Black Hole Apocalypse

PBS Airdate: January 10, 2018

JANNA LEVIN (Astrophysicist, Barnard College of Columbia University): Of all the objects in the cosmos,…

RAINER WEISS (Massachusetts Institute of Technology): …planets

EILAT GLIKMAN (Middlebury College): …stars,…

PETER GALISON (Harvard University): …galaxies.

JANNA LEVIN: …none are as strange, mysterious or powerful as black holes.

NEIL DEGRASSE TYSON (American Museum of Natural History): Black holes are the most mind-blowing things in the universe.

PRIYAMVADA NATARAJAN (Yale University): They could swallow a star completely intact.

FERYAL ÖZEL (University of Arizona): Black holes have these powerful jets that just spew matter out.

JANNA LEVIN: First discovered on paper,…

PETER GALISON: …on the back of an envelope, some squiggles of the pen.

JANNA LEVIN: …the bizarre solution to a seemingly unsolvable equation.

PRIYAMVADA NATARAJAN: …mathematical enigma.

JANNA LEVIN: Einstein himself could not accept black holes as real.

RAI WEISS: People didn't even believe, for many years, that they existed.

MARCIA BARTUSIAK (Massachusetts Institute of Technology): Nature doesn't work that way.

JANNA LEVIN: Yet slowly, as scientists investigate black holes by observing the effect they have on their surroundings, evidence begins to mount.

ANDREA GHEZ (University of California, Los Angeles, Galactic Center Group): That is the proof of a black hole!

NEIL DEGRASSE TYSON: …millions of times the mass of the sun.

JANNA LEVIN: Cutting edge discoveries show…

DAVID REITZE (Laser Interferometer Gravitational-Wave Observatory): We did it!

JANNA LEVIN: …black holes are very real.

KIP THORNE (California Institute of Technology): I thought it was crazy.

RAI WEISS: I said, "Holy !"

JANNA LEVIN: But what exactly are they? If we could visit one, what we might we see? With their immense power, do black holes somehow shape the very structure of the universe?

Is it possible we might not exist without them?

PETER GALISON: It's quite a journey…

JANNA LEVIN: Black Hole Apocalypse, right now, on NOVA.

There are apocalyptic objects in the universe: engines of destruction, menacing and mysterious: black holes. Even scientists who study them find them astonishing.

EILAT GLIKMAN: Black holes can sort of blow your mind.

PRIYAMVADA NATARAJAN: I'm amazed that these objects actually exist.

JANNA LEVIN: Black holes defy our understanding of nature.

SHEP DOELEMAN (Director, Event Horizon Telescope): Black holes are the greatest mystery in the universe.

JANNA LEVIN: They're completely invisible, yet powerful beyond imagining. They can tear a star to shreds.

FERYAL ÖZEL: Black holes actually will eat anything that comes in their path.

NEIL DEGRASSE TYSON: You really want to avoid them at all cost.

JANNA LEVIN: Black holes even slow time.

Once thought too strange to be real, black holes shatter our very understanding of physics. But we're learning they may somehow be necessary for the universe we know to exist.

PRIYAMVADA NATARAJAN: They might well be the key players in the universe.

JANNA LEVIN: What are these strange, powerful objects? Outrageous and surprising, where are they, and how do they control the universe?

The search for black holes is on. And it will be a wild ride across the cosmos to places where everything you think you know is challenged, where space and time, even reality, are stranger than fiction.

And we're starting that journey at a very unlikely place: here, at a remote location in Washington State, where, for the first time, a radical new experiment has detected black holes.

It originated over 50 years ago, when a few visionary scientists imagine a technology that hasn't yet been invented, searching for something no one is certain can be found. The experiment is daring and risky; failure could mark their lives forever.

But they don't fail. Right here, in these facilities, they make a remarkable discovery. In the early hours of September 14, 2015, they record a message.

It looks and sounds like this. Just a little chirp, but that chirp is epic, monumental. The signal traveled over a billion light years to reach us. It started far, far away. And what it tells us is this: somewhere in the cosmos, over a billion years ago, two massive black holes circle each other in a fatal encounter. Closer and closer they come, swirling faster and faster until finally, they slam together.

The black holes create waves that spread outward, just like vibrations on a drum: a ringing in the fabric of space itself. The collision creates a massive blast, putting out 50 times as much power as the entire visible universe. It sends out a wave not of heat or light or sound, but of gravity.

NEIL DEGRASSE TYSON: This gravity wave is moving its way through the universe at the speed of light.

JANNA LEVIN: The wave races by stars. On the young earth, supercontinents are forming, microscopic organisms have just appeared.

NEIL DEGRASSE TYSON: Washing over one galaxy after another, after another.

JANNA LEVIN: Dinosaurs roam the earth.

NEIL DEGRASSE TYSON: The wave is still moving.

JANNA LEVIN: It zooms through clouds of dust.

NEIL DEGRASSE TYSON: And then it nears the Milky Way galaxy.

JANNA LEVIN: The Ice Age is just beginning.

NEIL DEGRASSE TYSON: We're troglodytes, drawing in caves.

JANNA LEVIN: The wave reaches nearby stars; Albert Einstein is in the sixth grade. The wave approaches as close as Alpha Centauri. At midnight, on September 13th, 2015, it is as close as Saturn.

Finally, over a billion years after the black holes collide, the wave reaches us. It strikes a pair of revolutionary new observatories, the sites of the daring experiment.

This is LIGO, the Laser Interferometer Gravitational-Wave Observatory.

The experiment 50 years in the making has finally hit the jackpot and opened an entirely new way of exploring the universe. For 400 years, almost everything we've observed in space has come to us in some form of electromagnetic energy. That little chirp is different. What hits the earth in September 2015 is a gravitational wave, a squeezing and stretching of the very fabric of space.

It produced no light; no telescope could ever see the collision. We needed an entirely new kind of observatory to detect it. That wave is new and direct evidence of one of the strangest mysteries in our universe, black holes.

Most of us have heard of black holes. They're invisible, powerful…

PRIYAMVADA NATARAJAN: We are talking about things that are a billion times the mass of the sun.

JANNA LEVIN: …bizarre.

EILAT GLIKMAN: A physical entity with infinite density…

ANDREA GHEZ: No beginning, no end.

JANNA LEVIN: They pull things in and warp light. Approach one and time itself begins to change.

PRIYAMVADA NATARAJAN: The gravity is so intense that a moving clock will tick slower.

NEIL DEGRASSE TYSON: Time will become so slow for you that you will watch the entire future of the universe unfold before your very eyes.

JANNA LEVIN: Fall in and you'd be squeezed as thin as a noodle.

NEIL DEGRASSE TYSON: You'll be extruded through the fabric of space and time like toothpaste through a tube.

JANNA LEVIN: Today, we know more about black holes than ever before, but the more we learn, the more mysterious they become.

ANDREA GHEZ: They're the most exotic objects in the universe. We don't have the physics to describe them.

PRIYAMVADA NATARAJAN: No matter how well you understand them, they remain unreachable, in some sense.

NARRATION OF MOVIE, The Black Hole: Now, man is about to enter The Black Hole!

JANNA LEVIN: So black holes have a pretty fierce reputation. And if you want a villain for a sci-fi movie, cast a black hole.

But in reality, what exactly is a black hole? And where do they come from? You might think a black hole is like this: an object.

But it's not. It's a hole in the fabric of space, a place where there is nothing; nothing except gravity, gravity at its most intense and overwhelming.

So if black holes are all about gravity, gravity at its most extreme, what exactly is gravity?

We're all familiar with gravity; it rules our lives. But even so, for a very long time, how gravity actually works was one of the greatest mysteries. Over 300 years ago, Isaac Newton was fascinated with the behavior of moving objects. Eventually, he figured out his laws of motion. They work so well, we still use them today.

But Newton's laws can only describe gravity's effects, not explain what it is. And here's where Albert Einstein comes in. Like Newton, he thinks about objects in motion and he wonders what gravity actually is: is it a force, or could it be something else?

Here's what concerns Einstein. Take this apple. I can't move it without touching it, but if I drop the apple, it moves towards Earth. But what if I take my hand away, and the floor and the basement and the floor below that? Then what happens? The apple just keeps falling.

Einstein realized that gravity had something to do with falling.

Now, if I throw the apple, it falls along a curved path. But imagine I could get the apple moving much faster.

Eventually, if I get the apple moving really, really fast, say, 17,000 miles an hour, its curved path matches the curve of Earth. The apple is in orbit, falling freely, just like the International Space Station and the astronauts inside it.

According to Einstein, the apple and the Space Station and the astronauts are all falling freely along a curved path in space. And what makes that path curved? The mass of the earth.

PETER GALISON: Einstein came up with a supremely simple concept; and that is that space and time is bent by the earth and by the sun and by all the objects in the world.

JANNA LEVIN: So, according to Einstein, the mass of every object causes the space around it to curve.

PETER GALISON: And that was Einstein's conception. There are no forces any more. There's just objects bending space-time and other objects following the straightest line through it.

JANNA LEVIN: All objects in motion follow the curves in space.

So, how does the earth move the apple without touching it? The earth curves space and the apple falls freely along those curves. That, according to Einstein's general theory of relativity, is gravity: curved space.

And that understanding of gravity, that an object causes the space around it to curve, leads directly to black holes. But it's not Albert Einstein who first makes the connection between gravity and black holes; it's another scientist.

MARCIA BARTUSIAK: Karl Schwarzschild was a German astronomer, head of the Potsdam Observatory, in Germany.

PETER GALISON: Ever since he was a teenager, he had been calculating complicated features of planetary orbits.

JANNA LEVIN: As Einstein unveils his theory of gravity, in 1915, Karl Schwarzschild is in the German army, calculating artillery trajectories in World War I.

MARCIA BARTUSIAK: And just weeks after Einstein presented his papers, Schwarzschild, then on the Russian front, quickly got a copy, and was mapping the gravitational field around a star.

PETER GALISON: Einstein had gotten at it through a series of approximations. But Schwarzschild, sitting on the front with bullets and bombs flying, calculated an exact solution to Einstein's theory and sent it to Einstein. Einstein was astonished. He hadn't even imagined that you could solve these equations exactly.

JANNA LEVIN: But Schwarzschild isn't done. In his solution to Einstein's equations, he discovers something Einstein himself had not anticipated.

PETER GALISON: Schwarzschild said, "I can calculate this strange distance from a gravitating object that represents a kind of a boundary."

JANNA LEVIN: Schwarzschild mathematically concentrates a mass, for example, a star, into a single point. Then he calculates how that mass would bend space and curve rays of light passing nearby.

MARCIA BARTUSIAK: As he, through his mathematics, aimed particles of light or matter towards this point, there was this boundary surrounding the point, at which the particles would just stop. The particles disappeared. Time stopped.

JANNA LEVIN: Schwarzschild has discovered that a concentration of mass will warp space to such an extreme that it creates a region of no return. Anything that enters that region will be trapped, unable to escape, even light.

PETER GALISON: It's like those roach motels. You can check in, but you can't check out. Once you go across that boundary, even if you can sail through, there's nothing you can do to get out, there's nothing you can do to signal out. It becomes this strange cut off portion of space-time.

JANNA LEVIN: What Karl Schwarzschild has discovered is that any mass compressed into a small enough space creates what we today call a black hole.

But Albert Einstein, whose own theory of gravity predicts such a thing, cannot believe it can happen in the real world.

MARCIA BARTUSIAK: Einstein didn't think that nature would act like this.

PETER GALISON: He didn't like this idea.

JANNA LEVIN: Karl Schwarzschild becomes ill and dies before he has a chance to further investigate his own discovery.

Two and a half years later, in November 1918, World War I ends.

The strange theoretical sphere discovered by Karl Schwarzschild seems destined to be forgotten, nothing but a curious historical footnote. But in the coming decades, physicists learn more about the atom, and about how fusing atoms powers stars, a process called "nuclear fusion."

Some begin to wonder if something like a black hole could actually come from a star, but not just any star, it would have to be big.

EILAT GLIKMAN: Stars are born in litters, and you get a distribution of sizes and masses, thousands of little stars and a few big stars, very big stars, incredibly massive.

NIA IMARA (Harvard University): Stars are, in many ways, similar to living creatures. Like humans, they have life cycles.

JANNA LEVIN: Investigating stars' life cycles in the 1930s, two visionaries, Subrahmanyan Chandrasekhar and Robert Oppenheimer, discover that the most massive stars end their lives very differently from smaller ones.

NIA IMARA: The life cycle of a star really depends on its mass. The mass of a star determines what's going to happen after it finishes burning its hydrogen fuel.

JANNA LEVIN: All stars start out burning hydrogen, the lightest atom, fusing hydrogen atoms into helium, working their way up to heavier elements. Gravity wants to crush the entire mass of the star, but the enormous energy released by fusion pushes outward, preventing the star from collapsing.

NIA IMARA: Stars are stable because you have an outward-moving pressure due to nuclear fusion, and that's balancing with the inward force of gravity.

JANNA LEVIN: Smaller stars can't fuse elements heavier than helium, but in the most massive stars, fusion crushes heavier and heavier atoms, all the way up to iron.

EILAT GLIKMAN: Iron is such a massive element, it has so many protons in it, that by the time you fuse iron, you don't get any energy back out.

JANNA LEVIN: Iron is a dead-end for stars. Fusing atoms larger than iron doesn't release enough energy to support the star, and without enough energy from fusion keeping the star inflated, there's nothing to fight gravity.

EILAT GLIKMAN: And gravity wins. And so the entire star collapses.

JANNA LEVIN: Very rapidly, trillions of tons of material come crashing down, hit the dense core and bounce back out, blowing off the outer layers of the star in a massive explosion, a "supernova."

The more mass, the more gravity, so if the remaining core is massive enough, gravity becomes unstoppable.

NEIL DEGRASSE TYSON: There's no known force to prevent the collapse to an infinitesimally small dot.

JANNA LEVIN: Gravity crushes the stellar core down, smaller and smaller and smaller, until all its mass is compressed in an infinitely small point, a black hole.

The theory makes sense, but most physicists remain skeptical about black holes.

PRIYAMVADA NATARAJAN: Einstein and Eddington, all the, sort of, you know, preeminent astrophysicists in the 1930s through 1950s, did not believe that they were actually real. It remained a solution, a mathematical enigma for a very long time, so it took a long time for people to even start looking for them.

JANNA LEVIN: It's not until the 1960s that the idea of a supernova creating a black hole is taken seriously. Princeton physicist John Wheeler, who had originally been a skeptic, begins to use a name from history for these invisible objects: black hole.

PRIYAMVADA NATARAJAN: The term "black hole" actually originates in India; the Black Hole was the name of an infamous prison in Calcutta.

JANNA LEVIN: Still, no one has ever detected any sign of a black hole.

Then, in 1967, graduate student Jocelyn Bell discovers a strange, extremely tiny dead star that gives off very little light, a neutron star. The cold remains of a stellar collapse, the neutron star gives astronomers more confidence that black holes, much heavier dead stars, might also exist.

A half-century after Karl Schwarzschild mathematically showed that black holes are theoretically possible, scientists have identified a natural process that might create them: the death of large stars.

EILAT GLIKMAN: So these giant supernova explosions of extremely massive stars make black holes.

PRIYAMVADA NATARAJAN: Any star that is born with a mass that's about 10 times the mass of the sun, or higher, will end in a black hole. So our galaxy is replete with little black holes, which are the stellar corpses of generations of stars that have come and gone.

JANNA LEVIN: So, what are these invisible stellar corpses like?

Imagine I'm exploring space, with some advanced technology for interstellar travel, so that we could visit a black hole, maybe one in our own galactic neighborhood.

This particular black hole isn't very big, only about 10 solar masses, meaning, 10 times the mass of the sun. And like all black holes, it has an event horizon, a distinct edge to the darkness. That's the boundary Karl Schwarzschild first discovered, where gravity is so strong that nothing can escape, not even light.

And that's where we're going.

As we get closer, some very strange things begin to happen. Look at the edge of the black hole. See how the image of distant stars is distorted and smeared into a circle? That's gravitational lensing. The black hole's extreme gravity bends the path of light passing by, so that a single point of light, like a star, briefly appears as a ring around the event horizon.

I'm now deep in the black hole's gravity well, and we're going to start experiencing the effects.

The extreme gravity actually slows down time relative to the earth. From their point of view, I appear to be slooowwwing dooowwwn, but from my point of view, time on Earth is speeding up.

Now, let's say I want to get even closer, by taking a spacewalk. The way the black hole slows down time is about to get even more pronounced. To keep track of the changes I'm about to experience, I'm turning on this strobe light. It'll blink once a second.

From here, I can see the shadow of the event horizon approaching and my light blinking normally. But watching from the ship, the closer I move toward the black hole, the more slowly I appear to move. The pulses are nearly infinitely spaced, so it looks as though I'm frozen in time.

For me, everything is completely normal. Even when I reach the event horizon.

If you waited long enough, maybe millions or billions of years, the ship would finally see me disappear. And that's the last you'd see of me.

What's inside a black hole? That's still a mystery. And even if I find out, I can never go back and tell you. But I can say this: black holes may be dark from the outside, but inside, they can be bright. I can watch the light from the galaxy that's fallen in behind me, and that's the last thing I'll ever see. Unfortunately, the fun is about to end.

Now that I've crossed the event horizon, I'm falling toward the center, where all of the mass of the black hole is concentrated, and I'm beginning to get stretched. As I fall in, the gravitational pull at my feet is stronger than at my head, and my body is starting to get pulled apart. I'll be stretched as long and thin as a noodle—spaghettified—and, ultimately, I'll end up completely disintegrating into my fundamental particles, which are then crushed to an infinitely small point, a "singularity," where everything we understand about space and time breaks down.

Or maybe the black hole, less than 40 miles across on the outside, is as big as a universe on the inside, and as I pass through, my particles will join the primordial soup of a new beginning.

So, that's what theory tells us we might experience if we could travel to a black hole. But how can we know for sure? How do you investigate something you can't even see?

There are ways to investigate if something is happening somewhere, even if I can't see that thing directly.

Take Yankee Stadium: what's happening inside there? Is there a game going on? I can't see the field; I can't see any players or baseballs or bats; but I can definitely tell if there's activity around the park. It's pretty clear something is going on.

It might seem obvious, but whatever it is, I can learn a lot just by observing the happenings around the stadium. And these do look a lot like baseball fans.

And that's the way we investigate black holes: by observing the effect they have on their surroundings.

But what sort of effects? How might a black hole reveal itself?

Starting just before World War II, two monumental discoveries are about to radically change astronomy. In 1931, Bell Labs engineer Karl Jansky picks up mysterious radio waves emanating from deep space.

Then, the sky gets even stranger, when scientists mount Geiger counters on captured German rockets and discover the cosmos is also full of X-rays.

These discoveries give astronomers important new tools that will revolutionize the hunt for black holes and dramatically expand our vision.

MARCIA BARTUSIAK: What our eyes can perceive is a very narrow part of the electromagnetic spectrum.

JANNA LEVIN: If the electromagnetic spectrum were laid out along the Brooklyn Bridge, the portion we can see with our eyes would be just a few feet wide.

Electromagnetic radiation includes waves of many different frequencies: radio waves, microwaves, infrared and ultraviolet light, X-rays and gamma rays.

Radio and X-ray astronomy open up the sky, revealing dim or even invisible objects, blasting out powerful energy no one knew was there.

RAI WEISS: They began to realize that this very placid thing that we see out there, all this very quiet thing that looks like nothing is happening and the only thing that's moving is the planets, found out that there was madness going out there. It was chaos out there.

JANNA LEVIN: X-rays come from the high-energy end of the spectrum. What is creating all this energy? This much is certain: whatever the source, it is invisible to ordinary optical telescopes. And it is hot.

PAUL MURDIN (Institute of Astronomy, University of Cambridge): X-rays come from things which are at temperatures of millions of degrees, even tens of millions.

JANNA LEVIN: One of the first of these X-ray sources to catch the attention of astronomers is named "Cygnus X-1."

PAUL MURDIN: "Cygnus," it was in the constellation Cygnus; "X," it was an X-ray source; "1," it was the first one you found.

JANNA LEVIN: In 1970, Paul Murdin is a young English astronomer, trying to secure his next job.

PAUL MURDIN: I was a research fellow. I was coming to the end of my three-year contract, and I thought, "What can I contribute to finding out what these things are?"

JANNA LEVIN: Murdin works in a 15th-century castle surrounded by telescopes, the Royal Observatory. Using the largest telescope in England, he begins searching the area of the constellation Cygnus, the Swan. He decides to hunt for pairs of stars.

Pairs of stars are called binaries. They may sound exotic, but they're not at all uncommon. Many of the stars we see, perhaps half, are actually binaries: pairs of orbiting stars, locked together by gravity.

But Murdin wonders: is it possible there are binaries where only one of the stars is visible?

PAUL MURDIN: I thought that maybe there was a kind of a star system in which there was a star, one ordinary star that made light, and then there was another star nearby that made X-rays.

JANNA LEVIN: The telltale sign of a binary is that the stars are moving around each other. So, Murdin begins searching for a visible star that shows signs of motion.

PAUL MURDIN: Sometimes it's coming towards you, sometimes it's coming away. Sometimes it's coming towards you, sometimes it's coming away.

JANNA LEVIN: When the star is moving toward us, it appears more blue, as the wavelength of its light gets shorter. Moving away, it appears more red, as the wavelength of its light gets longer. This is known as "Doppler shift."

After looking for color changes in hundreds of stars in the area of Cygnus, Murdin spots a possible suspect, a visible star whose light is shifting, as though moving around.

PAUL MURDIN: It very clearly was a binary star, a double star. The star was moving around and around, with a period going round once every 5.6 days.

JANNA LEVIN: But whatever it's going around can't be seen.

PAUL MURDIN: There was no trace in the spectrum of the second star. There was one star there; there wasn't the second star there.

JANNA LEVIN: Murdin has a binary pair in which only one star is visible. The second object emits X-rays, has enough mass and gravity to dramatically move a star, but gives off no light.

Could it be the corpse of a star massive enough to become a black hole?

KIP THORNE: The crucial issue in deciding whether Cygnus X-1 was a black hole was to measure the mass of the X-ray-emitting object.

JANNA LEVIN: It would have to be very massive; at least three times the mass of our sun. If not, it's probably just a neutron star, a collapsed star that's dense but not heavy enough to be a black hole.

KIP THORNE: So, the observers needed to come up with a conclusion that the dark object, the X-ray-emitting object in Cygnus X-1, was heavier, hopefully substantially heavier, than three solar masses.

JANNA LEVIN: From his observations, Murdin is able to make an estimate of the mass of the invisible partner.

PAUL MURDIN: And the answer came out to be six times the mass of the sun. So, there was a story, then, that Cygnus X-1 was a black hole. And the key to the argument was that the mass of the star you couldn't see was more than three solar masses.

When I'd finished writing it all out, I sat back and thought, "It's a black hole."

JANNA LEVIN: This would be the first actual detection of a black hole. It's a huge claim, and Murdin will have to convince skeptics, starting with his boss.

PAUL MURDIN: The Astronomer Royal, Sir Richard Woolley, he didn't really go for black holes. "It's all fanciful…It's kind of… A lot of people in California were talking about this." And there are a lot of funny people in California, you know, a lot of hippie-type people.

JANNA LEVIN: People like theorist Kip Thorne.

PAUL MURDIN: So, I was nervous about it. I was nervous about the scale of the discovery, and, actually, so were other people all around me.

I was working with a fellow scientist, Louise Webster. And we were modest about the claim that we were making, because we knew what people would think of it. And if you look at the paper we published, it just mentions the word "black hole" once, right at the end. "We think this might be a black hole."

JANNA LEVIN: The Paul Murdin-Louise Webster paper appears in September 1971. Other astronomers agree it could be a black hole, but no one knows for sure.

Three years later, Kip Thorne and the noted British physicist Stephen Hawking make a now-famous wager about Cygnus X-1.

KIP THORNE: We made a bet as to whether Cygnus X-1 really was a black hole or not.

JANNA LEVIN: The bet is partly in jest. Both men hope it is a black hole. But Hawking, not wanting to jinx it, bets against his own wishes.

KIP THORNE: Stephen claims that Cygnus X-1 is not a black hole, and I claim it is a black hole. And so we signed that bet in December 1974.

And, gradually, the case that it really was a black hole became stronger and stronger and stronger. So, in June of 1990, Stephen broke into my office and he thumbprinted off on this bet, conceded the bet in my absence. I came back from Russia and discovered that he had conceded.

JANNA LEVIN: Now, by 1990, the evidence of Cygnus X-1's mass may be strong enough to settle a bet between two friends, but the original estimate wasn't precise enough to be definitive.

In order to calculate mass, Paul Murdin had to rely on rough estimates of the distance to Cygnus X-1, which varied by a factor of 10. And the question wouldn't be answered for another 20 years, until astronomer Mark Reid became intrigued by the puzzle.

Reid is an astronomer at the Harvard-Smithsonian Center for Astrophysics when he sets out to conclusively prove that Cygnus X-1 is a black hole, by measuring its precise mass.

But how can you measure the mass of an invisible object?

Using laws developed by German astronomer Johannes Kepler in the 1600s, it's possible to calculate the mass of a celestial object, but only if you know its distance.

MARK REID (Harvard-Smithsonian Center for Astrophysics): Distance in astronomy is absolutely fundamental. If you don't know distance, you don't know what the object is. It could be a very nearby firefly-like thing. It could be a very distant, huge star, much, much bigger than the sun.

JANNA LEVIN: So, to get the true, precise mass of Cygnus X-1 and confirm that it is a black hole, Reid needs to know how far away it is. But how can he measure the distance to a star?

The secret lies in a familiar phenomenon: parallax. It's what our eyes and brains use to see in three dimensions.

MARK REID: You can put your finger up at arm's length, look at it and close one eye. I'm closing my left eye, and I'm looking at my finger relative to the wall in the background there. And now, if I open my eye, close my right eye, I see my finger has appeared to move with respect to the original position. And that's because our eyes are separated and we view from different vantage points.

JANNA LEVIN: To use parallax to measure distance to an object in the sky, astronomers let the motion of the earth provide the two different vantage points.

MARK REID: Imagine Cygnus X-1 is right here, and the earth and the sun are over there. Now, the earth goes around the sun once a year. And in the springtime, the earth ends up on one side of the sun, and we observe Cygnus X-1 along a ray path like this. And six months later, the earth goes around the sun to the other side, we get a different vantage point from Cygnus X-1.

JANNA LEVIN: Now he has a triangle that goes between the earth at its two positions, and Cygnus X-1. We know the base of the triangle: the diameter of Earth's orbit. And the principles of geometry tell us that all we need to calculate the distance is the size of the angle at the top.

MARK REID: And we measure this very small angle here, at the point at Cygnus X-1. And then, from direct geometry, we can calculate the distance to Cygnus X-1, and from that infer a very accurate mass.

JANNA LEVIN: The concept is simple, but Cygnus X-1 is so far away that the angle to be measured is miniscule, a tiny fraction of one degree. It's smaller than the angle spanned by Abraham Lincoln's nose, on a penny in San Francisco, viewed from New York. Because the angle is so very tiny, it can't be measured by any one telescope. But Reid's team has a solution.

MARK REID: We take 10 radio telescopes that are spread across the continental U.S. and to Hawaii and to St. Croix in the Virgin Islands. We use these telescopes simultaneously, and we synthesize, in a computer, a telescope that has the diameter of the size of the earth. That gives you incredible angular resolution.

JANNA LEVIN: Using this technique, Reid's team determines that Cygnus X-1 is 6,000 light years away.

MARK REID: With the new distance we got, the 6,000 light year distance, we're able to determine that the mass is about 15 solar masses, easily a black hole.

JANNA LEVIN: Forty years after it was identified as a possibility, Cygnus X-1 is now widely accepted as the first confirmed black hole.

PAUL MURDIN: It's an understated paper, and the fact that my name was on it and Louise Webster's was on it, did us a lot of good in our careers. I think, as a result of this discovery, I got offered a permanent job. And it was a great celebration for the family. So it worked out very well for me, as well as getting the intellectual satisfaction of solving a problem.

JANNA LEVIN: So, finally, after years of speculation, we have a real black hole, not only that, but a black hole that's blasting out X-rays and has a companion star. If we could visit in my imaginary spaceship, what would we see?

The distance to Cygnus X-1 has been established at 6,000 light years from Earth, and its mass is 15 solar masses, or 15 times the mass of the sun.

And Cygnus X-1 is surrounded by an accretion disk, a disk-shaped cloud of gas and dust, outside its event horizon, the point of no return.

As gravity pulls matter toward the black hole, the cloud starts rotating, just like water being pulled down a drain. Within that accretion disk, particles closest to the black hole whip around at half the speed of light. It's like a giant particle accelerator in space.

But why does it emit X-rays? As those particles race around, they collide, which heats them up to millions of degrees.

When they get that hot, particles blast out X-rays. And it's those X-rays that first led astronomer Paul Murdin to investigate this black hole nearly five decades ago.

And there's something else about Cygnus that's different: it has a companion star. This blue supergiant star orbits the black hole once every 5.6 days. It orbits so close to Cygnus X-1 that the black hole strips material off the star and pulls it into the accretion disk.

Some of that material will cross the event horizon and get swallowed up, but not all of it.

FERYAL ÖZEL: Some of the stuff actually comes back out before ever entering the black hole, kind of like a toddler eating: half the pasta ends up on the floor, half of it may be on the ceiling and some of it in the mouth.

JANNA LEVIN: One of the most striking and enigmatic features of Cygnus X-1 is its enormous jets. These beams of particles and radiation stream outward from Cygnus's north and south poles, perpendicular to the accretion disk.

There's still a lot we don't know about these jets, but they are tightly focused and extremely powerful, blasting out at nearly the speed of light, and extending well beyond Cygnus.

FERYAL ÖZEL: When gas gets to these high temperatures and produces the light, there's also a little bit of a magnetic field that forms around them. And we don't understand exactly how, but these magnetic fields help collimate these massive outflows from black holes, powerful hoses, if you will, that just spew matter out.

JANNA LEVIN: So that's Cygnus X-1, if we could see it up close: a growing, feeding black hole with huge jets blasting particles way out into the universe.

PRIYAMVADA NATARAJAN: They're almost these breathing, fire eating demons, if you will. They flicker, they have bursts; it's a very violent fireball, very active.

JANNA LEVIN: What was once a bizarre mathematical curiosity has now become quite real. After decades of skepticism, scientists now accept that burned-out corpses of large stars can trap light inside them, warp space and time around them, attract matter and accelerate it to mind-boggling speeds.

PETER GALISON: Black holes seemed like such a radical idea that we shouldn't accept it. But bit by bit, the evidence for black holes has gotten stronger and stronger. And we've seen these amazing things.

JANNA LEVIN: At least 20 black holes have been found in our galaxy, X-ray binaries, like Cygnus X-1. And there are probably millions more of these massive stellar corpses in our galaxy alone.

Still, a stunning surprise awaits.

Everything astronomers think they know about black holes, and much of what they believe about the universe itself will be upended by a shocking discovery.

The revelations begin when radio telescope surveys of the sky detect mysterious hot spots emitting radio energy.

MARCIA BARTUSIAK: They were coming from what looked like stars.

JANNA LEVIN: Because these objects resemble stars, but were discovered through radio signals, astronomers name them "quasi-stellar radio sources," "quasars."

But are they stars or not? The first step in investigating them is to figure out what they're made of. To do that, astronomers analyze the electromagnetic energy they emit. Every element has a unique spectral fingerprint. For example, carbon, helium, hydrogen, these lines reveal the chemical makeup of a star. But the spectrum of a quasar turns out to be incomprehensible.

MARCIA BARTUSIAK: They looked at it and it was gibberish. It didn't look like there were any emissions from elements that they knew!

JANNA LEVIN: What are they missing? There has to be a clue somewhere.

Finally, in 1963, Caltech astronomer Maarten Schmidt finds it hiding in plain sight. Buried in the quasar's spectrum is the fingerprint of hydrogen.

MARCIA BARTUSIAK: He noticed something familiar, but it was in the wrong place. The fingerprints of hydrogen had been shifted way off to the red.

JANNA LEVIN: It was hard to spot because the spectral lines of hydrogen were radically shifted toward the lower frequency end of the spectrum. And that could only mean one thing: the quasar is moving away from us at fantastic speed.

But astronomers have never before seen light shifted to such an extreme. Like a familiar sound shifting too low to understand, the light from quasars has shifted to such a degree that hydrogen is unrecognizable. This extreme amount of shift means quasars are racing away from us at blinding speeds.

The reason? It's the legacy of an event that occurred almost 14-billion years ago: the Big Bang, the beginning of our universe. And ever since, the universe has been expanding, carrying with it all the objects it contains, including quasars.

EILAT GLIKMAN: No one had ever seen anything moving away at that high speed. This made this object the furthest away thing that had ever been seen, which meant the thing itself had to be so luminous, and you had to account for that.

MARCIA BARTUSIAK: Two-billion light years away, putting out the energy of a trillion suns each second, what could possibly create that?

PETER GALISON: No one had any idea what could be powering these things. Where could all of this energy come from?

MARCIA BARTUSIAK: If you work out through calculations, it can't be chemical energy. They knew it couldn't be nuclear energy.

JANNA LEVIN: There's no way a quasar could be a star. No amount of nuclear fusion could produce that much star power.

MARCIA BARTUSIAK: The only engine that could possibly put out that much energy is gravity.

JANNA LEVIN: Gravity.

In everyday life, we can overcome gravity easily, but when concentrated to an extreme by a black hole, gravity is overwhelmingly powerful.

A handful of scientists start wondering: could quasars perhaps be powered by gravity engines? What if the energy blasting out from quasars is coming from bright accretion disks around black holes?

PRIYAMVADA NATARAJAN: To produce that kind of energy, that kind of brightness, it has to involve a black hole.

JANNA LEVIN: But not just any black hole.

KIP THORNE: Whatever was the source of the emission from a quasar had to be massive. How massive? Well, millions or billions of times heavier than the sun.

JANNA LEVIN: Millions or billions of times heavier than the sun. Cygnus X-1 is only 15 times the mass of the sun. The black holes powering quasars are an entirely different category of black hole, "supermassives." And they seem to be located in the centers of galaxies.

But what about our own galaxy? Could there be any supermassive black holes closer to home? The center, where any supermassive would be found, lies in the direction of the constellation Sagittarius, the Archer.

Now, Sagittarius isn't just any constellation. It's in the direction of the center of our own Milky Way galaxy. But since we live inside the Milky Way, we can't see the galaxy the way a space traveler would.

But I can use my trusted imaginary star machine to show us the galaxy from the outside. Our home is a spiral galaxy, hundreds of billions of stars, drawn together into a gigantic disk. It's wide, about 100,000 light years across; but it's relatively thin, only about a thousand light years thick. And the whole spiral slowly rotates.

Our solar system is here.

And here, 26,000 light years from the earth, is the center, which we see in the direction of Sagittarius. In this dense center, there are millions of stars and lots and lots of dust and gas.

So, that's the view of our galaxy from the outside, thanks to my imaginary technology. But since we live inside the Milky Way, when we look towards the center, we're looking through much of our own galaxy, which means it appears to us as a band of stars and dust across the sky, a "milky" way.

Deep inside this band of stars and dust, could a supermassive black hole be lurking? In the 1990s, astronomers grow determined to solve the mystery, to peer through the murky Milky Way and learn what, if anything, is at its center. One of them is Andrea Ghez.

Ghez takes on a daunting challenge. She will try to track individual stars orbiting the center of the galaxy.

ANDREA GHEZ: The essence of this experiment comes from watching stars orbit the center of the galaxy. So you want to find the stars that are as close to the center of the galaxy as possible. Which means that I want to get access to the largest telescope I can possibly get my hands on.

JANNA LEVIN: And that means coming here. The summit of Mauna Kea, a dormant volcano almost 14,000 feet above the beaches of Hawaii. High altitude and low humidity make this the ideal place for astronomy.

The instrument Ghez uses is Mauna Kea's Keck Observatory, one of the largest in the world. But despite its size, Keck has the same problem as all telescopes on Earth: atmospheric distortion.

ANDREA GHEZ: Think about looking at a pebble at the bottom of a river. The river is moving very quickly, and your view of that pebble is distorted.

JANNA LEVIN: Like a river, the earth's atmosphere is constantly changing, bending light like a funhouse mirror.

To compensate for this, Keck pioneers the scientific use of a declassified military technology called "adaptive optics." First, they shine a laser into the sky creating an artificial guide star. The turbulent atmosphere distorts the guide star, but the computer knows what it should look like and adjusts the telescope mirror accordingly.

ANDREA GHEZ: So, if you look at yourself in a circus funhouse mirror, you look completely distorted. And the goal of the adaptive optics system is to introduce a second mirror that's the exact opposite shape and make you look flat again.

JANNA LEVIN: Buried deep inside the telescope, the deformable mirror changes shape up to 2,000 times a second to reverse the atmosphere's distortion.

ANDREA GHEZ: And it has allowed us to take the sharpest images ever obtained of the center of the galaxy.

JANNA LEVIN: The sharpness of those images allows Ghez to make out individual stars near the center, a huge advance in astronomy. She begins recording their positions in 1995.

ANDREA GHEZ: And every year since then we've taken an image. Just take a picture.

JANNA LEVIN: Putting those annual snapshots together creates a time-lapse movie of stellar orbits, and what those movies reveal is astounding. The stars are whipping around the center of the Milky Way at phenomenal speeds.

ANDREA GHEZ: These things are moving at several thousand, up to 10,000 kilometers per second, or 10,000,000 miles per hour. They're, they're really hauling.

JANNA LEVIN: To go that fast, the stars must be orbiting something extremely massive.

ANDREA GHEZ: The mass that we infer is four-million times the mass of the sun!

JANNA LEVIN: What could be four-million times the mass of the sun, yet be completely invisible?

ANDREA GHEZ: That is the proof of a black hole!

JANNA LEVIN: And not just any black hole: a supermassive, silent and sleeping right in the center of our own galaxy.

ANDREA GHEZ: In fact, this is the best evidence to date that we have for the existence of supermassive black holes, not only in the center of our own galaxy, but anywhere in the universe.

JANNA LEVIN: A supermassive black hole, four-million times the mass of the sun, in the very center of our own Milky Way galaxy. From a cosmic perspective, it's right next door.

And it raises a profound question. There are billions of galaxies out there. If ours has a supermassive black hole at its center, and if quasars are found at the centers of their galaxies, what about the others?

TOD LAUER (National Optical Astronomy Observatory): Are there black holes at the centers of galaxies? If they are, how common are they? We simply didn't know.

JANNA LEVIN: Could astronomers ever hope to find what lurks at the centers of other galaxies, millions of light years away, as Ghez did in our Milky Way?

SPACE SHUTTLE MISSION CONTROL (Film Clip): Five, four, three, …

JANNA LEVIN: It would take another innovation in astronomy to make that possible.

SPACE SHUTTLE MISSION CONTROL (Film Clip): And liftoff of the space shuttle Discovery with the Hubble Space Telescope, our window on the universe!

JANNA LEVIN: When the Hubble Space Telescope starts delivering clear images of distant galaxies, a team of astronomers gets to work. They become known as "the Nukers," because their focus is galactic nuclei, the centers of galaxies. One of them is Tod Lauer.

TOD LAUER: Step one, we take a picture of the galaxy with the Hubble Space Telescope. It shows us where the stars in the galaxy are. It tells us its structure in exquisite resolution.

JANNA LEVIN: The key to finding supermassive black holes is to learn how fast the stars in the galaxy are moving. Galaxies outside our own are much too far away to measure the speed of individual stars. But by analyzing the way light is shifted from blue to red at different points in the galaxy, astronomers can put together an average speed of stars orbiting the center.

It's accurate enough to create a replica in a computer.

TOD LAUER: The second step, where the real work begins, is to try to model the observations. And we actually do that by building models of galaxies in the computer.

JANNA LEVIN: It's known as "Schwarzschild's method," developed by Princeton astronomer Martin Schwarzschild, son of Karl Schwarzschild, whose mathematics first described the possibility of black holes.

TOD LAUER: Martin Schwarzschild's trick was he would actually build up a model of the galaxy that not only had where the mass was, but it also had how the stars were moving.

JANNA LEVIN: For each galaxy they investigate, the Nukers painstakingly build a computer model and then, using trial and error, adjust the parameters of mass and velocity, trying to make the model match the original observations they got from the Hubble.

TOD LAUER: And we say, "let's try a star here; let's try one over here; let's have it go around this way; let's have this one go around that way." And we do this thousands and thousands of times until we build up a library of how stars can orbit in this galaxy.

Success is when observations of the model match the observations taken with the Hubble Space Telescope.

JANNA LEVIN: But that doesn't happen. The models are missing something.

TOD LAUER: We try it again and again and again, all with no black hole yet. And you say, "Gee, we really can't get the observations explained by a model."

JANNA LEVIN: Only when they add an enormous invisible mass at the galaxy's center, does the model match the Hubble observations.

TOD LAUER: Almost always, we have to put in a black hole at the center. We can't match the observations without a black hole in the model.

JANNA LEVIN: Of roughly three dozen galaxies that the Nukers investigate, virtually all of them require a supermassive black hole. And since then, other observations have made us even more certain that supermassives and galaxies go together.

NEIL DEGRASSE TYSON: Every galaxy we've looked for one, we have found a supermassive black hole in its center.

JANNA LEVIN: It's a stunning revelation. Supermassives, once an entirely unexpected category of black holes, may be common, not only at the center of our galaxy, but of all galaxies.

Take galaxy M31, also known as the great Andromeda Galaxy. It's two-and-a-half million light years away. On a clear night, you can see it from Earth, but even with the Hubble Space Telescope, we can't make out precise details of its center. Still, we're pretty sure there's something extremely massive hiding there.

What if we could take a closer look? What if we could visit a galaxy far, far away?

As we enter the outer part of Andromeda, we're still too far away to see what's lurking at the center, but we can make out a dense cluster of stars in the core, and that could be a sign that there's a giant black hole nearby.

Billions of years ago, it would have been surrounded by gas and stars and other small black holes. The black hole may have powered a quasar, feeding mad and blasting out blinding radiation. Over hundreds of millions of years, it would have consumed all of the available gas and the closest stars.

These days it's relatively quiet. But it has some distinctive features we've never seen before. First, it's colossal. If it were dropped in our solar system, Mercury, Venus, Earth and Mars would all be trapped inside the event horizon.

That's big, but it's nothing compared to the sheer mass: a hundred-million times the mass of the sun. And the destruction won't end there. Jupiter won't last long. The gravitational field of the supermassive will grab hold and swallow it whole. Eventually, Saturn will suffer the same fate.

The outer planets might survive, but in cold and dark orbits.

This black hole rotates rapidly, distorting and dragging the fabric of space-time. Like all black holes, the event horizon is completely featureless. Remember, there's nothing there; it's just a boundary that conceals the interior.

But the accretion disk can tell us a lot about what's going on. That's the fiery ring of gas and dust around the black hole.

Imagine if we could release a swarm of autonomous robots to explore the accretion disk.

The disk is spinning at an incredible speed, as much as half the speed of light. If Jupiter moved that fast, it would complete its entire orbit in a few hours. The region around the black hole is a cosmic tornado. Now our swarm is caught in the whirlwind too.

They're like tracers dropped into the storm to map the movement. The middle robot can send us images. It's following the leader like a racecar speeding around the track. From here, the extreme warping of space-time around the black hole plays crazy tricks on our eyes.

It looks like there's one accretion disk whipping around the equator, and another arcing over and under the poles, but that's an illusion. The black hole's extreme gravity bends the path of light emitted behind the black hole, and makes it look like the accretion disk is both above and below. There's actually nothing around the poles, it's just the passing light rays. That's gravitational lensing again.

Drawing much closer to the event horizon, the gravitational lensing would become so extreme that one of my robots could look straight ahead and eventually see its own back, the light forever trapped in an eternal circle.

So, that's our tour of the supermassive black hole at the center of the Andromeda galaxy. Pretty amazing.

Also amazing, nothing in the mathematics led scientists to imagine that black holes could get that big. As strange as they are, ordinary stellar mass black holes were at least predicted by theory; supermassives are a complete surprise.

ANDREA GHEZ: For the stellar mass black holes, people thought about them from a theoretical perspective, and then we found them observationally. The supermassive black holes, the story has been inverted. We actually found evidence of them observationally first. And now we're working on the theory of "How did these things come into being?"

JANNA LEVIN: We already know that stars can collapse to create ordinary black holes. But supermassives are bigger by many orders of magnitude. Cygnus X-1 is 15 times as big as our sun. The supermassive at the center of our Milky Way is 4,000,000 times as big as our sun. The one in the Andromeda galaxy is 100-million times as big as our sun, and it's not the biggest, not even close. There are supermassives 10-, even 20-billion times the mass of our sun.

How is it possible to make such gigantic black holes? Could supermassives have come from collapsed stars? That seems very unlikely; we don't know any stars billions of times bigger than the sun.

NEIL DEGRASSE TYSON: We know about black holes you might get from a dying star. They have several times the mass of the sun contained within them. But millions of times the mass of the sun? If that's the case, a dying star cannot have possibly made it.

JANNA LEVIN: So, do these supermassives, millions or even billions of times heavier than the sun, somehow just grow, packing it on like voracious giants?

SHEP DOELEMAN: The wild thing about black holes is that they feed. They're constantly devouring anything that comes within their sphere of influence, so they grow.

JANNA LEVIN: But how exactly do they grow? What do they eat, and where do they find it?

PRIYAMVADA NATARAJAN: We believe that black holes grow by accretion of gas. And the way this works is that you have a lot of gas around in the center of a galaxy, and this gas would then assemble and form an accretion disk.

JANNA LEVIN: The accretion disk is made up of hydrogen, helium and other elements in a gaseous form. The immense gravity of the black hole pulls the gas in toward it. As it swirls around, it orbits closer and closer to the black hole, and the feeding begins.

PRIYAMVADA NATARAJAN: The stuff in the inner regions would get slowly pulled in, sped up, will reach the event horizon, and then that's it.

JANNA LEVIN: Whatever gas crosses the event horizon disappears forever. The black hole has absorbed that material.

PRIYAMVADA NATARAJAN: So, it actually adds to the mass of the black hole.

JANNA LEVIN: So, this is one way a black hole can grow, gradually nibbling gas and dust, but it's not the only way. Cygnus X-1 has been slowly stripping material off a nearby star, a process that will likely go on for thousands or millions of years.

But what if a black hole could rip an entire star apart in just a matter of years or even weeks? That would be a very violent event. And a team of space explorers is on the lookout.

This is the Operations Control Center for a space telescope…

SCIENTIST #1: I've got 5 by 5. We show beginning of track as 0330.

JANNA LEVIN: …the Chandra X-ray Observatory.

SCIENTIST #2: Sixteen minutes, one, six.

JANNA LEVIN: Orbiting up to 86,000 miles above the earth, Chandra takes high-resolution images of objects that emit X-rays.

This is one, a short-lived, extremely violent event called a "transient," which fascinates James Guillochon.

JAMES GUILLOCHON (Harvard-Smithsonian Center for Astrophysics): Supernovae, the destruction of planets by their host stars, yeah, I'm just fascinated with destroying things for science.

JANNA LEVIN: James is investigating a mystery discovered by a colleague, Dacheng Lin. This blur on James's screen is actually a massive sudden burst of X-ray energy, caught by accident.

JAMES GUILLOCHON: This little smudge popped up in the background of this image, and, given its great distance, it's actually tremendously bright.

JANNA LEVIN: Could it be a black hole caught in the act of being born, in the violent collapse of a huge star? A supernova? Perhaps. But the intense radiation released by a supernova would only linger for a few months. So, how long has this mystery object been blasting out X-rays?

To find out, they look at images of that same part of the sky taken at earlier dates: 2015, 2011, 2008, 2005-July, 2005-April: no X-rays detected.

But the X-rays are there, just three months later, in July. And the powerful, bright signal has continued for more than 10 years, from July 2005 to the present, far too long to be a supernova. So what could it be?

A black hole that's not feeding is quiet and completely dark, it won't show up on any telescope. But a black hole that is feeding is different, when it feeds, it blasts out X-rays.

So, could this be a black hole that's suddenly begun devouring something big?

NEIL DEGRASSE TYSON: What effect will this have on anything that comes near? What would it do to a star that wanders too close? Well, it will flay a star layer by layer, ultimately devouring the entire star.

JANNA LEVIN: Unlike Cygnus X-1, this is no mere nibbling. This is a 10-year feeding frenzy, a massive black hole devouring an entire star in a cosmic blink of an eye. It's the result of a chance collision, when an unlucky star wanders too close and the black hole's extreme gravity actually rips it apart.

JAMES GUILLOCHON: The gravity from the black hole will progressively get stronger and stronger as the star gets near. And at that point, the star will begin to deform.

JANNA LEVIN: It's called "tidal disruption." It's similar to the way our moon's gravity easily moves all the world's oceans. The tides caused by a black hole would be billions of times stronger and much more violent.

PRIYAMVADA NATARAJAN: You have these events where a star could be ripped apart by the black hole, so you would see, sort of, a plume of light from the last gasp of the material in the star.

JANNA LEVIN: But there is a chance for some part of the star to escape, as James illustrates.

JAMES GUILLOCHON: As the star is elongated by the black hole's tidal forces, it will essentially be feeding the black hole at the same time as half of it is trying to escape.

So, everything above this point, approximately, will have the chance of leaving the galaxy. It's moving that rapidly. And everything below this point will fall back onto the black hole and eventually be consumed by it.

JANNA LEVIN: So, this is another way for a black hole to gain weight. Unlike the slow steady nibbling of Cygnus X-1, this black hole is devouring most of an entire star in one gulp.

But whether a black hole feeds suddenly, by swallowing half a star, or steadily, through accretion, astronomers still face a problem when they try to understand how supermassives got so big: the timing problem.

The trouble begins with the very oldest supermassives: quasars, those very bright, very distant and ancient objects, first discovered in the early 1960s.

PRIYAMVADA NATARAJAN: The conundrum was when we started finding these quasars, very bright quasars, very early on in the universe.

DALE KOCEVSKI (Colby College): They're giving off so much energy that they have to have very massive supermassive black holes at their center.

JANNA LEVIN: But quasars are extremely far away, which means they're part of the very early universe, which began nearly 14-billion years ago.

PRIYAMVADA NATARAJAN: Bright quasars, six-hundred-million years after the Big Bang, a fraction of today's age.

JANNA LEVIN: And they're enormous.

PRIYAMVADA NATARAJAN: So, billion solar mass black holes, these behemoths, had to be in place when the universe was about 550-million years old.

EILAT GLIKMAN: Now, you have a problem, because you have to grow something really big, really fast. And you are bumping up against, sort of, physical limits.

JANNA LEVIN: Whether a black hole is nibbling or gulping down its meal, it turns out that accretion, how black holes feed, has a speed limit.

Named after English astronomer Arthur Eddington, the "Eddington limit" will not allow a black hole to feed too fast, because of the light blasting out from its own accretion disk.

EILAT GLIKMAN: Light has a pressure. So, photons can impart a force on something. We see this in winds from stars; light is pushing out gas. So, there's a limit to how fast you can feed a black hole before its own luminosity quenches its own growth.

JANNA LEVIN: So, given this speed limit, how did early supermassives, quasars, get so big, so fast? Could there be a way to bypass the speed limit entirely?

PRIYAMVADA NATARAJAN: The problem is still time itself. How do you grow them to a billion times the mass of the sun? What are the conditions that you need for that kind of growth?

JANNA LEVIN: Some scientists are now asking, "What if there's a way to create a black hole that's already much more massive from birth, giving it a head start?"

PRIYAMVADA NATARAJAN: If there was a physical mechanism that would allow you to make a black hole seed, which was much more massive from the get-go, then the timing crunch is not as much of an issue and the growing problem is not as acute.

JANNA LEVIN: The answer, some believe, is to create a black hole directly from a cloud of gas: a scenario called "direct collapse."

It starts with gas clouds made of hydrogen, helium and other elements, the same raw materials from which stars are born.

EILAT GLIKMAN: The denser clouds will start to collapse under their own gravity. And as they collapse, parts that are more dense will collapse more quickly. And so, what happens is the cloud fragments.

JANNA LEVIN: Those fragments continue collapsing until the hydrogen atoms within them begin to merge, nuclear fusion begins and stars are created.

But what if a giant gas cloud collapsed without making stars?

PRIYAMVADA NATARAJAN: We realized that there are a set of physical conditions that would allow you to form a very large gas disk prior to the formation of any stars. So, this gas disk starts getting unstable, that would allow the mass to sort of flow into the center very, very rapidly, and make a very massive black hole.

JANNA LEVIN: It's something we've all seen in nature, from tornadoes to bathtubs: a vortex, but on a supermassive scale.

PRIYAMVADA NATARAJAN: If you're in a bathtub, and you pull the plug out and you see the water flowing in in a vortex, very fast down to the center, that's exactly what happens.

JANNA LEVIN: Direct collapse might be a way to create very large black holes, early in the universe, from enormous gas clouds, completely skipping the star stage.

Because they would be so large already, at birth, these direct collapse black holes would have a head start, helping them to quickly grow into the enormous young supermassives we see in the distant universe.

PRIYAMVADA NATARAJAN: You could, potentially, have these direct collapse black holes, so black holes whose original masses, seed masses, the initial masses, are about 10,000 to maybe a hundred-thousand times the mass of the sun, and that they form from the get-go with that mass.

JANNA LEVIN: Direct collapse may explain how enormous early supermassives got their start. But there's another fundamental question about supermassives. What is their role in the universe? Is their existence just a matter of chance? Or are they connected in some larger way to the very structure of the cosmos?

Supermassive black holes don't exist in isolation; they seem to live in partnership with galaxies. Collections of millions, billions or even trillions of stars, bound together by gravity, galaxies are the fundamental building blocks of our universe. So are the supermassive black holes at their centers somehow fundamental to their very existence?

NEIL DEGRASSE TYSON: We now just assume every galaxy, even ones we have yet to confirm, will have a supermassive black hole in their center.

DALE KOCEVSKI: It could be that instead of simply being oddities, that they are a key component to galaxies, a key component to the universe.

SHEP DOELEMAN: We've come, in a very short time, to realize that they likely inhabit the centers of all the galaxies, and that can really only happen if there's some symbiotic relationship between the evolution of a galaxy and the supermassive black hole in its core.

JANNA LEVIN: What could that relationship be? One intriguing clue relates to size.

CHUNG-PEI MA (University of California, Berkeley): The bigger the galaxy is, the more massive the black hole appears to be. So, these black holes at the center seem to know about their larger scale environment.

JANNA LEVIN: So, which comes first: the galaxy or the supermassive black hole? It's not that simple. It appears they somehow grow in tandem.

ANDREA GHEZ: It's hard for one to form first and affect the other, so today we think that whatever formed one had to form the other as a byproduct of that process, and that there has to be some feedback mechanism between the black hole and the galaxy that keeps the growth of the two in lock sync.

JANNA LEVIN: The way galaxies grow is by forming new stars from clouds of hydrogen gas.

ELIZABETH MCGRATH (Colby College): Gas is essentially the fuel for star formation, just like gas is the fuel for our cars. And so, if you run out of gas, you run out of new stars.

JANNA LEVIN: So are supermassive black holes somehow interfering with star formation?

EILAT GLIKMAN: When a black hole is growing, a tremendous amount of energy is being liberated and sent out into the galaxy. And so, we think that some of that energy goes to warm up gas, and gas that's too warm will not form stars anymore.

JANNA LEVIN: The heat produced by a growing black hole makes it impossible for stars to form nearby.

EILAT GLIKMAN: And so, one way that a growing black hole can influence its host galaxy is by quenching the star formation.

JANNA LEVIN: In effect, the growth of the supermassive determines whether or not its host galaxy grows or stagnates.

PETER GALISON: They have a kind of eating phase and then a quiescent phase. So they seem to be involved with the formation of the galaxy in that way, and then stabilizing of the galaxy at the same time.

JANNA LEVIN: So these mysterious supermassives may actually control the building of the universe, not so much by their size, but by the way the energy they generate shapes galaxies.

PRIYAMVADA NATARAJAN: By mass, if you count up all the black holes in the universe, the tiny ones as well as the supermassive ones, the ultra-massive ones, black holes are nothing. However, energetically, how much power the galaxy gets and at what time, as it assembles, seems to be dictated by the central black hole. So, they might well be the key players in the universe.

JANNA LEVIN: In the next two years, NASA plans to launch the James Webb Space Telescope. Humanity's most powerful telescope ever, the James Webb is designed to look in the infrared, allowing it to see farther back in time than Hubble, getting a look at the first stars and galaxies that formed after the Big Bang. Hopes are high that the James Webb Space Telescope will help solve many of the remaining mysteries about the earliest supermassive black holes.

NEIL DEGRASSE TYSON: The James Webb Space Telescope is tuned specifically to observe the early universe, when galaxies were being born. That could give us deeper understanding of how you end up with a supermassive black hole in your galaxy to begin with.

ANDREA GHEZ: Technology is moving really fast, and as result we have really fundamental new views of the universe. I think we are really living in a golden era of astronomy.

JANNA LEVIN: And the James Webb Space Telescope isn't the only new development that promises to solve some of the mysteries around black holes.

SCIENTIST: I believe have infrared components.

JANNA LEVIN: A group of scientists, led by Shep Doeleman, is now attempting the impossible: to take a picture of a black hole.

SHEP DOELEMAN: It's interesting that we can say something about the accretion flow near the black hole at all.

FERYAL ÖZEL: And if some of this linear behavior survives, maybe we'll have a way of interpreting it.

JANNA LEVIN: The project is called the Event Horizon Telescope.

SHEP DOELEMAN: The basic goal of the Event Horizon Telescope is really to see the unseeable. It's to bring into focus something that science has told us for many, many years is precisely something we can't observe, the, the black hole.

JANNA LEVIN: Their primary target is Sagittarius A*, the supermassive in the center of our Milky Way galaxy. They're using a global network of radio telescopes.

SHEP DOELEMAN: We need good weather at eight different telescopes around the world, and that is a tall order.

JANNA LEVIN: But if black holes are invisible, what exactly do they hope to photograph?

FERYAL ÖZEL: What we are trying to photograph, really, is the shadow. So, as this gas around the black hole swirls inwards and actually hits the event horizon, it leaves a silhouette, a very well defined shadow on the surrounding light. So, really, it should look like a doughnut with its very well defined hole. And that's the picture that we're after.

SCIENTIST: I can into frequencies I get a 2-pi square there.

JANNA LEVIN: The team has conducted their first observing run, and is processing the data now.

SHEP DOELEMAN: And you're saying the velocity…

JANNA LEVIN: It's hoped that these new technologies will give us an unprecedented view of black holes in our universe.

But there is one new technology that is already delivering results.

And that brings us back here to LIGO—a key player in the black hole drama—to an idea that took root way ahead of its time: gravitational waves.

With general relativity, his theory of gravity, Einstein predicts that when an object moves, it can create ripples in space and time, an actual squeezing and stretching of space itself.

One of the Holy Grails of 20th century physics was to detect these gravitational waves.

RAI WEISS: That was not easy to do with general relativity, because all the effects that you could think of were infinitesimally small, very, very difficult to measure.

JANNA LEVIN: The thinking was if gravitational waves could be measured, it would confirm Einstein's prediction. And there could be an added benefit: it might also prove the existence of black holes and help solve the mystery of how supermassives grow.

But how to detect gravitational waves? In 1970, the problem caught the attention of a young experimental physicist, Rai Weiss. Rai had the perfect background to hunt for gravitational waves. For decades, he'd been working with more familiar waves, sound waves.

RAI WEISS: We were immigrants; we were German Jews, and a lot of our friends were very, very interested in music.

JANNA LEVIN: Rai devoted himself to coaxing every subtle nuance he could out of recorded music.

RAI WEISS: Those records had a terrible problem. When the music was loud, it sounded wonderful; when the music was real quiet and slow, what you heard was this "shhhh," like that, a hissing noise. And that was so annoying.

JANNA LEVIN: The lessons he learns trying to eliminate noise in recordings will pay off later, when Rai turns his attention to detecting gravitational waves.

RAI WEISS: You have to understand how a gravitational wave does its dirty work.

JANNA LEVIN: As a physics problem, gravitational waves are not unlike sound waves.

RAI WEISS: Suppose the wave comes from something that is in some way moving and oscillating.

JANNA LEVIN: A sound wave compresses and expands air, a gravitational wave compresses and expands space and everything in it.

RAI WEISS: If a wave came through the earth, it would cause space to expand momentarily and then contract again. It keeps doing it, so it's just a thing that goes "blip, blip, blip, blip," right along, like that.

JANNA LEVIN: So, how to measure the extremely tiny expansion and contraction of space? Rai's idea was to use light. Send a beam of light from one place to another and measure the time it takes to get there.

That's how the exact distance to the moon was calculated: bouncing a laser beam from the earth off a mirror left behind by Apollo 11 astronauts. From the duration of the round trip, scientists could determine the distance.

Rai came up with an ingenious design for an instrument that uses lasers and mirrors to detect the faint expansions and contractions of space that would be caused by a gravitational wave. It's called a laser interferometer.

It works by firing a laser into a splitter. Half of the light continues straight ahead towards one mirror, while the other half is sent towards another mirror. The distant mirrors bounce the light beams back, where they rejoin at a photo detector. If the distances the two beams travel are exactly the same, the system is designed so the two beams cancel each other out; the detector sees nothing.

RAI WEISS: You've set the trap to measure the gravitational wave. Now, comes the gravitational wave that's coming, let's say, at this structure.

JANNA LEVIN: If a gravitational wave passes through, it briefly changes the length of the arms. The light beams no longer arrive back at the same time to cancel each other out.

A gravitational wave hits, light appears at the detector. The trap has sprung.

RAI WEISS: That's the basic idea. It's a very straightforward measurement.

JANNA LEVIN: A clever idea and simple in principle, but the devil—and the Nobel Prize—lie in the details. The difference in length between the two arms would be tiny beyond imagining.

KIP THORNE: How tiny? Well, take the size of an atom, it's less than that. Go down by a factor of a hundred thousand—that's the nucleus of an atom—it's less than that. It was a hundred times below that. So we're talking about really itsy bitsy, teeny weeny.

I thought it was crazy. I think everybody's initial reaction to the idea was, "This is going to be impossible."

JANNA LEVIN: In 1973, Kip Thorne puts his skepticism on the record in a classic textbook, doubting it will ever work. But Kip has never heard Rai Weiss explain his plan in detail, and when he does…

RAI WEISS: We spent the whole night talking. And so I said "No, no, no, it's very possible." And within no time at all, 20 minutes, maybe half an hour, Kip was solidly understanding this thing, and he says, "Yup!"

KIP THORNE: And I ate crow the rest of my career, because once I had talked with Ray about it in detail, I decided I would spend a large fraction of the rest of my career helping the experimenters.

JANNA LEVIN: But it will take 40 years and enormous sums of money to bring Rai and Kip's vision to reality.

FRED RAAB (LIGO Hanford Observatory / CalTech): Getting LIGO funded was extremely controversial: hundreds of millions of dollars to detect a signal that had never been seen before? There were many people who feared that LIGO would suck the money out of the room. And so there was a lot of controversy. What everybody could agree on was this was extremely difficult.

JANNA LEVIN: With such a sensitive instrument, one of the biggest challenges is Rai Weiss's old hi-fi nemesis: noise.

RAI WEISS: Ground motion, the seismic motion of the earth, acoustics noise, sounds, everything would tend to move that mirror.

JANNA LEVIN: Turns out, even the emptiness of a total vacuum creates a potentially crippling problem. At subatomic distances, the weird randomness of the quantum world causes a ruckus in the mirrors.

KIP THORNE: This quantum noise is due to quantum fluctuations. These mirrors are doing what an electron does inside an atom: they're jiggling around.

JANNA LEVIN: Exquisite sensitivity, extreme vacuum, hundreds of thousands of electronic circuits: LIGO is one of the most complex instruments in the history of science. And, as a final means of eliminating false signals, they build not one, but two complete installations, one in Washington State and another in Louisiana.

NEIL DEGRASSE TYSON: And so the LIGO designers did it right. They designed more than one detector, separated from one another by great distances, so that if you detect something in one and not in the other, then, you know, go back and check your electronics. Check to see if it was April Fools' Day, and somebody didn't just tweak the knobs.

JANNA LEVIN: Early fall, 2015: Both locations are operating but the first official science run has not yet begun. They're still testing.

In the early hours of Sunday, September 14, 2015, a scientist in Louisiana makes a fateful decision.

Robert Schofield has been working all weekend, doing final calibrations.

ROBERT SCHOFIELD (University of Oregon/Laser Interferometer Gravitational-Wave Observatory, Hanford): Alrighty, let's take a spectrum…

JANNA LEVIN: He has one last test.

ROBERT SCHOFIELD: So, let's see where this computer's getting its power.

JANNA LEVIN: But it's late and the equipment is not cooperating.

ROBERT SCHOFIELD: It was about four or so in the morning, and we still had about another hour of work to do. And we were like, "Yeah, things aren't working so well, and I'm really tired. Let's not do this last hour or so of work."

JANNA LEVIN: They call it a night.

And 40 minutes later, in the silence of their inactivity, they open the door to history: a powerful gravitational wave rumbles through both detectors, Louisiana and Washington.

Had Robert Schofield worked 40 more minutes that night, with the instruments in test mode, a signal that had been on its way for 1.3-billion years would never have been recorded.

ROBERT SCHOFIELD: So, I like to say, you know, one of my biggest contributions to LIGO has been my laziness that day.

RAI WEISS: I got an email from somebody here saying, "Hey, look, look at this place on the web."

I looked at that, and I said, "Holy !"

KIP THORNE: It was so strong that you could see it by eye in the data. It was too good to be true.

JANNA LEVIN: But it was true. In fact, it was loud and surprisingly clear.

RAI WEISS: And it just sang at you. There it was, standing out.

JANNA LEVIN: The signal lasted less than a second, but, in that briefest of moments, it delivered a cosmically profound message, more than a billion years in the making, proving the existence of black holes.

KIP THORNE: So, what we saw in the signal involved oscillations of the mirrors that were slow at first, became faster and faster and faster. And this was precisely the kind of behavior that you would expect from gravitational waves caused by two black holes going around each other, spiraling together.

JANNA LEVIN: Two massive black holes, one 29 times the mass of the sun, the other 36 times the mass of the sun, whipping around each other hundreds of times a second, finally completing their act of mutual destruction by merging, creating a single, larger black hole of 62 solar masses.

The violent merger converts some of the mass into an apocalyptic release of energy, beyond anything ever before witnessed.

KIP THORNE: The collision, in effect, creates a very, a veritable storm in the fabric or the shape of space and time. As though you had taken three suns, you had annihilated them completely, converted it into gravitational waves. The power was 50 times higher than the output power of all the stars in the universe put together, in a fraction of a second, but the most powerful explosion that humans have ever had any evidence for, with the exception of the Big Bang.

JANNA LEVIN: Since that very first signal in September 2015, LIGO has detected several more collisions of black holes. In October 2017, Rai Weiss, Kip Thorne and LIGO's former director, Barry Barish, received the Nobel Prize.

The LIGO discoveries prove that black holes can merge, one way they can grow bigger quickly.

PETER GALISON: More and more evidence of these merging black holes tells us there are a lot of these stellar black holes around, that they can find each other and, and merge.

JANNA LEVIN: And, the discovery opened an entirely new way of observing the universe.

PRIYAMVADA NATARAJAN: We've always thought of astronomy as an observational field where we are looking at radiation. We are seeing things. But this is not radiation. This is something much more fundamental. These are, sort of, fundamental tremors in space-time itself. We can now hear the universe.

JANNA LEVIN: For the first time, astronomers have simultaneously seen and heard a cosmic event.

In August 2017, LIGO detected gravitational waves from a collision of two neutron stars. Black holes are empty space, but neutron stars are dense dead stars that can crash together and light up the skies. When telescopes and satellites around the globe pointed in the direction of the sound, the world saw fireworks in an explosive collision and afterglow. Possibly, the collision resulted in the creation of a new black hole.

But unless we observe the formation of a black hole, there is much we will never know. Because so much about black holes is irretrievably out of our reach, we can never know where they came from, what's inside or their history.

But we can imagine their future. The number of black holes in the universe is increasing, and they're getting bigger.

Stars collapse, black holes feed and merge, new ones form.

Could it be that one day everything will end up inside them and they will rule the universe?

Untold trillions upon trillions of years after this happens, and the last bits of matter cross their event horizons, black holes themselves may radiate away and vanish from this reality.

Their mysteries are many, and we're just starting to unlock the secrets of these strange, powerful places. But one thing is certain: black holes will continue to intrigue us, tantalize us and challenge both our science and our imaginations.