0:00:09 > 0:00:10Under the cover of darkness,
0:00:11 > 0:00:15the world lies hidden from view.
0:00:17 > 0:00:23Without light, I've no idea what lies beyond my immediate surroundings.
0:00:27 > 0:00:30I'm closed in, enveloped on all sides
0:00:30 > 0:00:33by the unknown.
0:00:40 > 0:00:42For much of human history,
0:00:42 > 0:00:45when the sun went down and the dark set in,
0:00:45 > 0:00:47we were at the mercy of the night.
0:00:47 > 0:00:49But over the centuries,
0:00:49 > 0:00:53we've developed our own sources of illumination.
0:01:07 > 0:01:11We've lit our homes, our streets, our cities,
0:01:11 > 0:01:15and doing so, we've banished the darkness into the shadows.
0:01:28 > 0:01:32And just as we've used light to illuminate our world,
0:01:32 > 0:01:36the more we've discovered about light's properties,
0:01:36 > 0:01:39the more of the Universe it's shown us.
0:01:43 > 0:01:46We've seen into the depths of space...
0:01:47 > 0:01:51..and back to the beginning of time.
0:01:56 > 0:01:58But as we've looked deeper,
0:01:58 > 0:02:03we've come to realise how little we've seen
0:02:03 > 0:02:06and that the cosmos's greatest mysteries
0:02:06 > 0:02:10remain hidden in the dark.
0:02:11 > 0:02:14Light and dark is essentially the story
0:02:14 > 0:02:16of everything we know
0:02:16 > 0:02:21and everything we don't know about our Universe.
0:02:47 > 0:02:51And it all begins with light.
0:02:51 > 0:02:56It's such an integral part of the way we perceive the world,
0:02:56 > 0:02:59it's easy to take it for granted.
0:03:00 > 0:03:04But for centuries, understanding what light really is
0:03:04 > 0:03:09has been one of science's most enduring questions.
0:03:11 > 0:03:15The first steps toward understanding the properties of light
0:03:15 > 0:03:17were made in the third century BC
0:03:17 > 0:03:20by the renowned Greek mathematician Euclid.
0:03:20 > 0:03:23He did it by thinking about something so obvious,
0:03:23 > 0:03:27most of us don't give it any thought at all.
0:03:41 > 0:03:44Placing the tiny chair very close to the camera
0:03:44 > 0:03:47produces a large image on the retina,
0:03:47 > 0:03:51and because we're not used to seeing tiny chairs in everyday life,
0:03:51 > 0:03:53our brains are tricked into thinking
0:03:53 > 0:03:56it's a normal-sized chair in the middle of the room.
0:03:56 > 0:03:58The reason this illusion works at all
0:03:58 > 0:04:01is because, to judge distances,
0:04:01 > 0:04:04our brains rely on a simple fact -
0:04:04 > 0:04:09the further away things are, the smaller they appear to the eye.
0:04:26 > 0:04:29And it was by focusing on exactly why
0:04:29 > 0:04:32distant objects could appear the same size
0:04:32 > 0:04:35as much smaller ones closer up...
0:04:35 > 0:04:42..that led Euclid to discover of one of light's most fundamental properties.
0:04:45 > 0:04:50Obviously the London Eye is much bigger than my fingers, I know that,
0:04:50 > 0:04:52and yet to me they look the same size.
0:04:52 > 0:04:55So, how do we explain this?
0:04:55 > 0:04:57Well, Euclid came up with an elegant solution.
0:04:57 > 0:05:00For my finger to appear at the top of the wheel,
0:05:00 > 0:05:03my eye, my finger and the top of the wheel
0:05:03 > 0:05:07must all lie on the same line.
0:05:14 > 0:05:18But Euclid's insight didn't just explain the tricks of perspective,
0:05:18 > 0:05:23it revealed a basic truth about light itself.
0:05:23 > 0:05:27Euclid had discovered that light travels in straight lines.
0:05:29 > 0:05:32Realising how it travels
0:05:32 > 0:05:36marks the beginning of our scientific understanding of light.
0:05:36 > 0:05:42And it also meant that if we could divert it from its straight-line path,
0:05:42 > 0:05:45we could change the way we see the world.
0:06:04 > 0:06:08But that leap wouldn't happen for another 2,000 years.
0:06:08 > 0:06:12It was eventually made in Renaissance Italy
0:06:12 > 0:06:16by one of the founding fathers of modern science.
0:06:20 > 0:06:23In the summer of 1609,
0:06:23 > 0:06:29Galileo Galilei made the short but fateful journey from his home in Padua
0:06:29 > 0:06:33to Venice, capital of the Venetian Republic.
0:06:39 > 0:06:43Galileo had flame-red hair, a full beard,
0:06:43 > 0:06:47and was well-known for his love of fine wines and generous hospitality,
0:06:47 > 0:06:51and also for his anti-establishment views.
0:06:51 > 0:06:56By this time, he'd also built up a reputation as a natural philosopher and mathematician
0:06:56 > 0:07:01and he was regarded as a valuable asset to the Venetian Republic.
0:07:03 > 0:07:07But although, as a professor, he had a regular income,
0:07:07 > 0:07:12Galileo was never far from financial troubles.
0:07:12 > 0:07:15When his father died in 1591,
0:07:15 > 0:07:18Galileo, the eldest of four surviving siblings,
0:07:18 > 0:07:20became the head of the household
0:07:20 > 0:07:24and, effectively, took on responsibility for supporting his brother,
0:07:24 > 0:07:26a poor itinerant musician,
0:07:26 > 0:07:29and for paying his sisters' dowries.
0:07:29 > 0:07:30By the time he came to Venice,
0:07:30 > 0:07:34he still owed a significant amount of money to his two brothers in law
0:07:34 > 0:07:38and so was always on the lookout for a money-making scheme.
0:07:42 > 0:07:44That summer,
0:07:44 > 0:07:47Venice was abuzz with rumours of a device
0:07:47 > 0:07:50that appeared to do the impossible...
0:07:51 > 0:07:53..a Dutch spyglass
0:07:53 > 0:07:57that could bring distant objects closer.
0:07:58 > 0:08:02It was just opportunity Galileo was looking for.
0:08:07 > 0:08:09Back in the 17th Century,
0:08:09 > 0:08:12the spyglass was cutting-edge technology
0:08:12 > 0:08:16and the details of how it worked were a closely-guarded secret.
0:08:16 > 0:08:21All Galileo knew was that it consisted of two lenses arranged in a tube,
0:08:21 > 0:08:26and so when he developed his own, he kept it very secret, as well.
0:08:27 > 0:08:29But we do know from a shopping list
0:08:29 > 0:08:33that he got his glass from the small island of Murano, out in the lagoon,
0:08:33 > 0:08:37and because no tools existed, he had to improvise,
0:08:37 > 0:08:39for instance, buying an artillery ball
0:08:39 > 0:08:43to grind the curved surfaces of the lenses.
0:08:51 > 0:08:54It had been known since the first spectacles were produced,
0:08:54 > 0:08:57in the middle of the 13th century,
0:08:57 > 0:09:01that glass had the strange property of bending light.
0:09:01 > 0:09:03But unlike spectacles,
0:09:03 > 0:09:06the spyglass, an early telescope,
0:09:06 > 0:09:09required a combination of lenses
0:09:09 > 0:09:12in a very specific arrangement.
0:09:12 > 0:09:15This is how Galileo's telescope works.
0:09:15 > 0:09:18Rays of light come in from a distant object
0:09:18 > 0:09:22so they're almost parallel where they meet his first lens.
0:09:22 > 0:09:26This is the objective lens, and it's plano-convex,
0:09:26 > 0:09:29which means it's flat on one side and curved on the other.
0:09:29 > 0:09:33It's the sort of lens used to treat long-sightedness.
0:09:33 > 0:09:37What it does is bend the rays of light towards each other
0:09:37 > 0:09:41so that they would meet at a point.
0:09:41 > 0:09:43But before this focal point,
0:09:43 > 0:09:48Galileo places his second lens, the ocular lens, which is plano-concave,
0:09:48 > 0:09:52and this bends the rays of light back out again
0:09:52 > 0:09:55so they emerge parallel, where they enter the eye,
0:09:55 > 0:09:59and then the eye's lens focuses them on the retina.
0:09:59 > 0:10:03Now the magnification of a telescope depends on the ratio
0:10:03 > 0:10:06of the focal lengths of the two lenses -
0:10:06 > 0:10:11the distances F1 and F2.
0:10:12 > 0:10:14The difficulty for Galileo
0:10:14 > 0:10:18was grinding down the convex surface of his objective lens
0:10:18 > 0:10:20to make it as shallow as possible
0:10:20 > 0:10:22in order to maximise the length F1,
0:10:22 > 0:10:25because the longer he could make that,
0:10:25 > 0:10:29the greater the magnification of his telescope.
0:10:30 > 0:10:32Produced in just a few weeks,
0:10:32 > 0:10:37Galileo's telescope had a magnification of eight times
0:10:37 > 0:10:41and was far more powerful than the original spyglass.
0:10:41 > 0:10:43All he needed to do now
0:10:43 > 0:10:47was cash in on his new invention.
0:10:47 > 0:10:51Ever the showman, on the 21st August, 1609,
0:10:51 > 0:10:54Galileo climbed one of the city's bell towers.
0:10:54 > 0:10:56BELLS CHIME
0:10:56 > 0:11:00LIFT MUZAK: "The Girl from Ipanema"
0:11:00 > 0:11:03Obviously, he would've used the stairs!
0:11:08 > 0:11:13At the top, in front of an assembled group of Venetian noblemen and senators,
0:11:13 > 0:11:17Galileo demonstrated his telescope.
0:11:20 > 0:11:24It was a sensation.
0:11:35 > 0:11:40Using it, the Venetians would be able to see approaching ships
0:11:40 > 0:11:44two hours earlier than with naked eye.
0:11:45 > 0:11:50The military and economic advantage of knowing who was sailing over the horizon
0:11:50 > 0:11:55was lost on no-one watching that day.
0:11:55 > 0:11:58Three days later, as a grand gesture,
0:11:58 > 0:12:02Galileo presented his telescope to the duke as a gift.
0:12:02 > 0:12:05In return, he was guaranteed his job for life,
0:12:05 > 0:12:08at double his salary.
0:12:20 > 0:12:23With his finances now secure,
0:12:23 > 0:12:28Galileo went on to develop a more powerful telescope,
0:12:28 > 0:12:31and with it, use the ability to bend light
0:12:31 > 0:12:36to change our perspective on the cosmos.
0:12:36 > 0:12:40This is the book Galileo published in 1610.
0:12:40 > 0:12:42It's called "Sidereus Nuncius",
0:12:42 > 0:12:46which in Latin means "The Starry Messenger".
0:12:46 > 0:12:49In it, he recorded his first observations of the night sky
0:12:49 > 0:12:51the first anyone had ever made
0:12:51 > 0:12:54using anything other than the naked eye.
0:12:54 > 0:12:57Today, it's hard to imagine
0:12:57 > 0:13:01how anything contained in this little book was controversial,
0:13:01 > 0:13:03but you have to remember that when it was written,
0:13:03 > 0:13:07the nature of heavens was thought to be knowable only to God
0:13:07 > 0:13:12and the Earth was considered to be at the centre of the Universe.
0:13:18 > 0:13:20These are his drawings of the moon.
0:13:20 > 0:13:25Since ancient times, all heavenly bodies were thought to be perfect spheres,
0:13:25 > 0:13:30but with his telescope, Galileo saw texture in the surface of the moon,
0:13:30 > 0:13:32deep craters and mountains
0:13:32 > 0:13:35that, from the shadows they cast across the lunar surface,
0:13:35 > 0:13:40he estimated to be some six kilometres tall.
0:13:45 > 0:13:49As well as showing the heavens to be imperfect...
0:13:50 > 0:13:56..his telescope began to uncover their true extent,
0:13:56 > 0:14:00revealing ten-times more stars
0:14:00 > 0:14:03than are visible to the naked eye.
0:14:03 > 0:14:05And in the final chapters,
0:14:05 > 0:14:08Galileo reports the discovery of four stars
0:14:08 > 0:14:11that appeared to form a straight line
0:14:11 > 0:14:13near the planet Jupiter.
0:14:13 > 0:14:18His drawings show how their positions change from night to night.
0:14:18 > 0:14:22Although they moved, they always did so along the same straight line,
0:14:22 > 0:14:27and from that, Galileo deduced that they had to be orbiting Jupiter.
0:14:27 > 0:14:30They weren't stars at all, they were moons.
0:14:36 > 0:14:38Through his telescope,
0:14:38 > 0:14:39Galileo had seen evidence
0:14:39 > 0:14:42that overturned the accepted dogma
0:14:42 > 0:14:44that the Earth was the fulcrum
0:14:44 > 0:14:48about which everything in the Universe revolved.
0:14:52 > 0:14:54Seeing moons in orbit around Jupiter
0:14:54 > 0:14:57meant that not everything went round the Earth.
0:14:57 > 0:15:00So, far from being the centre of the Universe,
0:15:00 > 0:15:03the Earth was just another planet.
0:15:15 > 0:15:18The telescope had allowed Galileo
0:15:18 > 0:15:23to glimpse the true nature of the cosmos
0:15:23 > 0:15:26and our place within it.
0:15:30 > 0:15:33But this way of manipulating light
0:15:33 > 0:15:36had another powerful application,
0:15:36 > 0:15:42one that would allow us to see into another world.
0:15:44 > 0:15:47BELLS CHIME
0:15:52 > 0:15:54In 17th-century London,
0:15:54 > 0:15:56one of the most prominent scientists of the age
0:15:56 > 0:16:00was using lenses in a very different way.
0:16:06 > 0:16:11Robert Hooke had taken the basic principle of the telescope
0:16:11 > 0:16:15and used it to build a microscope.
0:16:24 > 0:16:28Galileo uses the telescope to discover a new world in the heavens,
0:16:28 > 0:16:31and Hooke uses the microscope to discover a new world
0:16:31 > 0:16:34in the very, very small.
0:16:34 > 0:16:38But there's a difference, because what Galileo had presented
0:16:38 > 0:16:42was a world that was bigger and more plentiful,
0:16:42 > 0:16:45but it was a world that people were at least vaguely familiar with
0:16:45 > 0:16:47because you can look up in the sky and see the stars,
0:16:47 > 0:16:50whereas the world that Hooke presented
0:16:50 > 0:16:52was really something spectacular and new.
0:16:52 > 0:16:56It was a world inside the tiniest particles of matter
0:16:56 > 0:17:00that no-one had ever imagined to be there before.
0:17:00 > 0:17:02People didn't even realise
0:17:02 > 0:17:06that there was a microscopic world there to reveal.
0:17:10 > 0:17:12Hooke trained his microscope
0:17:12 > 0:17:16on a huge range of materials and living things.
0:17:20 > 0:17:26But it was his drawings of the exquisite detail he saw in the bodies of insects
0:17:26 > 0:17:29that would become famous.
0:17:31 > 0:17:36Up here, you can see a human flea, Pulex irritans, a very tiny creature,
0:17:36 > 0:17:39and here we've got the plate from "Micrographia",
0:17:39 > 0:17:43which is a huge image of the flea that Hooke produced,
0:17:43 > 0:17:46and it's really something spectacular.
0:17:46 > 0:17:49This would've folded out in the book, so it was really very large.
0:17:49 > 0:17:51Some people said it was as big as a cat.
0:17:51 > 0:17:56It's a work of art, really. I mean, there's so much intricate detail in there.
0:17:56 > 0:17:58It is, and there was nothing like it before Hooke.
0:17:58 > 0:18:01They really were unprecedented
0:18:01 > 0:18:06and the shading and the quality of the images is just superb.
0:18:06 > 0:18:09- And it's accurate. I mean, it's... - It is, it's absolutely accurate.
0:18:09 > 0:18:15I was looking yesterday at images of, er, photographs of the flea and, er, there's really -
0:18:15 > 0:18:17made with an electron microscope -
0:18:17 > 0:18:20and there's really nothing to chose between Hooke and, er,
0:18:20 > 0:18:23the current images.
0:18:24 > 0:18:27This is an image of the compound eye of a fly,
0:18:27 > 0:18:30which Hooke shows in amazing detail for the first time.
0:18:30 > 0:18:33This is an image of the foot of a fly.
0:18:33 > 0:18:36Hooke shows you the foot has little spikes in it
0:18:36 > 0:18:41that allow it to clasp into the pores on a surface.
0:18:41 > 0:18:45This image looks less interesting, less intricate than the others.
0:18:45 > 0:18:47It doesn't look terribly interesting
0:18:47 > 0:18:50but, actually, it's really quite a profound picture,
0:18:50 > 0:18:55because what Hooke is looking at here is a very thin slice of cork,
0:18:55 > 0:18:57which he cut with a penknife,
0:18:57 > 0:19:01and he's looking at the little individual components that make it up.
0:19:01 > 0:19:05And he calls them pores, and then he calls them caverns,
0:19:05 > 0:19:08he calls them boxes and then he calls them cells,
0:19:08 > 0:19:11and cell, of course, is the term that stuck.
0:19:11 > 0:19:15These are the little constituent parts, not just of cork, but of all living things,
0:19:15 > 0:19:18and so it's a profoundly important discovery
0:19:18 > 0:19:23and a name that has become standard in biology.
0:19:51 > 0:19:54Using glass to bend light
0:19:54 > 0:19:57revealed our true place in the Universe...
0:20:03 > 0:20:08..and the intricate architecture of the microscopic world.
0:20:15 > 0:20:17The more we looked,
0:20:17 > 0:20:20the more we saw.
0:20:20 > 0:20:24With each new insight into the nature of light
0:20:24 > 0:20:28came a fresh understanding of the cosmos.
0:20:33 > 0:20:35And the next discovery
0:20:35 > 0:20:38would take us far further...
0:20:39 > 0:20:45..and enable us to read the story of the stars.
0:20:57 > 0:21:03And it began with something Hooke had glimpsed through his microscope.
0:21:04 > 0:21:08This is Robert Hooke's book The Micrographia,
0:21:08 > 0:21:11published in 1664,
0:21:11 > 0:21:14350 years ago.
0:21:15 > 0:21:18It's full of...
0:21:18 > 0:21:20..his famous diagrams.
0:21:20 > 0:21:22Here's his picture of the flea.
0:21:22 > 0:21:25It's incredible seeing it in its original form.
0:21:25 > 0:21:28It really is the size of a cat!
0:21:28 > 0:21:33These images really captured the public imagination and they made the book a sensation,
0:21:33 > 0:21:37but for me, The Micrographia is about much more than that.
0:21:37 > 0:21:39The chapter that interests me as a physicist
0:21:39 > 0:21:44is one that contains hardly any images at all.
0:21:44 > 0:21:47And it's this one here -
0:21:47 > 0:21:48"Of the Colours observable
0:21:48 > 0:21:52"in Muscovy Glass, and other thin Bodies".
0:21:52 > 0:21:56Here Hooke describes the iridescent patterns of rainbow colours
0:21:56 > 0:21:58he sees through his microscope
0:21:58 > 0:22:01as light passes through thin materials,
0:22:01 > 0:22:05like soap bubbles and Muscovy-glass,
0:22:05 > 0:22:09a silicate mineral that's made up of lots of thin layers.
0:22:09 > 0:22:13At the time, it was thought that white light, like sunlight, was pure,
0:22:13 > 0:22:15that it came directly from God,
0:22:15 > 0:22:18and so Hooke concluded that the colours he was seeing
0:22:18 > 0:22:21must have somehow been added to the light,
0:22:21 > 0:22:23that they were effectively created
0:22:23 > 0:22:27as the light passed through the materials.
0:22:29 > 0:22:32But Hooke's theory about coloured light
0:22:32 > 0:22:36was about to be challenged by his greatest rival.
0:22:41 > 0:22:46Isaac Newton is one of the world's most revered scientists...
0:22:48 > 0:22:53..best known for his theory of universal gravitation.
0:23:05 > 0:23:08And just like his laws of gravity,
0:23:08 > 0:23:11Newton's discoveries about the nature of light
0:23:11 > 0:23:15are among his most celebrated achievements.
0:23:16 > 0:23:21But the story of how that work began is much less familiar,
0:23:21 > 0:23:26And this time, there was no fruit involved.
0:23:26 > 0:23:28This is Stourbridge Common,
0:23:28 > 0:23:33a sleepy riverside meadow on the banks of the River Cam.
0:23:33 > 0:23:37But when Newton's visited in 1664, it would've been very different.
0:23:37 > 0:23:42For over 700 years, every September this place would be transformed
0:23:42 > 0:23:46into what was, at its height, the largest fair in Europe.
0:23:46 > 0:23:49For several weeks each year, people would descend on the common
0:23:49 > 0:23:54for an annual festival of commerce and debauchery.
0:23:54 > 0:23:57DOGS BARK & SHOUTING
0:23:57 > 0:24:00SWORDS CLASH & APPLAUSE
0:24:03 > 0:24:08This whole common would've been packed with make-shift stalls -
0:24:08 > 0:24:12farming produce, brandy houses, goldsmiths, silk merchants.
0:24:12 > 0:24:16There'd have been slack-rope dancing, puppet shows, music,
0:24:16 > 0:24:19temptations of every kind,
0:24:19 > 0:24:25packed into row upon row of wooden booths and tents.
0:24:32 > 0:24:37Stourbridge Fair was a place you could buy anything you could imagine,
0:24:37 > 0:24:41but when Newton came here, it's said he bought just one thing -
0:24:41 > 0:24:43a prism.
0:24:47 > 0:24:50He bought it because it performed the same magic
0:24:50 > 0:24:53Hooke had seen with his microscope.
0:25:04 > 0:25:07Newton would later write that, using his new purchase,
0:25:07 > 0:25:12he would "try the celebrated phenomena of colours"...
0:25:12 > 0:25:14..a rather understated introduction
0:25:14 > 0:25:20to work that would produce one of the most profound insights into the nature of light.
0:25:28 > 0:25:31Newton devised an ingenious experiment
0:25:31 > 0:25:36to discover precisely how these rainbow colours were produced
0:25:36 > 0:25:38and to put Hooke's theory -
0:25:38 > 0:25:43that they were created by the prism itself - to the test.
0:25:46 > 0:25:52This is Newton's own drawing of what he called his "Crucial Experiment".
0:25:52 > 0:25:56In it, he arranged a prism so that sunlight -
0:25:56 > 0:26:00coming in from a small hole he'd made in the shutters of his bedroom window -
0:26:00 > 0:26:04passed through it and projected coloured light onto a screen.
0:26:04 > 0:26:08Well, here's my light source
0:26:08 > 0:26:09and here's my prism
0:26:09 > 0:26:13which, if I arrange carefully,
0:26:13 > 0:26:18I can get projected onto the back pillar.
0:26:18 > 0:26:20Of course, none of this was new.
0:26:20 > 0:26:23People knew that prisms produced coloured light,
0:26:23 > 0:26:27but what Newton did next had never been done before.
0:26:27 > 0:26:33He first isolated one of the colours using a slit,
0:26:33 > 0:26:36so in this case,
0:26:36 > 0:26:38the orange light.
0:26:38 > 0:26:42He then passed that orange light through a second prism.
0:26:42 > 0:26:45Now, if Hooke was right,
0:26:45 > 0:26:48then this prism should add the other colours to the orange
0:26:48 > 0:26:52and reproduce the rainbow.
0:26:57 > 0:27:00But all Newton saw was orange light.
0:27:00 > 0:27:03The prism wasn't adding any extra colour.
0:27:03 > 0:27:08He concluded that the colours must be contained in the white light in the first place,
0:27:08 > 0:27:10that white light wasn't pure
0:27:10 > 0:27:12and prisms don't add anything to it.
0:27:12 > 0:27:18Instead, they split it up into its constituent parts.
0:27:23 > 0:27:27Newton named the colours that make up white light
0:27:27 > 0:27:29"the spectrum",
0:27:29 > 0:27:32and when this discovery was combined with the telescope
0:27:32 > 0:27:35it would show us something remarkable.
0:27:35 > 0:27:37The spectrum would reveal
0:27:37 > 0:27:42precisely what it was we were looking at out in space.
0:27:43 > 0:27:45This is a spectroscope.
0:27:45 > 0:27:49As sunlight comes in, it's broken up into its constituent colours
0:27:49 > 0:27:53and spread out much more finely than you'd get with a simple prism.
0:27:53 > 0:27:55Now, with this camera,
0:27:55 > 0:27:59I should be able to show you what I can see.
0:27:59 > 0:28:02I'll just check that it's working.
0:28:02 > 0:28:05Yes. OK.
0:28:07 > 0:28:11When scientists first did this in the middle of the 19th century...
0:28:11 > 0:28:14I'm placing the spectroscope on top.
0:28:14 > 0:28:18..they saw something completely unexpected.
0:28:19 > 0:28:23You can see the colours of the spectrum as Newton would've seen them,
0:28:23 > 0:28:26but if you look more closely, you can see something else.
0:28:26 > 0:28:28It's not continuous,
0:28:28 > 0:28:32it's broken up by lots of thin black lines.
0:28:32 > 0:28:34These are gaps in the spectrum.
0:28:34 > 0:28:37It was soon realised that these gaps
0:28:37 > 0:28:41were due to atoms in the outer atmosphere of the sun
0:28:41 > 0:28:45absorbing certain wavelengths of light coming from its interior,
0:28:45 > 0:28:47and that they could be used
0:28:47 > 0:28:51to work out the chemical composition of the sun.
0:29:01 > 0:29:06Every element absorbs a unique pattern of wavelengths -
0:29:06 > 0:29:09an optical fingerprint
0:29:09 > 0:29:11that can be used to determine the chemicals
0:29:11 > 0:29:16that make up any bright object you can see in the sky.
0:29:18 > 0:29:20And in Rome,
0:29:20 > 0:29:24one man was using this technique to study light
0:29:24 > 0:29:28whose origins lay far beyond the sun.
0:29:28 > 0:29:31Father Angelo Secchi was no ordinary priest.
0:29:31 > 0:29:36He was charismatic and viewed as something of a heretic by his fellow Jesuits.
0:29:36 > 0:29:40That's because he was also a professor of physics,
0:29:40 > 0:29:44with a evangelical passion for astronomy.
0:29:52 > 0:29:53In 1852,
0:29:53 > 0:29:58Secchi was appointed Director of the Vatican Observatory.
0:29:59 > 0:30:03Within a year, he'd built a new observatory
0:30:03 > 0:30:08on the roof of St Ignatius Church, in the heart of the city.
0:30:11 > 0:30:15At the time, most astronomers were interested in mapping the positions of the stars
0:30:15 > 0:30:18and charting their motions across the heavens.
0:30:18 > 0:30:20But Secchi was different.
0:30:20 > 0:30:23He wanted to know what they actually were.
0:30:23 > 0:30:27So from his vantage point, high above the streets of the Eternal City,
0:30:27 > 0:30:31he began to meticulously analyse their light.
0:30:43 > 0:30:46Fitting a spectroscope to the observatory's telescope,
0:30:46 > 0:30:50Father Secchi laboriously recorded the spectra
0:30:50 > 0:30:53of more than 4,000 stars.
0:31:00 > 0:31:03This is Secchi's book "Le Stelle", The Stars,
0:31:03 > 0:31:06which he published in 1877.
0:31:06 > 0:31:08And flicking through it,
0:31:08 > 0:31:12you can see many of the observations that he made.
0:31:12 > 0:31:14This one in particular is interesting.
0:31:14 > 0:31:16It shows some of the spectra he recorded.
0:31:16 > 0:31:19The top one here is from the sun,
0:31:19 > 0:31:22but the second one is starlight.
0:31:22 > 0:31:25It's from Sirius A, the Dog Star,
0:31:25 > 0:31:29which is the brightest star in the night sky.
0:31:36 > 0:31:38It's 8.6 light years from Earth
0:31:38 > 0:31:42and over 20 times as luminous as the sun.
0:31:44 > 0:31:48You can see from its spectrum this clear sequence of bands,
0:31:48 > 0:31:50which is the signature of hydrogen,
0:31:50 > 0:31:52because it's a relatively young star.
0:31:59 > 0:32:03The Universe's hottest, brightest stars
0:32:03 > 0:32:06have spectra rich in the two lightest elements -
0:32:06 > 0:32:09hydrogen and helium.
0:32:11 > 0:32:13But as they age, they cool,
0:32:13 > 0:32:18and their spectra reveal the presence of many heavier elements.
0:32:21 > 0:32:24This third one is from the star Betelgeuse,
0:32:24 > 0:32:27which is a red supergiant.
0:32:27 > 0:32:29It's near the end of its life
0:32:29 > 0:32:31and so you can see from the many bands here
0:32:31 > 0:32:35that it's composed of lots of different elements.
0:32:41 > 0:32:43What's remarkable about this image is that,
0:32:43 > 0:32:47I mean, it really is one of the key moments in the history of astronomy,
0:32:47 > 0:32:51that we can learn so much about what distant stars are made of
0:32:51 > 0:32:54just by examining their light.
0:33:06 > 0:33:13But because Secchi had catalogued the spectra of so many stars of different ages,
0:33:13 > 0:33:18his observations led to something even more profound -
0:33:18 > 0:33:21that by analysing starlight,
0:33:21 > 0:33:25we can determine the stars' life cycles...
0:33:27 > 0:33:30..when they were born...
0:33:32 > 0:33:35..and when they'll die.
0:33:41 > 0:33:44Understanding the spectrum
0:33:44 > 0:33:48had allowed us to read the story of the stars.
0:33:50 > 0:33:53It's quite incredible to think
0:33:53 > 0:33:57that what began as a simple experiment in a darkened room
0:33:57 > 0:34:02could reveal so much about the Universe,
0:34:02 > 0:34:06that the scant light from those tiny points in the night sky
0:34:06 > 0:34:12could contain within it the epic drama of the heavens.
0:34:21 > 0:34:25But that wasn't all the spectrum could tell us.
0:34:26 > 0:34:32We know that it's made up of light of many different wavelengths,
0:34:32 > 0:34:38and that those wavelengths extend way beyond the range we can see.
0:34:38 > 0:34:42The spectrum, from the longest wavelengths used in radio communications,
0:34:42 > 0:34:45to the very shortest wavelength, gamma rays,
0:34:45 > 0:34:49covers a range of 30 orders of magnitude.
0:34:49 > 0:34:52The longest are 1-followed-by-30-zeros
0:34:52 > 0:34:54bigger than the shortest.
0:34:54 > 0:34:57That's the same as a spread in range of weights
0:34:57 > 0:35:00from that of a single grain of sand
0:35:00 > 0:35:04to the weight of all the water in all the oceans on the planet.
0:35:04 > 0:35:06And within that vast spread,
0:35:06 > 0:35:09visible light - the frequencies we can see -
0:35:09 > 0:35:12covers a factor of just two.
0:35:12 > 0:35:14That's the same as the difference in weight
0:35:14 > 0:35:19between this pebble and one twice its size.
0:35:23 > 0:35:25- Are we all set, Doctor? - Yes, I think so.
0:35:28 > 0:35:31And throughout the 20th century,
0:35:31 > 0:35:34opening our eyes to the full spectrum
0:35:34 > 0:35:37revealed even more of the Universe.
0:35:37 > 0:35:42If you had infrared eyes, here's how the sky would look.
0:35:42 > 0:35:48Infrared allowed us to see the Universe's coolest stars,
0:35:48 > 0:35:50while radio telescopes,
0:35:50 > 0:35:52sensitive to the longest wavelengths,
0:35:52 > 0:35:56revealed a cosmos in turmoil...
0:35:56 > 0:35:59It's the violent events that are picked up,
0:35:59 > 0:36:02exploded stars and galaxies.
0:36:02 > 0:36:05..and satellites scoured the heavens
0:36:05 > 0:36:08for short-wavelength ultra violet.
0:36:08 > 0:36:12The OAO picks the ultra-violet light from hot stars,
0:36:12 > 0:36:15which the atmosphere cuts off from ground telescopes.
0:36:15 > 0:36:19And here's the very latest window - gamma rays -
0:36:19 > 0:36:23which are like very energetic x-rays.
0:36:23 > 0:36:25Seeing beyond the visible
0:36:25 > 0:36:29has allowed us to peer deep into the cosmos.
0:36:29 > 0:36:31I was cock-a-hoop about this.
0:36:31 > 0:36:35I, too, was wildly excited when I heard of this discovery.
0:36:42 > 0:36:46But the very fact that light had proved such a useful tool
0:36:46 > 0:36:48for exploring the Universe
0:36:48 > 0:36:53depended on one of its most mysterious properties.
0:36:53 > 0:36:55Light behaves like a wave,
0:36:55 > 0:36:58but if it is a wave, what is it a wave in?
0:36:58 > 0:37:02Waves are carried across the ocean by the water.
0:37:02 > 0:37:06The sound you can hear now is due to waves in the air.
0:37:06 > 0:37:07In the vacuum of space, there is no air
0:37:07 > 0:37:09so there is no sound.
0:37:09 > 0:37:13But the reason you can see me is because I'm lit by sunlight
0:37:13 > 0:37:17that has travelled 150 million kilometres
0:37:17 > 0:37:19through empty space.
0:37:19 > 0:37:20So, what is light,
0:37:20 > 0:37:24and how can you have a wave in nothing?
0:37:24 > 0:37:29Answering that question would not only reveal what light is,
0:37:29 > 0:37:31it would ultimately allow us to glimpse
0:37:31 > 0:37:34the beginning of the Universe.
0:37:47 > 0:37:49And the first part of the solution
0:37:49 > 0:37:53was a discovery that challenged our most basic assumptions
0:37:53 > 0:37:56about how we see the world.
0:37:58 > 0:38:01To our eyes, light appears to be everywhere,
0:38:01 > 0:38:04instantaneously.
0:38:04 > 0:38:07When I look out at the view, there seems to be no time lag,
0:38:07 > 0:38:11no delay, while I wait for the light to reach me.
0:38:11 > 0:38:13But towards the end of the 17th century,
0:38:13 > 0:38:17it was discovered that our senses are mistaken.
0:38:23 > 0:38:28In 1672, the Danish astronomer Ole Romer arrived in Paris
0:38:28 > 0:38:32to begin work at the city's observatory
0:38:32 > 0:38:37and to continue his observations of the moons of Jupiter.
0:38:47 > 0:38:48For more than a decade
0:38:48 > 0:38:51Giovanni Cassini, the observatory's director,
0:38:51 > 0:38:56had been documenting their orbits in minute detail.
0:39:17 > 0:39:20Jupiter's innermost moon Io
0:39:20 > 0:39:23is known to make a complete circuit around the gas giant
0:39:23 > 0:39:27once every 1.77 Earth days
0:39:27 > 0:39:30that's every 42.5 hours.
0:39:30 > 0:39:34Now, from Earth, we can see it disappear behind Jupiter
0:39:34 > 0:39:37and then re-emerge round the other side
0:39:37 > 0:39:39as it travels around in its orbit.
0:39:39 > 0:39:42But here in Paris in the 1660s,
0:39:42 > 0:39:44Giovanni Cassini had noticed
0:39:44 > 0:39:49that the timing of these eclipses seemed to vary,
0:39:49 > 0:39:53sometimes sooner, sometimes later than expected.
0:39:59 > 0:40:01Soon after he arrived in Paris,
0:40:01 > 0:40:06Romer noticed that these fluctuations weren't happening at random.
0:40:06 > 0:40:10When the Earth was closer to Jupiter,
0:40:10 > 0:40:14Io would be seen to disappear and re-emerge earlier.
0:40:14 > 0:40:16But as the year went by
0:40:16 > 0:40:19and the Earth moved in its orbit around the sun
0:40:19 > 0:40:21so that it was further away from Jupiter,
0:40:21 > 0:40:26then the eclipses appeared to happen later than expected.
0:40:30 > 0:40:35Romer knew the moon always took the same time to travel around Jupiter.
0:40:35 > 0:40:40His great insight was to realise that the variations were due to the fact
0:40:40 > 0:40:45that light itself takes time to travel through space.
0:40:49 > 0:40:50Here's how it works...
0:40:50 > 0:40:54The eclipses of Io appear later than expected
0:40:54 > 0:40:56when the Earth is further from Jupiter,
0:40:56 > 0:41:01because light takes a longer time to cover the greater distance,
0:41:01 > 0:41:05but they appear earlier when the Earth is closer
0:41:05 > 0:41:10because light needs less time to reach the Earth.
0:41:11 > 0:41:14Light isn't instantaneous.
0:41:14 > 0:41:18It travels at a finite speed.
0:41:25 > 0:41:30Today, we've not only measured light's speed with incredible accuracy,
0:41:30 > 0:41:33we've seen it in motion.
0:41:33 > 0:41:36This is a video made by scientists at MIT,
0:41:36 > 0:41:41using a camera designed to monitor extremely fast, chemical reactions.
0:41:41 > 0:41:45It has a shutter speed of around a picosecond.
0:41:45 > 0:41:48That's a millionth of a millionth of a second
0:41:48 > 0:41:53the time it takes light to travel just a third of a millimetre.
0:41:53 > 0:41:57Now, look what happens when I press play.
0:42:09 > 0:42:12What you can see here is a pulse of laser light
0:42:12 > 0:42:14moving through a water-filled bottle.
0:42:14 > 0:42:19To us, this would appear as the briefest of flashes,
0:42:19 > 0:42:23but the camera reveals how the pulse travels through the bottle,
0:42:23 > 0:42:28scattering and bouncing around as it hits the water molecules.
0:42:35 > 0:42:37Light travels so fast -
0:42:37 > 0:42:40300,000 kilometres per second -
0:42:40 > 0:42:43that slowed down by the same amount,
0:42:43 > 0:42:46a bullet would take an entire year
0:42:46 > 0:42:49to travel the length of the bottle.
0:42:51 > 0:42:54It's one thing to know that light travels at a finite speed,
0:42:54 > 0:42:59quite another to actually see it move.
0:43:02 > 0:43:07The discovery of the speed of light was hugely significant.
0:43:08 > 0:43:11Not least because it proved crucial
0:43:11 > 0:43:15to uncovering what light actually is.
0:43:22 > 0:43:24Born in the summer of 1831,
0:43:24 > 0:43:28James Clerk Maxwell would become one of the leading lights
0:43:28 > 0:43:30of 19th-century physics.
0:43:30 > 0:43:33GASPS & APPLAUSE
0:43:33 > 0:43:36His work on electricity and magnetism
0:43:36 > 0:43:40was one of the greatest achievements of the age.
0:43:43 > 0:43:46This is Glenlair in south-west Scotland,
0:43:46 > 0:43:49Maxwell's family home.
0:43:49 > 0:43:54While he was growing up here, he developed an insatiable curiosity about the world around him,
0:43:54 > 0:43:56a desire to understand nature
0:43:56 > 0:43:59that he would never lose.
0:44:10 > 0:44:14The young Maxwell seems to have taken great delight
0:44:14 > 0:44:17in tormenting his parents and his nanny
0:44:17 > 0:44:21by constantly asking them how things worked.
0:44:21 > 0:44:23"What's the go o'that?" he'd say.
0:44:23 > 0:44:25If anyone ventured an answer,
0:44:25 > 0:44:28the young Maxwell would only be satisfied for a moment
0:44:28 > 0:44:30before asking them how they knew.
0:44:36 > 0:44:40Of course, none of this is particularly unusual for a child,
0:44:40 > 0:44:42but what sets Maxwell apart
0:44:42 > 0:44:44is that he was just 14 years old
0:44:44 > 0:44:47when he wrote his first scientific paper.
0:44:47 > 0:44:49So young, that a friend of the family
0:44:49 > 0:44:54had to present it to the Royal Society of Edinburgh on his behalf.
0:45:03 > 0:45:06Maxwell was one of the greatest scientists who ever lived
0:45:06 > 0:45:11and it was here that he carried out his most important work.
0:45:12 > 0:45:14During the 1860s,
0:45:14 > 0:45:18Maxwell produced a virtuoso piece of mathematics
0:45:18 > 0:45:21that showed electricity and magnetism
0:45:21 > 0:45:25were different aspects of the same thing.
0:45:26 > 0:45:29But his calculations would show something else.
0:45:29 > 0:45:35Quite by accident, they would reveal the true nature of light.
0:45:35 > 0:45:37These are Maxwell's four famous equations
0:45:37 > 0:45:43that describe the relationship between electric and magnetic fields.
0:45:43 > 0:45:45Curl of E
0:45:45 > 0:45:49is minus DB by DT.
0:45:49 > 0:45:52E is the electric field, B is the magnetic field.
0:45:52 > 0:45:55Curl of B over mu nought,
0:45:55 > 0:45:58div of E equals zero,
0:45:58 > 0:46:02equals epsilon nought equals nought.
0:46:02 > 0:46:04With a bit of algebra and manipulation,
0:46:04 > 0:46:08these four equations can be combined to give one single equation.
0:46:08 > 0:46:11So the way it's done is like this...
0:46:11 > 0:46:13We take the curl of curl of E...
0:46:13 > 0:46:16Hidden deep within his mathematics
0:46:16 > 0:46:20was something that even Maxwell didn't expect.
0:46:20 > 0:46:23..epsilon nought... Grad E 2 div...
0:46:23 > 0:46:26This second term is zero
0:46:26 > 0:46:30and I'm left with Del squared of E...
0:46:30 > 0:46:34..minus mu nought, epsilon nought
0:46:34 > 0:46:36D 2 E...
0:46:36 > 0:46:39..by DT squared.
0:46:41 > 0:46:43This is the wave equation.
0:46:43 > 0:46:46It tells us how an electromagnetic field
0:46:46 > 0:46:48travels through space.
0:46:48 > 0:46:51Now, the important bit is this here -
0:46:51 > 0:46:53mu nought, epsilon nought -
0:46:53 > 0:46:57because it's related to the speed that the wave is travelling.
0:46:57 > 0:47:00In fact, the speed is given...
0:47:00 > 0:47:06..by one over the square root of mu nought epsilon nought.
0:47:06 > 0:47:08And if you work that out, you arrive at...
0:47:08 > 0:47:13..3 times 10 to the power 8 metres per second,
0:47:13 > 0:47:16or 300,000 kilometres per second -
0:47:16 > 0:47:19the speed of light.
0:47:20 > 0:47:24If electromagnetic waves moved at the speed of light,
0:47:24 > 0:47:27it could only mean one thing.
0:47:27 > 0:47:31Maxwell knew this had to be more than just a coincidence.
0:47:31 > 0:47:38It meant that light itself had to be an electromagnetic wave.
0:47:49 > 0:47:53The discovery that light is an electromagnetic wave
0:47:53 > 0:47:58explains one of its most puzzling properties.
0:47:58 > 0:48:00What Maxwell's equations show
0:48:00 > 0:48:05is that light consists of electric and magnetic waves travelling through space.
0:48:05 > 0:48:10So light is simply electric and magnetic vibrations
0:48:10 > 0:48:14feeding off one another as they move.
0:48:16 > 0:48:22And we now know that these electromagnetic waves have a remarkable property -
0:48:22 > 0:48:25they don't need to be waves in anything,
0:48:25 > 0:48:28they can travel through empty space.
0:48:44 > 0:48:49I remember first learning about this when I was in my second year at university.
0:48:49 > 0:48:54I was in lecture hall 33AC21 of the physics department at the University of Surrey,
0:48:54 > 0:48:56the lecturer was Dr Chivers,
0:48:56 > 0:48:59and I remember turning to my friend next to me
0:48:59 > 0:49:03and remarking on how incredible I thought this was.
0:49:03 > 0:49:07I could tell by his reaction that he thought I was a bit of a geek.
0:49:07 > 0:49:12But, actually, it is incredible that in just a few lines of algebra,
0:49:12 > 0:49:16you can tell what light really is.
0:49:18 > 0:49:22And the fact that light travels at a finite speed
0:49:22 > 0:49:26has enabled us to do something else.
0:49:26 > 0:49:30It allows us to look into the past.
0:49:32 > 0:49:34Looking at a mirror one metre away,
0:49:34 > 0:49:38you see yourself as you were six nanoseconds ago.
0:49:38 > 0:49:43From Earth, the moon appears as it was one second ago
0:49:43 > 0:49:46and the sun eight minutes in the past.
0:49:46 > 0:49:51The further you look out in space, the further you look back in time.
0:49:52 > 0:49:56Light from the cosmos's most distant objects
0:49:56 > 0:50:00has taken billions of years to reach the Earth.
0:50:00 > 0:50:04But there's one source that has taken us so far back in time,
0:50:04 > 0:50:09we've reached the very limit of what can be seen with light.
0:50:14 > 0:50:19In 1964, while converting a strange-looking horn antenna
0:50:19 > 0:50:22designed for early satellite communications
0:50:22 > 0:50:25to make astronomical observations...
0:50:27 > 0:50:30..Arno Penzias and Robert Wilson
0:50:30 > 0:50:35began to pick up a mysterious signal they couldn't explain.
0:50:44 > 0:50:48Here, we had purposely picked a portion of the spectrum,
0:50:48 > 0:50:49a wavelength of seven centimetres,
0:50:49 > 0:50:52where we expected nothing or almost nothing,
0:50:52 > 0:50:55no radiation at all from the sky.
0:50:59 > 0:51:02Instead, what happened is that we found radiation
0:51:02 > 0:51:05coming into our antenna from all directions.
0:51:05 > 0:51:09It's just flooding in at us and, um,
0:51:09 > 0:51:14clearly was orders of magnitude more than we expected from the galaxy.
0:51:19 > 0:51:22At first, they dismissed it as noise,
0:51:22 > 0:51:26something unwanted, generated by the antenna itself.
0:51:26 > 0:51:28Now, we had some suspicion
0:51:28 > 0:51:33because the throat of the antenna came into the cab and was a little bit warmer,
0:51:33 > 0:51:36and that was an attractive place for pigeons,
0:51:36 > 0:51:41at least a pair of pigeons who liked to stay there, especially in the cold winter.
0:51:41 > 0:51:44We didn't mind that because they flew away when we came,
0:51:44 > 0:51:48except that they had coated the surface with a white sticky material
0:51:48 > 0:51:52which might not only absorb radio waves but emit radio waves,
0:51:52 > 0:51:56which could be part or maybe all of our result.
0:52:02 > 0:52:04With the antenna cleaned, and the pigeons -
0:52:04 > 0:52:07well, it didn't end well for the pigeons -
0:52:07 > 0:52:11Penzias and Wilson began searching for an astronomical explanation.
0:52:11 > 0:52:15But the signal wasn't coming from anything in our own galaxy.
0:52:15 > 0:52:18Nor did it appear to be coming from any other galaxy either.
0:52:18 > 0:52:23It seemed to be coming from everywhere.
0:52:25 > 0:52:28No matter when we looked, day or night, winter or summer,
0:52:28 > 0:52:32this background of radiation appeared everywhere in the sky.
0:52:34 > 0:52:39It was not tied to our galaxy or any other known source of radio waves.
0:52:39 > 0:52:42It was rather as if the whole Universe had been warmed up
0:52:42 > 0:52:45to a temperature about three degrees above absolute zero.
0:52:50 > 0:52:54And so we were left with the astonishing result
0:52:54 > 0:52:57that this radiation was coming from somewhere
0:52:57 > 0:52:59in really deep cosmic space...
0:53:02 > 0:53:04..beyond any radio sources
0:53:04 > 0:53:09that any of us knew about or even dreamed existed.
0:53:21 > 0:53:25What they'd discovered was light so ancient,
0:53:25 > 0:53:27it had been stretched out into microwaves
0:53:27 > 0:53:33and cooled to just a few scant degrees above absolute zero,
0:53:33 > 0:53:34light that had been travelling to Earth
0:53:34 > 0:53:38for almost the entire age of the Universe.
0:53:38 > 0:53:40It hadn't come from a distant galaxy
0:53:40 > 0:53:43and it was far older than any star.
0:53:43 > 0:53:48Penzias and Wilson had discovered that the entire Universe was awash with light
0:53:48 > 0:53:52from the embers of the Big Bang itself.
0:54:05 > 0:54:09Called the Cosmic Microwave Background,
0:54:09 > 0:54:15it was released when the Universe was just 370,000 years old
0:54:15 > 0:54:20and it gives us a snapshot of the cosmos in its infancy.
0:54:24 > 0:54:26And here it is,
0:54:26 > 0:54:29the latest image of the Cosmic Microwave Background,
0:54:29 > 0:54:33taken by the Planck satellite and published in early 2013.
0:54:33 > 0:54:38The different colours are fluctuations in temperature in the early Universe
0:54:38 > 0:54:41and the information they contain
0:54:41 > 0:54:44has proved priceless to cosmologists.
0:54:45 > 0:54:51The tiny variations in temperature are caused by matter clumping together
0:54:51 > 0:54:56into what will eventually become stars and galaxies.
0:54:57 > 0:55:00But what's truly remarkable about this image
0:55:00 > 0:55:03is that it's not just light from the early Universe,
0:55:03 > 0:55:08it's the very first light there ever was.
0:55:31 > 0:55:34During the first era of its life,
0:55:34 > 0:55:39the Universe was a fireball of hot dense plasma
0:55:39 > 0:55:43that trapped light, preventing it from moving.
0:55:45 > 0:55:49Then, as the cosmos cooled, the plasma condensed,
0:55:49 > 0:55:53forming the first atoms...
0:55:58 > 0:56:00..and the first light,
0:56:00 > 0:56:04light that would become the Cosmic Microwave Background,
0:56:04 > 0:56:08was released into the Universe.
0:56:21 > 0:56:27It's sort of hard to express what an astonishing achievement this is,
0:56:27 > 0:56:31that from our small planet, orbiting an unremarkable star,
0:56:31 > 0:56:34we've reached out into the Universe
0:56:34 > 0:56:40and seen as far as it's possible to see with light.
0:56:55 > 0:56:58The discovery of the Cosmic Microwave Background
0:56:58 > 0:57:03appeared to complete our picture of the Universe,
0:57:03 > 0:57:06the final chapter in our use of light
0:57:06 > 0:57:09to explore the cosmos.
0:57:10 > 0:57:16Understanding the nature of light has allowed us to illuminate our world.
0:57:16 > 0:57:20We've captured it from the depths of space and the beginning of time.
0:57:20 > 0:57:22At the smallest scales, light has uncovered
0:57:22 > 0:57:25the microscopic structure of living things,
0:57:25 > 0:57:29and at the largest, it's shown us our place in the cosmos
0:57:29 > 0:57:32and told us the story of the stars.
0:57:32 > 0:57:35Virtually everything we know about the Universe,
0:57:35 > 0:57:39we know because it's been revealed by light.
0:57:45 > 0:57:48But just as it seemed light would lead us
0:57:48 > 0:57:52to a complete understanding of everything...
0:57:53 > 0:57:56..in the last 30 years,
0:57:56 > 0:57:59it's shown us something disturbing.
0:58:00 > 0:58:06The vast majority of the cosmos can't be seen at all.
0:58:11 > 0:58:14Far from being a Universe of light,
0:58:14 > 0:58:17much of it is hidden in the dark.
0:58:21 > 0:58:23Next time,
0:58:23 > 0:58:26how scientists came to the realisation
0:58:26 > 0:58:29that more than 99 percent of the Universe
0:58:29 > 0:58:33lies concealed in the shadows,
0:58:33 > 0:58:36and the extraordinary quest
0:58:36 > 0:58:40to uncover what's out there in the dark.
0:58:43 > 0:58:45Whether you want to step into the light
0:58:45 > 0:58:48or explore the mysteries of the dark,
0:58:48 > 0:58:51let the Open University inspire you. Go to...
0:58:54 > 0:58:57and follow links to The Open University.
0:58:57 > 0:59:00Subtitles by Red Bee Media Ltd