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Exploring Black Holes: Frozen Stars and Gravitational Dynamos

Black holes are gravitational superheroes. Here is their origin story, including World War I, magnificent mustaches, and Albert Einstein.

Part 1 of a four-part series on black holes. This part defines what a black hole is, goes back to the beginnings of black hole research, and shows how physicists came to terms with the existence of these weird objects. Part 2 will talk about the role black holes play in galaxies and how invisible objects can become some of the brightest things in the cosmos. Part 3 will explain how astronomers observe black holes using the most sophisticated observatories around. Part 4 will get into the weird side of black hole science: how these objects challenge our fundamental theories of physics.

February 11, 2016, was a landmark day. After many decades of searching, scientists announced they had detected gravitational waves for the first time: disturbances in the structure of space-time that travel at light speed. But there was a second triumph of physics hiding inside that one. The waves gave us the best evidence so far for the existence of some of the most fascinating objects in our universe: black holes.

Few scientists these days doubt that black holes exist. But in a way, all our evidence for them is circumstantial. Black holes, by their very nature, are difficult to observe. All light falling on them is absorbed, rendering them nearly invisible.

On the other hand, black holes are the strongest gravitational powerhouses possible. When they strip matter off stars or out of interstellar gas clouds, that material heats up and shines brightly. It’s a seeming paradox: invisible objects that end up being some of the brightest things in the universe. The black holes known as quasars can be seen billions of light-years away.

That’s not the same as seeing the black holes directly. The gravitational wave detection from 2016, however, comes from black holes and nothing else. The signal seen by the Laser Interferometer Gravitational-Wave Observatory (LIGO) was literally made only by gravity, letting scientists reconstruct what kind of objects made the waves without interference from stars or gas or anything else.

Artist’s impression of the two black holes that LIGO detected. Note how their gravity distorts the light of stars behind them. Image: The SXS (Simulating eXtreme Spacetimes) Project

LIGO saw two objects, respectively 36 and 29 times the mass of the sun, which orbited each other several times per second — a substantial fraction of the speed of light — before colliding in a burst of gravitational waves. Only black holes can simultaneously be that massive and small enough to whip around each other at that breathtaking speed.

Of course, scientists are greedy: We want even more direct observations than LIGO’s. As I write this, astronomers are crunching data from the Event Horizon Telescope (EHT), an observatory consisting of dozens of telescopes across the planet that observed the Milky Way’s central black hole for two weeks. They hope in the end to provide the very first direct image of a black hole.

LIGO and EHT are the culmination of a century of research into black holes and the start of a whole new era of discoveries. In this series, I’ll summarize what we’ve learned about these deeply weird objects and what we still have to learn. To get started, it’s helpful to go all the way back to the beginning…

The story of black holes, like Wonder Woman, begins in the deep past but really gets going in World War I. Several 18th-century physicists speculated about “dark stars,” objects compact enough that light couldn’t escape their gravitational pull. However, their theories were incomplete without a full understanding of both light and gravity. That required Albert Einstein’s general theory of relativity, first published in 1915 and finalized the following year.

Karl Schwarzschild (1873–1916) was a prominent German astrophysicist and owner of magnificent facial hair. His early work on general relativity led to the prediction of black holes. Photo: Berlin-Brandenburgische Akademie der Wissenschaften — Archive

Astrophysicist and mustache champion Karl Schwarzschild was one of the first people to read and understand Einstein’s work. While serving in the German army, Schwarzschild used general relativity to describe the gravitational field both inside and outside a spherical, nonspinning star. Just a few months after he sent his work to Einstein, Schwarzschild died of an infection, like many other soldiers on both sides of the war.

The “Schwarzschild solution,” as it’s called in the literature, was originally based on the incomplete version of general relativity, but other physicists translated it to the final form of the theory. Today, it’s often the first thing baby physicists cut their teeth on when studying relativity. But one prediction of Schwarzschild’s solution was controversial.

Gravity is a property of both an object’s mass — how much matter it contains — and its size. The star Betelgeuse, for example, is nearly eight times the mass of the sun, but because it occupies a lot more space than the sun, the gravity on its surface is much smaller. However, if you compacted the same amount of mass in Betelgeuse into a sphere smaller than roughly 48 kilometers across (30 miles), the Schwarzschild solution tells us that something weird happens. Specifically, nothing getting close to that massive-but-small object would ever escape again.

That “nothing” includes light.

Think of gravity like a current in a river. When that current is gentle, you can row against it; your planet or spaceship won’t crash into the sun. Under most conditions, the current will tug you toward the sun or a star or planet, which results in the orbits we observe. Light is also subject to the gravitational current.

A black hole’s current is more like rapids, and at some point, no matter how hard you row, the current will draw you in. Past a kind of gravitational waterfall, you’d have to “row” faster than the speed of light, which is against the laws of physics. The boundary between the part of the current you can still escape and the part where it’s too strong is called the “event horizon.”

Artist’s impression of hot matter swirling around a black hole. Even though the black hole is invisible, it gives itself away by the effects that its powerful gravity has on surrounding material. The dark circle is the black hole’s “event horizon,” where no matter or light can escape from. Image: NASA/JPL-Caltech

An object with an event horizon is a black hole. Most astronomical and even everyday objects don’t have such a boundary—they’re too big. Your own event horizon would be smaller than an atom, so you’d have to compress all your mass into that tiny volume to become a black hole, something most doctors don’t recommend.

The term “black hole” is probably from physicist John Archibald Wheeler, who liked to make up terms for things and just use them as though those terms had always existed. Literally translating this term is dicey: It’s rude or scatological in many languages. In the Soviet Union, where physicists developed black hole theory independently thanks to policies on both sides of the Iron Curtain, black holes were known as “frozen stars,” a more evocative term, if one that’s not any more accurate.

The idea of event horizons worried some physicists, including Einstein. Just because general relativity describes black holes doesn’t mean they exist in the real universe. After all, the theory describes a number of things that probably don’t exist, like “white holes,” the inverted version of black holes, which spew things out rather than pulling them in. The trick for a black hole is forming it in the first place: If the universe doesn’t allow gravity to compress things small enough, black holes would be nothing but a mathematical oddity.

Albert Einstein and J. Robert Oppenheimer in 1950. This was a publicity photo, since the physicists didn’t really have much to do with each other, but you can’t have a black hole story without an Einstein picture. Oppenheimer’s work helped show that black holes were an inevitable result of gravitational collapse of very massive stars. Photo: U.S. Government Defense Threat Reduction Agency

For that reason, a lot of physicists — like J. Robert Oppenheimer, who later ran the scientific wing of the Manhattan Project — tried to work out how to make black holes out of collapsing stars. Ordinarily, gas pressure driven by a star’s hot core balances gravity. Oppenheimer and others showed that if a star exceeds a certain mass, when it runs out of nuclear fuel, it can’t maintain enough pressure to hold it back from collapsing into a black hole.

The real clincher came in the 1960s, when physicists, including Stephen Hawking and Roger Penrose, showed that once a certain tipping point is reached in gravitational collapse, nothing can stop a black hole from forming. It was a powerful mathematical proof, but some people still thought black holes might not exist in the real world — or even if they did, they might be undetectable using telescopes.

In the next installment of this series, I’ll explore how black holes went from a theoretical physics toy to something astronomers observe regularly.