Wonders Of The Universe
Falling
1. 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.
2. 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.
3. 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.
4. Push to me, push to me! 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.
5. 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.
6. 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".
7. 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.
8. 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.
9. 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.
10. 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. 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.
11. 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. And computer simulations suggest that they will collide together in around three billion years' time.
12. 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.