0:00:11 > 0:00:14Why are we here? Where do we come from?
0:00:14 > 0:00:18These are the most enduring of questions, and it's an essential
0:00:18 > 0:00:21part of human nature to want to find the answers.
0:00:28 > 0:00:32And we can trace our ancestry back hundreds of thousands of years
0:00:32 > 0:00:36to the dawn of humankind, but in reality,
0:00:36 > 0:00:39our story extends far further back in time.
0:00:39 > 0:00:43Our story starts with the beginning of the universe.
0:00:47 > 0:00:51It began 13.7 billion years ago.
0:00:55 > 0:00:59And today, it's filled with over 100 billion galaxies,
0:00:59 > 0:01:03each containing hundreds of billions of stars.
0:01:08 > 0:01:13In this series, I want to tell that story because, ultimately,
0:01:13 > 0:01:15we are part of the universe.
0:01:15 > 0:01:20So its story is our story.
0:01:22 > 0:01:25The force at the heart of this story is gravity.
0:01:29 > 0:01:34This fundamental force of nature built everything we see.
0:01:34 > 0:01:37It creates shape and order,
0:01:37 > 0:01:41and it initiates patterns that repeat across the heavens.
0:01:44 > 0:01:49But gravity also forges some of the most alien worlds in the cosmos,
0:01:49 > 0:01:52worlds that defy belief.
0:01:54 > 0:01:59The quest to understand this fundamental force of nature
0:01:59 > 0:02:03has unleashed a golden age of creativity,
0:02:03 > 0:02:05exploration and discovery.
0:02:06 > 0:02:10And it's led to a far deeper understanding
0:02:10 > 0:02:13of our place in the universe.
0:02:30 > 0:02:32Every moment of our lives,
0:02:32 > 0:02:36we experience a force that we can't see or touch.
0:02:38 > 0:02:43Yet this force is able to keep us firmly rooted to the ground.
0:02:43 > 0:02:46It is, of course, gravity.
0:02:50 > 0:02:55But despite its intangible nature, we always know it's with us.
0:02:57 > 0:02:58If I was to ask you,
0:02:58 > 0:03:01"How do you know that there's gravity around here?"
0:03:01 > 0:03:04Then you might say, "Well, it's obvious."
0:03:04 > 0:03:07You know, I can just do an experiment, I can drop something.
0:03:09 > 0:03:15Well, yes, but actually, gravity is a little bit more subtle than that.
0:03:15 > 0:03:18But to really experience it, to understand it,
0:03:18 > 0:03:21you have to do something pretty extreme.
0:03:27 > 0:03:31And this plane has been modified to help me do it.
0:03:31 > 0:03:36Thanks to its flight plan, it's known as the Vomit Comet.
0:03:50 > 0:03:53Once we've climbed to 15,000 metres,
0:03:53 > 0:03:58this plane does something no ordinary flight would do.
0:03:58 > 0:04:03Its engines are throttled back, and the jet falls to Earth.
0:04:04 > 0:04:08And then, something quite amazing happens.
0:04:11 > 0:04:13SCREAMS AND CHEERS
0:04:13 > 0:04:16Push to me, push to me! Oh!
0:04:24 > 0:04:27I'm now plummeting towards the ground just like
0:04:27 > 0:04:31someone's cut the cable in a lift, and you see that I'm not moving.
0:04:31 > 0:04:35Relative to Einstein, we're all just floating.
0:04:53 > 0:04:56By simply falling at the same rate as the plane,
0:04:56 > 0:05:03for a few fleeting moments, we are all free of gravity's grip.
0:05:15 > 0:05:17But this isn't just a joyride.
0:05:24 > 0:05:27There's something very profound here,
0:05:27 > 0:05:32because although I'm falling towards the ground, as you can see,
0:05:32 > 0:05:35gravity has completely gone away.
0:05:37 > 0:05:39Gravity is not here any more.
0:05:44 > 0:05:48I've cancelled gravity out just by falling.
0:05:48 > 0:05:52If you understand that, then you'll understand gravity.
0:05:58 > 0:06:02So it is possible, by the simple act of falling,
0:06:02 > 0:06:06to get a very different experience of gravity.
0:06:06 > 0:06:08But this force of nature does more
0:06:08 > 0:06:11than just bring us back down to Earth.
0:06:12 > 0:06:16Gravity also plays a role on the grandest of stages,
0:06:16 > 0:06:20because across the universe, from the smallest mote of dust
0:06:20 > 0:06:25to the most massive star, gravity is the great sculptor
0:06:25 > 0:06:28that created order out of chaos.
0:06:33 > 0:06:38Since the beginning of time, gravity has been at work in our universe.
0:06:41 > 0:06:45From the primordial cloud of gas and cosmic dust,
0:06:45 > 0:06:48gravity forged the stars.
0:06:54 > 0:06:57It sculpted the planets and moons,
0:06:57 > 0:07:01and set them in orbit around the newly formed suns.
0:07:03 > 0:07:08And gravity connects these star systems together in vast galaxies,
0:07:08 > 0:07:13and steers them on their journey through unbounded space.
0:07:16 > 0:07:21Over the centuries, our quest to understand gravity has allowed us
0:07:21 > 0:07:24to explain some of the true wonders of the universe.
0:07:24 > 0:07:29But at a deeper level, that quest has also allowed us to ask questions
0:07:29 > 0:07:33about the origin and evolution of the universe itself.
0:07:42 > 0:07:47To understand how gravity works across the universe,
0:07:47 > 0:07:51we need look no further than the ground beneath our feet.
0:07:57 > 0:08:00Well, the first scientist to really think about it
0:08:00 > 0:08:04was Isaac Newton back in the 1680s, and he said this -
0:08:04 > 0:08:10"Gravity is a force of attraction between all objects".
0:08:10 > 0:08:14Now, the force of attraction between these two rocks
0:08:14 > 0:08:17is obviously very small, almost impossible to measure,
0:08:17 > 0:08:20and that's because the force is proportional
0:08:20 > 0:08:22to the masses of the objects.
0:08:22 > 0:08:24These things are not very massive.
0:08:24 > 0:08:28But there is a more massive rock around here.
0:08:28 > 0:08:30It's the one I'm standing on, planet Earth.
0:08:36 > 0:08:40The mass of our Earth generates a gravitational pull
0:08:40 > 0:08:45strong enough to sculpt the entire surface of the planet.
0:08:45 > 0:08:50It causes water to gouge out vast canyons.
0:08:50 > 0:08:54It sets the limit for how high mountains can soar,
0:08:54 > 0:08:57and it shapes whole continents.
0:08:58 > 0:09:03But this invisible force does more than just shape our world.
0:09:09 > 0:09:14The skies are always changing, and the constellations rise and fall
0:09:14 > 0:09:16in different places every night,
0:09:16 > 0:09:21and the planets wander across the background of the fixed stars.
0:09:21 > 0:09:23But throughout human history,
0:09:23 > 0:09:27there's been one constant up there in the night sky,
0:09:27 > 0:09:32because every human that's ever lived has gazed up at the moon
0:09:32 > 0:09:35and seen one face shining back at us.
0:09:39 > 0:09:42The reason why we never see the dark side of the moon
0:09:42 > 0:09:46is all down to the subtlety with which gravity operates.
0:09:49 > 0:09:53Millions of years ago, the moon rotated rapidly.
0:09:53 > 0:09:59But from the moment it was born, our companion felt the tug of gravity.
0:10:02 > 0:10:06Just as the moon creates great tides in our oceans,
0:10:06 > 0:10:11the Earth caused a vast tide to sweep across the surface of the moon.
0:10:14 > 0:10:16But this tide wasn't in water.
0:10:16 > 0:10:18It was in rock.
0:10:21 > 0:10:27Imagine that this is the moon, and over there is the Earth.
0:10:27 > 0:10:32The Earth's gravity acts on the moon and stretches it out
0:10:32 > 0:10:34into a kind of rugby ball shape.
0:10:34 > 0:10:39Now, the size of that tidal bulge facing the Earth is something like
0:10:39 > 0:10:44seven metres in rock and then, as the moon rotates,
0:10:44 > 0:10:49that bulge sweeps across the lunar surface.
0:10:49 > 0:10:51I mean, imagine what that would look like here.
0:10:51 > 0:10:54You'd see a tidal wave sweep
0:10:54 > 0:10:59across this landscape, with the rock rising and falling by seven metres.
0:11:02 > 0:11:05This massive wave acted like a brake,
0:11:05 > 0:11:09and gradually slowed the moon down.
0:11:09 > 0:11:13Eventually, the tidal bulge became aligned with the Earth,
0:11:13 > 0:11:16locking the speed of the moon's rotation.
0:11:16 > 0:11:19So the time it takes the moon to spin once
0:11:19 > 0:11:24is almost the same as the time it takes to orbit the Earth.
0:11:29 > 0:11:32So there is no dark side of the moon,
0:11:32 > 0:11:36just a side that gravity hides from our view.
0:11:50 > 0:11:54The bond that gravity creates between the Earth and the moon
0:11:54 > 0:11:56is repeated across the cosmos.
0:11:59 > 0:12:04It's the glue that holds the planets in orbit around the sun.
0:12:06 > 0:12:09And it binds our solar system
0:12:09 > 0:12:12and countless other solar systems together,
0:12:12 > 0:12:15to form galaxies like our own Milky Way.
0:12:17 > 0:12:21But gravity's influence can be felt even further
0:12:21 > 0:12:25because it controls the fate of galaxies.
0:12:45 > 0:12:49When you look up into the night sky and you see the universe
0:12:49 > 0:12:52as it looks in visible light, with the glowing
0:12:52 > 0:12:56of the stars and the galaxies, but that's only part of the story,
0:12:56 > 0:13:00because the universe is full of dust and gas
0:13:00 > 0:13:03which you can't see with a conventional telescope,
0:13:03 > 0:13:06but you can see with a telescope like this.
0:13:10 > 0:13:14Radio telescopes, like the very large array in New Mexico,
0:13:14 > 0:13:16are able to peer deep into space
0:13:16 > 0:13:20and reveal the incredible attractive power of gravity.
0:13:27 > 0:13:29This is Andromeda,
0:13:29 > 0:13:34a spiral galaxy roughly the same size and mass as the Milky Way.
0:13:38 > 0:13:41This island of over a trillion stars
0:13:41 > 0:13:44sits over 2.5 million light years away,
0:13:44 > 0:13:50but every hour that gap shrinks by half a million kilometres.
0:13:52 > 0:13:56Whilst most galaxies have been rushing away from each other
0:13:56 > 0:13:59ever since they formed just after the Big Bang,
0:13:59 > 0:14:03some galaxies formed so close together that they are locked
0:14:03 > 0:14:05in a gravitational embrace,
0:14:05 > 0:14:10and the Milky Way and Andromeda are two such galaxies.
0:14:10 > 0:14:14Computer simulations suggest that they will collide together
0:14:14 > 0:14:17in around three billion years' time.
0:14:25 > 0:14:29Look at that. That's a simulation of the Milky Way galaxy
0:14:29 > 0:14:33and the Andromeda galaxy colliding together,
0:14:33 > 0:14:38and all these wisps of smoke getting thrown out are stars.
0:14:38 > 0:14:41These are star systems getting ripped out of the galaxy
0:14:41 > 0:14:44and thrown off into interstellar space.
0:14:52 > 0:14:55These two islands of hundreds of billions of suns
0:14:55 > 0:15:00have flown through each other, and gravity has exerted its grasp
0:15:00 > 0:15:02and dragged them back again.
0:15:05 > 0:15:08And just remember that we are one of those dots.
0:15:08 > 0:15:11You know, our sun and the Earth and the solar system
0:15:11 > 0:15:16are either going to be flung out into interstellar space,
0:15:16 > 0:15:19or they're going to be in here,
0:15:19 > 0:15:21in this maelstrom of hundreds of billions of suns
0:15:21 > 0:15:27swirling around each other and forming the core of a new galaxy.
0:15:41 > 0:15:44Just imagine what it would be like
0:15:44 > 0:15:48to gaze up at the sky as Andromeda approached.
0:15:48 > 0:15:52The sky would be ablaze with the light of hundreds of billions
0:15:52 > 0:15:56of suns, and the imminent collision would provide the energy
0:15:56 > 0:16:00to generate the births of hundreds of millions more.
0:16:00 > 0:16:04What a magnificent sight it would be.
0:16:10 > 0:16:16But far more magnificent is the immense scale of gravity's embrace.
0:16:20 > 0:16:25It holds galaxies together across hundreds of billions of kilometres
0:16:25 > 0:16:29and, in doing so, it creates the most magnificent structures.
0:16:32 > 0:16:38Our own Milky Way is part of one of these, the Virgo cluster.
0:16:40 > 0:16:46Every point of light in this image is not a star, but a galaxy.
0:16:47 > 0:16:50There are 2,000 galaxies in this cluster,
0:16:50 > 0:16:54and they're all bound together by gravity,
0:16:54 > 0:16:58making it the largest structure in our intergalactic neighbourhood.
0:17:03 > 0:17:08There seems to be no limit to the reach or power of gravity.
0:17:09 > 0:17:14Its influence can be felt across the vast expanses of space and time.
0:17:14 > 0:17:17But there's something very interesting about gravity,
0:17:17 > 0:17:22because it is by far the weakest force of nature. I mean, look.
0:17:22 > 0:17:27I can...pick this rock up off the ground even though
0:17:27 > 0:17:33there's an entire planet, planet Earth, trying to pull it down.
0:17:33 > 0:17:36So if gravity is so weak,
0:17:36 > 0:17:38how come it's so influential?
0:17:48 > 0:17:51Gravity may be weak here on Earth,
0:17:51 > 0:17:55but it's not so weak across the cosmos.
0:17:55 > 0:18:00This invisible force varies on all the planets in the solar system
0:18:00 > 0:18:04and on the exo-planets we've discovered orbiting other suns.
0:18:08 > 0:18:13To experience what gravity feels like on these worlds,
0:18:13 > 0:18:14I need to go for a spin.
0:18:20 > 0:18:22This is a centrifuge.
0:18:22 > 0:18:26It was built in the 1950s to test whether fighter pilots
0:18:26 > 0:18:29had the right stuff, but it's going to allow me to
0:18:29 > 0:18:34feel what it'd be like to stand on the surface of any of the planets
0:18:34 > 0:18:37in the solar system that are more massive than the Earth,
0:18:37 > 0:18:41and, in fact, also what it would be like to stand on some of the planets
0:18:41 > 0:18:44that we've found around distant stars.
0:18:51 > 0:18:54Right, I'll have to strap you in, first of all.
0:19:00 > 0:19:05This is an emergency switch in case something happens.
0:19:05 > 0:19:08When you release it, the centrifuge will stop.
0:19:08 > 0:19:12I was just told by the F-16 fighter pilot, who's just been in here,
0:19:12 > 0:19:15that it's a hundred times more uncomfortable
0:19:15 > 0:19:17than being in a jet fighter.
0:19:17 > 0:19:20I was kind of confident because I've been in jet fighters
0:19:20 > 0:19:23and didn't find it too uncomfortable, but apparently,
0:19:23 > 0:19:24this is a hundred times worse!
0:19:47 > 0:19:50Doors closed again. Profile is there. Doctor is ready.
0:19:50 > 0:19:56We'll start up the centrifuge, Brian, and bring you in orbit,
0:19:56 > 0:20:00and it happens in three...two...one second from now.
0:20:12 > 0:20:15'The first planet I'm travelling to is Neptune.
0:20:15 > 0:20:20'Its gravity is just fractionally stronger than here on Earth.'
0:20:20 > 0:20:22So this is the gravitational field
0:20:22 > 0:20:24on Neptune and you feel, you know what?
0:20:24 > 0:20:26I could probably get used to this.
0:20:26 > 0:20:28I could probably live on the surface of Neptune.
0:20:28 > 0:20:30Can you lift your hands a little?
0:20:30 > 0:20:32- There we go.- Yeah, and down.
0:20:32 > 0:20:36And it is actually quite an effort. It is noticeably heavier.
0:20:36 > 0:20:40It's like having a reasonably heavy weight in your hand.
0:20:40 > 0:20:42Are you ready to go to 2.5G?
0:20:42 > 0:20:45Yes, so now we'll move... move from Neptune to Jupiter.
0:20:45 > 0:20:47Let's go there.
0:20:47 > 0:20:51Jupiter is over 1,300 times more massive than the Earth,
0:20:51 > 0:20:56but because it's mostly gas, it's not very dense, so its gravity
0:20:56 > 0:20:58is just over twice as strong at its surface.
0:20:58 > 0:21:05Well, now actually, it is quite difficult to lift my hand.
0:21:05 > 0:21:08And that's 2.5G. I wouldn't want to sit here for half an hour.
0:21:08 > 0:21:12Can you lift...lift both of your hands above your head?
0:21:12 > 0:21:17- See what happens there.- Let's see, so actually...just about,
0:21:17 > 0:21:21but actually, it's an immense amount of hard work.
0:21:21 > 0:21:25- So it would be hard work living on Jupiter.- Let's go to 4G.
0:21:32 > 0:21:35Actually, this is heading to a planet around...
0:21:35 > 0:21:38a planet called Ogle-2TRL9B,
0:21:38 > 0:21:42which is around a star in the constellation of Carina.
0:21:42 > 0:21:45It's one of the exo-planets we've discovered.
0:21:46 > 0:21:48Oh, and there we go.
0:21:53 > 0:21:55Now, that is actually
0:21:55 > 0:21:59beginning to feel quite unpleasant.
0:21:59 > 0:22:01Can you describe what you're feeling?
0:22:01 > 0:22:02Very heavy face.
0:22:02 > 0:22:04My head is extremely heavy.
0:22:04 > 0:22:07How about your lungs, inhaling, exhaling, breathing?
0:22:07 > 0:22:09It's much harder work.
0:22:09 > 0:22:11I can't lift my hand off my leg.
0:22:11 > 0:22:14- OK.- And that's at 4G?- Yeah.
0:22:16 > 0:22:20Well, my head and my face feel very, very heavy.
0:22:20 > 0:22:22It's quite an unpleasant feeling.
0:22:22 > 0:22:26We'll go to five, and let me know if you have any visual disturbances.
0:22:28 > 0:22:34'I'm now en route to a newly discovered exo-planet, Wasp-8B.'
0:22:34 > 0:22:364.4.
0:22:38 > 0:22:42'This world sits in the small and faint constellation of Sculptor.'
0:22:45 > 0:22:47Quite hard to speak.
0:22:50 > 0:22:53'It has a gravitational force
0:22:53 > 0:22:56'nearly five times that of the Earth.'
0:22:56 > 0:22:59Right, we'll go to 5G.
0:22:59 > 0:23:00- Very foggy.- OK.
0:23:04 > 0:23:06- Very foggy.- Very foggy?
0:23:14 > 0:23:16- Still foggy?- Yeah.
0:23:16 > 0:23:17Right.
0:23:17 > 0:23:20- Take it down. - OK, we'll take you down.
0:23:34 > 0:23:36Very interesting.
0:23:36 > 0:23:39It was, wasn't it?
0:23:39 > 0:23:42My face felt a bit saggy, though.
0:23:42 > 0:23:44Well, you looked a little different.
0:24:00 > 0:24:04It was quite unpleasant that time, actually.
0:24:04 > 0:24:07It went very quickly up to 5G and what happens is -
0:24:07 > 0:24:10for me, anyway - vision becomes very, very foggy.
0:24:10 > 0:24:14The whole thing just blurs and blurs and blurs.
0:24:14 > 0:24:20So you realise that we're, obviously, very finely tuned to live
0:24:20 > 0:24:25on a planet that has an acceleration due to gravity of 1G.
0:24:25 > 0:24:28When you go to 2G, it's difficult.
0:24:28 > 0:24:33When you go to 3G and 4G, it becomes unpleasant
0:24:33 > 0:24:37and 5G anyway, for me, was on the border of being
0:24:37 > 0:24:40so unpleasant that you pass out.
0:24:44 > 0:24:47So, although gravity feels weak here on Earth,
0:24:47 > 0:24:52it certainly isn't weak everywhere across the universe,
0:24:52 > 0:24:54and that's because gravity is an additive force.
0:24:54 > 0:24:59It scales with mass, so the more massive the planet or star,
0:24:59 > 0:25:02the stronger its gravity.
0:25:06 > 0:25:11The body with the strongest gravity in our solar system is the sun.
0:25:13 > 0:25:19Our star has so much mass packed inside a relatively small space that
0:25:19 > 0:25:24it has a gravitational pull at its surface 28 times that of the Earth.
0:25:28 > 0:25:31If I were able to set foot on this world, all the blood would be
0:25:31 > 0:25:36poled out of my upper body, and I would die in less than a minute.
0:25:46 > 0:25:52But our sun's gravitational force is nothing compared to the extreme G
0:25:52 > 0:25:57found at the surface of one of the strangest places in the universe.
0:25:57 > 0:26:01Imagine the gravity on a world with more mass than our sun,
0:26:01 > 0:26:05crammed into a sphere just 20 kilometres across.
0:26:05 > 0:26:10We first detected such a wonder just 40 years ago, but the story
0:26:10 > 0:26:14of its discovery begins over a thousand years earlier.
0:26:43 > 0:26:47This is Chaco Canyon in New Mexico in the south western United States,
0:26:47 > 0:26:51and it was home to what's become known as the Chacoan civilisation.
0:27:02 > 0:27:06Well, this is Pueblo Bonito, one of the so-called Chacoan great houses.
0:27:06 > 0:27:11Back in the 1100s, this place had over 600 rooms.
0:27:11 > 0:27:16It's thought that this building must have been ceremonial
0:27:16 > 0:27:19or religious, a cathedral, if you like.
0:27:23 > 0:27:29The Chacoan great houses are aligned with interesting objects in the sky,
0:27:29 > 0:27:32so the points at which the sun and moon rise
0:27:32 > 0:27:34at important times of the year.
0:27:45 > 0:27:49So it seems that by constructing these grand buildings,
0:27:49 > 0:27:52the Chacoans were not only trying to place themselves
0:27:52 > 0:27:54at the heart of local culture,
0:27:54 > 0:27:58but also to place themselves at the heart of the cosmos.
0:28:02 > 0:28:06Very little is known about the Chacoan culture,
0:28:06 > 0:28:10because no written text has ever been discovered.
0:28:10 > 0:28:14But in another part of the canyon, there is a record of a spectacular
0:28:14 > 0:28:18event that they witnessed in the sky in 1054.
0:28:25 > 0:28:29Now, I've known about this place since I was 12 or 13 years old,
0:28:29 > 0:28:34and the reason is this book, and the television series Cosmos,
0:28:34 > 0:28:36Carl Sagan's masterpiece,
0:28:36 > 0:28:40probably the most important reason that I got interested in astronomy.
0:28:40 > 0:28:45And on page 232, there's a picture that's always fascinated me
0:28:45 > 0:28:51and captured my imagination and it's a photograph of that wall of rock,
0:28:51 > 0:28:56and in particular a painting that's on the overhang.
0:28:56 > 0:28:59Because it's thought that that painting is a record of one of
0:28:59 > 0:29:04the most spectacular and magical events in the cosmos.
0:29:11 > 0:29:17On 4th July 1054AD, a bright new star appeared,
0:29:17 > 0:29:22and it outshone every other star in the night sky for over three weeks.
0:29:22 > 0:29:25It was so bright that it was visible in the daytime,
0:29:25 > 0:29:30and it's thought that this painting is the Chacoan people's record
0:29:30 > 0:29:33of that astronomical event.
0:29:33 > 0:29:37The reason we think that is that using modern computer techniques,
0:29:37 > 0:29:41you can wind back the night sky and say,
0:29:41 > 0:29:44"Where would the moon have been? Where would the stars have been?"
0:29:44 > 0:29:47And you find that in that direction,
0:29:47 > 0:29:50the moon would have risen and tracked across the night sky,
0:29:50 > 0:29:54and the new star would have been very, very close
0:29:54 > 0:29:56to the crescent moon.
0:29:59 > 0:30:04We now know that that new star was in fact the explosive death
0:30:04 > 0:30:07of an old star, a supernova explosion,
0:30:07 > 0:30:12a star, literally, blowing itself apart at the end of its life.
0:30:17 > 0:30:22Throughout a star's life, there is a constant battle between energy
0:30:22 > 0:30:24pushing out and gravity pushing in.
0:30:30 > 0:30:34As long as the star burns, the two forces balance each other out.
0:30:39 > 0:30:44But when it runs out of fuel, gravity wins and the star collapses,
0:30:44 > 0:30:48and then explodes with the brightness of a billion suns.
0:30:53 > 0:30:57We can no longer see the supernova the Chacoans saw,
0:30:57 > 0:31:00but we can still marvel at what it left behind.
0:31:06 > 0:31:09This is the Crab Nebula, the remains of that
0:31:09 > 0:31:16exploding star that the Chacoans saw in these skies a thousand years ago.
0:31:16 > 0:31:20It's an expanding cloud of gas and dust, the remains
0:31:20 > 0:31:24of that dying star, and the colours are different chemical elements,
0:31:24 > 0:31:28so the orange is hydrogen, the red is nitrogen
0:31:28 > 0:31:31and those filaments of green are oxygen.
0:31:37 > 0:31:41While the explosion blew most of the stellar material out into the cosmos
0:31:41 > 0:31:43to form this vast nebula,
0:31:43 > 0:31:46we now know that this wasn't the end of the story.
0:31:48 > 0:31:54At the centre of the nebula lies the remnant of the star, its core,
0:31:54 > 0:31:56crushed by the force of gravity.
0:32:00 > 0:32:02That is a neutron star,
0:32:02 > 0:32:06an image taken by the Chandra X-ray satellite.
0:32:06 > 0:32:11The central blob there is only about 20 kilometres across,
0:32:11 > 0:32:16but it's got the mass of our sun, a star the size of a city.
0:32:16 > 0:32:22It's spinning at a rate of over 30 times a second,
0:32:22 > 0:32:261,800 revolutions per minute,
0:32:26 > 0:32:29and it really is an astonishingly alien world.
0:32:41 > 0:32:45As the neutron star spins, jets of particles
0:32:45 > 0:32:49stream out from the poles at almost the speed of light.
0:32:54 > 0:33:00These jets are powerful beams that sweep around as the star rotates.
0:33:06 > 0:33:09When the beams sweep across the Earth,
0:33:09 > 0:33:14they can be heard as regular pulses, so we call them pulsars.
0:33:20 > 0:33:25But it's not this rhythmic noise that makes the Crab Pulsar a wonder.
0:33:25 > 0:33:30It's the extraordinary nature of gravity on this alien world.
0:33:35 > 0:33:39If I were to be on its surface, then the gravitational pull on me
0:33:39 > 0:33:45would be a hundred thousand million times that that I feel on Earth.
0:33:45 > 0:33:49That means that if I were to jump from the top of that
0:33:49 > 0:33:52projection screen, by the time I hit the ground,
0:33:52 > 0:33:55I'd be travelling at over four million miles an hour.
0:33:55 > 0:33:57That's a lot of gravity.
0:34:00 > 0:34:03Pulsars have such extreme gravity
0:34:03 > 0:34:06because they're made of incredibly dense matter.
0:34:06 > 0:34:11To understand why, we have to look at what gravity can do to matter
0:34:11 > 0:34:13at the very smallest scales.
0:34:36 > 0:34:39Everything in the universe is made of atoms,
0:34:39 > 0:34:41and until the turn of the 20th century,
0:34:41 > 0:34:45it was thought that they were the smallest building blocks of matter.
0:34:45 > 0:34:48I mean, the word itself comes from the Greek "atomos",
0:34:48 > 0:34:50which means indivisible.
0:34:50 > 0:34:54But we now know that atoms are made of much smaller stuff.
0:34:59 > 0:35:04Atoms consist of an atomic nucleus surrounded by a cloud of electrons.
0:35:04 > 0:35:08And whilst almost all of the mass is contained in the nucleus,
0:35:08 > 0:35:12it is incredibly tiny compared to the size of an atom.
0:35:12 > 0:35:17If this were a nucleus, then the cloud of electrons would stretch out
0:35:17 > 0:35:20to something like a kilometre away.
0:35:20 > 0:35:22I mean, that's from here to that rock.
0:35:22 > 0:35:26And electrons on this scale are incredibly tiny.
0:35:26 > 0:35:30They're just like specks of dust and they're aren't many of them.
0:35:30 > 0:35:35So imagine a giant sphere centred on the atomic nucleus stretching out
0:35:35 > 0:35:38all the way to that rock and beyond,
0:35:38 > 0:35:42with just a few points of dust in it.
0:35:42 > 0:35:44That's an atom.
0:35:44 > 0:35:49So that means that matter is almost entirely empty space.
0:35:49 > 0:35:53I'm full of empty space. The Earth is full of empty space.
0:35:53 > 0:35:56Everything you can see in the universe
0:35:56 > 0:35:59is pretty much just empty space.
0:36:04 > 0:36:07So if everything in the universe is made up of atoms,
0:36:07 > 0:36:14and atoms are 99.9999% empty space, then most of the universe is empty.
0:36:17 > 0:36:22But in the Crab Pulsar, the force of gravity is so extreme
0:36:22 > 0:36:27that the empty space inside the atoms is squashed out of existence,
0:36:27 > 0:36:30so all you're left with is incredibly dense matter.
0:36:33 > 0:36:36Imagine this was matter taken from a neutron star -
0:36:36 > 0:36:40then it would weigh more than Mount Everest.
0:36:40 > 0:36:45Or to put it another way, if I took every human being on the planet
0:36:45 > 0:36:50and squashed them so they were as dense as neutron star matter,
0:36:50 > 0:36:53then we would all fit inside that.
0:36:53 > 0:36:58And if I were to drop my neutron star stuff to the ground,
0:36:58 > 0:37:01then it would slice straight through the Earth
0:37:01 > 0:37:03like a knife through butter.
0:37:11 > 0:37:15Wherever we look in the universe, we see gravity at work.
0:37:15 > 0:37:18It creates shape and structure.
0:37:18 > 0:37:23It governs the orbits of every planet, star and galaxy
0:37:23 > 0:37:26in ways we thought we were able to predict.
0:37:26 > 0:37:30But there was a flaw in our understanding of this force,
0:37:30 > 0:37:33and it was exposed by one of our close neighbours.
0:37:42 > 0:37:45This is Mercury.
0:37:45 > 0:37:47For thousands of years, we've marvelled
0:37:47 > 0:37:52as this fleet-footed planet races across the face of the sun.
0:37:54 > 0:37:56But 150 years ago,
0:37:56 > 0:38:01astronomers noticed something strange about Mercury's orbit.
0:38:17 > 0:38:23Imagine that this rock is the sun, and this is Mercury.
0:38:23 > 0:38:25Now Mercury has quite a complex orbit.
0:38:25 > 0:38:28For one thing it's not a perfect circle,
0:38:28 > 0:38:30it's quite an elongated ellipse.
0:38:30 > 0:38:32So at its closest approach to the sun,
0:38:32 > 0:38:34it's around 46 million kilometres away,
0:38:34 > 0:38:39and then it drifts out to something just under 70 million kilometres.
0:38:39 > 0:38:43But you can calculate Mercury's orbit very precisely
0:38:43 > 0:38:46using only Newton's laws of gravity.
0:38:49 > 0:38:53So astronomers used to predict the exact time when you could look up
0:38:53 > 0:38:55into the sky, look at the sun
0:38:55 > 0:38:59and see the tiny disc of Mercury pass across its face.
0:39:04 > 0:39:07But the thing was, they never got it right.
0:39:07 > 0:39:11They predicted it time and time again, and every time it happened,
0:39:11 > 0:39:15they got it slightly wrong, which was an immense embarrassment.
0:39:15 > 0:39:19So what they did was that, rather than question Newton,
0:39:19 > 0:39:22they invented another planet, and they called it Vulcan,
0:39:22 > 0:39:26and they said that there must be another planet somewhere
0:39:26 > 0:39:29in the solar system, which is always invisible from Earth
0:39:29 > 0:39:32but which perturbed Mercury's orbit a bit,
0:39:32 > 0:39:36and so that was the reason their calculations were wrong.
0:39:40 > 0:39:44For decades, astronomers searched and searched for Vulcan.
0:39:45 > 0:39:51But they never found it, because Vulcan didn't exist.
0:39:53 > 0:39:55The explanation, the real explanation,
0:39:55 > 0:39:59was even more interesting than inventing the planet Vulcan,
0:39:59 > 0:40:02because it required a modification,
0:40:02 > 0:40:07in fact, a complete re-writing of Newton's law of gravity.
0:40:10 > 0:40:15Gravity is NOT a force pulling us towards the centre of the Earth
0:40:15 > 0:40:17like a giant magnet.
0:40:17 > 0:40:22In a sense, gravity isn't really a force at all.
0:40:36 > 0:40:40Describing the nature of gravity turned out to be one of the great
0:40:40 > 0:40:42intellectual challenges,
0:40:42 > 0:40:47but almost 200 years after Newton's death, a new theory emerged.
0:40:51 > 0:40:54The new theory, called general relativity,
0:40:54 > 0:40:58was published in 1915 by Albert Einstein after ten years of work,
0:40:58 > 0:41:01and it stands to this day as one of the great achievements
0:41:01 > 0:41:03in the history of physics.
0:41:03 > 0:41:07You see, not only was it able to explain with absolute precision
0:41:07 > 0:41:09the strange behaviour of Mercury,
0:41:09 > 0:41:13but it explains to this day everything we can see
0:41:13 > 0:41:17out there in the universe that has anything to do with gravity.
0:41:17 > 0:41:24And, most importantly of all, it explains how gravity actually works.
0:41:35 > 0:41:39Gravity is the effect that the stars, planets and galaxies
0:41:39 > 0:41:42have on the very space that surrounds them.
0:41:46 > 0:41:51According to Einstein, space is not just an empty stage -
0:41:51 > 0:41:54it's a fabric called space-time.
0:41:57 > 0:42:01This fabric can be warped, bent and curved
0:42:01 > 0:42:07by the enormous mass of the planet's stars and galaxies.
0:42:13 > 0:42:17You see, all matter in the universe bends.
0:42:17 > 0:42:23The very fabric of the universe itself - matter - bends space.
0:42:23 > 0:42:27I bend space, these mountains bend space,
0:42:27 > 0:42:31but by the tiniest of tiniest of amounts.
0:42:31 > 0:42:36But when you get onto the scale of planets and stars, galaxies,
0:42:36 > 0:42:40then they bend and curve the fabric of the universe
0:42:40 > 0:42:43by a very large amount indeed.
0:42:49 > 0:42:51And here is the key idea.
0:42:51 > 0:42:55Everything moves in straight lines
0:42:55 > 0:42:58over the curved landscape of space-time.
0:42:58 > 0:43:03So what we see as a planet's orbit is simply the planet
0:43:03 > 0:43:10falling into the curved space-time created by the huge mass of a star.
0:43:11 > 0:43:16This is able to explain Mercury's erratic orbit.
0:43:16 > 0:43:19Because of the planet's proximity to our sun,
0:43:19 > 0:43:23the effects of the curvature of space-time matter far more
0:43:23 > 0:43:27for Mercury than for any other planet in the solar system.
0:43:34 > 0:43:39This idea of curved space is difficult to imagine,
0:43:39 > 0:43:42but if you could only step outside of it,
0:43:42 > 0:43:47if we could only float above space-time and look down on it,
0:43:47 > 0:43:51this is what our universe would look like.
0:44:14 > 0:44:17You would see the mountains and valleys.
0:44:17 > 0:44:20You would see the little peaks and troughs
0:44:20 > 0:44:22created by planets and moons,
0:44:22 > 0:44:28and you would see these vast, deep valleys created by the galaxies.
0:44:53 > 0:44:58And you would see planets and moons and stars circling the peaks
0:44:58 > 0:45:01as they follow their straight-line paths
0:45:01 > 0:45:05through the curved landscape of space-time.
0:45:15 > 0:45:17So one way to think about gravity
0:45:17 > 0:45:22is that everything in the universe is just falling through space-time.
0:45:24 > 0:45:29The moon is falling into the valley created by the mass of the Earth.
0:45:29 > 0:45:34The Earth is falling into the valley created by the sun,
0:45:34 > 0:45:38and the solar system is falling into the valley in space-time
0:45:38 > 0:45:39created by our galaxy.
0:45:43 > 0:45:49And our galaxy is falling towards other galaxies in the universe.
0:45:58 > 0:46:02Einstein's theory of general relativity is so profound
0:46:02 > 0:46:06and so beautiful that it can describe the structure and shape
0:46:06 > 0:46:07of the universe itself.
0:46:07 > 0:46:12But remarkably, the theory can also predict its own demise,
0:46:12 > 0:46:18because it predicts the existence of objects so dense and so powerful
0:46:18 > 0:46:23that they warp and stretch and bend the structure of space-time so much
0:46:23 > 0:46:28that they can stop time, and that they can swallow light.
0:46:28 > 0:46:30These are objects so powerful
0:46:30 > 0:46:35that they can tear all the other wonders of the universe apart.
0:46:42 > 0:46:45Since the dawn of civilisation, we've peered at the stars
0:46:45 > 0:46:50in the night sky and tracked the movements of the planets.
0:46:53 > 0:46:57We see these familiar patterns repeated across the whole universe.
0:47:02 > 0:47:05But when we train our telescopes to the stars that orbit around
0:47:05 > 0:47:10the centre of our galaxy, we see something very unusual.
0:47:13 > 0:47:18Well, this is one of the most fascinating and important movies
0:47:18 > 0:47:22made in astronomy over the last ten or 20 years. This is real data.
0:47:22 > 0:47:25Every point of light in this movie
0:47:25 > 0:47:29is a star orbiting around the centre of our galaxy.
0:47:29 > 0:47:31They're known as the S stars.
0:47:34 > 0:47:38Our sun takes around 200 million years to make its way
0:47:38 > 0:47:40around the Milky Way.
0:47:40 > 0:47:44One of these S stars takes only 15 years to go around
0:47:44 > 0:47:47the centre of the galaxy.
0:47:47 > 0:47:52It's travelling at 3,000 or 4,000 kilometres per second.
0:47:53 > 0:47:55Now, by tracking the orbits,
0:47:55 > 0:48:00it's possible to work out the mass of the thing at the centre.
0:48:00 > 0:48:05The answer took astronomers by surprise, I think it's fair to say,
0:48:05 > 0:48:08because the object in the centre of our galaxy
0:48:08 > 0:48:12is four million times as massive as the sun,
0:48:12 > 0:48:17and it fits into a space smaller than our solar system.
0:48:17 > 0:48:22Now there's only one thing that anyone knows of that can be so small
0:48:22 > 0:48:26and yet so massive, and that's a black hole.
0:48:26 > 0:48:32So what we're looking at here is stars swarming like bees
0:48:32 > 0:48:37around a super-massive black hole at the centre of the Milky Way galaxy.
0:48:43 > 0:48:47We think black holes can be smaller than an atom,
0:48:47 > 0:48:50or a billion times more massive than our sun.
0:48:50 > 0:48:54Some are born when a star dies.
0:49:06 > 0:49:11When a star around 15 times the mass of our sun collapses...
0:49:21 > 0:49:24..all the matter in its core is crushed
0:49:24 > 0:49:29into an infinite void of blackness known as a stellar mass black hole.
0:49:44 > 0:49:49Black holes are the most extreme example of warped space-time.
0:49:49 > 0:49:55They have such enormous mass crammed into such a tiny space
0:49:55 > 0:50:00that they curve space-time more than any other object in the universe.
0:50:13 > 0:50:19The immense gravitational pull of these monsters can rip a star apart.
0:50:19 > 0:50:23They tear matter from its surface and drag it into orbit.
0:50:28 > 0:50:32This super-heated matter spins around the mouth of the black hole,
0:50:32 > 0:50:36and great jets of radiation fire from the core.
0:50:41 > 0:50:45Although these jets can be seen across the cosmos,
0:50:45 > 0:50:48the core itself remains a mystery.
0:50:51 > 0:50:55Black holes curve space-time so much that nothing,
0:50:55 > 0:50:58not even light, can escape.
0:50:58 > 0:51:01So their interior is for ever hidden from us.
0:51:06 > 0:51:10But because we understand how matter curves the fabric of space,
0:51:10 > 0:51:13it is possible to picture what is happening.
0:51:40 > 0:51:46Near a black hole, space and time do some very strange things,
0:51:46 > 0:51:49because black holes are probably the most violent places
0:51:49 > 0:51:52we know of in the universe.
0:51:52 > 0:51:56This river provides a beautiful analogy for what happens
0:51:56 > 0:52:00to space and time as you get closer and closer to the black hole.
0:52:04 > 0:52:08Now, upstream, the water is flowing pretty slowly.
0:52:08 > 0:52:11Let's imagine that it's flowing at three kilometres per hour,
0:52:11 > 0:52:13and I can swim at four,
0:52:13 > 0:52:17so I can swim faster than the flow and can easily escape.
0:52:32 > 0:52:37But as you go further and further downstream towards the waterfall
0:52:37 > 0:52:40in the distance, the river flows faster and faster.
0:53:00 > 0:53:04Imagine I was to decide to jump into the river just there,
0:53:04 > 0:53:05on the edge of the falls -
0:53:05 > 0:53:09the water is flowing far faster than I could swim.
0:53:09 > 0:53:13So no matter what I did, no matter how hard I tried,
0:53:13 > 0:53:16I would not be able to swim back upstream.
0:53:16 > 0:53:19I would be carried inexorably towards the edge,
0:53:19 > 0:53:22and I would vanish over the falls.
0:53:37 > 0:53:41Well, it's the same close to a black hole, because space
0:53:41 > 0:53:46flows faster and faster and faster towards the black hole.
0:53:46 > 0:53:50Literally, this stuff, my space that I'm in,
0:53:50 > 0:53:53flowing over the edge into the black hole.
0:53:53 > 0:53:58And at the very special point called the event horizon,
0:53:58 > 0:54:03space is flowing at the speed of light into the black hole.
0:54:07 > 0:54:11Light itself, travelling at 300,000 kilometres per second
0:54:11 > 0:54:13is not going fast enough to escape the flow,
0:54:13 > 0:54:17and light itself will plunge into the black hole.
0:54:32 > 0:54:36Well, as you fall into a black hole, across the event horizon,
0:54:36 > 0:54:39then if you were going feet first,
0:54:39 > 0:54:42your feet would be accelerating faster than your head,
0:54:42 > 0:54:45so you would be stretched,
0:54:45 > 0:54:48and you would be quite literally spaghettified.
0:54:54 > 0:54:56Now as you get right to the centre,
0:54:56 > 0:55:00then our understanding of the laws of physics breaks down.
0:55:00 > 0:55:02Our best theory of space and time,
0:55:02 > 0:55:04Einstein's theory of general relativity,
0:55:04 > 0:55:08says that space and time become infinitely curved,
0:55:08 > 0:55:12that the centre of the hole becomes infinitely dense.
0:55:12 > 0:55:15That place is called the singularity,
0:55:15 > 0:55:22and it is the place where our understanding of the universe stops.
0:55:46 > 0:55:52Gravity is the great creator, the constructor of worlds.
0:55:57 > 0:56:00That's because it's the only force in the universe
0:56:00 > 0:56:04that can reach out across the vast expanses of space
0:56:04 > 0:56:07and pull matter together to make the planets,
0:56:07 > 0:56:12the moons, the stars and the galaxies.
0:56:12 > 0:56:17But gravity is also the destroyer, because it's relentless,
0:56:17 > 0:56:20and for the most massive objects in the universe,
0:56:20 > 0:56:25for the most enormous stars, and the centres of galaxies,
0:56:25 > 0:56:30gravity will eventually crush matter out of existence.
0:56:46 > 0:56:49Now, the word beautiful is probably over-used in physics.
0:56:49 > 0:56:52I probably over-use it.
0:56:52 > 0:56:55But I don't think there is any scientist who would disagree
0:56:55 > 0:57:00with its use in the context of Einstein's theory of gravity.
0:57:00 > 0:57:04Because here is a theory that describes a universe that
0:57:04 > 0:57:08is bent and curved out of shape by every moon, every star
0:57:08 > 0:57:10and every galaxy in the sky.
0:57:13 > 0:57:17And everything in the universe has to follow those curves,
0:57:17 > 0:57:23from the most massive black hole to the smallest mote of dust,
0:57:23 > 0:57:25even to beams of light.
0:57:25 > 0:57:29But the most tantalising thing about Einstein's theory of gravity
0:57:29 > 0:57:31is we know that it's not complete.
0:57:31 > 0:57:34We know that it's not the ultimate description
0:57:34 > 0:57:38of the structure and shape of the universe.
0:57:38 > 0:57:42And that, for a scientist, is the most beautiful place to be,
0:57:42 > 0:57:46on the border between the known and the unknown.
0:57:46 > 0:57:50That is the true wonder of the universe -
0:57:50 > 0:57:53there's so much more left of it to explore.
0:58:20 > 0:58:22Subtitles by Red Bee Media Ltd
0:58:22 > 0:58:24E-mail subtitling@bbc.co.uk