♪♪ [ Film projector clicks, whirring ] ♪♪ [ Clock ticking ] -Time -- we think of it as regular as clockwork... ...ticking out the steady progress of the universe.

But time is not constant.

And that holds the key to the secrets of creation.

And the two people who helped us unlock those mysteries are joined by a cosmic coincidence of timing.

♪♪ On March 14, 1879, Albert Einstein was born in Ulm, Germany.

And on March 14, 2018, Stephen Hawking died in Cambridge, England.

They are undoubtedly the most recognized scientists in the world.

And between them, they have transformed our understanding of everything.

♪♪ -If Albert Einstein had not lived, it is hard to imagine what the 20th century would've been like.

[ Train whistle blows ] -In the early 1900s, Albert Einstein developed an idea so revolutionary it changed the course of history.

It was called relativity.

♪♪ -Many of Einstein's ideas are out there.

At best, they're counterintuitive.

Others are simply mind-bending.

[ Pop ] -His giant leaps of imagination gave us a new understanding of the fabric of reality... ♪♪ ...and redefined our concepts of time and space.

♪♪ And, crucially, his work inspired Stephen Hawking... ♪♪ ...who would develop Einstein's ideas to reveal the most extraordinary phenomena in the universe.

♪♪ -Black holes are stranger than anything dreamed of by science-fiction writers, but they are firmly matters of science fact.

♪♪ -Hawking is incredibly innovative, able to make discoveries that seemed totally counterintuitive and totally impossible.

♪♪ -Right up to his death, he was redefining our understanding of the universe.

-He said he hadn't been this excited in 40 years.

When I miss him the most is when we figure something out.

I'd like to tell him.

♪♪ -This is the story of how these two remarkable scientists showed us that the universe is stranger and more wonderful than we ever imagined.

♪♪ ♪♪ [ Bells tolling ] [ Clock ticking ] The story starts in this apartment in Bern, Switzerland.

Einstein lived here with his first wife and fellow physicist, Mileva, for two years while he did his greatest work.

We can only imagine the conversations these walls have heard, the moments of genius they have witnessed.

But when he arrived here in 1903, there was no sign at all that Einstein was destined for greatness.

In fact, Einstein's progress had been distinctly underwhelming.

♪♪ -Albert Einstein was a rather unusual child.

It was very late in his early life that he began to talk, and so his parents, in fact, were worried about him for quite a while.

-So, Einstein, from a very early age, was independent-minded, focused on things that captured his imagination, but very quick to ignore things in school that he considered less important.

♪♪ He often fought with his professors.

He barely squeaked through his university education and had a great deal of difficulty finding a position after university.

♪♪ So, the one position Einstein could actually arrange was as an entry-level patent clerk in the patent office in Bern, Switzerland.

And Einstein entered literally at the bottom.

♪♪ -This was Einstein's lot in 1903 -- an academic underachiever in a dead-end job.

♪♪ But it was also a blessing in disguise.

-It was a perfect job for him in some ways, because he could go through the day and get paid and then spend his important time doing physics on the side.

-He just was able to live within his mind, and that, for all of humanity, was a great boon.

♪♪ -On January 8, 1942, Stephen Hawking was born in Oxford, England, coincidentally, on the 300th anniversary of the death of Galileo.

Unlike Einstein, he excelled academically from a young age and, although a respected scientist in his own right, he first hit the headlines in 1984 with the publication of "A Brief History of Time"... which tried to bring the theories of Albert Einstein to a wider audience.

♪♪ -Hawking is such a fascinating human being.

You can't really exaggerate the power of that intellect.

And a complicated man, you know, a complicated character.

♪♪ -Stephen was a lover of life.

He took me with him to Antarctica.

He, of course, went up in a suborbital flight, so he was weightless.

He enjoyed life to the full.

♪♪ -As a teenager, he was known to his classmates as "Einstein."

And at university, he often delighted in doing as little work as possible.

♪♪ -The physics course at Oxford at that time was ridiculously easy.

You didn't need to remember many facts, just a few equations.

♪♪ -But in 1962, when he was just 20, Hawking's happy-go-lucky lifestyle came to a grinding halt.

He was diagnosed with motor neuron disease and given just two years to live.

-Its first effect was to depress me.

I seemed to be getting worse fairly rapidly.

-But Hawking was not one to let adversity stand in his way.

-Stephen was the most stubborn man I have ever met.

He just absolutely refused to let his physical disability get in the way of doing anything.

♪♪ -Through the power of his mind, Stephen Hawking journeyed through the universe to the furthest corners of space and time.

-I certainly am happier now.

Before I got motor neuron disease, I was bored with life.

But the prospect of an early death made me realize life was really worth living.

♪♪ -Stephen Hawking and Albert Einstein never met, but they were joined together by an ability to conceptualize the universe in new and radical ways.

♪♪ [ Bell tolling ] And this new vision of reality began in Switzerland in 1905.

[ Bell tolling ] ♪♪ -1905 -- that's the year that we physicists refer to as "The Year of Miracles."

He wrote four papers that year.

Any one of them was worthy of a Nobel Prize.

♪♪ -In May of that year, Einstein wrote to a friend.

-I promise you four papers.

The first deals with the radiation and energy properties of light.

The second is a determination of the true size of atoms.

-The third explained the motion of particles in a liquid.

But it was the fourth paper, his theory of special relativity, that would change the world.

♪♪ -The fourth is a modification of the theory of space and time.

[ Bell tolls ] ♪♪ -Einstein wrote about how this medieval clock tower in Bern inspired him to think about the nature of time.

[ Clicking ] ♪♪ Time -- the thing we use to mark out the days and years, the one thing we rely on to be constant and unchanging.

♪♪ Einstein was going to show how we'd been getting that all wrong.

♪♪ He published his paper on September 26, 1905.

It was the beginning of his relativity revolution, and nothing would ever be the same again.

-There's before relativity and after relativity.

Once you learn it, your whole world view changes.

And that was the impact on the scientific community -- that the whole world changed.

-That's, you know, a spectacular moment for the species, when we can see what a human mind is capable of when all of the pieces come together in the right way.

♪♪ -Many of Einstein's ideas are out there.

At best, they're counterintuitive.

♪♪ Others are simply mind-bending.

♪♪ But that's just the reflection of a fact that Einstein understood very well, which is -- what is going on at the fundamental structure of our universe is very different from what our eyes and ears are telling us.

♪♪ -To understand how Einstein changed our concept of time... we first need to understand something fundamental about light and the way we see the world.

♪♪ -In the everyday world around us, everything that's not light moves in pretty much the same way.

And I'm gonna use this tennis-ball machine here to show you exactly what I mean.

♪♪ If I fire off these balls from the back of this stationary pickup truck, they fly off at 25 miles per hour.

But here's the thing -- no matter if I'm on the back of this pickup truck or if there's a person standing on the ground, everyone sees the same thing -- the balls flying off into the distance in the same way.

♪♪ ♪♪ But that changes if the truck starts to move.

As we accelerate, the motion of the truck affects the apparent trajectory of the balls.

If someone is standing on the back of the truck, the balls would appear to fly off with the same speed and take the same path through the air.

But for someone on the ground, they see something very different.

The balls don't travel nearly as far.

♪♪ And when the speed of the truck matches the speed of the balls, it doesn't look like they travel anywhere.

For the stationary observer, the balls just drop straight to the ground.

♪♪ As they say, it's all relative.

♪♪ -Everything in the universe behaves like this -- everything, except light.

Because, no matter where you are or how fast you're moving, you will always see light traveling at exactly the same speed.

♪♪ And Einstein use this fact to redefine our notion of time.

♪♪ -Well, the first problem that really launched Einstein's thinking had to do with light and its speed.

The speed of light seemed not to follow the traditional behavior of ordinary speeds of baseballs or any object that you typically throw.

-And Einstein worried that we maybe don't really understand light as clearly as -- as we should.

[ Film projector clicks, whirring ] ♪♪ -Einstein knew that, if he could understand the properties of light, then he could understand the way the universe really worked.

And he did it with a deceptively simple thought experiment.

[ Train whistle blows ] He imagined himself standing on a train platform, holding a clock that uses light to tell the time.

♪♪ The clock ticks as a single photon of light bounces back and forth between two mirrors.

On a nearby train, an identical clock ticks at an identical rate.

♪♪ Einstein then imagined that the train starts to pull away.

♪♪ And he realized that, as it did so, he would see the clock on the train start to behave very differently.

When the train is moving, as well as seeing the photon bouncing up and down, he also sees it moving forwards.

The photon is traveling further, and since the speed of light is constant, it means that each tick appears to take longer.

♪♪ For Einstein, the explanation was simple.

♪♪ Time is passing more slowly on the moving train.

♪♪ And the faster the train goes, the slower time passes.

♪♪ It was a staggering conclusion.

Einstein's claim was that time does not run at a fixed rate.

It is relative.

It can speed up and slow down, depending on how you are moving.

♪♪ -We're all used to an intuitive notion of time, which ticks off the same regardless of where you are or what you're doing, how fast you're moving.

That has no impact on the rate at which your clock ticks forward second after second.

And Einstein said, "Sure, that's our everyday experience of time, but that everyday experience is completely misleading."

♪♪ -But Einstein's genius went beyond simple thought experiments.

He was able to back his theories up with maths.

♪♪ -In this mathematical equation, Einstein was able to capture the effect of moving clocks slowing down.

And this has really, really dramatic effects.

If you look at the graph, you see that, if the speed of the train is small, compared to the speed of light, there's basically no difference.

The two times are equivalent.

But if you get closer and closer to the speed of light, you see this curve is shooting up, and the effect becomes really, really dramatic.

So, at 50%, one day on the train lasts 1.15 days on the platform.

At 75%, it's already 1 1/2 days.

At 99%, it lasts a week.

And if you get to a speed of 99 point -- and, here, I added twelve 9s -- as a percentage of the speed of light -- then one day on the train lasts 20,000 years on the platform.

♪♪ -This time-dilation effect doesn't just apply to Einstein's light clocks.

Exactly the same would apply to us, if we could travel fast enough.

[ Camera shutters clicking ] Einstein explained it by using twins as an example.

He reasoned that, if one twin left the Earth in a spaceship traveling at very high speed, they would experience time passing more slowly and would age much less than their twin left behind on Earth.

-If you do hop in the spaceship and move toward the nearest star at some high fraction of the speed of light -- 99%, right?

-- to you, it doesn't seem like that long.

The downside of that is that, if we came back, all of our friends back here on earth would be dead because many, many years would have passed for them.

♪♪ -It sounds like a crazy idea, but we've actually done this experiment for real.

NASA have identical-twin astronauts -- Scott and Mark Kelly.

-What's that?

You want us to -- we actually switched the name tags.

-[ Laughter ] -Actually... -And one of you shaved off the mustache.

-...he was the guy in space.

♪♪ -In 2015, Scott spent a year on the International Space Station, orbiting at over 25,000 kilometers per hour, while Mark stayed on the planet below.

When he returned to earth, Scott was younger than his twin by a full five milliseconds.

-In principle, time dilation should help us explore the universe.

It would allow us to travel arbitrarily large distances without aging and dying before we got there.

So, in principle, time dilation is a benefit.

♪♪ -Einstein's theory of special relativity could be the vital step that allows us to leave the Earth and explore the cosmos.

♪♪ But to do that, we would need to develop a way to travel at close to the speed of light, and that is a concept that was close to Stephen Hawking's heart.

♪♪ ♪♪ -Good afternoon.

We are here today to talk about Breakthrough Starshot and our future in space.

-In 2016, along with internet billionaire Yuri Milner, Hawking launched a venture called Breakthrough Starshot.

-I believe what makes us unique is transcending our limits.

The limit that confronts us now is the great void between us and the stars.

But now we can transcend it.

Today, we commit to this next great leap into the cosmos, because we are human, and our nature is to fly.

-The aim of Breakthrough Starshot is to develop the first spacecraft that can travel at a significant fraction of the speed of light.

♪♪ ♪♪ -In Virginia, aerospace engineer Zac Manchester is trying to turn Stephen Hawking's dream into a reality.

♪♪ In the '60s, the Wallops Island Flight Facility was used to test the capsules for the first manned space flights that would lead the way to the Moon.

-We choose to go to the Moon.

[ Applause ] We choose to go to the Moon in this decade and do the other things -- not because they are easy, but because they are hard.

-So, this is the Antares rocket and, at the top, behind that American flag, is a vehicle called the Cygnus.

At the back of the Cygnus, in kind of a trunk, we have a spacecraft that I built.

Unfortunately, as you can tell, the weather's not great and the last 2 or 3 times I've tried watching rocket launches, it's been the same thing.

I've never actually gotten to see something I've built launched.

♪♪ -Zac's spacecraft will be the first test of a completely new concept in space travel and will bring Stephen Hawking's dream just a little closer.

♪♪ -On this mission, we're testing this little guy.

We call this the Sprite.

It's currently the world's smallest spacecraft.

It's 3.5 by 3.5 centimeters and weighs 4 grams.

It's basically just a little circuit board.

And, on here, we've got a tiny computer we call a microcontroller, a radio transceiver, an antenna here for the radio, and a couple of sensors.

So, the plan here, if this rocket flies, is to deploy 100 of these in low-Earth orbit.

They're gonna network with each other, talk to each other, and then transmit a bunch of sensor data back down to the ground.

♪♪ -If it is successful, within a few years, we could be sending Sprites like these across the universe, traveling 1,000 times faster than anything we have built before.

♪♪ [ Squawking ] -Eventually, we'd like to be able to send spacecraft out to our neighboring solar systems -- right?

-- out to the stars.

And the closest star to us is called Proxima Centauri.

But Proxima Centauri is 4 light-years away from us, and with conventional rockets, it might take us 10,000 years to get there.

So we have in mind a different plan.

We're gonna build a giant laser on the ground.

This laser would be a kilometer across and have 100 gigawatts of power.

And then we're gonna build a really, really small, lightweight spacecraft to ride on it.

If we push a Sprite with this 100-gigawatt laser, we can accelerate it about as fast as a shell coming out of a cannon -- 60,000 G's.

And, within a few minutes, we could push this up to 20% the speed of light.

♪♪ -At 20% the speed of light, the time-dilation effects described by Einstein start to come into play.

♪♪ Time would travel more slowly on the spacecraft than it does for observers on Earth, who would be waiting a long time to see the results.

-Keep in mind, at 4 light-years, it takes the signals four years to get back to earth.

So, altogether, this mission might take 25 years.

That's not actually that long, compared to missions we've sent to the outer solar system already.

♪♪ -This is Mission Control, Houston.

Everything is set, ready to go for an on-time launch today.

-A few hours later, the clouds cleared and the rocket took to the skies.

-3, 2, 1...

Launch has been initiated.

-And we have liftoff of the NG-10 mission, taking Cygnus to the ISS.

♪♪ -And with it, we may have taken our first giant leap towards the stars, a journey that may, one day, allow us to send human explorers into the universe, a journey championed by Stephen Hawking and inspired by Albert Einstein.

♪♪ ♪♪ In 1909, Einstein's reputation was growing and he was made a professor at the University of Zurich.

♪♪ It was here he set to work on the next stage of his mission to revolutionize our understanding of the universe.

♪♪ He was going to rewrite the laws of gravity with his new theory of general relativity.

♪♪ Einstein knew that space and time were part of the same structure, that the whole universe was filled with a four-dimensional fabric, which combined time with the three dimensions of space.

It was called space-time and Einstein realized its key feature was that it could bend and flex.

♪♪ -Einstein had a stroke of genius when he saw that general relativity relates to the curvature of space-time, so general relativity gives space and time a shape.

♪♪ -Einstein's basic idea is that we need to think of space kind of like a fabric and that fabric is nice and flat when there's no matter or energy, which means that objects moving through space will move in a nice, straight trajectory.

Nothing too complicated about that at all.

But then the key idea is that if we have a massive object, like the sun, in space, merely by virtue of being within the environment, this massive object will warp the fabric.

And then another object -- think of it like a planet... ♪♪ ...will now go into a nice curved trajectory, will go into orbit... ♪♪ ...because it's rolling along a valley in the curved environment that the sun creates.

♪♪ And that's the way the force of gravity works -- space itself pushing planets to go into these nice, curved trajectories.

♪♪ This, in essence, is the essential idea of Einstein's general theory of relativity.

♪♪ -It seems a simple concept, easy to visualize in two dimensions, but space isn't a two-dimensional fabric.

It's not even in three dimensions.

It's a warped four-dimensional fabric that creates complex shapes that are seemingly impossible for humans to visualize.

♪♪ Once again, to complete his theory, Einstein used maths.

-So, Einstein is finally able to capture the curvature of space and time in a mathematical equation, and it's quite a beautiful and elegant equation.

On the left-hand side is the curvature, the geometry of space-time.

On the right-hand side is the distribution of energy and matter.

Basically, on the left-hand side is mathematics.

On the right-hand side is physics.

And it's quite remarkable that both are connected in one equation.

And the way to read this equation is that matter tells space-time how to curve, and space-time tells matter how to move.

-I think it's just one of those reminders about how really elegant and simple the universe is at its heart.

♪♪ -However, when Einstein published his paper in 1915, almost no one took any notice.

♪♪ -By that point, Europe had been engulfed by World War for more than a year.

[ Explosions ] Communication lines had been severed between, for example, Britain and Germany.

Trench warfare had dug in.

The war looked interminable.

So it was not, at first, seen as any particular great breakthrough, partly because many people had difficulty learning the news at all.

♪♪ -Einstein's theory may have been lost for decades, if it had not been for the work of a pioneering English astronomer -- Arthur Eddington.

♪♪ In 1919, Eddington took a telescope to the tropical island of Príncipe, in the Atlantic Ocean, to photograph a solar eclipse.

Einstein's theory predicted the passage of light would also be affected by the warping of space-time and that massive objects, like our Sun, could act like a magnifying glass by bending light around them.

[ Clicks ] The photos Eddington took of the eclipse showed that stars in the Hyades cluster appeared to be in the wrong positions.

The light coming from them had been bent.

♪♪ Einstein's theory was proved and, this time, the timing was perfect.

-The next day, Einstein is a worldwide celebrity.

There are famous front-page stories about this in The London Times, in The New York Times, in India, in Japan.

[ Crowd cheering ] It's really that announcement, in November 1919, that makes Einstein the character that we've come to seem so familiar to us.

♪♪ That's why we can buy T-shirts and coffee mugs and all kinds of swag with Einstein's face.

♪♪ -Einstein had become a worldwide celebrity, the most instantly recognizable scientist to have ever lived.

♪♪ With his theory of a universe constructed of warped space-time, Albert Einstein had transformed our understanding of the cosmos.

♪♪ [ Clocks ticking ] And, in the 1960s and '70s, a group of young scientists, including Stephen Hawking, started using those ideas to explain the most extreme phenomena in the universe.

-Einstein invented his theory of general relativity back in 1915, right?

But it was still kind of a tiny, little part of theoretical physics for a long time.

It wasn't until the 1960s, when people like Stephen Hawking really trained their mathematical muscles on understanding how general relativity works, that we understood all sorts of features of the theory that Einstein never knew.

-When I first met Stephen, I think it was in June of 1965.

He was in the midst of his PhD work at the time.

He was walking with a cane.

His speech was mildly affected, but not strongly affected.

He gave a seminar about his research.

♪♪ He held forth describing his insights about the birth of the universe.

♪♪ There was speculation that there was a Big Bang, based on observations, but no real proof, and Stephen was able to do the mathematics to give a firm proof that this had to have been.

♪♪ ♪♪ -The Big Bang theory had been raging for decades.

It had started soon after Einstein had given his theory of general relativity to the world.

-This is one of the wonderful things about physics, is that, once you invent a correct theory of physics, you don't own it anymore.

The theory is out there, right?

It's not like writing a novel, where it's your novel, once and forever.

Once Einstein invents his theory, I think, actually, there's no question that many people have understood and used Einstein's equations better than he did.

♪♪ -Once other people started exploring Einstein's theory, they found it was more powerful than he had ever imagined.

[ Wind whistling ] ♪♪ It didn't just tell us about the structure of the universe.

It could also tell us its history.

♪♪ In 1927, a Catholic priest and mathematician called Georges Lemaître wanted to know what Einstein's equations said about the shape of the entire universe.

♪♪ To do that, he needed to know where all the mass in the universe was located.

♪♪ -The easiest way to work with the entire universe is to make a big assumption.

And that is that the mass of the universe is pretty much uniformly distributed.

Every patch of the universe is like every other, kind of like this field of sand dunes stretching out behind us.

One patch of sand is similar to any other patch of sand.

And based on what we've observed, that's a very good assumption, because on the larger scales, the galaxies of the universe are uniformly distributed.

♪♪ -But when Lemaître put his monotonous universe through Einstein's equations, he made a very surprising discovery.

Whatever values he put in, there were no stable solutions.

The universe was either contracting or, more likely, expanding.

-Lemaître's work had enormous implications.

The universe is growing.

But if we run the clock backwards, we see that, in the past... [ Air hissing ] ...the universe must have been smaller.

And Lemaître realized that if you run the clock backwards far enough, all of the matter in the universe would have been concentrated into a single point.

Lemaître called this the cosmic egg, and that represented the beginning of the universe.

Today, we call it the Big Bang.

[ Pops ] [ Wind rushing ] -By discovering the Big Bang, Lemaître, the Catholic priest, had given the universe a moment of creation.

Lots of people hated this idea.

One of them was Albert Einstein.

-Einstein, like most everybody else, had hidden suppositions about what they considered to be natural in the universe, what they considered to be sensible.

-All of the world's greatest astronomers and cosmologists were absolutely convinced that we lived in a static universe -- no expansion, no contraction.

-The universe, unlike other things, wouldn't have a beginning and a middle and an end.

-Because once one has a beginning, one must ask, "What happened before the beginning?"

Or, "What caused things to start?"

And Einstein thought, "Let's avoid that entire discussion by saying there was no beginning.

There always was and always shall be a universe."

-And when his equation seemed to suggest that the universe, much like everything else, evolves and changes and may have had a beginning and -- who knows?

-- may have an end, he resisted it.

♪♪ -The big question of the 1920s was, "Which universe did we live in?"

Lemaître's expanding universe, or Einstein's stable one?

♪♪ -The answer came in 1929 from what was then the largest telescope in the world, on Mount Wilson, near Los Angeles.

American astronomer Edwin Hubble had been studying distant galaxies and had discovered that nearly all of them were moving away from us.

The further away they were, the faster they were receding.

It was comprehensive proof that the universe was expanding.

-And at that moment, Einstein kind of hits himself in the head and says to himself, like, "Why didn't I believe what the equations of general relativity were saying?"

In principle, he could have predicted this a dozen years before it actually was discovered.

♪♪ -When presented with the evidence, Einstein conceded his error.

It had been, he said, his greatest mistake.

[ Film projector clicking ] -He often said, "Oh, don't worry about my name on wrong papers.

My name is on plenty of wrong papers."

He wasn't that scared of being wrong.

I think he called it his greatest blunder maybe because this resistance was more out of character for him.

♪♪ -Stephen Hawking had burst onto the world stage with a paper giving theoretical proof the universe must have begun with the Big Bang.

♪♪ But his greatest work was reserved for the most remarkable phenomena in the cosmos... ♪♪ ...black holes.

♪♪ -It is said that fact is sometimes stranger than fiction.

But nowhere is that more true than in the case of black holes.

♪♪ They are quite literally holes in space that stuff can fall into but not get out of.

♪♪ They are places where the gravitational field is so strong that nothing, not even light, can get away.

♪♪ -Black holes had first been described as far back as 1916.

♪♪ While serving on the front line in the First World War, a German artillery officer called Karl Schwarzschild had been playing with Einstein's equations to see how far he could stretch space-time.

He discovered that the denser matter became, the more space-time warped, until, if you took a mass the size of our Sun and crushed it into a sphere just 2 miles across, it would create a hole in space-time that was infinitely deep.

Within that hole, the laws of physics break down.

Time stops... and space goes on forever.

But it seemed inconceivable that so much matter could be squeezed into such a tiny space.

-Albert Einstein wrote a paper in 1939 claiming matter could not be compressed beyond a certain point.

-I think Einstein went to his grave thinking that black holes were not real.

♪♪ -But just before the Second World War, scientists worked out what happened to stars at the end of their lives.

♪♪ -During most of the life of a normal star, over many billions of years, it will separate itself against its own gravity by thermal pressure, caused by nuclear processes which convert hydrogen into helium.

Eventually, however, the star will exhaust its nuclear fuel.

♪♪ -When a star runs out of fuel, it can no longer fight against its own gravity.

♪♪ Medium-sized stars would collapse in a massive explosion called a supernova... leaving behind a super-dense remnant.

These neutron stars have the same mass as our Sun, but compressed into a ball just 10 kilometers across.

But these were not the most extreme objects out there, because there are many much larger stars that will also one day die.

♪♪ -What would be the fate of those countless stars with greater mass than the neutron star, when they had exhausted nuclear fuel?

They would constrict to a single point of infinite density, a black hole.

♪♪ -Here, in the death of giant stars, was a mechanism through which black holes could form.

It suggested there should be millions of them out there, yet no one had ever seen a black hole or even a neutron star.

They were so small and far away, they would be invisible to conventional telescopes, but a new technology was developing.

♪♪ [ Trilling ] ♪♪ This is the Mullard Radio Astronomy Observatory.

In the early '60s, radio telescopes like these had started to reveal whole new areas of space we couldn't see with visible light.

♪♪ And in 1965, a 24-year-old graduate student called Jocelyn Bell started building a new radio telescope called the Interplanetary Scintillation Array.

[ Trilling ] -There were about half a dozen of us that spent two years building it.

A large number of wooden posts, over a thousand.

[ Trilling ] Probably about 150 miles of wire and cable.

It was very physical work.

I could swing a sledgehammer by the end of my PhD.

[ Chuckles ] ♪♪ -The data from the telescope were fed into this hut, where it was printed out on long rolls of paper -- over 200 feet of recordings every day.

What Jocelyn Bell found on those rolls changed the way we understood the universe forever.

-This is a piece of the original chart, and it's virtually impossible to see what's going on.

So, the first thing we decided to do was to spread this out by basically running the paper faster under the pen.

Finally got this string of pulses -- pulse, pulse, pulse, pulse, pulse, pulse, pulse, pulse, pulse, pulse.

-The challenge then was to figure out what could possibly be generating these mysterious radio signals.

-The pulses are short and quite steep, so these things have to be small.

The other thing that we very quickly established was that the pulse period was very, very constant.

It wasn't getting tired, which means it's got large reserves of energy, which means it's big.

So it's small, and it's big.

[ Laughs ] so... You have to be more precise in what you're saying.

It's small in width.

It's big in mass.

So high density.

[ Sharp pulsating ] That's the first pulsar.

CP1919.

So that's what it sounds like if you listen to the radio signal.

[ Pulse continues ] So it's spinning.

[ Pulse continues ] ♪♪ -Jocelyn had discovered a new, exotic class of star.

Tiny, super dense objects -- they emit no visible light... ...but rotate at enormous speed, sending out beams of radio waves as they do so.

They were named pulsars.

[ Pulse continues ] -So that's the whole star spinning at that rate.

And there are some that are even faster.

[ Pulse continues ] If you listened to the fastest one, it would sound like your kitchen blender -- it's going at 700 revs per second.

[ Pulse continues ] They're hard to believe.

[ Recording stops ] ♪♪ -But the key realization was that the existence of pulsars had been predicted.

[ Pulse continues ] ♪♪ They were neutron stars that were formed from the collapse of dying suns.

♪♪ It was a massive discovery.

♪♪ -This was incredibly exciting to people because the existence of a neutron star, which was a dead star, an incredibly dense state of matter, was a suggestion that if they're real, if you can see them, maybe black holes are too.

Because black holes are like the next step in the kind of astronomical graveyard.

♪♪ -For the first time, it seemed that black holes might really exist.

And it ignited the interest of Stephen Hawking and the next generation of physicists.

-It was a Golden Age of Relativity as we sorted out, under Hawking's leadership, the theory of black holes.

♪♪ -So we come to Einstein's most famous equation -- E equals mc squared.

[ Explosion ] -That's the power of E equals mc squared.

-And Hawking made the stunning discovery that black holes could evaporate.

-That was an in-your-face, shocking idea.

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