema to scientists now. Black holes are a gift, both physically and theoretically. They are detectable on the farthest reaches of the observable universe. They anchor galaxies, providing a center for our own galactic pinwheel and possibly every other island of stars. And theoretically, they provide a laboratory for the exploration of the farthest reaches of the mind. Black holes are the ideal fantasy scape on which to play out thought experiments that target the core truths about the cosmos.

When in pursuit of a black hole, you are not looking for a material object. A black hole can masquerade as an object, but it is really a place, a place in space and time. Better: A black hole is a spacetime.

Imagine an empty universe. You have never seen or experienced such a pristine place, a vast nothingness that is the same everywhere—vast and bare. And flat—still three-dimensional, but everywhere flat.

Here is the sense in which an empty universe is a flat space: If you were in flat space you would float on a straight line. Despite the fact that you are not falling in the colloquial sense, free motion is called free fall. You are in free fall as long as you do not fire any rockets, get pulled or pushed—essentially as long as there is only gravity. Just surrender to space. If free motion can be traced by straight lines, and lines that begin parallel never cross, then the geometry of space is flat.

Chances are surpassingly bad that you are in free fall right now. Chances are also surpassingly bad that you are in a flat space, since there is no such place anywhere in our galaxy. Sit on a chair and you are not in free fall. The chair pushes on you to stop your fall. Stand on a floor and you are not in free fall. The floor pushes against your feet to prevent your plummet to street level. Lying in bed we feel heavy. We say gravity pulls us down. But we have it all wrong. Totally inverted. What you feel is not gravity but rather the atoms in the mattress pushing against your atoms. If only the bed would get out of your way, and the floor, and all the lower floors, you would fall, and falling is the purest uninterrupted experience of gravity. Only in the fight against gravity do you feel its pull, an inertia, a resistance, a heaviness. Give in to gravity, and the feeling of a force disappears.

The classic setting for the idea of free fall is an elevator. You are high up in an apartment building in an elevator. You feel a force against your feet. That force is between your feet and the elevator floor and keeps you in the cab. It’s a force between matter. Now, if you are interested in pure gravity, without the interference of interactions between matter, you must get rid of the elevator cab somehow. So you enlist someone to cut the cable. The elevator falls and you with it. During the descent, since you and the floor fall at the same rate, you would float in the elevator. You don’t fall to the floor, because the floor is falling too. You can push off the walls and tumble in the air. You seem weightless, as though you were an astronaut in the space station. You can pour water out of your water bottle and drink the droplets from the air, like astronauts do. You can release a pen, a phone, a rock in front of you, and these float too. Einstein called this profoundly simple observation—that we experience weightlessness when we fall—the happiest thought of his life.

The spoiler is that your atoms interact with the atoms in the Earth’s surface, and that would ensure an unhappy end to your free fall when you hit the ground. But that’s not gravity’s fault. The shattering of your bones would be due to other forces, like forces between atoms. (If you were made of dark matter, you could fall right through the Earth’s crust and sail on down.)

We can’t run the elevator experiment for long before the Earth gets in the way. So instead imagine you float far from the Earth, far from the Milky Way. Imagine a fictitious empty space, except for you and your space suit. If you sent tracers, threw a projectile in each of the three spatial directions, those projectiles would free-fall. Now imagine each projectile leaves a helpful trail illuminating its path; soon, a grid of straight lines would be visible. You could see plainly that space was flat—free-falling objects follow straight lines—and that space was empty, except for you and your space suit and your tracers and the luminous trails charting the grid. Gravity is so weak that none of these little pieces have a noticeable impact on the flat emptiness of it all.

The universe is not empty. We are very aware that we are bound to the Earth. The Earth is bound to the Sun and the Sun to the Milky Way galaxy. The Milky Way is bound to the neighboring galaxy Andromeda, both residing in the Virgo supercluster of galaxies. And the Virgo supercluster senses all the other galaxies and all the accumulated energy in our observable universe. So we don’t live in a flat, empty spacetime.

Astronauts also don’t float in empty space. They can see the Earth spin and the Sun roll along. They are falling and weightless, but on a path we’ve been accustomed to calling an orbit, an orbit around the Earth in orbit around the Sun in a glacially long orbit around the galaxy. Their paths aren’t straight. Their paths are curved into a circle around the Earth sewn into the circle around the Sun sewn into the path around the galaxy, because free-fall paths are curved when the sky isn’t empty. Because space is curved by the presence of matter and energy.

Curved Space

You can prove that you live in a curved space and not a flat space from the comfort of your couch by throwing things. Throw something and watch the arc it traces. The projectile will not travel in a straight line. The path paints a curve in space, an arc. All the objects we throw follow curved paths toward the Earth. We could travel around the globe throwing projectiles and all objects we throw from international couches will bow to the ground. We could document the results and draw a three-dimensional grid of the curved paths and thereby construct a map of the shape of the space around the Earth. The lesson: The Earth deforms the shape of space. And you can map that shape by drawing free-fall paths.

Free fall depends on the speed at which you toss something around the planet. Drop a wrench to the Earth and it follows a line straight down. Throw the wrench across the room, and the descent is along an arc, the same arc a car would descend along if thrown at the same speed and in the same direction as the wrench. Throw the wrench faster and the arc gets longer. Throw the wrench fast enough and it will clear the curve of the Earth and launch into orbit. Throw the wrench faster still and it will float away from the Earth forever until caught on the curve of another celestial object, like Jupiter or the Sun, and tumble on a different path.

Planets fall around the Sun. No engine pushes them. They trace ovals, always clearing the atmosphere. The Earth falls freely around the Sun, and the Moon falls freely around the Earth.

How can the Earth pull on the Moon without touching it? It doesn’t. It doesn’t pull on the Moon at all. It exerts no force. Instead, the Earth bends space. And the Moon tumbles freely.

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We put lots of human-made objects into orbits, which are just free-fall paths. Once the launched spacecraft get where they need to be, the engines are turned off and they can fall forever in orbit around the Sun or, more commonly, the Earth. Some mission specialists fight against the inclusion of thrusters in the spacecraft designs, in case a decline in funding encourages the space agency to maneuver the satellite out of orbit and incinerate the bounty in the atmosphere. Defunct satellites haunt space, ghostly litter in orbit for the lifetime of the Solar System.

The International Space Station (ISS) falls freely around the Earth. The astronauts in the space station float because they are falling like the ill-fated elevator occupants, not because they don’t feel the gravitational effect of the Earth. They do. The station is only a few hundred kilometers high and very much under the Earth’s influence. The gravitational effect is traced by curves in the shape of space, and the ISS follows one such natural curve, an ever-falling circular orbit. The astronauts and the space station travel nearly 28,000 kilometers per hour to complete a full circle every 90 minutes, out of the sunlight and into the Earth’s shadow and out again into the Sun’s radiance. They move fast enough that they always clear the Earth’s atmosphere while falling and thereby never crash into the surface.

From Einstein’s happiest thought (we fall in space weightlessly), we deduce with the most ordinary observations (tossed projectiles from the vantage of our Earth-covering couches) that free-fall paths are curves in space. Gravitation is curved spacetime. And that insight was Einstein’s greatest.

Einstein’s unabashed devotion to simplicity shows in the childlike wondrousness of his unembellished, spare thought experiments. He knew he did not understand standard- issue grade-school gravity—how can the Earth pull on the Moon, when they are not touching? Einstein did not understand gravity and neither did anyone else, but other scientists of his time either didn’t appreciate the severity of the failing or they didn’t pause to consider the implications. But because he did not understand gravity and he admitted as much, Einstein challenged the most accepted and elementary aspects of reality.

Before Einstein, it was customary to consider gravity to be a force of one body acting on another, but a force that mysteriously did not require actual contact. After Einstein, the language changed. Gravity was cast as a curved spacetime. How can the Earth pull on the Moon without touching it? It doesn’t. It doesn’t pull on the Moon at all. It exerts no force. Instead, the Earth bends space. And the Moon tumbles freely.

Black Holes Are a Space

Far from a black hole, the curves in space are the same species as those around the Sun or the Moon or the Earth. If the Sun were replaced by a black hole tomorrow, our orbit would be unchanged. The curve we would fall along around a black-hole sun is nearly identical to the curve we fall along around the actual Sun. Of course, the perpetual dusk would be apocalyptically cold and dark. But our orbit would be just fine.

The Earth is on average about 150 million kilometers from the Sun, which is about 1.4 million kilometers across. In comparison, a black hole the mass of the Sun would be 6 kilometers across. You can approach much closer to a black hole and remain unharmed than you can to the Sun, despite a black hole’s reputation for voraciously consuming all and sundry. You only really notice a radical difference between the space around a black hole and the space around the Sun when you get within a few hundred kilometers of the center of each. And you can’t get that close to the Sun without incinerating.

Bore inside the solar plasma and the gravitational pull of the Sun eases off. As you approach the center, you leave behind some of the Sun’s mass. The curving of space inside the Sun’s atmosphere becomes more gradual as the mass beneath you diminishes.

By contrast, no matter how close you approach to a black hole, the source never diminishes. The curving only gets sharper. Black holes are special because you leave none of the black hole’s mass behind you; it is as though all of the mass is concentrated ahead of you. Always. You can approach infinitesimally close to the center of a black hole and still feel all that mass in front of you.

Keep a safe distance from an unobtrusive black hole and you will neither be torn apart nor sucked up. Black holes just are not the catastrophic engines of destruction they’re portrayed to be, at least not until you veer recklessly close, not until you cross the point of no return, and then admittedly circumstances can get harrowing. Even if you approach boldly close, within several widths of the black hole, you could set up your space station, shut off the engines, fall freely in a stable orbit that takes mere hours to complete, and enjoy the scenery for as long as supplies last.

Excerpted from Black Hole Survival Guide, by Janna Levin. Copyright © 2020 by Janna Levin. Excerpted by permission of Alfred A. Knopf, a division of Penguin Random House LLC. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.