"Why are we here?" "Where do we come from?" "These are the most enduring of questions, and it's an essential part of human nature to want to find the answers." "And we can trace our ancestry back hundreds of thousands of years to the dawn of humankind, but in reality, our story extends far further back in time." "Our story starts with the beginning of the universe." "It began 13.7 billion years ago." "And today, it's filled with over 100 billion galaxies, each containing hundreds of billions of stars." "In this series, I want to tell that story because, ultimately, we are part of the universe." "So its story is our story." "The force at the heart of this story is gravity." "This fundamental force of nature built everything we see." "It creates shape and order, and it initiates patterns that repeat across the heavens." "But gravity also forges some of the most alien worlds in the cosmos, worlds that defy belief." "The quest to understand this fundamental force of nature has unleashed a golden age of creativity, exploration and discovery." "And it's led to a far deeper understanding of our place in the universe." "Every moment of our lives, we experience a force that we can't see or touch." "Yet this force is able to keep us firmly rooted to the ground." "It is, of course, gravity." "But despite its intangible nature, we always know it's with us." "If I was to ask you," ""How do you know that there's gravity around here?"" "Then you might say, "Well, it's obvious."" "You know, I can just do an experiment, I can drop something." "Well, yes, but actually, gravity is a little bit more subtle than that." "But to really experience it, to understand it, you have to do something pretty extreme." "And this plane has been modified to help me do it." "Thanks to its flight plan, it's known as the Vomit Comet." "Once we've climbed to 15,000 metres, this plane does something no ordinary flight would do." "Its engines are throttled back, and the jet falls to earth." "And then, something quite amazing happens." "SCREAMS AND CHEERS" "Push to me, push to me!" "Oh!" "I'm now plummeting towards the ground just like someone's cut the cable in a lift, and you see that I'm not moving." "Relative to Einstein, we're all just floating." "By simply falling at the same rate as the plane, for a few fleeting moments, we are all free of gravity's grip." "But this isn't just a joyride." "There's something very profound here, because although I'm falling towards the ground, as you can see, gravity has completely gone away." "Gravity is not here any more." "I've cancelled gravity out just by falling." "If you understand that, then you'll understand gravity." "So it is possible, by the simple act of falling, to get a very different experience of gravity." "But this force of nature does more than just bring us back down to earth." "Gravity also plays a role on the grandest of stages, because across the universe, from the smallest mote of dust to the most massive star, gravity is the great sculptor that created order out of chaos." "Since the beginning of time, gravity has been at work in our universe." "From the primordial cloud of gas and cosmic dust, gravity forged the stars." "It sculpted the planets and moons, and set them in orbit around the newly formed suns." "And gravity connects these star systems together in vast galaxies, and steers them on their journey through unbounded space." "Over the centuries, our quest to understand gravity has allowed us to explain some of the true wonders of the universe." "But at a deeper level, that quest has also allowed us to ask questions about the origin and evolution of the universe itself." "To understand how gravity works across the universe, we need look no further than the ground beneath our feet." "Well, the first scientist to really think about it was Isaac Newton back in the 1680s, and he said this " ""Gravity is a force of attraction between all objects"." "Now, the force of attraction between these two rocks is obviously very small, almost impossible to measure, and that's because the force is proportional to the masses of the objects." "These things are not very massive." "But there is a more massive rock around here." "It's the one I'm standing on, planet Earth." "The mass of our earth generates a gravitational pull strong enough to sculpt the entire surface of the planet." "It causes water to gouge out vast canyons." "It sets the limit for how high mountains can soar, and it shapes whole continents." "But this invisible force does more than just shape our world." "The skies are always changing, and the constellations rise and fall in different places every night, and the planets wander across the background of the fixed stars." "But throughout human history, there's been one constant up there in the night sky, because every human that's ever lived has gazed up at the moon and seen one face shining back at us." "The reason why we never see the dark side of the moon is all down to the subtlety with which gravity operates." "Millions of years ago, the moon rotated rapidly." "But from the moment it was born, our companion felt the tug of gravity." "Just as the moon creates great tides in our oceans, the Earth caused a vast tide to sweep across the surface of the moon." "But this tide wasn't in water." "It was in rock." "Imagine that this is the moon, and over there is the Earth." "The Earth's gravity acts on the moon and stretches it out into a kind of rugby ball shape." "Now, the size of that tidal bulge facing the Earth is something like seven metres in rock and then, as the moon rotates, that bulge sweeps across the lunar surface." "I mean, imagine what that would look like here." "You'd see a tidal wave sweep across this landscape, with the rock rising and falling by seven metres." "This massive wave acted like a brake, and gradually slowed the moon down." "Eventually, the tidal bulge became aligned with the Earth, locking the speed of the moon's rotation." "So the time it takes the moon to spin once is almost the same as the time it takes to orbit the Earth." "So there is no dark side of the moon, just a side that gravity hides from our view." "The bond that gravity creates between the Earth and the moon is repeated across the cosmos." "It's the glue that holds the planets in orbit around the sun." "And it binds our solar system and countless other solar systems together, to form galaxies like our own Milky Way." "But gravity's influence can be felt even further because it controls the fate of galaxies." "When you look up into the night sky and you see the universe as it looks in visible light, with the glowing of the stars and the galaxies, but that's only part of the story, because the universe is full of dust and gas" "which you can't see with a conventional telescope, but you can see with a telescope like this." "Radio telescopes, like the very large array in New Mexico, are able to peer deep into space and reveal the incredible attractive power of gravity." "This is Andromeda, a spiral galaxy roughly the same size and mass as the Milky Way." "This island of over a trillion stars sits over 2.5 million light years away, but every hour that gap shrinks by half a million kilometres." "Whilst most galaxies have been rushing away from each other ever since they formed just after the Big Bang, some galaxies formed so close together that they are locked in a gravitational embrace, and the Milky Way and Andromeda are two such galaxies." "Computer simulations suggest that they will collide together in around three billion years' time." "Look at that." "That's a simulation of the Milky Way galaxy and the Andromeda galaxy colliding together, and all these wisps of smoke getting thrown out are stars." "These are star systems getting ripped out of the galaxy and thrown off into interstellar space." "These two islands of hundreds of billions of suns have flown through each other, and gravity has exerted its grasp and dragged them back again." "And just remember that we are one of those dots." "You know, our sun and the Earth and the solar system are either going to be flung out into interstellar space, or they're going to be in here, in this maelstrom of hundreds of billions of suns" "swirling around each other and forming the core of a new galaxy." "Just imagine what it would be like to gaze up at the sky as Andromeda approached." "The sky would be ablaze with the light of hundreds of billions of suns, and the imminent collision would provide the energy to generate the births of hundreds of millions more." "What a magnificent sight it would be." "But far more magnificent is the immense scale of gravity's embrace." "It holds galaxies together across hundreds of billions of kilometres and, in doing so, it creates the most magnificent structures." "Our own Milky Way is part of one of these, the Virgo cluster." "Every point of light in this image is not a star, but a galaxy." "There are 2,000 galaxies in this cluster, and they're all bound together by gravity, making it the largest structure in our intergalactic neighbourhood." "There seems to be no limit to the reach or power of gravity." "Its influence can be felt across the vast expanses of space and time." "But there's something very interesting about gravity, because it is by far the weakest force of nature." "I mean, look." "I can...pick this rock up off the ground even though there's an entire planet, planet Earth, trying to pull it down." "So if gravity is so weak, how come it's so influential?" "Gravity may be weak here on Earth, but it's not so weak across the cosmos." "This invisible force varies on all the planets in the solar system and on the exo-planets we've discovered orbiting other suns." "To experience what gravity feels like on these worlds," "I need to go for a spin." "This is a centrifuge." "It was built in the 1950s to test whether fighter pilots had the right stuff, but it's going to allow me to feel what it'd be like to stand on the surface of any of the planets in the solar system that are more massive than the Earth," "and, in fact, also what it would be like to stand on some of the planets that we've found around distant stars." "Right, I'll have to strap you in, first of all." "This is an emergency switch in case something happens." "When you release it, the centrifuge will stop." "I was just told by the F-16 fighter pilot, who's just been in here, that it's a hundred times more uncomfortable than being in a jet fighter." "I was kind of confident because I've been in jet fighters and didn't find it too uncomfortable, but apparently, this is a hundred times worse!" "Doors closed again." "Profile is there." "Doctor is ready." "We'll start up the centrifuge, Brian, and bring you in orbit, and it happens in three...two...one second from now." "'The first planet I'm travelling to is Neptune." "'Its gravity is just fractionally stronger than here on Earth'." "So this is the gravitational field on Neptune and you feel, you know what?" "I could probably get used to this." "I could probably live on the surface of Neptune." "Can you lift your hands a little?" "There we go." "Yeah, and down." "And it is actually quite an effort." "It is noticeably heavier." "It's like having a reasonably heavy weight in your hand." "Are you ready to go to 2.5G?" "Yes, so now we'll move, move from Neptune to Jupiter." "Let's go there." "Jupiter is over 1300 times more massive than the Earth, but because it's mostly gas, it's not very dense, so its gravity is just over twice as strong at its surface." "Well, now actually, it is quite difficult to lift my hand." "And that's 2.5G. I wouldn't want to sit here for half an hour." "Can you lift, lift both of your hands above your head?" "See what happens there." "Let's see, so actually...just about, but actually, it's an immense amount of hard work." "So it would be hard work living on Jupiter." "Let's go to 4G." "Actually, this is heading to a planet around, a planet called Ogle-2TRL9B, which is around a star in the constellation of Carina." "It's one of the exo-planets we've discovered." "Oh, and there we go." "Now, that is actually beginning to feel quite unpleasant." "Can you describe what you're feeling?" "Very heavy face." "My head is extremely heavy." "How about your lungs, inhaling, exhaling, breathing?" "It's much harder work." "I can't lift my hand off my leg." "OK." "And that's at 4G?" "Yeah." "Well, my head and my face feel very, very heavy." "It's quite an unpleasant feeling." "We'll go to five, and let me know if you have any visual disturbances." "'I'm now en route to a newly discovered exo-planet, Wasp-8B'." "4.4." "'This world sits in the small and faint constellation of Sculptor'." "Quite hard to speak." "'It has a gravitational force nearly five times that of the Earth'." "Right, we'll go to 5G." "Very foggy." "OK." "Very foggy." "Very foggy?" "Still foggy?" "Yeah." "Right." "Take it down." "OK, we'll take you down." "Very interesting." "It was, wasn't it?" "My face felt a bit saggy, though." "Well, you looked a little different." "It was quite unpleasant that time, actually." "It went very quickly up to 5G and what happens is - for me, anyway - vision becomes very, very foggy." "The whole thing just blurs and blurs and blurs." "So you realise that we're, obviously, very finely tuned to live on a planet that has an acceleration due to gravity of 1G." "When you go to 2G, it's difficult." "When you go to 3G and 4G, it becomes unpleasant and 5G anyway, for me, was on the border of being so unpleasant that you pass out." "So, although gravity feels weak here on Earth, it certainly isn't weak everywhere across the universe, and that's because gravity is an additive force." "It scales with mass, so the more massive the planet or star, the stronger its gravity." "The body with the strongest gravity in our solar system is the sun." "Our star has so much mass packed inside a relatively small space that it has a gravitational pull at its surface 28 times that of the Earth." "If I were able to set foot on this world, all the blood would be poled out of my upper body, and I would die in less than a minute." "But our sun's gravitational force is nothing compared to the extreme G found at the surface of one of the strangest places in the universe." "Imagine the gravity on a world with more mass than our sun, crammed into a sphere just 20 kilometres across." "We first detected such a wonder just 40 years ago, but the story of its discovery begins over a thousand years earlier." "This is Chaco Canyon in New Mexico in the south western United States, and it was home to what's become known as the Chacoan civilisation." "Well, this is Pueblo Bonito, one of the so-called Chacoan great houses." "Back in the 1100s, this place had over 600 rooms." "It's thought that this building must have been ceremonial or religious, a cathedral, if you like." "The Chacoan great houses are aligned with interesting objects in the sky, so the points at which the sun and moon rise at important times of the year." "So it seems that by constructing these grand buildings, the Chacoans were not only trying to place themselves at the heart of local culture, but also to place themselves at the heart of the cosmos." "Very little is known about the Chacoan culture, because no written text has ever been discovered." "But in another part of the canyon, there is a record of a spectacular event that they witnessed in the sky in 1054." "Now, I've known about this place since I was 12 or 13 years old, and the reason is this book, and the television series Cosmos," "Carl Sagan's masterpiece, probably the most important reason that I got interested in astronomy." "And on page 232, there's a picture that's always fascinated me and captured my imagination and it's a photograph of that wall of rock, and in particular a painting that's on the overhang." "Because it's thought that that painting is a record of one of the most spectacular and magical events in the cosmos." "On 4th July 1054AD, a bright new star appeared, and it outshone every other star in the night sky for over three weeks." "It was so bright that it was visible in the daytime, and it's thought that this painting is the Chacoan people's record of that astronomical event." "The reason we think that is that using modern computer techniques, you can wind back the night sky and say," ""Where would the moon have been?" "Where would the stars have been?"" "And you find that in that direction, the moon would have risen and tracked across the night sky, and the new star would have been very, very close to the crescent moon." "We now know that that new star was in fact the explosive death of an old star, a supernova explosion, a star, literally, blowing itself apart at the end of its life." "Throughout a star's life, there is a constant battle between energy pushing out and gravity pushing in." "As long as the star burns, the two forces balance each other out." "But when it runs out of fuel, gravity wins and the star collapses, and then explodes with the brightness of a billion suns." "We can no longer see the supernova the Chacoans saw, but we can still marvel at what it left behind." "This is the Crab Nebula, the remains of that exploding star that the Chacoans saw in these skies a thousand years ago." "It's an expanding cloud of gas and dust, the remains of that dying star, and the colours are different chemical elements, so the orange is hydrogen, the red is nitrogen and those filaments of green are oxygen." "While the explosion blew most of the stellar material out into the cosmos to form this vast nebula, we now know that this wasn't the end of the story." "At the centre of the nebula lies the remnant of the star, its core, crushed by the force of gravity." "That is a neutron star, an image taken by the Chandra X-ray satellite." "The central blob there is only about 20 kilometres across, but it's got the mass of our sun, a star the size of a city." "It's spinning at a rate of over 30 times a second, 1,800 revolutions per minute, and it really is an astonishingly alien world." "As the neutron star spins, jets of particles stream out from the poles at almost the speed of light." "These jets are powerful beams that sweep around as the star rotates." "When the beams sweep across the Earth, they can be heard as regular pulses, so we call them pulsars." "But it's not this rhythmic noise that makes the Crab Pulsar a wonder." "It's the extraordinary nature of gravity on this alien world." "If I were to be on its surface, then the gravitational pull on me would be a hundred thousand million times that that I feel on Earth." "That means that if I were to jump from the top of that projection screen, by the time I hit the ground," "I'd be travelling at over four million miles an hour." "That's a lot of gravity." "Pulsars have such extreme gravity because they're made of incredibly dense matter." "To understand why, we have to look at what gravity can do to matter at the very smallest scales." "Everything in the universe is made of atoms, and until the turn of the 20th century, it was thought that they were the smallest building blocks of matter." "I mean, the word itself comes from the Greek "atomos", which means indivisible." "But we now know that atoms are made of much smaller stuff." "Atoms consist of an atomic nucleus surrounded by a cloud of electrons." "And whilst almost all of the mass is contained in the nucleus, it is incredibly tiny compared to the size of an atom." "If this were a nucleus, then the cloud of electrons would stretch out to something like a kilometre away." "I mean, that's from here to that rock." "And electrons on this scale are incredibly tiny." "They're just like specks of dust and they're aren't many of them." "So imagine a giant sphere centred on the atomic nucleus stretching out all the way to that rock and beyond, with just a few points of dust in it." "That's an atom." "So that means that matter is almost entirely empty space." "I'm full of empty space." "The Earth is full of empty space." "Everything you can see in the universe is pretty much just empty space." "So if everything in the universe is made up of atoms, and atoms are 99.9999% empty space, then most of the universe is empty." "But in the Crab Pulsar, the force of gravity is so extreme that the empty space inside the atoms is squashed out of existence, so all you're left with is incredibly dense matter." "Imagine this was matter taken from a neutron star - then it would weigh more than Mount Everest." "Or to put it another way, if I took every human being on the planet and squashed them so they were as dense as neutron star matter, then we would all fit inside that." "And if I were to drop my neutron star stuff to the ground, then it would slice straight through the earth like a knife through butter." "Wherever we look in the universe, we see gravity at work." "It creates shape and structure." "It governs the orbits of every planet, star and galaxy in ways we thought we were able to predict." "But there was a flaw in our understanding of this force, and it was exposed by one of our close neighbours." "This is Mercury." "For thousands of years, we've marvelled as this fleet-footed planet races across the face of the sun." "But 150 years ago, astronomers noticed something strange about Mercury's orbit." "Imagine that this rock is the sun, and this is Mercury." "Now Mercury has quite a complex orbit." "For one thing it's not a perfect circle, it's quite an elongated ellipse." "So at its closest approach to the sun, it's around 46 million kilometres away, and then it drifts out to something just under 70 million kilometres." "But you can calculate Mercury's orbit very precisely using only Newton's laws of gravity." "So astronomers used to predict the exact time when you could look up into the sky, look at the sun and see the tiny disc of Mercury pass across its face." "But the thing was, they never got it right." "They predicted it time and time again, and every time it happened, they got it slightly wrong, which was an immense embarrassment." "So what they did was that, rather than question Newton, they invented another planet, and they called it Vulcan, and they said that there must be another planet somewhere in the solar system, which is always invisible from Earth" "but which perturbed Mercury's orbit a bit, and so that was the reason their calculations were wrong." "For decades, astronomers searched and searched for Vulcan." "But they never found it, because Vulcan didn't exist." "The explanation, the real explanation, was even more interesting than inventing the planet Vulcan, because it required a modification, in fact, a complete re-writing of Newton's law of gravity." "Gravity is NOT a force pulling us towards the centre of the Earth like a giant magnet." "In a sense, gravity isn't really a force at all." "Describing the nature of gravity turned out to be one of the great intellectual challenges, but almost 200 years after Newton's death, a new theory emerged." "The new theory, called general relativity, was published in 1915 by Albert Einstein after ten years of work, and it stands to this day as one of the great achievements in the history of physics." "You see, not only was it able to explain with absolute precision the strange behaviour of Mercury, but it explains to this day everything we can see out there in the universe that has anything to do with gravity." "And, most importantly of all, it explains how gravity actually works." "Gravity is the effect that the stars, planets and galaxies have on the very space that surrounds them." "According to Einstein, space is not just an empty stage - it's a fabric called space-time." "This fabric can be warped, bent and curved by the enormous mass of the planet's stars and galaxies." "You see, all matter in the universe bends." "The very fabric of the universe itself - matter - bends space." "I bend space, these mountains bend space, but by the tiniest of tiniest of amounts." "But when you get onto the scale of planets and stars, galaxies, then they bend and curve the fabric of the universe by a very large amount indeed." "And here is the key idea." "Everything moves in straight lines over the curved landscape of space-time." "So what we see as a planet's orbit is simply the planet falling into the curved space-time created by the huge mass of a star." "This is able to explain Mercury's erratic orbit." "Because of the planet's proximity to our sun, the effects of the curvature of space-time matter far more for Mercury than for any other planet in the solar system." "This idea of curved space is difficult to imagine, but if you could only step outside of it, if we could only float above space-time and look down on it, this is what our universe would look like." "You would see the mountains and valleys." "You would see the little peaks and troughs created by planets and moons, and you would see these vast, deep valleys created by the galaxies." "And you would see planets and moons and stars circling the peaks as they follow their straight-line paths through the curved landscape of space-time." "So one way to think about gravity is that everything in the universe is just falling through space-time." "The moon is falling into the valley created by the mass of the Earth." "The Earth is falling into the valley created by the sun, and the solar system is falling into the valley in space-time created by our galaxy." "And our galaxy is falling towards other galaxies in the universe." "Einstein's theory of general relativity is so profound and so beautiful that it can describe the structure and shape of the universe itself." "But remarkably, the theory can also predict its own demise, because it predicts the existence of objects so dense and so powerful that they warp and stretch and bend the structure of space-time so much that they can stop time, and that they can swallow light." "These are objects so powerful that they can tear all the other wonders of the universe apart." "Since the dawn of civilisation, we've peered at the stars in the night sky and tracked the movements of the planets." "We see these familiar patterns repeated across the whole universe." "But when we train our telescopes to the stars that orbit around the centre of our galaxy, we see something very unusual." "Well, this is one of the most fascinating and important movies made in astronomy over the last ten or 20 years." "This is real data." "Every point of light in this movie is a star orbiting around the centre of our galaxy." "They're known as the S stars." "Our sun takes around 200 million years to make its way around the Milky Way." "One of these S stars takes only 15 years to go around the centre of the galaxy." "It's travelling at 3,000 or 4,000 kilometres per second." "Now, by tracking the orbits, it's possible to work out the mass of the thing at the centre." "The answer took astronomers by surprise, I think it's fair to say, because the object in the centre of our galaxy is four million times as massive as the sun, and it fits into a space smaller than our solar system." "Now there's only one thing that anyone knows of that can be so small and yet so massive, and that's a black hole." "So what we're looking at here is stars swarming like bees around a super-massive black hole at the centre of the Milky Way galaxy." "We think black holes can be smaller than an atom, or a billion times more massive than our sun." "Some are born when a star dies." "When a star around 15 times the mass of our sun collapses..." "..all the matter in its core is crushed into an infinite void of blackness known as a stellar mass black hole." "Black holes are the most extreme example of warped space-time." "They have such enormous mass crammed into such a tiny space that they curve space-time more than any other object in the universe." "The immense gravitational pull of these monsters can rip a star apart." "They tear matter from its surface and drag it into orbit." "This super-heated matter spins around the mouth of the black hole, and great jets of radiation fire from the core." "Although these jets can be seen across the cosmos, the core itself remains a mystery." "Black holes curve space-time so much that nothing, not even light, can escape." "So their interior is for ever hidden from us." "But because we understand how matter curves the fabric of space, it is possible to picture what is happening." "Near a black hole, space and time do some very strange things, because black holes are probably the most violent places we know of in the universe." "This river provides a beautiful analogy for what happens to space and time as you get closer and closer to the black hole." "Now, upstream, the water is flowing pretty slowly." "Let's imagine that it's flowing at three kilometres per hour, and I can swim at four, so I can swim faster than the flow and can easily escape." "But as you go further and further downstream towards the waterfall in the distance, the river flows faster and faster." "Imagine I was to decide to jump into the river just there, on the edge of the falls - the water is flowing far faster than I could swim." "So no matter what I did, no matter how hard I tried," "I would not be able to swim back upstream." "I would be carried inexorably towards the edge, and I would vanish over the falls." "Well, it's the same close to a black hole, because space flows faster and faster and faster towards the black hole." "Literally, this stuff, my space that I'm in, flowing over the edge into the black hole." "And at the very special point called the event horizon, space is flowing at the speed of light into the black hole." "Light itself, travelling at 300,000 kilometres per second is not going fast enough to escape the flow, and light itself will plunge into the black hole." "Well, as you fall into a black hole, across the event horizon, then if you were going feet first, your feet would be accelerating faster than your head, so you would be stretched, and you would be quite literally spaghettified." "Now as you get right to the centre, then our understanding of the laws of physics breaks down." "Our best theory of space and time," "Einstein's theory of general relativity, says that space and time become infinitely curved, that the centre of the hole becomes infinitely dense." "That place is called the singularity, and it is the place where our understanding of the universe stops." "Gravity is the great creator, the constructor of worlds." "That's because it's the only force in the universe that can reach out across the vast expanses of space and pull matter together to make the planets, the moons, the stars and the galaxies." "But gravity is also the destroyer, because it's relentless, and for the most massive objects in the universe, for the most enormous stars, and the centres of galaxies, gravity will eventually crush matter out of existence." "Now, the word beautiful is probably over-used in physics." "I probably over-use it." "But I don't think there is any scientist who would disagree with its use in the context of Einstein's theory of gravity." "Because here is a theory that describes a universe that is bent and curved out of shape by every moon, every star and every galaxy in the sky." "And everything in the universe has to follow those curves, from the most massive black hole to the smallest mote of dust, even to beams of light." "But the most tantalising thing about Einstein's theory of gravity is we know that it's not complete." "We know that it's not the ultimate description of the structure and shape of the universe." "And that, for a scientist, is the most beautiful place to be, on the border between the known and the unknown." "That is the true wonder of the universe - there's so much more left of it to explore." "Subtitles by Red Bee Media Ltd" "E-mail subtitling@bbc.co.uk"