Wonders of the Universe


Wonders of the Universe

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We all have an intuitive understanding of time.

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It seems obvious to us that things change,

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and the future will be different to the past.

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But why are we compelled to travel into the future?

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The answer to that question can be seen

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in how the world around us is always changing.

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This is Kolmanskop,

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an abandoned diamond mining town in southern Namibia.

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For half a century,

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it's fallen into disrepair

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as it's slowly reclaimed by the sands.

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The process at play here at Kolmanskop

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is happening everywhere in the universe - decay.

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Or in the language of physics, an increase in entropy.

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Entropy explains why,

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left to the mercy of the elements,

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mortar crumbles, glass shatters

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and buildings collapse.

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And a good way to understand how

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is to think of objects not as single things, but as being made up

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of many constituent parts, like the individual grains

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that make up this pile of sand.

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Now, entropy is a measure of how many ways

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I can re-arrange those grains and still keep the sand piled the same,

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and there are trillions and trillions and trillions

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of ways of doing that.

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I mean, pretty much anything I do to this sand pile,

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if I mess the sand around and move it around,

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then it doesn't change the shape or the structure at all.

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So, in the language of entropy, this sand pile has high entropy,

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because there are many, many ways

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that I can re-arrange its constituents and not change it.

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But now, let me create some order in the universe.

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Now, there are approximately as many sand grains in this sandcastle

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as there are in the sand pile.

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But now, virtually anything I do to it will mess it up,

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will remove the beautiful order from this structure

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and because of that, the sandcastle has a low entropy.

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It's a much more ordered state.

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So, many ways of re-arranging the sand grains

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without changing the structure - high entropy.

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Very few ways of re-arranging the sand grains

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without changing the structure, without disordering it -

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low entropy.

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Now, imagine I was to leave this castle in the desert all day,

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the desert winds are going to blow the sand around

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and this castle is going to disintegrate.

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It's going to become less ordered.

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But there's nothing fundamental

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in the laws of physics that says

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that the wind couldn't pick up some sand from over here,

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deposit it here and deposit it in precisely the shape of a sandcastle.

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You know, in principle,

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the wind could spontaneously build a sandcastle out of a pile of sand.

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There's no reason why that couldn't happen.

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It's just extremely, extremely unlikely,

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because there are very few ways of organising this sand

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so that it looks like a castle.

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It's overwhelmingly more likely

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that when the wind blows the sand around,

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it will take the low entropy structure, the castle,

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and turn it into a high entropy structure, the sand pile.

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So, entropy always increases. Why is that?

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Because it's overwhelmingly more likely that it will.

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So, everything tends from order to disorder.

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That means that there is a difference between the future

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and the past, and that's one reason why time travels in one direction.

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Everything that we see on Earth, from the grandest mountain

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to the most fleeting cloud,

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is made from the same set of building blocks.

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They're called the chemical elements.

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We now know that the Earth is made of 92 chemical elements

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and that's pretty amazing if you think of the complexity

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that we see around us. We also know that everything beyond Earth,

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everything we can see in the universe,

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is made of those same 92 elements,

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and notice I didn't say "we think" that's what they're made of.

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I said "we know" that's what they're made of, because we can prove it.

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The chemistry set we have on Earth extends far beyond the planet.

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We have set foot on the moon

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and know that it's rich in helium, silver and water.

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And discovered that Mars is rich in iron.

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And we know that Venus's thick atmosphere is full of sulphur.

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But what of the rest of the universe?

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It seems impossible that we could discover what the stars are made of,

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because they're so far away.

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Even the nearest star, Proxima Centauri, is 10,000 times

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more distant than Neptune, 4.2 light years from Earth.

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Yet despite these vast distances, these alien worlds are constantly

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sending us signals, telling us exactly what they're made of.

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Our only contact with the distant stars is their light

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that has journeyed across the universe to reach us,

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and encoded in that light is the key to understanding

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what the universe is made of,

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and it's all down to a particular property of the chemical elements.

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You see, when you heat the elements, when you burn them,

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then they give off light

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and each element gives off its own unique set of colours.

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So this is strontium

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and it burns with a beautiful red colour.

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Sodium is yellow.

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Potassium is lilac.

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And copper is blue.

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Each element has its own characteristic colour.

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It's this property that tells us what the stars are made of.

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But it's a little more complicated

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than simply looking at the colour of the light that each star emits.

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You can see why by looking at the light

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from our nearest star - the sun.

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This is a spectrum of the light taken from our sun,

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and at first glance, it looks very familiar.

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It looks like a stretched-out rainbow.

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But if you look a bit more closely,

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then you see that this spectrum is covered in black lines.

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These are called absorption lines.

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Each element within our sun not only emits light of a certain colour,

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it also absorbs light of the same colour.

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By looking for these black lines in the sun's light,

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we can simply read off a list of its constituent elements,

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like a barcode.

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For example, these two black lines

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in the yellow bit of the spectrum are sodium.

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You can see iron. Right down here, you can see hydrogen.

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So, by looking at these lines in precise detail,

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you can work out exactly what elements are present in the sun,

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and it turns out that that's about 70% hydrogen,

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28% helium, and 2% the rest.

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And you can do this not only for the sun but for any of the stars

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you can see in the sky, and you can measure exactly what they're made of.

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So, that star there is Polaris, the pole star, and you can see that

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because all the other stars in the night sky appear to rotate around it.

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Now, it's 430 light years away.

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But we know, just by looking at the light,

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that it's got about the same heavy element abundance as our sun,

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but it's got markedly less carbon and a lot more nitrogen.

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And the same applies for other stars.

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Sirius, the dog star, contains three times as much iron as the sun.

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And Proxima Centauri is rich in magnesium.

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But although the quantities of the elements may vary,

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wherever we look across space,

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we only ever find the same 92 elements that we find on Earth.

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We are made of the same stuff as the stars and the galaxies.

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Everything in the universe, from the most distant star or galaxy

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to our small planet,

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is made from just 92 chemical elements,

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and here on planet Earth,

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there's one element that defines it more than any other.

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Life is completely dependent on carbon.

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I mean, I'm made of about a billion, billion, billion carbon atoms,

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as is every human being out there, every living thing on the planet.

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Imagine how many carbon atoms that is.

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So where does all that carbon come from?

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Well, it comes from the only place in the universe

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where elements are made - stars.

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But in order for us to live, a star must die.

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Stars in the prime of their lives, like our sun,

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burn the element hydrogen, converting it into helium.

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But forming the other elements requires much higher temperatures,

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temperatures that can only be reached

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at the end of a star's life.

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Imagine this old prison in Rio is a dying star.

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Out there is the bright surface, shining off into space.

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As I descend deeper and deeper into the prison,

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the conditions would become hotter and hotter and denser and denser,

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until down there in the heart of the star is the core.

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Deep in its core,

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the star is fighting a futile battle against its own gravity.

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As it desperately tries

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to stop itself collapsing under its own weight,

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new elements are made in a sequence of separate stages.

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Stage one - while the star burns hydrogen to helium in the core,

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vast amounts of energy are released and that energy escapes,

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literally creating an outward pressure

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which balances the force of gravity

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and, well, it holds the star up and keeps it stable.

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But eventually, the hydrogen in the core will run out.

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Now, at that point,

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the core will start to collapse very rapidly,

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leaving a shell...

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of hydrogen and helium behind.

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Beneath this shell, as the core collapses,

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the temperature rises again,

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until at a hundred million degrees,

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stage two starts and helium nuclei begin to fuse together.

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A helium fusion does two things.

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Firstly, more energy is released and so, the collapse is halted.

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But secondly, two more elements are produced in that process -

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carbon...

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..oxygen. Two elements vital for life.

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So, this is where all the carbon in the universe comes from.

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You know, every atom of carbon in my hand,

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every atom of carbon in every living thing on the planet

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was produced in the heart of a dying star.

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But in only about a million years,

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the supply of helium in the core is used up

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and for stars as massive as the sun, that's where fusion stops,

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because there isn't enough gravitational energy

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to compress the core any further and restart fusion.

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But for massive stars, the fusion process can continue.

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Launching stage three, in which carbon fuses into magnesium,

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neon, sodium and aluminium.

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And so it goes on. Core collapse,

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followed by the next stage of fusion

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to create more elements, each stage hotter and shorter than the last.

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And eventually, in a final stage that lasts only a couple of days,

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the heart of the star is transformed into almost pure iron,

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whose chemical symbol is Fe,

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and this is where the fusion process stops.

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In its millions of years of life,

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the star has made all the common elements,

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the stuff that makes up 99% of the Earth.

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The core is now a solid ball of those elements,

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stacked on top of each other in layers.

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The star has only seconds left to live.

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When a star runs out of fuel,

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then it can no longer release energy through fusion reactions,

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and then, there's only one thing that can happen.

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LOUD EXPLOSIONS

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In about the same amount of time it takes this prison block to crumble,

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the entire star falls in on itself.

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Yet even the implosion of the star only forges the first 26 elements.

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For the remaining 66 elements,

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we have to look to some of the rarest conditions in the universe,

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the explosive death throes of the very largest stars,

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stars at least nine times the mass of our sun.

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It's called a supernova,

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the biggest explosion in the universe.

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Only these events can generate the enormous temperatures,

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hundreds of millions of degrees, necessary to fuse large amounts

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of the heaviest elements, elements like platinum, silver and gold.

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So, the most precious elements are created

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in the death throes of the most massive stars.

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For centuries, people thought that light travelled instantly from one place to another,

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but then, 350 years ago,

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one man's study of the planets and moons of the solar system

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revealed that it did in fact take time for light to travel.

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Ever since Galileo discovered that Jupiter had moons,

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astronomers realised that you could use Jupiter

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and its moons as a very precise clock in the sky.

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So here's the solar system, there's the sun, there's the Earth,

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here's Jupiter, and here is Jupiter's innermost moon, Io.

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Now it was known that Io takes precisely 42 and a half hours

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to orbit around Jupiter, so if from the Earth, you see Io emerge

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from behind Jupiter at say, midnight on a Tuesday, then you know

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that it should re-emerge again at half-past six

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on Thursday afternoon. Beautiful.

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Now, one of the men charged with making precise tables

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of exactly when Io should be seen to emerge from behind Jupiter

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was the Danish astronomer Ole Romer, but he noticed something surprising.

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You see, depending on the time of year,

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Io emerged later than expected or earlier than expected.

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Now, Romer's genius was to realise that had nothing to do at all

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with the orbit of Io around Jupiter.

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It was to do with the orbit of the Earth around the sun.

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You see, what Romer noticed was that

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when the Earth was in a position in its orbit so that it was close to Jupiter,

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then Io emerged earlier than it was expected to.

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Then, as the year passed and Earth moved around the sun

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and got further away from Jupiter, Romer noticed that Io

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then emerged later than it was expected to.

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Romer realised that it takes time for light to travel from Jupiter

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to the Earth, so when the Earth is far away from Jupiter,

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it takes longer for the light to travel and therefore you see

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Io emerge from behind Jupiter later than you would expect.

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Then, when the distance is small,

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it takes less time for the light to travel

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and you see Io emerge earlier than you might expect.

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So Romer had discovered that light doesn't travel instantaneously.

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It moves through space with a finite speed.

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This remarkable insight led to a measurement of the speed of light.

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We now know that light travels

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at precisely 299,792,458 metres per second.

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That means that in the time it takes for me to click my fingers,

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light has travelled around the Earth seven times...

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or that it travels ten million, million kilometres in one year.

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And that's the yardstick we use to measure the universe,

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because ten million, million kilometres is approximately one light year.

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It's easy to think that the universe has always existed,

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but our best scientific theory states that it emerged in one moment,

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from an event known as the Big Bang.

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And one of the most significant pieces of evidence for this theory

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comes from our understanding of light and colour.

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To reveal how colour can unlock the secrets of our universe's creation,

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I've come to one of the most spectacular natural wonders on Earth.

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This is Victoria Falls in Zambia.

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But I'm not here to marvel at the scale of this wonder.

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I've come to see a much more delicate feature that appears above the water.

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These magnificent rainbows are a permanent feature

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in the skies above Victoria Falls.

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Now, rainbows are a beautiful phenomena,

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but I think they're even more beautiful when you understand how they're made

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because they are a visual representation

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of the fact that light is made up of, well, all the colours of the rainbow.

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Just like light shining through a prism,

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rays of light from the sun are refracted as they enter the water droplets.

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The light beams then reflect off the back of the droplets

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and are bent for a second time as they leave.

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This bending and reflecting splits the light,

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and the colours hidden inside the white sunlight are revealed.

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What we see as different colours are actually different wavelengths of light,

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so blue light has a relatively short wavelength,

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and then you go through green and yellow all the way to the red end of the spectrum,

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which has a very large wavelength.

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Starlight is made up of countless different wavelengths,

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and when we look at the most distant stars

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and galaxies in the universe, their light appears redder,

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and it's this colouring that helps reveal that our universe had a beginning.

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When light is emitted by a distant star or galaxy,

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its wavelength doesn't have to stay fixed -

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it can be squashed or stretched and when light's stretched,

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its wavelength increases and it moves to the red end of the spectrum.

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So the interpretation of the fact that the most distant galaxies appear red,

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is that the space in-between them and us

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has stretched during the time it's taken the light to journey over that vast distance.

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That means that our entire universe is expanding.

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Now just think about what an expanding universe implies,

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because if the galaxies are all rushing away from each other,

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that means that if you re-wind time,

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then they must have been closer together in the past, and actually if you just keep re-winding,

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then you find that at some point in the past,

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all the galaxies we can see in the sky were quite literally on top of each other.

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The universe was squashed down to a point.

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That implies that the universe may have had a beginning

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and that is the Big Bang Theory.

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Life on Earth takes seemingly endless forms.

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Yet all creatures, however different,

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have evolved over billions of years from an ancient common ancestor.

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This connection is explained by the theory of evolution by natural selection,

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and some of the best evidence for evolution

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are in the preserved remains of ancient creatures found in fossil beds.

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This is one of them,

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the Burgess Shale in the Rocky Mountains of Canada.

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Well, this is one of the most important fossil sites in the world,

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but actually it's one of the most important scientific sites of any kind,

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and it's not just because of the number and diversity of animals

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you find fossilised in these rocks, it's because of their age.

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Over half a billion years old,

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they are some of the earliest fossils of complex life.

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I mean, it's as if at one instant in this time we call the Cambrian Era,

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complex multi-cellular life suddenly emerged almost intact on the planet.

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It's called the Evolutionary Big Bang.

0:28:470:28:50

So the Burgess Shale tells us that complex life seemed to emerge suddenly

0:28:530:28:58

and a new theory may also suggest what triggered this moment.

0:28:580:29:02

Well, this is one of the beautiful animals

0:29:080:29:10

you find up here in the fossil beds.

0:29:100:29:12

It's called a trilobite. It's a very complex organism.

0:29:120:29:16

It's got an external skeleton, it's got jointed limbs

0:29:160:29:19

but perhaps most remarkably these, because these are compound eyes.

0:29:190:29:26

They were very sophisticated

0:29:260:29:28

and this was one of the first predators to be able to detect shapes

0:29:280:29:31

and see movement and it could successfully chase its prey.

0:29:310:29:35

These creatures were among the first to harness the light that filled the universe.

0:29:370:29:43

Before they emerged, the rise and fall of the sun and the stars in the night sky simply went unnoticed.

0:29:430:29:51

Now there is a speculative theory that the emergence of the eye

0:29:510:29:55

actually triggered the Cambrian explosion,

0:29:550:29:58

this Evolutionary Big Bang, because once one species got eyes,

0:29:580:30:04

then other species had also to develop eyes to either chase them as predators

0:30:040:30:10

or evade them as prey, and that led to an evolutionary arms race.

0:30:100:30:16

More and more complex life forms developed.

0:30:160:30:20

So the evolution of the eye may have played a fundamental role

0:30:240:30:27

in the emergence of complex life on Earth...

0:30:270:30:30

..and could have led to the evolution of our species.

0:30:320:30:37

The light we can see is just a tiny fraction of the light in our universe.

0:30:480:30:53

Beyond the visible spectrum,

0:30:540:30:57

our world is also bathed in the light we can't see.

0:30:570:31:01

X-rays, infrared, ultraviolet are all types of light,

0:31:010:31:07

but although we can't see this light, we can still sense it.

0:31:070:31:11

This sand has been under the full glare of the sun all day

0:31:140:31:17

and I can feel the heat radiating off it.

0:31:170:31:21

Well, heat is nothing more than a form of light,

0:31:210:31:25

although we don't normally call it light.

0:31:250:31:28

It's actually infrared light and the only difference between infrared

0:31:280:31:33

and visible light is the wavelength.

0:31:330:31:35

Infrared has a longer wavelength than visible light.

0:31:350:31:39

Infrared isn't the end of the story.

0:31:410:31:43

There are even longer wavelengths of light,

0:31:430:31:48

and these can reveal something extraordinary about our universe.

0:31:480:31:54

To detect them, you don't need a billion-pound satellite

0:31:540:31:57

or a telescope built into the side of a mountain.

0:31:570:32:01

You just need one of these, a radio,

0:32:010:32:05

because when we tune a radio,

0:32:050:32:09

we're tuning into a form of light - radio waves.

0:32:090:32:14

MUSIC PLAYS ON RADIO

0:32:140:32:18

But detecting them and understanding them

0:32:200:32:23

provides the key to understanding the origin of the universe.

0:32:230:32:28

When you de-tune the radio a bit, you can just hear static

0:32:310:32:33

but about 1% of that static is music to the ears of a physicist

0:32:330:32:40

because that is stretched light from the Big Bang.

0:32:400:32:45

# Carry him home safely to me... #

0:32:450:32:50

The reason we can't see this ancient light

0:32:520:32:54

is because as the universe expanded, the light waves were stretched

0:32:540:32:59

and transformed into radio waves and microwaves.

0:32:590:33:04

This first light is called the Cosmic Microwave Background or CMB.

0:33:040:33:10

The CMB fills every part of the universe.

0:33:120:33:16

If my eyes could only see it, then the sky would be ablaze

0:33:170:33:22

with this primordial light both day and night.

0:33:220:33:26

Although we are not sensitive to this light, specialised cameras are,

0:33:330:33:38

and when they are pointed towards the heavens, something beautiful emerges.

0:33:380:33:43

These scattered colours are the fading embers,

0:33:540:33:57

the last remnants of light from the beginning of the universe.

0:33:570:34:03

Looking out into space, you might think that the cosmos

0:34:130:34:16

is a constant unchanging place, that the stars will always be there.

0:34:160:34:23

But in fact, the stars are only a temporary feature in the sky,

0:34:230:34:28

and though they may burn brightly for many millions or billions of years,

0:34:280:34:32

they can only live for as long as they have a supply of hydrogen to burn.

0:34:320:34:37

And when a star like our sun runs out of hydrogen, it begins to die.

0:34:400:34:44

But it doesn't go quietly.

0:34:470:34:48

At the end of its life, the sun won't simply fade away to nothing.

0:34:520:34:56

As it begins to run out of fuel, its core will collapse

0:34:590:35:03

and the extra heat this generates will cause its outer layers to expand.

0:35:030:35:08

In around a billion years' time,

0:35:150:35:17

this will have a catastrophic effect on our fragile world.

0:35:170:35:21

Gradually, the Earth will become hotter and hotter,

0:35:280:35:31

so there will be one last perfect day on Earth, but eventually,

0:35:310:35:37

the existence of all life on this planet will become impossible.

0:35:370:35:42

Long after life has disappeared, the sun will have grown so much,

0:35:460:35:49

it will fill the entire horizon.

0:35:490:35:53

It will have become a red giant, the last phase of its life.

0:35:590:36:05

Our planet might not survive to this point, but if it does,

0:36:140:36:17

little more than a scorched and barren rock will remain

0:36:170:36:21

to witness the final death throes of our star.

0:36:210:36:26

In six billion years, our sun will explode,

0:36:370:36:41

throwing vast amounts of gas and dust out into space to form a gigantic nebula.

0:36:410:36:46

At its heart will be a faintly glowing ember,

0:36:540:36:58

all that remains of our once-magnificent sun.

0:36:580:37:02

It will be smaller than the size of the Earth,

0:37:020:37:05

less than a millionth of its current volume, and a fraction of its brightness.

0:37:050:37:10

Our sun will have become a white dwarf.

0:37:100:37:13

With no fuel left to burn, a white dwarf's faint glow

0:37:230:37:27

comes from the last residual heat from its extinguished furnace.

0:37:270:37:31

The sun is now dead,

0:37:340:37:37

its remains slowly cooling in the freezing temperatures of deep space.

0:37:370:37:42

Looking at it from where the Earth is now,

0:37:460:37:48

it would only generate the same amount of light as the full moon on a clear night.

0:37:480:37:55

The fate of the sun is the same as for all stars.

0:37:590:38:03

One day they must all eventually die,

0:38:030:38:07

and the cosmos will be plunged into eternal night,

0:38:070:38:11

because this structured universe that we inhabit and all its wonders,

0:38:110:38:15

the stars and the planets and the galaxies, cannot last forever.

0:38:150:38:21

The cosmos WILL eventually fade and die.

0:38:210:38:26

Black holes are the most destructive places in the universe,

0:38:520:38:55

able to devour entire stars.

0:38:550:38:59

Yet we've never seen one.

0:39:000:39:03

It's because of their effects on the stars and galaxies,

0:39:030:39:06

and the dust and gas around them, that we know that they exist.

0:39:060:39:10

But the reason why black holes are invisible

0:39:150:39:18

can be demonstrated here on Earth.

0:39:180:39:20

Near a black hole, space and time do some very strange things

0:39:430:39:48

because black holes are probably the most violent places

0:39:480:39:51

we know of in the universe.

0:39:510:39:54

This river provides a beautiful analogy for what happens to space and time

0:39:540:39:59

as you get closer and closer to the black hole.

0:39:590:40:02

Now, upstream, the water is flowing pretty slowly.

0:40:050:40:09

Let's imagine that it's flowing at three kilometres per hour

0:40:090:40:12

and I can swim at four, so I can swim faster than the flow and can easily escape.

0:40:120:40:19

But as you go further and further downstream towards the waterfall in the distance,

0:40:270:40:33

the river flows faster and faster.

0:40:330:40:36

Imagine I decide to jump into the river just there on the edge of the falls.

0:40:550:41:00

The water is flowing far faster than I could swim, so no matter what I did,

0:41:000:41:05

no matter how hard I tried, I would not be able to swim back upstream.

0:41:050:41:10

I would be carried inexorably towards the edge

0:41:100:41:13

and I would vanish over the falls.

0:41:130:41:16

Well, it's the same close to a black hole

0:41:220:41:26

because space flows faster and faster and faster

0:41:260:41:29

towards the black hole -

0:41:290:41:31

literally this stuff,

0:41:310:41:34

my space that I'm in, flowing over the edge into the black hole.

0:41:340:41:37

And at the very special point called the event horizon,

0:41:370:41:42

space is flowing at the speed of light into the black hole.

0:41:420:41:47

Light itself, travelling at 300,000 kilometres per second,

0:41:520:41:56

is not going fast enough to escape the flow,

0:41:560:41:59

and light itself will plunge into the black hole.

0:41:590:42:03

So the fact that black holes can swallow light

0:42:100:42:13

means that they will for ever remain invisible to our eyes.

0:42:130:42:18

Gravity is the force that keeps our feet firmly rooted to our planet.

0:42:320:42:37

Yet although it may appear constant and unchanging,

0:42:370:42:40

this force varies on all the planets in the solar system

0:42:400:42:44

and on the exo-planets we've discovered orbiting other suns.

0:42:440:42:50

To experience the gravity on these worlds, I need to go for a spin.

0:42:500:42:54

This is a centrifuge.

0:43:030:43:05

It was built in the 1950s to test whether fighter pilots had the right stuff,

0:43:050:43:10

but it's going to allow me to feel what it would be like

0:43:100:43:13

to stand on the surface of any of the planets in the solar system

0:43:130:43:17

that are more massive than the Earth,

0:43:170:43:19

and in fact, also what it would be like

0:43:190:43:22

to stand on some of the planets that we've found around distant stars.

0:43:220:43:26

Three...two...one.

0:43:280:43:30

As the centrifuge rotates, it feels exactly as if gravity is increased.

0:43:330:43:39

The faster it spins, the greater the effect

0:43:390:43:42

and we measure this in multiples of the strength

0:43:420:43:47

of Earth's gravity, known as 1G.

0:43:470:43:50

The first planet I'm travelling to is Neptune.

0:43:520:43:56

Its gravity is just fractionally stronger than here on Earth.

0:43:560:44:00

So this is the gravitational field on Neptune

0:44:000:44:03

and you feel, "You know what? I could probably get used to this.

0:44:030:44:06

"I could probably live on the surface of Neptune."

0:44:060:44:09

-Can you lift your hands a little?

-There we go.

-Yeah, and down.

0:44:090:44:12

And it is actually quite an effort. It is noticeably heavier.

0:44:120:44:17

It's like having a reasonably heavy weight in your hand.

0:44:170:44:21

To go to 2.5G?

0:44:210:44:23

-Yes, so now we'll move, move from Neptune to Jupiter.

-Let's go there.

0:44:230:44:27

Jupiter is over 1,300 times more massive than the Earth,

0:44:300:44:33

but because it's mostly gas, it's not very dense

0:44:330:44:36

so its gravity is just over twice as strong at its surface.

0:44:360:44:39

Well, now actually it is quite difficult to lift my hand.

0:44:390:44:44

And that's 2.5G. I wouldn't want to sit here for half an hour.

0:44:450:44:49

Can you lift both of your hands above your head?

0:44:490:44:53

-See what happens there.

-Let's see, so actually...

0:44:530:44:56

just about, but actually it's an immense amount of hard work.

0:44:560:45:02

-So it would be hard work living on Jupiter.

-Let's go to 4G.

0:45:020:45:06

Actually, this is heading to a planet around...

0:45:130:45:16

A planet called Ogle 2TRL9B

0:45:160:45:19

which is around a star in the constellation of Carina.

0:45:190:45:23

It's one of the exo-planets we've discovered.

0:45:230:45:26

Oh, and there we go.

0:45:270:45:29

Now, that is actually...beginning to feel quite unpleasant.

0:45:330:45:39

-Can you describe what you're feeling?

-Very heavy face.

0:45:390:45:43

My head is extremely heavy.

0:45:430:45:45

How about your lungs, inhaling, exhaling, breathing?

0:45:450:45:47

-It's much harder work. I can't lift my hand off my leg.

-OK.

0:45:470:45:53

-And that's at 4G?

-Yeah.

0:45:530:45:55

Well, my head and my face feel very, very heavy.

0:45:550:46:00

It's quite an unpleasant feeling.

0:46:000:46:02

We'll go to 5 and let me know if you have any visual disturbances.

0:46:020:46:08

I am now en-route to a newly discovered exo-planet, Wasp 8b.

0:46:090:46:13

4.4.

0:46:150:46:17

This world sits in the small and faint constellation of Sculptor.

0:46:170:46:22

Quite hard to speak.

0:46:260:46:27

It has a gravitational force nearly five times that of the Earth.

0:46:310:46:35

Right, we'll go to 5G.

0:46:350:46:40

-Very foggy.

-OK.

0:46:400:46:41

-Very foggy.

-Very foggy?

0:46:450:46:47

-Still foggy?

-Yeah.

-Right.

0:46:550:46:57

-Take it down.

-OK, we'll take you down.

0:46:580:47:00

Very interesting.

0:47:150:47:17

-It was, wasn't it?

-My face felt a bit saggy, though.

0:47:180:47:22

Well, you looked a little different.

0:47:220:47:26

That was, um, quite unpleasant that time, actually.

0:47:400:47:45

So you realise that we're, obviously, very finely tuned

0:47:450:47:50

to live on a planet that has a gravitational,

0:47:500:47:54

an acceleration due to gravity of 1G.

0:47:540:47:57

When you go to 2G, it's difficult.

0:47:570:48:00

When you go to 3G and 4G it becomes unpleasant, and 5G, anyway,

0:48:000:48:05

for me, was on the border of being so unpleasant that you pass out.

0:48:050:48:11

So although gravity feels weak here on Earth,

0:48:180:48:21

it certainly isn't weak everywhere across the universe.

0:48:210:48:25

And that's because gravity is an additive force.

0:48:260:48:29

It scales with mass,

0:48:290:48:32

so the more massive the planet or star,

0:48:320:48:37

the stronger its gravity.

0:48:370:48:41

Every moment of our lives,

0:48:560:48:57

we experience a force that we can't see or touch.

0:48:570:49:01

Yet this force is able to keep us firmly rooted to the ground.

0:49:040:49:08

It is, of course, gravity.

0:49:080:49:12

But despite its intangible nature, we always know it's with us.

0:49:160:49:20

Now if I was to ask you, "How do you know that there's gravity around here?"

0:49:220:49:26

Then you might say, "Well, it's obvious.

0:49:260:49:28

"You know, I can just do an experiment, I can drop something."

0:49:280:49:31

Well, yes, but actually gravity is a little bit more subtle than that

0:49:330:49:39

but to really experience it, to understand it,

0:49:390:49:43

you have to do something pretty extreme.

0:49:430:49:46

And this plane has been modified to help me do it.

0:49:530:49:57

Thanks to its flight plan, it's known as the Vomit Comet.

0:49:570:50:02

Once we've climbed to 15,000 metres,

0:50:150:50:18

this plane does something no ordinary flight would do -

0:50:180:50:23

its engines are throttled back and the jet falls to Earth.

0:50:230:50:28

And then, something quite amazing happens.

0:50:290:50:33

THEY SQUEAL AND CHEER

0:50:350:50:37

Push to me, push to me!

0:50:380:50:40

I'm now plummeting towards the ground just like someone's cut the cable in a lift,

0:50:430:50:49

and you see, we're all just floating.

0:50:490:50:52

By simply falling at the same rate as the plane,

0:51:090:51:11

for a few fleeting moments, we are all free of gravity's grip.

0:51:110:51:18

But this isn't just a joy ride.

0:51:300:51:32

Now, look, there's something very profound here

0:51:390:51:42

because although I'm falling towards the ground,

0:51:420:51:47

as you see, gravity has completely gone away.

0:51:470:51:50

Gravity is not here any more.

0:51:500:51:54

'Because the aircraft is accelerating towards the ground at 1G,

0:51:580:52:02

'the effects of the Earth's gravity are completely cancelled out.'

0:52:020:52:08

So it is possible by the simple act of falling to get a very different experience of gravity.

0:52:140:52:22

Nothing can travel faster than the speed at which light travels,

0:52:320:52:39

but although light travels fast, it's not infinitely fast

0:52:390:52:42

so the further away an object is, the further back in time we see it.

0:52:420:52:46

The sun is 150 million kilometres away.

0:52:520:52:55

Now that's very close by cosmic standards,

0:52:580:53:01

but light travels at only 300,000 kilometres per second,

0:53:010:53:06

so that means that we're seeing the sun

0:53:060:53:09

as it was in the past, actually eight minutes in the past.

0:53:090:53:15

But when we look beyond our sun, to far more distant stars,

0:53:210:53:25

we reach further back in time.

0:53:250:53:27

As the sun dips below the horizon and night falls...

0:53:350:53:39

..the universe just fades into view.

0:53:410:53:45

And then, as it gets darker and darker, the Milky Way appears -

0:53:490:53:53

a vast swathe of billions and billions of suns as you look out

0:53:530:53:59

towards the centre of our Milky Way galaxy.

0:53:590:54:02

But I think for me, the most magical thing

0:54:060:54:09

you can see in the sky with the naked eye, is just below

0:54:090:54:13

the constellation of Cassiopeia, the W of stars in the sky.

0:54:130:54:18

There, look at that.

0:54:300:54:34

Actually, I've got to say, that's amazing.

0:54:350:54:38

You see, that misty patch of light is not a cloud in the sky,

0:54:400:54:44

it's not even gas and dust in our galaxy.

0:54:440:54:47

That...is another galaxy.

0:54:470:54:50

It's the Andromeda galaxy, which is roughly the same size as our own -

0:54:500:54:54

an island of hundreds of billions of stars,

0:54:540:54:57

25 million, million, million kilometres in that direction.

0:54:570:55:03

The light that I've just captured in my camera began its journey two and a half million years ago.

0:55:080:55:14

At that time on Earth, there were no humans.

0:55:140:55:17

Homo habilis, our distant ancestors,

0:55:170:55:20

were roaming the plains of Africa and as those light rays travelled

0:55:200:55:24

through the vastness of space, our species evolved

0:55:240:55:29

and thousands and thousands and thousands of generations of humans lived and died.

0:55:290:55:35

And then, two and a half million years after their journey began,

0:55:350:55:39

these messengers from the depths of space, and from way back in our past,

0:55:390:55:47

arrived here on Earth, and I just captured them and took that picture.

0:55:470:55:52

But by peering further than the naked eye will allow,

0:55:570:55:59

we can journey to a time way before human history.

0:55:590:56:04

In the last 20 years, powerful space telescopes have carried us

0:56:040:56:08

ever deeper into space and we have become virtual time travellers.

0:56:080:56:13

This is NGC 520 and it's the product of a cosmic collision,

0:56:170:56:22

but this galaxy is a hundred million light years away.

0:56:220:56:27

That means that the light began its journey from this galaxy

0:56:270:56:30

to my eye when the dinosaurs roamed the Earth.

0:56:300:56:33

But these spectacular galaxies

0:56:440:56:46

are not the end of our journey into the past.

0:56:460:56:49

In 2004, we peered further back in time than ever before

0:56:490:56:54

and captured the light from the most distant galaxies in the universe.

0:56:540:57:00

The image is called the Hubble Ultra Deep Field.

0:57:020:57:06

It's a picture taken by the Hubble Space Telescope

0:57:060:57:09

over a period of 11 days, and it focused its camera

0:57:090:57:13

on the tiniest piece of sky, just below the constellation of Orion.

0:57:130:57:19

Now, it's a piece of sky that you would cover

0:57:190:57:21

if you took your thumb, held it in front of your face

0:57:210:57:26

and then moved it 20 times further away.

0:57:260:57:29

But the Hubble captured the faintest lights from the most distant regions

0:57:320:57:37

of the universe and it took this photograph.

0:57:370:57:40

Now, almost every point of light in that image is not a star,

0:57:440:57:49

but a galaxy of over a hundred billion stars.

0:57:490:57:55

The most distant galaxies in that image are over 13 billion light years away.

0:57:550:58:01

That means that the faint light from those galaxies

0:58:010:58:05

began its journey to Earth 13 billion years ago.

0:58:050:58:10

That's over three times the age of the Earth.

0:58:100:58:15

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0:58:330:58:36

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0:58:360:58:39

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