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Wonders of the Universe Falling Gravity

where the force at the heart of this story is gravity 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. 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. But this tide wasn't in water. It was in rock. Imagine that this is the moon, and over there is the Earth. 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. 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.


  BBC Wonders of the Universe Falling where the force at the heart of this story is gravity
 
Milky Way and Androme are locked in a gravitational embrace
Milky Way and Androme are locked in a gravitational embrace
  The Earth's gravity acts on the moon and stretches it out into a kind of rugby ball shape
The Earth's gravity acts on the moon and stretches it out into a kind of rugby ball shape
  The Chacoan great houses are aligned with interesting objects in the sky
The Chacoan great houses are aligned with interesting objects in the sky
  Planet's orbit is simply the planet falling into the curved space-time created by the huge mass of a star
Planet's orbit is simply the planet falling into the curved space-time created by the huge mass of a star
 
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. 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.