♪♪ [ 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.

-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.

[ Train whistle blows ] -In the first program, we saw how Einstein discovered that time appears to slow down on moving objects... ...stretching so much that at close to the speed of light, a day can appear to last 20,000 years... ...how he realized the universe is made of a flexible fabric of space and time... ...that the force of gravity is created by the distortion of this space-time by stars and planets... ...and how those discoveries inspired Stephen Hawking to develop Einstein's ideas to reveal the most extraordinary phenomena in the universe.

-Black holes are stranger than anything dreamed up 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.

♪♪ -In this program, we will discover how Einstein's theories have been developed to give us the power to understand creation... -The point of the LHC is to try and understand how the universe came into its current state from the Big Bang.

-...or to unleash unimaginable destruction.

-And that's the power of E equals mc squared.

-And how right up to his death, Stephen Hawking 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.

♪♪ ♪♪ [ Radio telescopes beeping ] In the 1960s, radio telescopes around the world started to pick up strange signals from outer space.

They appeared to be coming from tiny objects of massive density, neutron stars.

These mysterious objects had been predicted by Einstein's theory of relativity, and their discovery meant other predictions of relativity might also exist -- objects like black holes.

Understanding these mysterious monsters of the universe kickstarted the careers of a young group of physicists, including Stephen Hawking.

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

♪♪ ♪♪ -It soon became clear that black holes were very peculiar places.

♪♪ ♪♪ -We think of a black hole as a thing, as an object, but really, it's a region of space-time.

A black hole is a region of space-time where gravity is so strong that once you enter that region, you just cannot ever leave it.

-Black holes are fissures in the fabric of the universe.

Surrounded by an invisible boundary called the event horizon, nothing that crosses that horizon can ever get away.

-Falling through the event horizon is a bit like going over Niagara Falls in a canoe.

If you are above the falls, you can get away if you paddle fast enough.

But once you are over the edge, you are lost.

-The black hole is really defined by that event horizon.

It separates us, fundamentally and forever, from the inside of the black hole.

You can fall in, but you cannot come out.

-The consequence is -- once something has fallen into a black hole, it appears to be lost from the universe forever.

-From the outside, you can't tell what is inside a black hole.

You can throw television sets, diamond rings, or your worst enemies into a black hole, and all the black hole will remember is the total mass and the state of rotation.

♪♪ -It was these monsters that Stephen Hawking was working on in Cambridge in the mid-1970s.

And in 1974, he made an enormous breakthrough.

-Up to 1974, everyone, including me, thought that nothing could get out of a black hole.

Then I discovered that the uncertainty principle of quantum mechanics allowed particles to leap out.

-By adding quantum mechanics, Hawking made the stunning discovery that, actually, black holes could evaporate.

-Black holes are not what you thought they were.

Black holes can actually radiate.

Yeah, that was an in-your-face, shocking idea that he put forward.

-Quantum mechanics implies that the whole of space is filled with pairs of virtual particles and antiparticles that are constantly materializing in pairs and annihilating each other.

Now, in the presence of a black hole, one member of a pair of virtual particles may fall into the hole, leaving the other member without a partner.

As particles escape from a black hole, the hole will lose mass and shrink.

Eventually, the black hole will lose all its mass and disappear.

♪♪ -The discovery that black holes emitted particles was soon named Hawking radiation.

It showed that black holes were even stranger than we had imagined.

-How can a black hole evaporate?

It seemed so peculiar and very counterintuitive, but it made sense.

The calculations were checked over and over again, and it made sense.

-This is completely counter to everything we thought that we understood about black holes in the year 1970.

[ Clocks ticking ] ♪♪ -By combining general relativity with quantum mechanics, Hawking had shown the enduring power of Einstein's theory.

♪♪ But as Einstein himself would discover, that theory also had a dark side.

It could be used to unleash devastation on the earth.

♪♪ Back in the 1920s, Einstein found that interest in his work had extended beyond the world of theoretical physics and into the realm of populist politics.

[ Crowd chanting "Sieg Heil!"

] ♪♪ -Long before there was even a formal Nazi party, as early as spring of 1920, there were rallies to denounce not just Albert Einstein, but rather, to denounce general relativity.

They would rent out sports arenas and opera houses and large, large stadiums -- thousands of people in attendance -- to basically chant, "Down with warping space-time."

It was seen as a symbol of all that had gone wrong with so-called Jewish physics.

It became clear it was not gonna be safe for him.

-After the Nazis came to power, Einstein fled Germany in 1933... and settled here, in Princeton, New Jersey, working at the Institute for Advanced Study.

And as a lifelong pacifist, he was appalled at the prospect of another war, especially since he knew his own work could have a devastating impact on its outcome.

-Einstein did not want to get that involved in the war effort.

I think everyone was extremely surprised that this 1905 paper -- E equals mc squared -- could have an implication for killing people.

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

And like many of his equations, it has this magical property of equating two things that you wouldn't expect to be equal.

On the left-hand side, we find "E," the energy.

On the right-hand side, "m," the mass.

And so this equation tells us that energy and mass are essentially the same.

They're equivalent.

They can be exchanged.

And the exchange rate is "c" squared -- "c" is the speed of light.

It's huge.

So "c" squared is a tremendously large number.

So, one way to read the equation is that a huge amount of energy can be converted into a small amount of matter.

But reading the equation from the opposite end, it tells us that a small amount of matter can be converted into a huge amount of energy.

♪♪ ♪♪ -The power of E equals mc squared went far beyond the theoretical.

In the early 1920s, Einstein's friend Arthur Eddington realized that this famous equation was the secret to how stars, like our Sun, produce such vast amounts of heat and light.

-Deep in the hearts of stars, pressures and temperatures are so high that hydrogen nuclei slam into each other, fusing, and eventually creating helium nuclei.

And in that process, a tiny amount of mass is converted into energy.

Albert Einstein realized that, perhaps, we could harness that process here on Earth and create energy of our own... ♪♪ ...or create a weapon.

♪♪ -Worried the Germans had already started a nuclear program, Einstein and fellow physicist Leo Szilard wrote to President Roosevelt, urging him to develop a bomb.

Roosevelt responded by setting up the Manhattan Project.

♪♪ They achieved their goal in just three years.

♪♪ -This is the Trinity Monument.

On July 16, 1945, the world's very first nuclear bomb was set off right here in this spot.

The bomb, nicknamed "Gadget," was winched to the top of a 100-foot tower that stood right here.

The observers retreated and, just before dawn, the bomb was detonated.

[ Explosion ] At 5:29 a.m., the device exploded with an energy equivalent to 20 kilotons of TNT.

[ Explosion ] The roar of the shock wave was felt over 100 miles away.

The surrounding mountains were illuminated brighter than daytime, and the mushroom cloud reached 7 1/2 miles in height.

All that power was released when, inside the bomb, just 0.9 grams of matter were converted into energy.

And that's the power of E equals mc squared.

-Just three weeks later, on August 6, 1945, another bomb was dropped, over the Japanese city of Hiroshima.

At exactly 8:15 in the morning, 0.6 grams of matter was converted into energy, incinerating 70,000 people.

♪♪ [ Explosion ] The devastating power of Einstein's equation had been unleashed.

♪♪ -Now, it's always been a mixed legacy for the history of physics, right?

Like, we like to think of ourselves as discovering secrets of the universe, not as blowing things up.

But there is this necessary interplay between the discoveries we make and the technological capabilities that we have here on Earth that physicists have to accept.

And, of course, Einstein always had mixed feelings about it later on, 'cause he was, at heart, a pacifist.

♪♪ -For the rest of his life, Einstein campaigned for nuclear disarmament and peace.

♪♪ ♪♪ -The nuclear bomb was a devastating demonstration of how Einstein's theories could be used to practical effect.

♪♪ Ever since then, humanity has been building increasingly complicated machines that use Einstein's theory, not for destruction, but to unlock the mysteries of the universe.

♪♪ ♪♪ This is the entrance to the Large Hadron Collider.

Buried deep below the Swiss-French border, it is the world's biggest machine.

♪♪ Sudan Paramesvaran is a scientist who uses the LHC to travel back in time.

♪♪ -The point of the LHC is to try and understand how the universe came into its current state from the Big Bang all those billions of years ago, and one way that we can do that is to try and re-create some of those early conditions.

-Sudan's work relies on the concept of time dilation developed by Albert Einstein.

Time dilation is only noticeable at close to the speed of light, but in the LHC, we can see it in action.

Beams of protons are fired through these tubes and smashed into each other at very high speed.

-In the middle of this pipe is where the actual protons are traveling.

We accelerate them using very powerful magnets.

They're actually traveling round the ring 11,000 times a second, very, very close to the speed of light.

♪♪ So when the protons collide, they actually smash up into lots of little different things.

A lot of new particles are created.

♪♪ -We think the particles created in these collisions are similar to those that existed in the moments after the Big Bang.

Studying them can help us understand the early universe, and that's where time dilation comes in.

-Okay, so what we have here is a visual representation of a collision.

The protons are colliding right in the middle of our detector, and then everything comes out from that point.

♪♪ -Each of these tracks represents an individual particle created in the collisions.

Many of them blink out of existence in a fraction of a second.

-This is actually the decay of a B meson.

And its typical lifetime is only a millionth of a millionth of a second, so it's tiny, and that would make it very difficult for us to actually investigate it.

It would decay very close to the primary point, here.

Now, because it's traveling close to the speed of light, its clock is running a little bit slower.

This means that it's actually able to travel this distance before decaying.

And so the fact that it undergoes time dilation gives us an avenue of physics to explore which we otherwise wouldn't be able to do.

♪♪ -The time dilation we see in the LHC is Einstein's theory of relativity in action.

♪♪ This time, for peaceful means.

♪♪ We have answered many mysteries about the universe using Einstein's theories.

♪♪ But even in the 21st century, there are still more discoveries to be made... ♪♪ ...discoveries that will elegantly tie together the work of Albert Einstein on relativity and Stephen Hawking on black holes.

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

♪♪ -Despite all our advances in technology, until recently, we had never seen a black hole.

And it is that search, for an elusive glimpse of a black hole, that has brought Dan Marrone to the mountains of Arizona.

♪♪ -Black holes are one of those things that I feel like everyone is interested in, right?

The fact that we've never even seen one is kind of amazing.

We're so confident they're there, but, yet, "Show me a picture," and you can't.

-Dan is on a mission to do something no one has ever done before -- take a picture of a black hole.

♪♪ Ordinarily, it would be impossible to photograph a black hole, like picking out a shadow on an already dark sky.

♪♪ But Dan is hoping to capture a black hole in a particular phase of life -- while it is feeding.

♪♪ The gravitational pull of black holes is so strong that they can consume entire stars.

♪♪ This simulation shows what we think happens as a star is ripped apart.

♪♪ -What you see is the bright splatter of the debris falling around the black hole from the neighboring star.

And it turns out, maybe even ironically, that the darkest objects fundamentally conceivable become the brightest beacons in the universe.

♪♪ -The event horizon, where the whirlpool is at its fiercest, is one of the most extreme environments in the universe.

♪♪ It is what Dan and his colleagues are trying to take a picture of.

♪♪ This is the Submillimeter Telescope on Mount Graham, in Arizona.

♪♪ A radio telescope, it can probe the depths of the universe in broad daylight.

♪♪ -We'd like to see that material circling the drain and disappearing.

We'd like to see the light itself not escaping a shadow in front of the black hole where the light is disappearing into it.

♪♪ -To find his black hole, Dan has set his sights on the very heart of galaxies.

♪♪ Astronomers have been tracking the motion of the stars around the core of our galaxy for more than 20 years.

♪♪ -How's the weather this evening?

-The weather's good.

-The enormous speed at which they are moving suggests that they are orbiting something huge and unseen.

They believe it is a supermassive black hole, over 40 million kilometers across and four million times the mass of our sun.

It is known as Sagittarius A-star.

♪♪ The event horizon around it should be one of the brightest spots in the galaxy.

♪♪ But it is still so small and so far away that to image it takes the largest telescope ever built.

♪♪ -So, right now, the telescope's pointed at Sagittarius A-star.

We have to look through the entire plane of the galaxy, and so, if you look with a visible-light telescope, you can't actually see the black hole.

All the dust in the way blocks the light and so we have to do this with a radio telescope.

♪♪ And, in fact, we're not just looking with one, but we're looking with a whole array of them scattered over the Earth.

♪♪ -The Event Horizon Telescope is a huge network.

As well as this dish on Mount Graham, there are dishes in Europe, on Hawaii, in Chile, and at the South Pole, all of which are simultaneously pointed at the same target.

♪♪ When the data from all of them is combined, it creates a telescope the size of the Earth.

But this telescope was not just trying to image the center of our galaxy.

They had also been studying a much larger galaxy, over 50 million light-years away, called M87.

And, in April 2019, they released this image -- the first view of a black hole and the material around it spiraling into oblivion.

But, while this image is remarkable, there was already another technology that could detect Stephen Hawking's beloved black holes and which may answer one of the final great mysteries of Einstein's theory of relativity.

♪♪ [ Elevator bell chimes ] Now, these are offices at LIGO.

We can use any of these rooms, if you want.

But, if you want my office, we're heading there.

[ Chuckles ] ♪♪ It's full of junk.

[ Laughs ] This is the office they give emeritus old professors.

That's what they give us.

[ Laughs ] So, a lot of my papers and junk is all around.

♪♪ There's a picture of my mentor right over there.

You can see him.

His name is Jerrold Zacharias.

He's the first guy who ever said to me that, you know, "You're not as dumb as you look."

That was very important.

You need somebody in your life to tell you that.

♪♪ -Rainer Weiss, "Rai" to his friends, is now a Nobel Prize-winning emeritus professor at M.I.T.

But in 1967, he was just a junior faculty member who was fascinated by a particular piece of Einstein's theory -- gravitational waves.

In 1916, Einstein himself had calculated that objects moving through the fabric of space-time should cause it to ripple, like waves on a pond.

At first, Einstein thought that these waves would be so tiny, they would be impossible to measure.

Later in life, he was not sure they existed at all.

♪♪ -If you ask me whether there are gravitational waves or not, I must answer that I don't know, but it is a highly interesting problem.

♪♪ -Rai thought gravitational waves did exist and has spent a lifetime trying to find them.

So he came up with a system that could potentially detect the most minuscule waves in space-time.

It was called laser interferometry.

-So, here's the idea.

There's a laser, which is this cylinder.

There's something called a beam splitter, which divides the light.

♪♪ Half of the light will get reflected to a mirror that's over here... and then sent back to this beam splitter.

And the other half hits this mirror and comes back to the beam splitter.

Then, two beams will head toward a photo detector, and you'll see something interesting.

No light goes to the photo detector.

Light cancels.

Now what happens is you change the length on one arm, make it longer, make the light shorter on the other arm.

You see light does go to the photo detector when the paths are not equal anymore.

And that, the fact that light goes through the photo detector here when the paths are no longer equal and they have been disturbed by that gravitational wave to do this, that's the way the detection is made.

It's really as simple as that.

♪♪ -If Rai's calculations were right, his system would be able to pick up movements in the mirrors of just a few trillionths of a meter.

-I heard about the idea.

I did some numbers, and it was obvious that Rai had gone crazy or something.

I just couldn't believe that he could really -- anyone could really pull this off.

And then I spent the rest of my career eating crow and trying to help him pull it off.

♪♪ -When Kip Thorne, a theoretical physicist and close friend of Stephen Hawking, joined the project, his job was to work out whether there were any events in the universe powerful enough to produce gravitational waves that Rai's system could detect.

♪♪ -As a theorist, I had the best handle anyone had.

It was not a great handle, but the best handle anyone had on the strengths of the sources.

♪♪ -What Kip needed was a source of gravitational energy so strong, it would shake the fabric of the entire universe.

The sort of energy that would be released if two black holes collided.

-You began with two massive stars.

This one collapsed, formed a black hole; and then, later, that one collapsed, formed a black hole.

And then, create a binary going around each other.

That binary emits gravitational waves, spirals together closer and closer and the black holes collide.

But we'd never seen pairs of black holes.

We couldn't see pairs of black holes because black holes don't emit any electromagnetic waves.

As they go around each other, they only produce gravitational waves, so the only way we would ever see them was with our detectors.

♪♪ -To try to detect the gravitational signals from colliding black holes, Kip Thorne and Rai Weiss embarked on one of the most expensive scientific ventures ever attempted.

♪♪ After years of planning, construction started in Livingston, Louisiana, in 1994.

♪♪ The lasers would be fired up these arms, each 4 kilometers long, before reflecting off the mirrors and returning to sensors so finely tuned, they could, in theory, detect a shortening of the arms caused by gravitational waves of just a million-million-millionth of a meter.

♪♪ 3,000 miles away, in Hanford, Washington, an identical facility was built.

♪♪ For any detection to be confirmed, it would have to be picked up by both detectors at the same time.

♪♪ The instruments were activated in 2002.

For eight years, they listened to the universe.

By 2010, they had heard absolutely nothing.

♪♪ For the LIGO team, the pressure was mounting.

-Yeah.

-We had convinced the NSF and Congress that they should spend $1 billion on this, and we had nothing to show for it.

♪♪ -Doubling down, they shut the project for another five years, to install an even more precise set of detectors.

♪♪ It was a high-stakes gamble.

♪♪ ♪♪ In September 2015, almost exactly 100 years after Einstein published his theory of general relativity, Rai Weiss' detectors were reactivated.

Just two days later, both sites recorded an unusual signal.

-Ladies and gentlemen, we have detected gravitational waves.

We did it.

[ Cheers and applause ] -After working on the problem for almost 50 years, it was a massive vindication for Rai and his friend Kip.

-I want to first remind you of Einstein's 1915 big discovery was really the formulation of these field equations.

He applied these field equations to the idea of gravitational waves.

♪♪ -Almost the first day of the real run, we saw what is in front of you on the screen here.

And let me play it for you.

[ Thumping ] That isn't much.

It's a little blip.

That's all you heard.

But let me change -- and this is the trick.

We made it so that you could hear it better by changing the frequency of everything.

[ Blips ] And that chirp is the characteristic of this particular source.

And that was something which was quite astounding to us.

[ Blips ] ♪♪ -Long, long ago, in a galaxy far, far away, a pair of black holes, each around 30 times the mass of the Sun, circled each other, moving faster and closer.

♪♪ -The point when we first see the gravitational waves, they're maybe 1,000 kilometers apart, each one roughly 100 kilometers in size.

They're going around and around, creating stronger and stronger warping of space and time.

As they near each other, they warp space and time in a wild way, very much like a storm at sea.

-And then, the black holes coalesce.

♪♪ In that instant, three times the mass of the Sun is converted into pure energy.

♪♪ But none of that energy is emitted as light.

It is all released as gravitational waves.

-So there's a huge amount of energy -- turns out to be 50 times higher than the total power output of all the stars in the universe put together.

♪♪ By far, the largest explosion, in terms of power output, except the Big Bang, that we ever had any evidence of.

♪♪ -As they travel through space at the speed of light, the waves dissipate, getting smaller and smaller, until, 1.3 billion years later... they passed through Rai's detectors.

[ Blips ] -I wish I could have told Einstein directly that we had seen a black hole doing this.

That would have been just a wonderful experience, to see the expression on his face after we'd told him that.

[ Blips ] -The discovery of gravitational waves is the ultimate triumph of Einstein's theory of relativity... ...a theory that has been repeatedly tested and proven to be correct.

♪♪ Einstein died in 1955.

In just half a century, he had reimagined the entire universe.

-Well, Albert Einstein enters the world when it's one way and, by the time he leaves the world, it's completely different.

-We now live in a world where we understand the universe had a beginning and we can tell how long ago it was.

Those are stunning achievements and stunning shifts, both in our scientific and our cultural perspective.

-He was a figure that is rare, you know, once in a hundred years, maybe once in 500 years.

♪♪ ♪♪ -Stephen Hawking's greatest contribution to physics was his discovery that black holes could evaporate.

If his theory of Hawking radiation was correct, it would have profound implications for the way we understand the entire universe.

♪♪ In Haifa, Israel, Jeff Steinhauer has spent a decade searching for evidence of Hawking radiation... using artificial black holes that he makes out of sound.

♪♪ -So, here we go.

All the equipment that you see... is for the sake of the small point right in there where the artificial black hole is.

We start out with these very cold atoms in this tube-like volume and it's only 0.1 millimeters long.

We then apply a blue laser beam which causes some of the atoms to be accelerated to supersonic speeds.

So, in this region, they're flowing slower than the speed of sound and, here, they're flowing faster than the speed of sound.

So, here, in the supersonic region, a sound wave trying to move against the flow will actually fall back.

It can't go forward against the very fast flow.

♪♪ -In his artificial black hole, Jeff has created an event horizon.

Once the sound wave is over that line, it cannot escape, just as light cannot escape a real black hole.

And, when Jeff takes photos of his sonic event horizon, he sees something very strange -- the telltale signs of Hawking radiation.

-You see this band?

This band is an observation of sound waves being emitted from the horizon of the black hole.

♪♪ This experiment is the first time that Hawking radiation's been observed.

♪♪ -It is an amazing discovery, which suggests that Hawking radiation may be a fundamental feature of black holes.

If that's true, it means that, given enough time, all black holes will evaporate.

And that prospect has troubling implications for the rest of physics.

[ Flames crackling ] ♪♪ ♪♪ -A central principle of all physics -- in fact, it's written into the laws of quantum theory -- is that information cannot be destroyed.

Consider this newspaper.

It has information on every page and, if I were to throw it on this fire, it's quickly consumed by the flames... and it looks like the information is destroyed.

But if I were able to take all the ashes, collect all the smoke and all the heat, and analyze and reconstruct the paths of all the particles, then I should be able to reconstruct what's on every page.

And this is because the laws of physics allow us to tell the entire future and the entire past of any system.

No one's saying it's easy, but it is possible, as long as you have all the information.

We call this determinism.

[ Flames crackling ] -Scientists believe that the principle of determinism applies to all systems, no matter how big or complex.

It should even apply to the entire universe.

-So, if I were to somehow know the position and state of every particle in the universe, I could apply the laws of physics and know the entire future of the universe and the entire past of the universe.

But there's a catch, because if, as Hawking predicted, black holes could evaporate in such a way that they leave behind no trace of the material that was inside, then information could be lost and, even if only a small amount of information is lost, then everything is lost.

You can't reconstruct the past and you can't predict the future.

It seems trivial, but, actually, it was a massive problem that shook the very foundations of physics, and this became known as the information paradox.

♪♪ -If determinism, the predictability of the universe, breaks down with black holes... it could break down in other situations.

Even worse, if determinism breaks down... ...we can't be sure of our past history, either.

♪♪ The history books and our memories could just be illusions.

It is the past that tells us who we are.

Without it, we lose our identity.

♪♪ -The information paradox has troubled physicists for decades.

If Hawking was right and black holes did destroy the information they contained, then they would also destroy almost everything we know about modern physics.

-It created this big rift between those who were adherents to quantum thinking and those who were adherents to relativity.

The quantum people saying, "The information must get out.

We don't know how, we don't see how, but it must."

And the relativists saying, "You know, maybe, maybe the information's just lost."

-I'm inclined to suspect that information is truly lost down black holes, but mine is the minority point of view.

-Who cares if black holes evaporate and swallow little bits of information as they go?

But, in the 40-plus years since then, it's kind of been like a snowball, the importance of this problem, and has grown and grown.

♪♪ -The information paradox was a problem that Stephen Hawking worked on for over four decades.

♪♪ And, in 2015, he came up with a major breakthrough.

♪♪ -Many scientists felt that information should not be lost, but no one could suggest a mechanism by which it could be preserved.

The arguments went on for years.

Finally, I found what I think is the answer.

I realized that a black hole can store the information in what is called supertranslations of the horizon.

♪♪ -Hawking's new idea was that, as objects fall into a black hole... ...they disturb the event horizon... ...leaving behind a pattern of turbulence, which could preserve the information about everything that's fallen inside.

♪♪ -I'm now working with my colleagues Malcolm Perry at Cambridge... and Andrew Strominger at Harvard on whether this can resolve the paradox.

-If information is stuck on the horizon, it has not fallen into the black hole.

And you can't directly observe it from the outside, but what will happen is it will influence the Hawking radiation that comes out of the black hole, and that may be enough to help you resolve the information paradox.

♪♪ -When we first came upon it, we were up until late at night.

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

♪♪ And he started working on it basically full-time.

♪♪ -Together, the scientists wrote three papers on the subject... ♪♪ ...edging closer to a solution to the information paradox.

♪♪ It was Hawking's last great idea.

♪♪ -Sadly, this is the last paper that Stephen wrote.

He was full engaged with it right till the very end.

♪♪ -Hawking died in March 2018 at the age of 76, over half a century after he had been given just two years to live.

♪♪ -He was kind of magical to be around.

When I miss him the most is when we figure something out, I'd like to tell him.

♪♪ -Such was his contribution to science, his ashes were interred in Westminster Abbey, alongside Isaac Newton and Charles Darwin... ♪♪ -Stephen was one of the great minds of the 20th century, in terms of his impact on our understanding of the laws that govern the universe.

I miss Stephen greatly.

It was a tremendous loss for me, personally.

♪♪ -...his gravestone engraved with the equation describing Hawking radiation.

♪♪ Stephen Hawking's legacy will clearly be a vital part in the unfolding story of putting gravity and quantum mechanics together.

♪♪ And, as we go forward, everything that we do, in some sense, will go right back to Stephen Hawking.

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

♪♪ But time is not constant.

It holds the key to the secrets of the universe.

♪♪ 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.

♪♪ -Einstein gave to us all a set of tools that literally help us organize the world around us to this day.

-And, on March 14, 2018, Stephen Hawking died.

♪♪ -I think Stephen should be remembered as one of the most remarkable people of the 21st century.

He is someone who, in a thousand years from now, will be remembered.

His name will be known.

That's not true for most of us.

♪♪ -Between them, they have transformed our understanding of the entire universe.

♪♪ -As a working physicist, I look at these two giants and I have nothing but gratitude that they revealed the world to be so much more than it would have been.

And, without them, who knows if we would've gotten to this wondrous stage in our understanding.

♪♪ -I have a real feeling of achievement that I have made a modest, but significant, contribution to human knowledge despite my condition.

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