0:00:23 > 0:00:26We all have an intuitive understanding of time.
0:00:26 > 0:00:29It seems obvious to us that things change,
0:00:29 > 0:00:32and the future will be different to the past.
0:00:36 > 0:00:39But why are we compelled to travel into the future?
0:00:39 > 0:00:42The answer to that question can be seen
0:00:42 > 0:00:46in how the world around us is always changing.
0:00:49 > 0:00:50This is Kolmanskop,
0:00:50 > 0:00:55an abandoned diamond mining town in southern Namibia.
0:01:00 > 0:01:02For half a century,
0:01:02 > 0:01:04it's fallen into disrepair
0:01:04 > 0:01:06as it's slowly reclaimed by the sands.
0:01:15 > 0:01:17The process at play here at Kolmanskop
0:01:17 > 0:01:21is happening everywhere in the universe - decay.
0:01:21 > 0:01:28Or in the language of physics, an increase in entropy.
0:01:30 > 0:01:32Entropy explains why,
0:01:32 > 0:01:35left to the mercy of the elements,
0:01:35 > 0:01:38mortar crumbles, glass shatters
0:01:38 > 0:01:39and buildings collapse.
0:01:42 > 0:01:44And a good way to understand how
0:01:44 > 0:01:48is to think of objects not as single things, but as being made up
0:01:48 > 0:01:52of many constituent parts, like the individual grains
0:01:52 > 0:01:55that make up this pile of sand.
0:01:58 > 0:02:00Now, entropy is a measure of how many ways
0:02:00 > 0:02:05I can re-arrange those grains and still keep the sand piled the same,
0:02:05 > 0:02:09and there are trillions and trillions and trillions
0:02:09 > 0:02:10of ways of doing that.
0:02:10 > 0:02:13I mean, pretty much anything I do to this sand pile,
0:02:13 > 0:02:16if I mess the sand around and move it around,
0:02:16 > 0:02:19then it doesn't change the shape or the structure at all.
0:02:19 > 0:02:24So, in the language of entropy, this sand pile has high entropy,
0:02:24 > 0:02:26because there are many, many ways
0:02:26 > 0:02:30that I can re-arrange its constituents and not change it.
0:02:30 > 0:02:35But now, let me create some order in the universe.
0:02:42 > 0:02:46Now, there are approximately as many sand grains in this sandcastle
0:02:46 > 0:02:49as there are in the sand pile.
0:02:49 > 0:02:53But now, virtually anything I do to it will mess it up,
0:02:53 > 0:02:56will remove the beautiful order from this structure
0:02:56 > 0:03:00and because of that, the sandcastle has a low entropy.
0:03:00 > 0:03:03It's a much more ordered state.
0:03:03 > 0:03:07So, many ways of re-arranging the sand grains
0:03:07 > 0:03:10without changing the structure - high entropy.
0:03:10 > 0:03:13Very few ways of re-arranging the sand grains
0:03:13 > 0:03:18without changing the structure, without disordering it -
0:03:18 > 0:03:19low entropy.
0:03:27 > 0:03:31Now, imagine I was to leave this castle in the desert all day,
0:03:31 > 0:03:34the desert winds are going to blow the sand around
0:03:34 > 0:03:37and this castle is going to disintegrate.
0:03:37 > 0:03:39It's going to become less ordered.
0:03:39 > 0:03:43But there's nothing fundamental
0:03:43 > 0:03:45in the laws of physics that says
0:03:45 > 0:03:49that the wind couldn't pick up some sand from over here,
0:03:49 > 0:03:55deposit it here and deposit it in precisely the shape of a sandcastle.
0:03:55 > 0:03:56You know, in principle,
0:03:56 > 0:04:01the wind could spontaneously build a sandcastle out of a pile of sand.
0:04:10 > 0:04:14There's no reason why that couldn't happen.
0:04:14 > 0:04:17It's just extremely, extremely unlikely,
0:04:17 > 0:04:21because there are very few ways of organising this sand
0:04:21 > 0:04:23so that it looks like a castle.
0:04:27 > 0:04:30It's overwhelmingly more likely
0:04:30 > 0:04:32that when the wind blows the sand around,
0:04:32 > 0:04:36it will take the low entropy structure, the castle,
0:04:36 > 0:04:40and turn it into a high entropy structure, the sand pile.
0:04:48 > 0:04:52So, entropy always increases. Why is that?
0:04:52 > 0:04:57Because it's overwhelmingly more likely that it will.
0:05:06 > 0:05:09So, everything tends from order to disorder.
0:05:09 > 0:05:13That means that there is a difference between the future
0:05:13 > 0:05:19and the past, and that's one reason why time travels in one direction.
0:05:33 > 0:05:36Everything that we see on Earth, from the grandest mountain
0:05:36 > 0:05:39to the most fleeting cloud,
0:05:39 > 0:05:42is made from the same set of building blocks.
0:05:42 > 0:05:45They're called the chemical elements.
0:05:49 > 0:05:53We now know that the Earth is made of 92 chemical elements
0:05:53 > 0:05:56and that's pretty amazing if you think of the complexity
0:05:56 > 0:06:00that we see around us. We also know that everything beyond Earth,
0:06:00 > 0:06:02everything we can see in the universe,
0:06:02 > 0:06:05is made of those same 92 elements,
0:06:05 > 0:06:09and notice I didn't say "we think" that's what they're made of.
0:06:09 > 0:06:14I said "we know" that's what they're made of, because we can prove it.
0:06:20 > 0:06:24The chemistry set we have on Earth extends far beyond the planet.
0:06:28 > 0:06:30We have set foot on the moon
0:06:30 > 0:06:34and know that it's rich in helium, silver and water.
0:06:36 > 0:06:40And discovered that Mars is rich in iron.
0:06:42 > 0:06:47And we know that Venus's thick atmosphere is full of sulphur.
0:06:49 > 0:06:51But what of the rest of the universe?
0:06:53 > 0:06:57It seems impossible that we could discover what the stars are made of,
0:06:57 > 0:07:00because they're so far away.
0:07:00 > 0:07:07Even the nearest star, Proxima Centauri, is 10,000 times
0:07:07 > 0:07:11more distant than Neptune, 4.2 light years from Earth.
0:07:13 > 0:07:18Yet despite these vast distances, these alien worlds are constantly
0:07:18 > 0:07:23sending us signals, telling us exactly what they're made of.
0:07:23 > 0:07:28Our only contact with the distant stars is their light
0:07:28 > 0:07:30that has journeyed across the universe to reach us,
0:07:30 > 0:07:34and encoded in that light is the key to understanding
0:07:34 > 0:07:36what the universe is made of,
0:07:36 > 0:07:40and it's all down to a particular property of the chemical elements.
0:07:40 > 0:07:43You see, when you heat the elements, when you burn them,
0:07:43 > 0:07:44then they give off light
0:07:44 > 0:07:49and each element gives off its own unique set of colours.
0:07:49 > 0:07:52So this is strontium
0:07:52 > 0:07:57and it burns with a beautiful red colour.
0:08:02 > 0:08:04Sodium is yellow.
0:08:06 > 0:08:08Potassium is lilac.
0:08:10 > 0:08:12And copper is blue.
0:08:16 > 0:08:19Each element has its own characteristic colour.
0:08:24 > 0:08:28It's this property that tells us what the stars are made of.
0:08:29 > 0:08:32But it's a little more complicated
0:08:32 > 0:08:37than simply looking at the colour of the light that each star emits.
0:08:37 > 0:08:41You can see why by looking at the light
0:08:41 > 0:08:42from our nearest star - the sun.
0:08:47 > 0:08:51This is a spectrum of the light taken from our sun,
0:08:51 > 0:08:53and at first glance, it looks very familiar.
0:08:53 > 0:08:56It looks like a stretched-out rainbow.
0:08:56 > 0:08:57But if you look a bit more closely,
0:08:57 > 0:09:01then you see that this spectrum is covered in black lines.
0:09:01 > 0:09:05These are called absorption lines.
0:09:05 > 0:09:10Each element within our sun not only emits light of a certain colour,
0:09:10 > 0:09:13it also absorbs light of the same colour.
0:09:13 > 0:09:17By looking for these black lines in the sun's light,
0:09:17 > 0:09:21we can simply read off a list of its constituent elements,
0:09:21 > 0:09:23like a barcode.
0:09:23 > 0:09:25For example, these two black lines
0:09:25 > 0:09:28in the yellow bit of the spectrum are sodium.
0:09:28 > 0:09:34You can see iron. Right down here, you can see hydrogen.
0:09:34 > 0:09:38So, by looking at these lines in precise detail,
0:09:38 > 0:09:42you can work out exactly what elements are present in the sun,
0:09:42 > 0:09:46and it turns out that that's about 70% hydrogen,
0:09:46 > 0:09:4828% helium, and 2% the rest.
0:09:48 > 0:09:53And you can do this not only for the sun but for any of the stars
0:09:53 > 0:09:59you can see in the sky, and you can measure exactly what they're made of.
0:10:03 > 0:10:08So, that star there is Polaris, the pole star, and you can see that
0:10:08 > 0:10:13because all the other stars in the night sky appear to rotate around it.
0:10:13 > 0:10:17Now, it's 430 light years away.
0:10:19 > 0:10:22But we know, just by looking at the light,
0:10:22 > 0:10:27that it's got about the same heavy element abundance as our sun,
0:10:27 > 0:10:34but it's got markedly less carbon and a lot more nitrogen.
0:10:38 > 0:10:42And the same applies for other stars.
0:10:45 > 0:10:49Sirius, the dog star, contains three times as much iron as the sun.
0:10:52 > 0:10:56And Proxima Centauri is rich in magnesium.
0:10:59 > 0:11:03But although the quantities of the elements may vary,
0:11:03 > 0:11:05wherever we look across space,
0:11:05 > 0:11:10we only ever find the same 92 elements that we find on Earth.
0:11:13 > 0:11:18We are made of the same stuff as the stars and the galaxies.
0:11:35 > 0:11:39Everything in the universe, from the most distant star or galaxy
0:11:39 > 0:11:41to our small planet,
0:11:41 > 0:11:44is made from just 92 chemical elements,
0:11:44 > 0:11:46and here on planet Earth,
0:11:46 > 0:11:50there's one element that defines it more than any other.
0:11:55 > 0:11:59Life is completely dependent on carbon.
0:11:59 > 0:12:04I mean, I'm made of about a billion, billion, billion carbon atoms,
0:12:04 > 0:12:10as is every human being out there, every living thing on the planet.
0:12:10 > 0:12:13Imagine how many carbon atoms that is.
0:12:13 > 0:12:15So where does all that carbon come from?
0:12:15 > 0:12:18Well, it comes from the only place in the universe
0:12:18 > 0:12:22where elements are made - stars.
0:12:22 > 0:12:27But in order for us to live, a star must die.
0:12:30 > 0:12:33Stars in the prime of their lives, like our sun,
0:12:33 > 0:12:37burn the element hydrogen, converting it into helium.
0:12:39 > 0:12:44But forming the other elements requires much higher temperatures,
0:12:44 > 0:12:46temperatures that can only be reached
0:12:46 > 0:12:49at the end of a star's life.
0:12:55 > 0:12:58Imagine this old prison in Rio is a dying star.
0:12:58 > 0:13:03Out there is the bright surface, shining off into space.
0:13:03 > 0:13:07As I descend deeper and deeper into the prison,
0:13:07 > 0:13:12the conditions would become hotter and hotter and denser and denser,
0:13:12 > 0:13:19until down there in the heart of the star is the core.
0:13:22 > 0:13:23Deep in its core,
0:13:23 > 0:13:28the star is fighting a futile battle against its own gravity.
0:13:29 > 0:13:31As it desperately tries
0:13:31 > 0:13:34to stop itself collapsing under its own weight,
0:13:34 > 0:13:38new elements are made in a sequence of separate stages.
0:13:43 > 0:13:50Stage one - while the star burns hydrogen to helium in the core,
0:13:50 > 0:13:54vast amounts of energy are released and that energy escapes,
0:13:54 > 0:13:57literally creating an outward pressure
0:13:57 > 0:13:59which balances the force of gravity
0:13:59 > 0:14:03and, well, it holds the star up and keeps it stable.
0:14:05 > 0:14:10But eventually, the hydrogen in the core will run out.
0:14:10 > 0:14:11Now, at that point,
0:14:11 > 0:14:15the core will start to collapse very rapidly,
0:14:15 > 0:14:18leaving a shell...
0:14:18 > 0:14:23of hydrogen and helium behind.
0:14:27 > 0:14:30Beneath this shell, as the core collapses,
0:14:30 > 0:14:34the temperature rises again,
0:14:34 > 0:14:37until at a hundred million degrees,
0:14:37 > 0:14:43stage two starts and helium nuclei begin to fuse together.
0:14:51 > 0:14:53A helium fusion does two things.
0:14:53 > 0:14:59Firstly, more energy is released and so, the collapse is halted.
0:14:59 > 0:15:06But secondly, two more elements are produced in that process -
0:15:06 > 0:15:07carbon...
0:15:10 > 0:15:15..oxygen. Two elements vital for life.
0:15:15 > 0:15:20So, this is where all the carbon in the universe comes from.
0:15:20 > 0:15:23You know, every atom of carbon in my hand,
0:15:23 > 0:15:28every atom of carbon in every living thing on the planet
0:15:28 > 0:15:32was produced in the heart of a dying star.
0:15:34 > 0:15:37But in only about a million years,
0:15:37 > 0:15:40the supply of helium in the core is used up
0:15:40 > 0:15:45and for stars as massive as the sun, that's where fusion stops,
0:15:45 > 0:15:48because there isn't enough gravitational energy
0:15:48 > 0:15:52to compress the core any further and restart fusion.
0:15:52 > 0:15:57But for massive stars, the fusion process can continue.
0:15:59 > 0:16:04Launching stage three, in which carbon fuses into magnesium,
0:16:04 > 0:16:09neon, sodium and aluminium.
0:16:09 > 0:16:12And so it goes on. Core collapse,
0:16:12 > 0:16:14followed by the next stage of fusion
0:16:14 > 0:16:21to create more elements, each stage hotter and shorter than the last.
0:16:23 > 0:16:29And eventually, in a final stage that lasts only a couple of days,
0:16:29 > 0:16:35the heart of the star is transformed into almost pure iron,
0:16:35 > 0:16:40whose chemical symbol is Fe,
0:16:40 > 0:16:43and this is where the fusion process stops.
0:16:45 > 0:16:47In its millions of years of life,
0:16:47 > 0:16:51the star has made all the common elements,
0:16:51 > 0:16:54the stuff that makes up 99% of the Earth.
0:16:57 > 0:17:00The core is now a solid ball of those elements,
0:17:00 > 0:17:03stacked on top of each other in layers.
0:17:06 > 0:17:09The star has only seconds left to live.
0:17:11 > 0:17:13When a star runs out of fuel,
0:17:13 > 0:17:17then it can no longer release energy through fusion reactions,
0:17:17 > 0:17:20and then, there's only one thing that can happen.
0:17:23 > 0:17:25LOUD EXPLOSIONS
0:17:36 > 0:17:40In about the same amount of time it takes this prison block to crumble,
0:17:40 > 0:17:43the entire star falls in on itself.
0:17:50 > 0:17:56Yet even the implosion of the star only forges the first 26 elements.
0:17:56 > 0:17:58For the remaining 66 elements,
0:17:58 > 0:18:02we have to look to some of the rarest conditions in the universe,
0:18:02 > 0:18:07the explosive death throes of the very largest stars,
0:18:07 > 0:18:10stars at least nine times the mass of our sun.
0:18:10 > 0:18:12It's called a supernova,
0:18:12 > 0:18:16the biggest explosion in the universe.
0:18:34 > 0:18:38Only these events can generate the enormous temperatures,
0:18:38 > 0:18:44hundreds of millions of degrees, necessary to fuse large amounts
0:18:44 > 0:18:50of the heaviest elements, elements like platinum, silver and gold.
0:18:55 > 0:18:58So, the most precious elements are created
0:18:58 > 0:19:01in the death throes of the most massive stars.
0:19:13 > 0:19:19For centuries, people thought that light travelled instantly from one place to another,
0:19:19 > 0:19:23but then, 350 years ago,
0:19:23 > 0:19:26one man's study of the planets and moons of the solar system
0:19:26 > 0:19:30revealed that it did in fact take time for light to travel.
0:19:33 > 0:19:36Ever since Galileo discovered that Jupiter had moons,
0:19:36 > 0:19:38astronomers realised that you could use Jupiter
0:19:38 > 0:19:42and its moons as a very precise clock in the sky.
0:19:42 > 0:19:45So here's the solar system, there's the sun, there's the Earth,
0:19:45 > 0:19:50here's Jupiter, and here is Jupiter's innermost moon, Io.
0:19:50 > 0:19:54Now it was known that Io takes precisely 42 and a half hours
0:19:54 > 0:20:00to orbit around Jupiter, so if from the Earth, you see Io emerge
0:20:00 > 0:20:05from behind Jupiter at say, midnight on a Tuesday, then you know
0:20:05 > 0:20:08that it should re-emerge again at half-past six
0:20:08 > 0:20:11on Thursday afternoon. Beautiful.
0:20:11 > 0:20:15Now, one of the men charged with making precise tables
0:20:15 > 0:20:20of exactly when Io should be seen to emerge from behind Jupiter
0:20:20 > 0:20:25was the Danish astronomer Ole Romer, but he noticed something surprising.
0:20:25 > 0:20:27You see, depending on the time of year,
0:20:27 > 0:20:33Io emerged later than expected or earlier than expected.
0:20:33 > 0:20:37Now, Romer's genius was to realise that had nothing to do at all
0:20:37 > 0:20:40with the orbit of Io around Jupiter.
0:20:40 > 0:20:44It was to do with the orbit of the Earth around the sun.
0:20:44 > 0:20:47You see, what Romer noticed was that
0:20:47 > 0:20:52when the Earth was in a position in its orbit so that it was close to Jupiter,
0:20:52 > 0:20:56then Io emerged earlier than it was expected to.
0:20:56 > 0:21:00Then, as the year passed and Earth moved around the sun
0:21:00 > 0:21:04and got further away from Jupiter, Romer noticed that Io
0:21:04 > 0:21:08then emerged later than it was expected to.
0:21:08 > 0:21:13Romer realised that it takes time for light to travel from Jupiter
0:21:13 > 0:21:17to the Earth, so when the Earth is far away from Jupiter,
0:21:17 > 0:21:21it takes longer for the light to travel and therefore you see
0:21:21 > 0:21:25Io emerge from behind Jupiter later than you would expect.
0:21:25 > 0:21:27Then, when the distance is small,
0:21:27 > 0:21:30it takes less time for the light to travel
0:21:30 > 0:21:35and you see Io emerge earlier than you might expect.
0:21:35 > 0:21:40So Romer had discovered that light doesn't travel instantaneously.
0:21:40 > 0:21:44It moves through space with a finite speed.
0:21:53 > 0:21:57This remarkable insight led to a measurement of the speed of light.
0:22:02 > 0:22:05We now know that light travels
0:22:05 > 0:22:10at precisely 299,792,458 metres per second.
0:22:10 > 0:22:14That means that in the time it takes for me to click my fingers,
0:22:14 > 0:22:17light has travelled around the Earth seven times...
0:22:17 > 0:22:23or that it travels ten million, million kilometres in one year.
0:22:23 > 0:22:29And that's the yardstick we use to measure the universe,
0:22:29 > 0:22:34because ten million, million kilometres is approximately one light year.
0:22:56 > 0:22:59It's easy to think that the universe has always existed,
0:22:59 > 0:23:04but our best scientific theory states that it emerged in one moment,
0:23:04 > 0:23:06from an event known as the Big Bang.
0:23:13 > 0:23:17And one of the most significant pieces of evidence for this theory
0:23:17 > 0:23:21comes from our understanding of light and colour.
0:23:24 > 0:23:29To reveal how colour can unlock the secrets of our universe's creation,
0:23:29 > 0:23:33I've come to one of the most spectacular natural wonders on Earth.
0:23:43 > 0:23:45This is Victoria Falls in Zambia.
0:24:00 > 0:24:03But I'm not here to marvel at the scale of this wonder.
0:24:03 > 0:24:08I've come to see a much more delicate feature that appears above the water.
0:24:10 > 0:24:14These magnificent rainbows are a permanent feature
0:24:14 > 0:24:17in the skies above Victoria Falls.
0:24:17 > 0:24:19Now, rainbows are a beautiful phenomena,
0:24:19 > 0:24:23but I think they're even more beautiful when you understand how they're made
0:24:23 > 0:24:26because they are a visual representation
0:24:26 > 0:24:32of the fact that light is made up of, well, all the colours of the rainbow.
0:24:34 > 0:24:37Just like light shining through a prism,
0:24:37 > 0:24:42rays of light from the sun are refracted as they enter the water droplets.
0:24:43 > 0:24:47The light beams then reflect off the back of the droplets
0:24:47 > 0:24:50and are bent for a second time as they leave.
0:24:51 > 0:24:54This bending and reflecting splits the light,
0:24:54 > 0:24:58and the colours hidden inside the white sunlight are revealed.
0:25:02 > 0:25:06What we see as different colours are actually different wavelengths of light,
0:25:06 > 0:25:11so blue light has a relatively short wavelength,
0:25:11 > 0:25:16and then you go through green and yellow all the way to the red end of the spectrum,
0:25:16 > 0:25:19which has a very large wavelength.
0:25:30 > 0:25:33Starlight is made up of countless different wavelengths,
0:25:33 > 0:25:36and when we look at the most distant stars
0:25:36 > 0:25:39and galaxies in the universe, their light appears redder,
0:25:39 > 0:25:46and it's this colouring that helps reveal that our universe had a beginning.
0:25:47 > 0:25:51When light is emitted by a distant star or galaxy,
0:25:51 > 0:25:54its wavelength doesn't have to stay fixed -
0:25:54 > 0:25:59it can be squashed or stretched and when light's stretched,
0:25:59 > 0:26:05its wavelength increases and it moves to the red end of the spectrum.
0:26:05 > 0:26:09So the interpretation of the fact that the most distant galaxies appear red,
0:26:09 > 0:26:12is that the space in-between them and us
0:26:12 > 0:26:20has stretched during the time it's taken the light to journey over that vast distance.
0:26:20 > 0:26:25That means that our entire universe is expanding.
0:26:27 > 0:26:31Now just think about what an expanding universe implies,
0:26:31 > 0:26:36because if the galaxies are all rushing away from each other,
0:26:36 > 0:26:38that means that if you re-wind time,
0:26:38 > 0:26:43then they must have been closer together in the past, and actually if you just keep re-winding,
0:26:43 > 0:26:46then you find that at some point in the past,
0:26:46 > 0:26:51all the galaxies we can see in the sky were quite literally on top of each other.
0:26:51 > 0:26:55The universe was squashed down to a point.
0:26:55 > 0:27:00That implies that the universe may have had a beginning
0:27:00 > 0:27:04and that is the Big Bang Theory.
0:27:19 > 0:27:24Life on Earth takes seemingly endless forms.
0:27:24 > 0:27:27Yet all creatures, however different,
0:27:27 > 0:27:32have evolved over billions of years from an ancient common ancestor.
0:27:41 > 0:27:46This connection is explained by the theory of evolution by natural selection,
0:27:46 > 0:27:49and some of the best evidence for evolution
0:27:49 > 0:27:54are in the preserved remains of ancient creatures found in fossil beds.
0:27:56 > 0:27:57This is one of them,
0:27:57 > 0:28:00the Burgess Shale in the Rocky Mountains of Canada.
0:28:03 > 0:28:08Well, this is one of the most important fossil sites in the world,
0:28:08 > 0:28:13but actually it's one of the most important scientific sites of any kind,
0:28:13 > 0:28:17and it's not just because of the number and diversity of animals
0:28:17 > 0:28:21you find fossilised in these rocks, it's because of their age.
0:28:23 > 0:28:25Over half a billion years old,
0:28:25 > 0:28:29they are some of the earliest fossils of complex life.
0:28:34 > 0:28:40I mean, it's as if at one instant in this time we call the Cambrian Era,
0:28:40 > 0:28:47complex multi-cellular life suddenly emerged almost intact on the planet.
0:28:47 > 0:28:50It's called the Evolutionary Big Bang.
0:28:53 > 0:28:58So the Burgess Shale tells us that complex life seemed to emerge suddenly
0:28:58 > 0:29:02and a new theory may also suggest what triggered this moment.
0:29:08 > 0:29:10Well, this is one of the beautiful animals
0:29:10 > 0:29:12you find up here in the fossil beds.
0:29:12 > 0:29:16It's called a trilobite. It's a very complex organism.
0:29:16 > 0:29:19It's got an external skeleton, it's got jointed limbs
0:29:19 > 0:29:26but perhaps most remarkably these, because these are compound eyes.
0:29:26 > 0:29:28They were very sophisticated
0:29:28 > 0:29:31and this was one of the first predators to be able to detect shapes
0:29:31 > 0:29:35and see movement and it could successfully chase its prey.
0:29:37 > 0:29:43These creatures were among the first to harness the light that filled the universe.
0:29:43 > 0:29:51Before they emerged, the rise and fall of the sun and the stars in the night sky simply went unnoticed.
0:29:51 > 0:29:55Now there is a speculative theory that the emergence of the eye
0:29:55 > 0:29:58actually triggered the Cambrian explosion,
0:29:58 > 0:30:04this Evolutionary Big Bang, because once one species got eyes,
0:30:04 > 0:30:10then other species had also to develop eyes to either chase them as predators
0:30:10 > 0:30:16or evade them as prey, and that led to an evolutionary arms race.
0:30:16 > 0:30:20More and more complex life forms developed.
0:30:24 > 0:30:27So the evolution of the eye may have played a fundamental role
0:30:27 > 0:30:30in the emergence of complex life on Earth...
0:30:32 > 0:30:37..and could have led to the evolution of our species.
0:30:48 > 0:30:53The light we can see is just a tiny fraction of the light in our universe.
0:30:54 > 0:30:57Beyond the visible spectrum,
0:30:57 > 0:31:01our world is also bathed in the light we can't see.
0:31:01 > 0:31:07X-rays, infrared, ultraviolet are all types of light,
0:31:07 > 0:31:11but although we can't see this light, we can still sense it.
0:31:14 > 0:31:17This sand has been under the full glare of the sun all day
0:31:17 > 0:31:21and I can feel the heat radiating off it.
0:31:21 > 0:31:25Well, heat is nothing more than a form of light,
0:31:25 > 0:31:28although we don't normally call it light.
0:31:28 > 0:31:33It's actually infrared light and the only difference between infrared
0:31:33 > 0:31:35and visible light is the wavelength.
0:31:35 > 0:31:39Infrared has a longer wavelength than visible light.
0:31:41 > 0:31:43Infrared isn't the end of the story.
0:31:43 > 0:31:48There are even longer wavelengths of light,
0:31:48 > 0:31:54and these can reveal something extraordinary about our universe.
0:31:54 > 0:31:57To detect them, you don't need a billion-pound satellite
0:31:57 > 0:32:01or a telescope built into the side of a mountain.
0:32:01 > 0:32:05You just need one of these, a radio,
0:32:05 > 0:32:09because when we tune a radio,
0:32:09 > 0:32:14we're tuning into a form of light - radio waves.
0:32:14 > 0:32:18MUSIC PLAYS ON RADIO
0:32:20 > 0:32:23But detecting them and understanding them
0:32:23 > 0:32:28provides the key to understanding the origin of the universe.
0:32:31 > 0:32:33When you de-tune the radio a bit, you can just hear static
0:32:33 > 0:32:40but about 1% of that static is music to the ears of a physicist
0:32:40 > 0:32:45because that is stretched light from the Big Bang.
0:32:45 > 0:32:50# Carry him home safely to me... #
0:32:52 > 0:32:54The reason we can't see this ancient light
0:32:54 > 0:32:59is because as the universe expanded, the light waves were stretched
0:32:59 > 0:33:04and transformed into radio waves and microwaves.
0:33:04 > 0:33:10This first light is called the Cosmic Microwave Background or CMB.
0:33:12 > 0:33:16The CMB fills every part of the universe.
0:33:17 > 0:33:22If my eyes could only see it, then the sky would be ablaze
0:33:22 > 0:33:26with this primordial light both day and night.
0:33:33 > 0:33:38Although we are not sensitive to this light, specialised cameras are,
0:33:38 > 0:33:43and when they are pointed towards the heavens, something beautiful emerges.
0:33:54 > 0:33:57These scattered colours are the fading embers,
0:33:57 > 0:34:03the last remnants of light from the beginning of the universe.
0:34:13 > 0:34:16Looking out into space, you might think that the cosmos
0:34:16 > 0:34:23is a constant unchanging place, that the stars will always be there.
0:34:23 > 0:34:28But in fact, the stars are only a temporary feature in the sky,
0:34:28 > 0:34:32and though they may burn brightly for many millions or billions of years,
0:34:32 > 0:34:37they can only live for as long as they have a supply of hydrogen to burn.
0:34:40 > 0:34:44And when a star like our sun runs out of hydrogen, it begins to die.
0:34:47 > 0:34:48But it doesn't go quietly.
0:34:52 > 0:34:56At the end of its life, the sun won't simply fade away to nothing.
0:34:59 > 0:35:03As it begins to run out of fuel, its core will collapse
0:35:03 > 0:35:08and the extra heat this generates will cause its outer layers to expand.
0:35:15 > 0:35:17In around a billion years' time,
0:35:17 > 0:35:21this will have a catastrophic effect on our fragile world.
0:35:28 > 0:35:31Gradually, the Earth will become hotter and hotter,
0:35:31 > 0:35:37so there will be one last perfect day on Earth, but eventually,
0:35:37 > 0:35:42the existence of all life on this planet will become impossible.
0:35:46 > 0:35:49Long after life has disappeared, the sun will have grown so much,
0:35:49 > 0:35:53it will fill the entire horizon.
0:35:59 > 0:36:05It will have become a red giant, the last phase of its life.
0:36:14 > 0:36:17Our planet might not survive to this point, but if it does,
0:36:17 > 0:36:21little more than a scorched and barren rock will remain
0:36:21 > 0:36:26to witness the final death throes of our star.
0:36:37 > 0:36:41In six billion years, our sun will explode,
0:36:41 > 0:36:46throwing vast amounts of gas and dust out into space to form a gigantic nebula.
0:36:54 > 0:36:58At its heart will be a faintly glowing ember,
0:36:58 > 0:37:02all that remains of our once-magnificent sun.
0:37:02 > 0:37:05It will be smaller than the size of the Earth,
0:37:05 > 0:37:10less than a millionth of its current volume, and a fraction of its brightness.
0:37:10 > 0:37:13Our sun will have become a white dwarf.
0:37:23 > 0:37:27With no fuel left to burn, a white dwarf's faint glow
0:37:27 > 0:37:31comes from the last residual heat from its extinguished furnace.
0:37:34 > 0:37:37The sun is now dead,
0:37:37 > 0:37:42its remains slowly cooling in the freezing temperatures of deep space.
0:37:46 > 0:37:48Looking at it from where the Earth is now,
0:37:48 > 0:37:55it would only generate the same amount of light as the full moon on a clear night.
0:37:59 > 0:38:03The fate of the sun is the same as for all stars.
0:38:03 > 0:38:07One day they must all eventually die,
0:38:07 > 0:38:11and the cosmos will be plunged into eternal night,
0:38:11 > 0:38:15because this structured universe that we inhabit and all its wonders,
0:38:15 > 0:38:21the stars and the planets and the galaxies, cannot last forever.
0:38:21 > 0:38:26The cosmos WILL eventually fade and die.
0:38:52 > 0:38:55Black holes are the most destructive places in the universe,
0:38:55 > 0:38:59able to devour entire stars.
0:39:00 > 0:39:03Yet we've never seen one.
0:39:03 > 0:39:06It's because of their effects on the stars and galaxies,
0:39:06 > 0:39:10and the dust and gas around them, that we know that they exist.
0:39:15 > 0:39:18But the reason why black holes are invisible
0:39:18 > 0:39:20can be demonstrated here on Earth.
0:39:43 > 0:39:48Near a black hole, space and time do some very strange things
0:39:48 > 0:39:51because black holes are probably the most violent places
0:39:51 > 0:39:54we know of in the universe.
0:39:54 > 0:39:59This river provides a beautiful analogy for what happens to space and time
0:39:59 > 0:40:02as you get closer and closer to the black hole.
0:40:05 > 0:40:09Now, upstream, the water is flowing pretty slowly.
0:40:09 > 0:40:12Let's imagine that it's flowing at three kilometres per hour
0:40:12 > 0:40:19and I can swim at four, so I can swim faster than the flow and can easily escape.
0:40:27 > 0:40:33But as you go further and further downstream towards the waterfall in the distance,
0:40:33 > 0:40:36the river flows faster and faster.
0:40:55 > 0:41:00Imagine I decide to jump into the river just there on the edge of the falls.
0:41:00 > 0:41:05The water is flowing far faster than I could swim, so no matter what I did,
0:41:05 > 0:41:10no matter how hard I tried, I would not be able to swim back upstream.
0:41:10 > 0:41:13I would be carried inexorably towards the edge
0:41:13 > 0:41:16and I would vanish over the falls.
0:41:22 > 0:41:26Well, it's the same close to a black hole
0:41:26 > 0:41:29because space flows faster and faster and faster
0:41:29 > 0:41:31towards the black hole -
0:41:31 > 0:41:34literally this stuff,
0:41:34 > 0:41:37my space that I'm in, flowing over the edge into the black hole.
0:41:37 > 0:41:42And at the very special point called the event horizon,
0:41:42 > 0:41:47space is flowing at the speed of light into the black hole.
0:41:52 > 0:41:56Light itself, travelling at 300,000 kilometres per second,
0:41:56 > 0:41:59is not going fast enough to escape the flow,
0:41:59 > 0:42:03and light itself will plunge into the black hole.
0:42:10 > 0:42:13So the fact that black holes can swallow light
0:42:13 > 0:42:18means that they will for ever remain invisible to our eyes.
0:42:32 > 0:42:37Gravity is the force that keeps our feet firmly rooted to our planet.
0:42:37 > 0:42:40Yet although it may appear constant and unchanging,
0:42:40 > 0:42:44this force varies on all the planets in the solar system
0:42:44 > 0:42:50and on the exo-planets we've discovered orbiting other suns.
0:42:50 > 0:42:54To experience the gravity on these worlds, I need to go for a spin.
0:43:03 > 0:43:05This is a centrifuge.
0:43:05 > 0:43:10It was built in the 1950s to test whether fighter pilots had the right stuff,
0:43:10 > 0:43:13but it's going to allow me to feel what it would be like
0:43:13 > 0:43:17to stand on the surface of any of the planets in the solar system
0:43:17 > 0:43:19that are more massive than the Earth,
0:43:19 > 0:43:22and in fact, also what it would be like
0:43:22 > 0:43:26to stand on some of the planets that we've found around distant stars.
0:43:28 > 0:43:30Three...two...one.
0:43:33 > 0:43:39As the centrifuge rotates, it feels exactly as if gravity is increased.
0:43:39 > 0:43:42The faster it spins, the greater the effect
0:43:42 > 0:43:47and we measure this in multiples of the strength
0:43:47 > 0:43:50of Earth's gravity, known as 1G.
0:43:52 > 0:43:56The first planet I'm travelling to is Neptune.
0:43:56 > 0:44:00Its gravity is just fractionally stronger than here on Earth.
0:44:00 > 0:44:03So this is the gravitational field on Neptune
0:44:03 > 0:44:06and you feel, "You know what? I could probably get used to this.
0:44:06 > 0:44:09"I could probably live on the surface of Neptune."
0:44:09 > 0:44:12- Can you lift your hands a little? - There we go.- Yeah, and down.
0:44:12 > 0:44:17And it is actually quite an effort. It is noticeably heavier.
0:44:17 > 0:44:21It's like having a reasonably heavy weight in your hand.
0:44:21 > 0:44:23To go to 2.5G?
0:44:23 > 0:44:27- Yes, so now we'll move, move from Neptune to Jupiter.- Let's go there.
0:44:30 > 0:44:33Jupiter is over 1,300 times more massive than the Earth,
0:44:33 > 0:44:36but because it's mostly gas, it's not very dense
0:44:36 > 0:44:39so its gravity is just over twice as strong at its surface.
0:44:39 > 0:44:44Well, now actually it is quite difficult to lift my hand.
0:44:45 > 0:44:49And that's 2.5G. I wouldn't want to sit here for half an hour.
0:44:49 > 0:44:53Can you lift both of your hands above your head?
0:44:53 > 0:44:56- See what happens there. - Let's see, so actually...
0:44:56 > 0:45:02just about, but actually it's an immense amount of hard work.
0:45:02 > 0:45:06- So it would be hard work living on Jupiter.- Let's go to 4G.
0:45:13 > 0:45:16Actually, this is heading to a planet around...
0:45:16 > 0:45:19A planet called Ogle 2TRL9B
0:45:19 > 0:45:23which is around a star in the constellation of Carina.
0:45:23 > 0:45:26It's one of the exo-planets we've discovered.
0:45:27 > 0:45:29Oh, and there we go.
0:45:33 > 0:45:39Now, that is actually...beginning to feel quite unpleasant.
0:45:39 > 0:45:43- Can you describe what you're feeling?- Very heavy face.
0:45:43 > 0:45:45My head is extremely heavy.
0:45:45 > 0:45:47How about your lungs, inhaling, exhaling, breathing?
0:45:47 > 0:45:53- It's much harder work. I can't lift my hand off my leg.- OK.
0:45:53 > 0:45:55- And that's at 4G?- Yeah.
0:45:55 > 0:46:00Well, my head and my face feel very, very heavy.
0:46:00 > 0:46:02It's quite an unpleasant feeling.
0:46:02 > 0:46:08We'll go to 5 and let me know if you have any visual disturbances.
0:46:09 > 0:46:13I am now en-route to a newly discovered exo-planet, Wasp 8b.
0:46:15 > 0:46:174.4.
0:46:17 > 0:46:22This world sits in the small and faint constellation of Sculptor.
0:46:26 > 0:46:27Quite hard to speak.
0:46:31 > 0:46:35It has a gravitational force nearly five times that of the Earth.
0:46:35 > 0:46:40Right, we'll go to 5G.
0:46:40 > 0:46:41- Very foggy.- OK.
0:46:45 > 0:46:47- Very foggy.- Very foggy?
0:46:55 > 0:46:57- Still foggy?- Yeah.- Right.
0:46:58 > 0:47:00- Take it down. - OK, we'll take you down.
0:47:15 > 0:47:17Very interesting.
0:47:18 > 0:47:22- It was, wasn't it? - My face felt a bit saggy, though.
0:47:22 > 0:47:26Well, you looked a little different.
0:47:40 > 0:47:45That was, um, quite unpleasant that time, actually.
0:47:45 > 0:47:50So you realise that we're, obviously, very finely tuned
0:47:50 > 0:47:54to live on a planet that has a gravitational,
0:47:54 > 0:47:57an acceleration due to gravity of 1G.
0:47:57 > 0:48:00When you go to 2G, it's difficult.
0:48:00 > 0:48:05When you go to 3G and 4G it becomes unpleasant, and 5G, anyway,
0:48:05 > 0:48:11for me, was on the border of being so unpleasant that you pass out.
0:48:18 > 0:48:21So although gravity feels weak here on Earth,
0:48:21 > 0:48:25it certainly isn't weak everywhere across the universe.
0:48:26 > 0:48:29And that's because gravity is an additive force.
0:48:29 > 0:48:32It scales with mass,
0:48:32 > 0:48:37so the more massive the planet or star,
0:48:37 > 0:48:41the stronger its gravity.
0:48:56 > 0:48:57Every moment of our lives,
0:48:57 > 0:49:01we experience a force that we can't see or touch.
0:49:04 > 0:49:08Yet this force is able to keep us firmly rooted to the ground.
0:49:08 > 0:49:12It is, of course, gravity.
0:49:16 > 0:49:20But despite its intangible nature, we always know it's with us.
0:49:22 > 0:49:26Now if I was to ask you, "How do you know that there's gravity around here?"
0:49:26 > 0:49:28Then you might say, "Well, it's obvious.
0:49:28 > 0:49:31"You know, I can just do an experiment, I can drop something."
0:49:33 > 0:49:39Well, yes, but actually gravity is a little bit more subtle than that
0:49:39 > 0:49:43but to really experience it, to understand it,
0:49:43 > 0:49:46you have to do something pretty extreme.
0:49:53 > 0:49:57And this plane has been modified to help me do it.
0:49:57 > 0:50:02Thanks to its flight plan, it's known as the Vomit Comet.
0:50:15 > 0:50:18Once we've climbed to 15,000 metres,
0:50:18 > 0:50:23this plane does something no ordinary flight would do -
0:50:23 > 0:50:28its engines are throttled back and the jet falls to Earth.
0:50:29 > 0:50:33And then, something quite amazing happens.
0:50:35 > 0:50:37THEY SQUEAL AND CHEER
0:50:38 > 0:50:40Push to me, push to me!
0:50:43 > 0:50:49I'm now plummeting towards the ground just like someone's cut the cable in a lift,
0:50:49 > 0:50:52and you see, we're all just floating.
0:51:09 > 0:51:11By simply falling at the same rate as the plane,
0:51:11 > 0:51:18for a few fleeting moments, we are all free of gravity's grip.
0:51:30 > 0:51:32But this isn't just a joy ride.
0:51:39 > 0:51:42Now, look, there's something very profound here
0:51:42 > 0:51:47because although I'm falling towards the ground,
0:51:47 > 0:51:50as you see, gravity has completely gone away.
0:51:50 > 0:51:54Gravity is not here any more.
0:51:58 > 0:52:02'Because the aircraft is accelerating towards the ground at 1G,
0:52:02 > 0:52:08'the effects of the Earth's gravity are completely cancelled out.'
0:52:14 > 0:52:22So it is possible by the simple act of falling to get a very different experience of gravity.
0:52:32 > 0:52:39Nothing can travel faster than the speed at which light travels,
0:52:39 > 0:52:42but although light travels fast, it's not infinitely fast
0:52:42 > 0:52:46so the further away an object is, the further back in time we see it.
0:52:52 > 0:52:55The sun is 150 million kilometres away.
0:52:58 > 0:53:01Now that's very close by cosmic standards,
0:53:01 > 0:53:06but light travels at only 300,000 kilometres per second,
0:53:06 > 0:53:09so that means that we're seeing the sun
0:53:09 > 0:53:15as it was in the past, actually eight minutes in the past.
0:53:21 > 0:53:25But when we look beyond our sun, to far more distant stars,
0:53:25 > 0:53:27we reach further back in time.
0:53:35 > 0:53:39As the sun dips below the horizon and night falls...
0:53:41 > 0:53:45..the universe just fades into view.
0:53:49 > 0:53:53And then, as it gets darker and darker, the Milky Way appears -
0:53:53 > 0:53:59a vast swathe of billions and billions of suns as you look out
0:53:59 > 0:54:02towards the centre of our Milky Way galaxy.
0:54:06 > 0:54:09But I think for me, the most magical thing
0:54:09 > 0:54:13you can see in the sky with the naked eye, is just below
0:54:13 > 0:54:18the constellation of Cassiopeia, the W of stars in the sky.
0:54:30 > 0:54:34There, look at that.
0:54:35 > 0:54:38Actually, I've got to say, that's amazing.
0:54:40 > 0:54:44You see, that misty patch of light is not a cloud in the sky,
0:54:44 > 0:54:47it's not even gas and dust in our galaxy.
0:54:47 > 0:54:50That...is another galaxy.
0:54:50 > 0:54:54It's the Andromeda galaxy, which is roughly the same size as our own -
0:54:54 > 0:54:57an island of hundreds of billions of stars,
0:54:57 > 0:55:0325 million, million, million kilometres in that direction.
0:55:08 > 0:55:14The light that I've just captured in my camera began its journey two and a half million years ago.
0:55:14 > 0:55:17At that time on Earth, there were no humans.
0:55:17 > 0:55:20Homo habilis, our distant ancestors,
0:55:20 > 0:55:24were roaming the plains of Africa and as those light rays travelled
0:55:24 > 0:55:29through the vastness of space, our species evolved
0:55:29 > 0:55:35and thousands and thousands and thousands of generations of humans lived and died.
0:55:35 > 0:55:39And then, two and a half million years after their journey began,
0:55:39 > 0:55:47these messengers from the depths of space, and from way back in our past,
0:55:47 > 0:55:52arrived here on Earth, and I just captured them and took that picture.
0:55:57 > 0:55:59But by peering further than the naked eye will allow,
0:55:59 > 0:56:04we can journey to a time way before human history.
0:56:04 > 0:56:08In the last 20 years, powerful space telescopes have carried us
0:56:08 > 0:56:13ever deeper into space and we have become virtual time travellers.
0:56:17 > 0:56:22This is NGC 520 and it's the product of a cosmic collision,
0:56:22 > 0:56:27but this galaxy is a hundred million light years away.
0:56:27 > 0:56:30That means that the light began its journey from this galaxy
0:56:30 > 0:56:33to my eye when the dinosaurs roamed the Earth.
0:56:44 > 0:56:46But these spectacular galaxies
0:56:46 > 0:56:49are not the end of our journey into the past.
0:56:49 > 0:56:54In 2004, we peered further back in time than ever before
0:56:54 > 0:57:00and captured the light from the most distant galaxies in the universe.
0:57:02 > 0:57:06The image is called the Hubble Ultra Deep Field.
0:57:06 > 0:57:09It's a picture taken by the Hubble Space Telescope
0:57:09 > 0:57:13over a period of 11 days, and it focused its camera
0:57:13 > 0:57:19on the tiniest piece of sky, just below the constellation of Orion.
0:57:19 > 0:57:21Now, it's a piece of sky that you would cover
0:57:21 > 0:57:26if you took your thumb, held it in front of your face
0:57:26 > 0:57:29and then moved it 20 times further away.
0:57:32 > 0:57:37But the Hubble captured the faintest lights from the most distant regions
0:57:37 > 0:57:40of the universe and it took this photograph.
0:57:44 > 0:57:49Now, almost every point of light in that image is not a star,
0:57:49 > 0:57:55but a galaxy of over a hundred billion stars.
0:57:55 > 0:58:01The most distant galaxies in that image are over 13 billion light years away.
0:58:01 > 0:58:05That means that the faint light from those galaxies
0:58:05 > 0:58:10began its journey to Earth 13 billion years ago.
0:58:10 > 0:58:15That's over three times the age of the Earth.
0:58:33 > 0:58:36Subtitles by Red Bee Media Ltd
0:58:36 > 0:58:39E-mail subtitling@bbc.co.uk