♪ ♪ JANNA LEVIN: Of all the objects in the cosmos... Planets... Stars... Galaxies... (explosion echoes) LEVIN: None are as strange, mysterious, or powerful as black holes.

♪ ♪ NEIL DEGRASSE TYSON: Black holes are the most mind-blowing things in the universe.

PRIYAMVADA NATARAJAN: They can swallow a star completely intact.

FERYAL OZEL: Black holes have these powerful jets that just spew matter out.

LEVIN: First discovered on paper... PETER GALISON: On the back of an envelope, some squiggles of the pen.

LEVIN: ...the bizarre solution to a seemingly unsolvable equation... NATARAJAN: A mathematical enigma... LEVIN: Einstein himself could not accept black holes as real.

People didn't even believe for many years that they existed.

Nature doesn't work that way.

♪ ♪ LEVIN: Yet slowly, as scientists investigate black holes by observing the effect they have on their surroundings, evidence begins to mount... ANDREA GHEZ: That is the proof of a black hole.

TYSON: Millions of times the mass of the sun.

LEVIN: Cutting-edge discoveries show... We did it!

(applause) LEVIN: ...black holes are very real.

I thought it was crazy.

I said, "Holy (bleep)!"

♪ ♪ LEVIN: But what exactly are they?

If we could visit one, what 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?

It's quite a journey.

♪ ♪ LEVIN: "Black Hole Apocalypse."

Right now on "NOVA."

♪ ♪ ♪ ♪ LEVIN: 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.

I'm amazed that these objects actually exist.

LEVIN: Black holes defy our understanding of nature.

Black holes are the greatest mystery in the universe.

LEVIN: They're completely invisible, yet powerful beyond imagining.

They can tear a star to shreds.

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

You really want to avoid them at all cost.

LEVIN: Black holes even slow time.

Once thought too strange to be real... (glass shatters) ...black holes shatter our very understanding of physics.

But we're learning they may somehow be necessary for the universe we know to exist.

They might well be the key players in the universe.

LEVIN: What are these strange, powerful objects, outrageous and surprising?

Where are they?

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.

♪ ♪ (loud whooshing, rumbling) 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.

(chirp) 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.

(drum beats) The black holes create waves that spread outward.

(drum beats) 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.

This gravity wave is moving its way through the universe at the speed of light.

♪ ♪ LEVIN: The wave races by stars.

On the young Earth, supercontinents are forming.

Microscopic organisms have just appeared.

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

LEVIN: Dinosaurs roam the Earth.

The wave is still moving.

LEVIN: It zooms through clouds of dust.

And then it nears the Milky Way Galaxy.

LEVIN: The Ice Age is just beginning.

We're troglodytes, drawing in caves.

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 13, 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.

(faint chirp) 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.

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

LEVIN: Bizarre.

GLIKMAN: A physical entity with infinite density.

No beginning, no end.

LEVIN: They pull things in... and warp light.

Approach one, and time itself begins to change.

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

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

(radio chatter) LEVIN: Fall in, and you'd be squeezed as thin as a noodle.

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

♪ ♪ LEVIN: Today, we know more about black holes than ever before.

But the more we learn, the more mysterious they become.

GHEZ: They're the most exotic objects in the universe.

We don't have the physics to describe them.

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

(static hisses, machine beeping) ANNOUNCER (on film): Now... MAN: Gravity's at max... ANNOUNCER (on film): Man is about to enter... the black hole!

(machine beeping) 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?

♪ ♪ (bell rings) (people chatting) We're all familiar with gravity.

(plates crash) Yep, it's Friday.

LEVIN: 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.

MAN (on film): Lift-off, we have lift-off at 9:34 a.m.

But Newton's laws can only describe gravity's effects, not explain what it is.

NEWTON (dramatized): Hm.

And here's where Albert Einstein comes in.

(camera clicking) 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 toward the 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.

(cannon firing) Eventually, if I get the apple moving really, really fast-- say, 17,000 miles an hour-- its curved path matches the curve of the 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.

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.

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

GALISON: And that was Einstein's conception.

There are no forces anymore.

There's just objects bending space-time and other objects following the straightest line through it.

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.

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

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

GALISON: Einstein had gotten at it through a series of approximations.

But Schwarzschild... (explosion echoes) 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.

LEVIN: But Schwarzschild isn't done.

In his solution to Einstein's equations, he discovers something Einstein himself had not anticipated.

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

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.

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.

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.

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.

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.

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

He didn't like this idea.

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

(thunder rumbles) LEVIN: 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.

(light clicks off) (airplane engine roars) (explosion echoes) But in the coming decades, physicists learn more about the atom... (explosion echoes) 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.

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: Stars are in many ways similar to living creatures.

Like humans, they have life cycles.

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

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.

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.

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.

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.

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.

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.

GLIKMAN: And gravity wins.

And so the entire star collapses.

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: (explosion roars) a supernova.

The more mass, the more gravity.

So if the remaining core is massive enough, gravity becomes unstoppable.

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

(explosion roars) 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.

NATARAJAN: Einstein and Eddington, all the sort of, you know, pre-eminent 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.

(explosion) 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.

The term "black hole" actually originates in India.

The Black Hole was the name of an infamous prison in Calcutta.

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.

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

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

NATARAJAN: Any star that is born with a mass that's about ten 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.

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 ten solar masses-- meaning ten 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.

(engine runs, machine chirps) ♪ ♪ (spacecraft engines roaring) ♪ ♪ LEVIN: 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.

♪ ♪ (spacecraft beeping) 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... (audio slows): I appear to be slowing down.

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.

♪ ♪ (spacecraft beeping, air hissing) 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.

(explosion) 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.

♪ ♪ (horn honks) (bat hits ball) (crowd cheers, organ plays) 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.

(Geiger counter crackles) ♪ ♪ These discoveries give astronomers important new tools that will revolutionize the hunt for black holes and dramatically expand our vision.

(machine beeping) BARTUSIAK: What our eyes can perceive is a very narrow part of the electromagnetic spectrum.

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... (low-pitched hum) microwaves, infrared and ultraviolet light... (higher-pitched hum) X-rays, and gamma rays.

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

BARTUSIAK: They discovered that the universe looked very different than it did in visible light.

It was much more violent, much more active.

Where in visible light a galaxy could look very serene, it was very violent and active in x-rays.

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!

LEVIN: X-rays come from the high-energy end of the spectrum.

(buzzing) What is creating all this energy?

This much thing is certain: whatever the source, it is invisible to ordinary telescopes.

And it is hot.

PAUL MURDIN: X-rays come from things which are at temperatures of millions of degrees.

Even tens of millions.

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

Cygnus, it was in the constellation Cygnus; X, it was an x-ray source; one, it was the first one you found.

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

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?"

♪ ♪ 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?

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.

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.

Sometimes it's coming towards you, sometimes it's coming away.

Sometimes it's coming towards you, sometimes it's coming away.

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.

MURDIN: It very clearly was a binary star, a double star.

The star was moving around and around with a period, going around once, every 5.6 days.

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

MURDIN: There was no trace in the spectrum of the second star.

There was one star there.

There wasn't the second star there.

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.

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.

THORNE: Neutron stars have a maximum mass.

We were pretty sure the maximum mass was less than three solar masses.

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.

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

And the answer came out to be six times the mass of the sun.

(buzzing) 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."

♪ ♪ 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.

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.

There are a lot of funny people in California.

(chuckles): You know, a lot of hippie-type people.

LEVIN: People like theorist Kip Thorne.

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."

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.

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

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.

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 thumb-printed off on this bet, conceded the bet in my absence.

I came back from Russia and discovered that he had conceded.

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 ten.

And the question wouldn't be answered for another 20 years, until astronomer Mark Reid became intrigued by the puzzle.

That range was something like three to 30 solar masses, times the mass of the sun.

So there wasn't really solid proof that it was so massive that it had to be a black hole.

LEVIN: 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.

REID: 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.

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.

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.

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.

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.

Then six months later, the Earth goes around the sun to the other side.

We get a different vantage point from Cygnus X-1.

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.

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.

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.

REID: We're generally talking about what we call milliarcseconds, which are one one-thousandth of a second of arc.

LEVIN: 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.

We take ten 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 a diameter of the size of the Earth.

That gives you incredible angular resolution.

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

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.

♪ ♪ LEVIN: 40 years after it was identified as a possibility, Cygnus X-1 is now widely accepted as the first confirmed black hole.

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.

♪ ♪ 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?

(spacecraft whooshes) ♪ ♪ 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.

(hissing) 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 super-giant 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.

OZEL: 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.

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.

OZEL: 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.

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.

(whooshing) 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.

(spacecraft whooshes) ♪ ♪ LEVIN: What was once a bizarre mathematical curiosity has now become quite real.

(explosion roars) 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.

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.

♪ ♪ LEVIN: At least 20 X-ray binaries that include a black hole like Cygnus X-1 have been found in the Milky Way.

♪ ♪ (explosion) And recently astronomers announced finding evidence of 10,000 single black holes near the center of our galaxy, leading scientists to believe there are millions more of these stellar corpses out there.

The proof is in.

Black holes are real.

♪ ♪ But where are they?

How big can they get?

Are black holes merely curiosities, or do they play a role in the universe?

Is it possible we might not exist without them?

They might well be the key players in the universe.

LEVIN: Ever stranger and closer to home, the search for black holes continues.

♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪