0:00:04 > 0:00:06Everything around us
0:00:06 > 0:00:12exists somewhere on a vast scale, from cold to hot.
0:00:13 > 0:00:18The tiniest insects, all of us, the Earth, the stars,
0:00:18 > 0:00:22even the universe itself - everything has a temperature.
0:00:25 > 0:00:27I'm Dr Helen Czerski.
0:00:27 > 0:00:31In this series, I'm going to unlock temperature's deepest mysteries.
0:00:35 > 0:00:37Across three programmes,
0:00:37 > 0:00:41I'm going to explore the extremes of the temperature scale,
0:00:41 > 0:00:44from some of the coldest temperatures
0:00:44 > 0:00:46to the very hottest,
0:00:46 > 0:00:49and everything in between.
0:00:49 > 0:00:52I'm a physicist, so my treasure map is woven
0:00:52 > 0:00:55from the fundamental physical laws of the universe,
0:00:55 > 0:00:58and temperature is an essential part of that.
0:00:59 > 0:01:03It's the hidden energy contained within matter,
0:01:03 > 0:01:08and the way that energy endlessly shifts and flows
0:01:08 > 0:01:11is the architect that has shaped our planet...
0:01:13 > 0:01:15..and the universe.
0:01:16 > 0:01:19It's not often that I get up at 5am to watch a pond,
0:01:19 > 0:01:21but this one's worth watching.
0:01:25 > 0:01:27In this first programme,
0:01:27 > 0:01:31I'm going to venture to the bottom of the temperature scale.
0:01:31 > 0:01:36I'll explore how cold has fashioned the world around us
0:01:36 > 0:01:39and why frozen doesn't mean what you might think.
0:01:39 > 0:01:40The salt looks like that here.
0:01:40 > 0:01:43It would look like that if I took it into a sauna
0:01:43 > 0:01:45because it's a frozen solid.
0:01:47 > 0:01:50And I'll descend to the very limits of cold
0:01:50 > 0:01:53where the everyday laws of physics break down
0:01:53 > 0:01:58and a new world of scientific possibility begins.
0:01:58 > 0:02:02Temperature is in every single story that nature has to tell,
0:02:02 > 0:02:04and, in this series, I'll be exploring why,
0:02:04 > 0:02:06what temperature means, how it works,
0:02:06 > 0:02:11and just how deep its influence on our lives and our world really is.
0:02:45 > 0:02:47This is Eldhraun, in the south of Iceland,
0:02:47 > 0:02:50and it's a great place to start the story of temperature
0:02:50 > 0:02:52because this weird landscape around me -
0:02:52 > 0:02:54all these lumps and bumps -
0:02:54 > 0:02:58this was sculpted by the interplay between hot and cold.
0:03:00 > 0:03:03Just over 230 years ago,
0:03:03 > 0:03:08a huge fissure in the ground opened up over there - 25km long -
0:03:08 > 0:03:11and a huge amount of lava flooded out
0:03:11 > 0:03:14in an event that lasted eight months.
0:03:15 > 0:03:18It's thought that over 500 square kilometres
0:03:18 > 0:03:21was covered in molten red rock.
0:03:24 > 0:03:27When the lava came out of the ground,
0:03:27 > 0:03:30it was at about 850 degrees Celsius,
0:03:30 > 0:03:34but it met cool air, and heat flowed from hot to cold
0:03:34 > 0:03:36because that's the way our universe works.
0:03:36 > 0:03:41And, as the lava cooled, it froze, and this landscape is what you get
0:03:41 > 0:03:45when the hot innards of the Earth meet cold
0:03:45 > 0:03:48and are fixed in a form that will last for millennia.
0:03:50 > 0:03:55But the mysterious ability of cold to create solid matter
0:03:55 > 0:03:58is something we've only recently uncovered.
0:03:58 > 0:04:01DOGS BARK
0:04:04 > 0:04:09We've always been familiar with the experience of cold and heat,
0:04:09 > 0:04:13but, until recently, we didn't understand what they actually were,
0:04:13 > 0:04:16and, as the era of modern science dawned,
0:04:16 > 0:04:20that lack of knowledge was becoming a barrier to progress.
0:04:24 > 0:04:27I'm here at the Radcliffe Observatory in Oxford
0:04:27 > 0:04:30and what it was built to observe is the cosmos.
0:04:30 > 0:04:32Back in the 18th century,
0:04:32 > 0:04:34this was one of the most foremost centres
0:04:34 > 0:04:36of the new science of astronomy.
0:04:36 > 0:04:38But, while looking up there,
0:04:38 > 0:04:42they discovered they had a problem that started down here.
0:04:48 > 0:04:53I'm meeting Amy Creese, who's a meteorological observer.
0:04:53 > 0:04:57It's a role that was created here over 200 years ago
0:04:57 > 0:05:01to solve a very specific problem caused by temperature.
0:05:02 > 0:05:05Early observers made quite meticulous records
0:05:05 > 0:05:06of the temperature
0:05:06 > 0:05:09and that was because it was important to know
0:05:09 > 0:05:12what the temperature was like in order to correct
0:05:12 > 0:05:14for something called atmospheric refraction,
0:05:14 > 0:05:18which is how much the light from a celestial object bends
0:05:18 > 0:05:20as it comes into the Earth's atmosphere,
0:05:20 > 0:05:22and that depends quite a lot on temperature.
0:05:22 > 0:05:25So, in order to make very accurate measurements
0:05:25 > 0:05:28of positions of stars, the observers found
0:05:28 > 0:05:30that they needed to measure temperature, as well,
0:05:30 > 0:05:32so they kept very good records of that.
0:05:32 > 0:05:35So, even those people who were looking up at the cosmos
0:05:35 > 0:05:37and thinking grand thoughts about the universe
0:05:37 > 0:05:39needed to know about this quite mundane thing down here,
0:05:39 > 0:05:41which was the temperature.
0:05:41 > 0:05:44And you've got a book there with some of the early recordings on it.
0:05:44 > 0:05:47I do. I have a book here from 1776.
0:05:47 > 0:05:50It's some of the original recordings from Thomas Hornsby,
0:05:50 > 0:05:53who founded this observatory.
0:05:53 > 0:05:56And several times a day - he was much more keen than I am -
0:05:56 > 0:05:59he came up here and took measurements
0:05:59 > 0:06:01of pressure and temperature,
0:06:01 > 0:06:03but he also made some quite funny notes in the margins.
0:06:03 > 0:06:07For example, on the 26th of January 1776,
0:06:07 > 0:06:10he wrote about how the wine in his study had started to freeze
0:06:10 > 0:06:12because it had got very cold that day.
0:06:12 > 0:06:14Which is a very important thing for a scientist to know about.
0:06:14 > 0:06:17Yeah, and I'm glad that he wrote about it!
0:06:18 > 0:06:19These are some of the earliest
0:06:19 > 0:06:23regular measurements of temperature ever made,
0:06:23 > 0:06:25and they were only possible thanks to one of the greatest
0:06:25 > 0:06:29scientific innovations of the 18th century -
0:06:29 > 0:06:31the modern thermometer.
0:06:32 > 0:06:35The first thermometers were simple tubes filled with liquid.
0:06:35 > 0:06:38If you put them in something warm, the liquid level would go up,
0:06:38 > 0:06:39and if you put them in something cold,
0:06:39 > 0:06:41the liquid level would go down.
0:06:41 > 0:06:43That's not much use if you're trying to establish
0:06:43 > 0:06:46a universal temperature scale that everyone can agree on.
0:06:48 > 0:06:52Every inventor had their own idea of what that scale should be
0:06:52 > 0:06:56and so no two thermometers were alike.
0:06:56 > 0:06:59The solution that was arrived at was really clever.
0:06:59 > 0:07:03It was to say that perhaps we can find fixed points.
0:07:03 > 0:07:04So, perhaps there are situations
0:07:04 > 0:07:08which are absolutely always the same temperature
0:07:08 > 0:07:10and then everyone can agree on those points on the scale,
0:07:10 > 0:07:13and then we can all calibrate our instruments.
0:07:13 > 0:07:16The choices that stuck were those made by Daniel Fahrenheit,
0:07:16 > 0:07:18who was a Polish physicist,
0:07:18 > 0:07:22and he chose three fixed points that everyone else then followed.
0:07:22 > 0:07:26So, the first one of his fixed points was this mixture here -
0:07:26 > 0:07:30ammonium chloride and liquid water and water ice.
0:07:30 > 0:07:32And that is a very interesting type of mixture
0:07:32 > 0:07:35because when you mix those three things together,
0:07:35 > 0:07:40they will find an equilibrium at a very specific temperature,
0:07:40 > 0:07:42and Fahrenheit choice that as his starting point.
0:07:42 > 0:07:46So, this is at zero degrees Fahrenheit.
0:07:46 > 0:07:51Fahrenheit's second fixed point was a mixture of water and ice,
0:07:51 > 0:07:54which will always settle at the same temperature -
0:07:54 > 0:07:5632 degrees Fahrenheit,
0:07:56 > 0:08:01more familiar to us these days as zero degrees Celsius.
0:08:01 > 0:08:03And then there was one more fixed point
0:08:03 > 0:08:06and Fahrenheit choice the temperature of the human body.
0:08:06 > 0:08:09So, if you put a thermometer under your arm or under your tongue,
0:08:09 > 0:08:13Fahrenheit said that was 96 on his scale,
0:08:13 > 0:08:16and that was the beginning of the Fahrenheit scale.
0:08:16 > 0:08:18All of those scientists and engineers
0:08:18 > 0:08:21could calibrate their thermometers using those same three points.
0:08:21 > 0:08:23They could divide up the temperature scale
0:08:23 > 0:08:25in exactly the same way,
0:08:25 > 0:08:29and, finally, the real science of temperature could begin.
0:08:32 > 0:08:36The thermometer opened up a whole world of possibilities
0:08:36 > 0:08:41for astronomy, meteorology, and, of course, medicine.
0:08:41 > 0:08:44But it also brought with it a paradox.
0:08:44 > 0:08:48While we now had a standard scale to record temperature,
0:08:48 > 0:08:51we still didn't have any scientific explanation
0:08:51 > 0:08:56of what temperature really was, of what made things hot or cold.
0:08:58 > 0:09:00Some of the earliest scientific theories propose
0:09:00 > 0:09:03that temperature was a physical substance.
0:09:10 > 0:09:14One idea was that heat was a weightless liquid called caloric
0:09:14 > 0:09:16that warmed things up.
0:09:16 > 0:09:18Another theory suggested that
0:09:18 > 0:09:22cold consisted of frigorific particles.
0:09:25 > 0:09:29These ideas persisted until the late 18th century
0:09:29 > 0:09:33when they were thrown into doubt by a discovery about heat
0:09:33 > 0:09:37that would ultimately transform our understanding of cold.
0:09:37 > 0:09:40In the 1790s, an American-born inventor
0:09:40 > 0:09:42working in Germany called Count Rumford
0:09:42 > 0:09:44applied his mind to the study of heat,
0:09:44 > 0:09:47and this is the report that he wrote on his work.
0:09:47 > 0:09:50And I love this document because it's written in a very human way.
0:09:51 > 0:09:54Count Rumford was overseeing the manufacture of cannons
0:09:54 > 0:09:56by German artillerymen
0:09:56 > 0:09:58when he noticed something very curious
0:09:58 > 0:10:01as they bored holes into the cold metal.
0:10:03 > 0:10:05To show you, I've got a battery-powered drill
0:10:05 > 0:10:08and an infrared camera that will reveal
0:10:08 > 0:10:11what happens to the temperature of the metal as I drill through it.
0:10:11 > 0:10:14And I'm just going to drill through this piece of metal here.
0:10:16 > 0:10:18And have a look on the infrared camera.
0:10:18 > 0:10:21You can see the spot around where I was drilling has warmed up
0:10:21 > 0:10:23and I can feel the heat with my finger.
0:10:23 > 0:10:25So, even a simple drilling experiment like this
0:10:25 > 0:10:27can generate heat.
0:10:28 > 0:10:32And this was exactly what Count Rumford observed
0:10:32 > 0:10:35as he watched the cannon-makers at work.
0:10:35 > 0:10:39As they bored through the metal, the cold iron got hotter.
0:10:41 > 0:10:43The other important thing that Count Rumford noticed
0:10:43 > 0:10:46was that the heat didn't run out.
0:10:46 > 0:10:49You could keep drilling and the metal just got hotter.
0:10:50 > 0:10:53And, in his opinion, that put a very big dent
0:10:53 > 0:10:54in the theory of caloric
0:10:54 > 0:10:58because if heat is a fluid flowing from a hot place to a cold place,
0:10:58 > 0:11:00at some point, that fluid is going to run out.
0:11:02 > 0:11:06Rumford had discovered something fundamental about temperature,
0:11:06 > 0:11:09of what makes matter hot or cold,
0:11:09 > 0:11:11yet it would be nearly a century
0:11:11 > 0:11:14before it was fully recognised and explained.
0:11:23 > 0:11:27And the first step towards an explanation would come
0:11:27 > 0:11:30from a completely different branch of science altogether.
0:11:36 > 0:11:39In 1827, Scottish botanist Robert Brown
0:11:39 > 0:11:42was deep into his research on flowering plants.
0:11:42 > 0:11:45It was an exciting time in biology because of the new realisation
0:11:45 > 0:11:48that inside the very tiny plant cell,
0:11:48 > 0:11:52there was even tinier machinery making everything work.
0:11:55 > 0:11:58Brown was particularly interested in pollen...
0:11:59 > 0:12:02..so he took pollen grains back to his laboratory,
0:12:02 > 0:12:04suspended them in drops of water
0:12:04 > 0:12:06and looked at them under his microscope.
0:12:08 > 0:12:12And what he saw was the pollen grains sitting in the water,
0:12:12 > 0:12:16but, from them, there were emerging even smaller particles,
0:12:16 > 0:12:18and when he watched those particles,
0:12:18 > 0:12:20they were moving, they were jiggling about.
0:12:20 > 0:12:24So, the first thing that Brown did was check whether they were alive,
0:12:24 > 0:12:27but they weren't, and he tried with lots of different materials,
0:12:27 > 0:12:30and what he saw was that every time there was a particle that small,
0:12:30 > 0:12:33just on the edge of what the microscope could see,
0:12:33 > 0:12:35it would always be just jiggling about,
0:12:35 > 0:12:40whatever it was made of, and he had no idea why that was.
0:12:40 > 0:12:43The answer didn't come until nearly 100 years later
0:12:43 > 0:12:47in a paper written in 1905 by Albert Einstein,
0:12:47 > 0:12:49and it's a really elegant paper.
0:12:51 > 0:12:55Einstein's paper drew together two crucial ideas.
0:13:00 > 0:13:03First, that all matter was made of atoms,
0:13:03 > 0:13:08and, second, that these atoms were constantly moving about.
0:13:09 > 0:13:14This finally solved the mystery of Robert Brown's jiggling particles.
0:13:14 > 0:13:16They were being bombarded by billions
0:13:16 > 0:13:20of smaller, invisible atoms,
0:13:20 > 0:13:25and Einstein's explanation depended on one fundamental point -
0:13:25 > 0:13:28that the movement of atoms was directly linked
0:13:28 > 0:13:30to their temperature.
0:13:30 > 0:13:33The physical existence of our universe
0:13:33 > 0:13:37is all about the relationship between matter and energy,
0:13:37 > 0:13:41and this paper was where that story really started.
0:13:41 > 0:13:45Einstein understood that heat is just the energy that atoms have
0:13:45 > 0:13:47due to their movement,
0:13:47 > 0:13:51and the measure of that movement energy is temperature.
0:13:53 > 0:13:56The more energy, the faster the movement
0:13:56 > 0:13:58and the higher the temperature.
0:13:58 > 0:14:01More than a century after Rumford had puzzled over
0:14:01 > 0:14:06what was heating up his cannons, Einstein had explained it.
0:14:06 > 0:14:09The very act of boring through the metal
0:14:09 > 0:14:12was adding energy to the atoms,
0:14:12 > 0:14:16increasing their movement and, so, making the metal hotter.
0:14:17 > 0:14:21This definition of heat also means something profound
0:14:21 > 0:14:24for our understanding of cold
0:14:24 > 0:14:26because if heat is the measure of energy,
0:14:26 > 0:14:28of the movement of atoms,
0:14:28 > 0:14:31then cold is simply an absence of energy,
0:14:31 > 0:14:32a lack of motion.
0:14:42 > 0:14:44And this is vital to understanding
0:14:44 > 0:14:49how every single solid thing in our universe came into being.
0:15:01 > 0:15:04To show you why, I'm back in Iceland -
0:15:04 > 0:15:07the perfect place to explore the relationship
0:15:07 > 0:15:09between cold and matter.
0:15:11 > 0:15:14This is Breidamerkurjokull Glacier.
0:15:14 > 0:15:19Here, matter exists side by side in three very different forms.
0:15:26 > 0:15:28This is made of water molecules,
0:15:28 > 0:15:31there's water molecules dripping off the roof here,
0:15:31 > 0:15:35and the air I'm breathing out also contains some water molecules.
0:15:35 > 0:15:39Billions upon billions of the same type of molecule
0:15:39 > 0:15:41all in the same place,
0:15:41 > 0:15:44but behaving in three different ways -
0:15:44 > 0:15:47as a solid, a liquid, and a gas.
0:15:50 > 0:15:55Each of these three states is a consequence of temperature,
0:15:55 > 0:15:58of how fast the molecules of water are moving...
0:15:59 > 0:16:01..and when the water reaches its freezing point
0:16:01 > 0:16:04and changes from a liquid to a solid,
0:16:04 > 0:16:06something extraordinary is happening
0:16:06 > 0:16:09in the hidden world of its molecules,
0:16:09 > 0:16:14something we can't see by looking at ice at this massive scale.
0:16:14 > 0:16:18To understand it, we need to look at something very much smaller,
0:16:18 > 0:16:23something that's also frozen, even if it might not look like it.
0:16:23 > 0:16:26This is table salt - sodium chloride.
0:16:26 > 0:16:28About as common as you can get.
0:16:28 > 0:16:33And, even here, you can see that the salt's a little bit sparkly.
0:16:33 > 0:16:38If I put it under the microscope, now you can see what's going on.
0:16:38 > 0:16:43Those tiny little grains of salt here have flat faces -
0:16:43 > 0:16:47they're little cubes - and every single grain is the same.
0:16:47 > 0:16:50Not a perfect cube, but they've all got a cubic shape,
0:16:50 > 0:16:52and it's those flat faces that are reflecting the light
0:16:52 > 0:16:54and making the salt sparkle.
0:16:54 > 0:16:57And that's an indication of something deeper down
0:16:57 > 0:16:59in the structure of the salt.
0:17:03 > 0:17:08Salt is made of equal numbers of sodium and chloride ions.
0:17:08 > 0:17:11The chloride ions are assembled in rows and columns,
0:17:11 > 0:17:14so that they sit on a square grid.
0:17:14 > 0:17:19The smaller sodium ions fit into the spaces in between.
0:17:19 > 0:17:22A salt crystal is just a giant grid, like this -
0:17:22 > 0:17:27a cube that's a million or so atoms long on each side.
0:17:27 > 0:17:31This is the hidden structure of a crystal.
0:17:31 > 0:17:35Its atoms are no longer free to move around each other.
0:17:35 > 0:17:38Each one is locked in its own place on the grid.
0:17:40 > 0:17:42So, the salt looks like that here.
0:17:42 > 0:17:46It would look like that if I took it into a sauna because it's frozen.
0:17:46 > 0:17:47It's a frozen solid.
0:17:49 > 0:17:53Freezing is simply what happens when the molecules of a substance
0:17:53 > 0:17:57no longer have enough energy to move past each other,
0:17:57 > 0:18:00and so they become fixed in position.
0:18:00 > 0:18:02And this doesn't always happen at a temperature
0:18:02 > 0:18:04that we would consider cold.
0:18:04 > 0:18:09For salt, it happens at about 800 degrees Celsius.
0:18:11 > 0:18:13We associate freezing with water ice,
0:18:13 > 0:18:17but that's just because water is important to us.
0:18:17 > 0:18:19The concept of freezing is far bigger,
0:18:19 > 0:18:21and the transition from liquid to solid
0:18:21 > 0:18:24can happen at a huge range of temperatures,
0:18:24 > 0:18:26depending on the substance.
0:18:28 > 0:18:31Liquid iron freezes to become a solid metal
0:18:31 > 0:18:34at around 1,500 degrees Celsius.
0:18:35 > 0:18:38Liquid tungsten turns into a solid
0:18:38 > 0:18:42at nearly 3,500 degrees Celsius.
0:18:44 > 0:18:46It's exactly the same process
0:18:46 > 0:18:50that transforms liquid water into solid ice
0:18:50 > 0:18:52at zero degrees Celsius.
0:18:54 > 0:18:57As with other liquids, the molecules in liquid water
0:18:57 > 0:19:00have enough energy to keep moving past each other.
0:19:00 > 0:19:04But, as they cool, the molecules slow down.
0:19:04 > 0:19:06As water reaches its freezing point,
0:19:06 > 0:19:10they arrange themselves in tightly fixed positions,
0:19:10 > 0:19:14forming a hexagonal lattice - a crystalline structure.
0:19:23 > 0:19:26The beautiful symmetry of snowflakes comes, in part,
0:19:26 > 0:19:29from this microscopic hexagonal form.
0:19:33 > 0:19:39Here, deep in this cave of ice, it exists on a massive scale,
0:19:39 > 0:19:42and, in fact, the very process of cooling and freezing is key
0:19:42 > 0:19:45to how the entire planet formed.
0:19:52 > 0:19:54Some 4 billion years ago,
0:19:54 > 0:19:56the Earth was covered in molten rock.
0:19:58 > 0:20:01As we've seen in the striking landscapes of Iceland,
0:20:01 > 0:20:06that lava eventually cooled and froze into solid rock,
0:20:06 > 0:20:09and, sometimes, the way it cooled
0:20:09 > 0:20:12created something truly extraordinary.
0:20:13 > 0:20:17The hexagonal columns of basalt at Reynisfjara
0:20:17 > 0:20:20are one of Earth's natural wonders
0:20:20 > 0:20:22and Professor Thor Thordarson,
0:20:22 > 0:20:25a volcanologist from the University of Iceland,
0:20:25 > 0:20:28is going to help me understand how they formed.
0:20:30 > 0:20:32There's lots of basalt in the world,
0:20:32 > 0:20:36but not all of it has this amazing structure here.
0:20:36 > 0:20:38So, here, we have these beautiful, regular columns,
0:20:38 > 0:20:41and these extend 10m, 15m up into the cliff edge.
0:20:41 > 0:20:44Columns like this are fairly unusual.
0:20:50 > 0:20:52These columns tell a story
0:20:52 > 0:20:56of how the intricacies of cooling and freezing
0:20:56 > 0:20:58have shaped the fabric of our planet.
0:21:01 > 0:21:05So, this column here, which is about 80cm in width here,
0:21:05 > 0:21:08this width is actually the function of the cooling.
0:21:09 > 0:21:14So, if you think of a lava flow, it starts cooling from the surface,
0:21:14 > 0:21:19and it also cools fastest when it is close...
0:21:19 > 0:21:21in contact with the atmosphere.
0:21:24 > 0:21:29As the lava cools and freezes, it also shrinks,
0:21:29 > 0:21:32as its molecules arrange themselves into a solid structure.
0:21:34 > 0:21:36This happens more quickly at the surface,
0:21:36 > 0:21:38where the lava meets the air,
0:21:38 > 0:21:42and more slowly underneath, where it stays warmer.
0:21:42 > 0:21:45And if the rate of shrinking is great enough,
0:21:45 > 0:21:50the cooling lava at the surface is under so much stress that it cracks,
0:21:50 > 0:21:52and often the most efficient way
0:21:52 > 0:21:54to dissipate this huge build-up of stress
0:21:54 > 0:21:58is to crack at an angle of 120 degrees -
0:21:58 > 0:22:00the angle that gives us a hexagon.
0:22:01 > 0:22:05As the rock beneath the surface also continues to cool,
0:22:05 > 0:22:07these cracks extend downwards,
0:22:07 > 0:22:11creating the colossal pillars we see today.
0:22:12 > 0:22:15Can you tell from the size of these how quickly these cooled?
0:22:15 > 0:22:18I mean, did these take a day to form, or a week, or a year?
0:22:18 > 0:22:20Can you tell?
0:22:20 > 0:22:23Not exactly, but I would guess between ten and 20 years.
0:22:27 > 0:22:31This landscape was formed because lava began to cool and freeze
0:22:31 > 0:22:37at just the right speed for the laws of physics to create a masterpiece.
0:22:37 > 0:22:41A little faster or slower, and these columns wouldn't exist.
0:22:43 > 0:22:46They stand as evidence that solid rock,
0:22:46 > 0:22:50the fabric of our world, is frozen,
0:22:50 > 0:22:54and the architect that sculpted it is temperature.
0:23:11 > 0:23:15And as we humans have built architectural wonders of our own,
0:23:15 > 0:23:19so we've learned to harness the potential of cooling,
0:23:19 > 0:23:22to change the very nature of matter.
0:23:27 > 0:23:30This is Ely Cathedral.
0:23:30 > 0:23:32It's been here for nearly 1,000 years,
0:23:32 > 0:23:34and, over the centuries,
0:23:34 > 0:23:37countless craftsmen have taken local raw materials,
0:23:37 > 0:23:39limestone and oak,
0:23:39 > 0:23:43and transformed them into this vast and intricate structure.
0:23:48 > 0:23:51But I'm not here because of those materials.
0:23:51 > 0:23:54I'm here to see something else.
0:23:54 > 0:23:57The stained glass windows here are breathtaking,
0:23:57 > 0:24:01and they only exist thanks to the unique properties of glass
0:24:01 > 0:24:03that emerge as it cools.
0:24:08 > 0:24:11It's only when you're right in close like this
0:24:11 > 0:24:14that you can really appreciate these fabulous windows.
0:24:14 > 0:24:15Each one of these panels
0:24:15 > 0:24:19is illuminating the cathedral with a story.
0:24:19 > 0:24:21But the story that you can see from down there
0:24:21 > 0:24:24is built of a thousand smaller stories
0:24:24 > 0:24:25that you can only see up here
0:24:25 > 0:24:29because every single one of these pieces of glass
0:24:29 > 0:24:32is carrying its own distinctive history
0:24:32 > 0:24:36of how cooling shaped it and locked in its properties.
0:24:45 > 0:24:47To understand why,
0:24:47 > 0:24:52I've come to meet someone who works with glass day in, day out.
0:24:52 > 0:24:54This is Walter Pinches,
0:24:54 > 0:24:57a glass-maker carrying on a tradition
0:24:57 > 0:25:00that's changed little in 800 years.
0:25:02 > 0:25:05- How hot is it in there? - 1,250-1,300.
0:25:05 > 0:25:061,300 degrees C?
0:25:08 > 0:25:10It's only 2m away!
0:25:10 > 0:25:13SHE LAUGHS
0:25:13 > 0:25:16Standing next to the fiery glow of the furnace,
0:25:16 > 0:25:20it's easy to think that the key to glass-making is heat.
0:25:20 > 0:25:22But the real key to this process is what happens
0:25:22 > 0:25:25when the glass comes out of the furnace and begins to cool.
0:25:27 > 0:25:30And the colour's just mixing into the liquid as you go along.
0:25:30 > 0:25:33The colour's already twisted in. You've already got your pattern.
0:25:34 > 0:25:37Cooling is a process that craftsmen like Walter
0:25:37 > 0:25:40learn to control precisely.
0:25:40 > 0:25:44When the hot glass first emerges, it's molten,
0:25:44 > 0:25:45so, like all liquids,
0:25:45 > 0:25:50its molecules are still free to move and slide over each other,
0:25:50 > 0:25:56and this gives Walter a brief window of time to manipulate its shape.
0:25:56 > 0:26:00But, with every passing second, the glass is cooling,
0:26:00 > 0:26:03especially at the surface, where it's in contact with the air.
0:26:04 > 0:26:07What's amazing about this is that the inside and the outside
0:26:07 > 0:26:10are different temperatures, and right in at a molecular level,
0:26:10 > 0:26:12everything in there is different.
0:26:12 > 0:26:16Everywhere is behaving differently because of its temperature.
0:26:18 > 0:26:22Starting at the surface, the glass begins to freeze.
0:26:22 > 0:26:26Its atoms slow down and come to rest in fixed positions,
0:26:26 > 0:26:30and they do so in a way that's unlike many other solids.
0:26:30 > 0:26:34This is my favourite bit - when it just blows up like a balloon.
0:26:36 > 0:26:39As we've seen, when other substances freeze,
0:26:39 > 0:26:42like water or salt, their atoms become fixed
0:26:42 > 0:26:45in the ordered structure of a crystal.
0:26:45 > 0:26:48But glass is different.
0:26:48 > 0:26:51It cools more quickly and so its atoms don't have time
0:26:51 > 0:26:54to arrange themselves in a regular pattern.
0:26:55 > 0:26:58Instead, they freeze in the disordered,
0:26:58 > 0:27:01chaotic arrangement of a liquid,
0:27:01 > 0:27:06and this gives glass one of its most valuable properties.
0:27:06 > 0:27:10Unconstrained by a rigid, crystalline structure,
0:27:10 > 0:27:15it can be worked and manipulated into an infinite number of forms.
0:27:20 > 0:27:22This is the clever bit -
0:27:22 > 0:27:24hot molecules at the bottom flowing quickly,
0:27:24 > 0:27:27cooler ones at the top flowing more slowly.
0:27:34 > 0:27:38By precisely controlling the heating and cooling of glass,
0:27:38 > 0:27:42craftsmen like Walter can create shapes and forms
0:27:42 > 0:27:44that are truly unique.
0:27:47 > 0:27:50Liquids are at their most beautiful when they're flowing freely,
0:27:50 > 0:27:52but they change so quickly
0:27:52 > 0:27:55that we almost never get to appreciate the details.
0:27:55 > 0:27:58But glass-blowing is this fabulous process
0:27:58 > 0:28:00of sculpting a moment in time
0:28:00 > 0:28:04and then catching it by cooling it for us all to admire.
0:28:12 > 0:28:15The modern world is built of solids like glass
0:28:15 > 0:28:19that we created by controlling the process of cooling and freezing.
0:28:32 > 0:28:37But that change from liquid to solid isn't the end of the story.
0:28:40 > 0:28:44As a solid becomes colder, it may outwardly look the same,
0:28:44 > 0:28:48but, in the hidden world of atoms and molecules,
0:28:48 > 0:28:50it can still be changing
0:28:50 > 0:28:55in ways that utterly transform how it behaves.
0:28:55 > 0:28:59And, occasionally, when we fail to understand these changes,
0:28:59 > 0:29:04our pursuit of progress has ended in catastrophe.
0:29:04 > 0:29:08Some events in history are so unexpected, so shocking
0:29:08 > 0:29:11that the mentality of an entire society is divided
0:29:11 > 0:29:14into before and after.
0:29:14 > 0:29:16And, for our nation's maritime history,
0:29:16 > 0:29:20that cusp came on the 15th of April 1912,
0:29:20 > 0:29:24when news filtered out from London and New York
0:29:24 > 0:29:29that the gigantic Titanic, that unsinkable symbol of luxury,
0:29:29 > 0:29:31had struck an iceberg and had sunk.
0:29:32 > 0:29:36There were 2,200 people on that ship,
0:29:36 > 0:29:38and 70% of them died that day.
0:29:46 > 0:29:50Titanic was built from state-of-the-art steel.
0:29:50 > 0:29:53As with glass, we'd learned over centuries
0:29:53 > 0:29:56to make steel incredibly strong
0:29:56 > 0:30:00through precisely honed processes of heating and cooling.
0:30:00 > 0:30:03Nobody doubted she was strong enough
0:30:03 > 0:30:07to stand up to the extreme cold of the Arctic.
0:30:08 > 0:30:10To understand what went wrong,
0:30:10 > 0:30:14I've come to the Cammell Laird shipyard in Merseyside
0:30:14 > 0:30:18where marine engineers are working on their latest project.
0:30:21 > 0:30:23I've been on a lot of ships,
0:30:23 > 0:30:25but I haven't ever been quite this excited
0:30:25 > 0:30:27to be on the back deck of a ship
0:30:27 > 0:30:29because this is the Royal Research Ship
0:30:29 > 0:30:33Sir David Attenborough in the process of being built.
0:30:33 > 0:30:35We're surrounded by the innards of a ship,
0:30:35 > 0:30:40all these individual pieces that will build the final structure.
0:30:40 > 0:30:42And what's brilliant about it is that the oceans are raw
0:30:42 > 0:30:45and the structures you need to sail on them are raw,
0:30:45 > 0:30:47and this is what's going on -
0:30:47 > 0:30:51steel being welded to build one of the most modern
0:30:51 > 0:30:53polar research ships in the world.
0:30:57 > 0:30:59Joining me on board the Sir David Attenborough
0:30:59 > 0:31:01is Captain Ralph Stevens.
0:31:01 > 0:31:06It will be his responsibility to navigate this huge vessel
0:31:06 > 0:31:08through icy polar waters.
0:31:09 > 0:31:12It's astonishing to me that we're still building ships of steel.
0:31:12 > 0:31:15You know, we associate steel with the Industrial Revolution
0:31:15 > 0:31:18150 years ago, and, yet, we are still building ships from steel.
0:31:18 > 0:31:19Why is it so good?
0:31:19 > 0:31:23Well, for us, it's quite a revolutionary material
0:31:23 > 0:31:26in that it allows us to take impacts.
0:31:26 > 0:31:30It's quite common for us to say some of the ice is as hard as steel.
0:31:30 > 0:31:33And some of the glacial ice, it's rock-hard
0:31:33 > 0:31:34and it's noticeably different.
0:31:34 > 0:31:37When you hit a piece, you'll hear a big clang throughout the ship.
0:31:37 > 0:31:40LOUD CLANG
0:31:40 > 0:31:43And so we want the hull to be able to take all of these forces
0:31:43 > 0:31:46that it's exposed to without cracking.
0:31:46 > 0:31:47And steel can do that job?
0:31:47 > 0:31:49Steel can do that. The right steel can do that.
0:31:51 > 0:31:56But, ironically, steel may actually have been Titanic's Achilles heel...
0:31:58 > 0:32:01..because what the engineers of the day didn't fully understand
0:32:01 > 0:32:04is that, under certain conditions,
0:32:04 > 0:32:07the behaviour of steel can fundamentally change.
0:32:09 > 0:32:12And the key to this change was cold.
0:32:16 > 0:32:19Steel, like many metals, is ductile.
0:32:19 > 0:32:22That means it can stretch when put under stress -
0:32:22 > 0:32:27a property that's useful in a huge structure like a ship.
0:32:27 > 0:32:32Few had imagined that, in the cold, this crucial property might change.
0:32:33 > 0:32:35Got a sample of shipbuilding steel here
0:32:35 > 0:32:37with a little notch in the bottom,
0:32:37 > 0:32:39and I'm going to do this experiment twice -
0:32:39 > 0:32:42once with this one, which is at room temperature,
0:32:42 > 0:32:44and once with an identical sample,
0:32:44 > 0:32:47which has been in the dry ice here at minus 80 Celsius.
0:32:47 > 0:32:50Very, very cold. The difference will be very obvious.
0:32:50 > 0:32:52So, here we go.
0:32:52 > 0:32:54First, the steel at room temperature.
0:33:07 > 0:33:11So, here's the cold one, down at minus 80 Celsius.
0:33:22 > 0:33:24This is the sample at room temperature,
0:33:24 > 0:33:27and you can see that it bent, absorbed the energy,
0:33:27 > 0:33:29absorbed the energy, but it didn't snap,
0:33:29 > 0:33:32whereas this one - this is the cold temperature one -
0:33:32 > 0:33:34and the surface looks really different.
0:33:34 > 0:33:37There's all this speckled pattern and that's a snap.
0:33:37 > 0:33:41This was brittle fracture. You don't want your ship doing this.
0:33:43 > 0:33:46Cold has changed the nature of the steel,
0:33:46 > 0:33:48making it more brittle.
0:33:50 > 0:33:52And it's this that some experts now think
0:33:52 > 0:33:56could have played a significant role in the Titanic disaster.
0:33:58 > 0:34:01Analysis of metal taken from the wreckage
0:34:01 > 0:34:05suggests that, rather than flexing on collision with the iceberg,
0:34:05 > 0:34:10the hull and rivets had become brittle and they fractured.
0:34:19 > 0:34:22With this in mind, modern shipbuilders are able
0:34:22 > 0:34:25to avoid the mistakes of their predecessors.
0:34:27 > 0:34:29We did some calculations.
0:34:29 > 0:34:32We went through the last ten years of temperatures
0:34:32 > 0:34:34our ships have been exposed to.
0:34:34 > 0:34:38We came to 25 degrees and then reduced it down to minus 35.
0:34:38 > 0:34:41So, the game is that you want the steel to give a little bit,
0:34:41 > 0:34:43- but not snap.- That's it.
0:34:43 > 0:34:45We don't... We can't afford to have it fracture,
0:34:45 > 0:34:47and if the worst came to the worst,
0:34:47 > 0:34:50you want that steel to deform rather than crack.
0:34:53 > 0:34:58The tragic irony of Titanic is that she was constructed from metals
0:34:58 > 0:35:00that we've been using for centuries.
0:35:03 > 0:35:05We thought we understood them...
0:35:07 > 0:35:12..but cold altered them in ways that no-one expected.
0:35:19 > 0:35:22Since then, we've been much more aware
0:35:22 > 0:35:25of the hidden changes that can occur within materials
0:35:25 > 0:35:30when they're cooled far below their freezing point.
0:35:30 > 0:35:33And, by pushing temperatures lower and lower,
0:35:33 > 0:35:36we're beginning to unlock some strange and exciting
0:35:36 > 0:35:38new properties of matter.
0:35:42 > 0:35:44This is a material with a very long name.
0:35:44 > 0:35:49It's yttrium barium copper oxide, and it doesn't look like very much.
0:35:49 > 0:35:53There's very strong magnets here and it's not responding to them.
0:35:53 > 0:35:55It doesn't conduct electricity.
0:35:55 > 0:35:56Doesn't seem very interesting,
0:35:56 > 0:35:59but, when you cool it down, it changes completely.
0:36:00 > 0:36:04Using liquid nitrogen, I'm reducing the temperature of the disc
0:36:04 > 0:36:08to minus 196 degrees Celsius.
0:36:08 > 0:36:12And, now, when I bring it close to the magnets,
0:36:12 > 0:36:14something unexpected happens.
0:36:19 > 0:36:21It's levitating.
0:36:23 > 0:36:27And it will scoot around on the little track here for quite a while.
0:36:27 > 0:36:29So, something's changed. We've cooled it down.
0:36:29 > 0:36:30The behaviour changed completely.
0:36:33 > 0:36:38And that's because cold has altered the material at the atomic scale.
0:36:40 > 0:36:45Materials conduct electricity when electrons travel through them,
0:36:45 > 0:36:49but the atoms in a conductor are an obstacle to the flow of electrons
0:36:49 > 0:36:53because, as electrons bump into them, they lose energy.
0:36:53 > 0:36:56At extremely low temperatures,
0:36:56 > 0:36:59the electrons can team up into pairs
0:36:59 > 0:37:02and then the attraction between the electron pairs
0:37:02 > 0:37:05helps them navigate through the atoms far more easily.
0:37:07 > 0:37:10So, when I bring the disc close to the magnetic track,
0:37:10 > 0:37:15a strong electric current begins to flow in the disc.
0:37:15 > 0:37:19This, in turn, generates its own magnetic field.
0:37:19 > 0:37:22The magnets in the track and the disc repel each other,
0:37:22 > 0:37:24and so the disc levitates.
0:37:24 > 0:37:27This is an example of superconductivity.
0:37:27 > 0:37:30Once it's cooled down below the critical temperature,
0:37:30 > 0:37:32the properties of the material change.
0:37:32 > 0:37:35It becomes able to conduct electrical currents
0:37:35 > 0:37:36without any resistance.
0:37:36 > 0:37:41And it also changes how it responds to magnets.
0:37:43 > 0:37:45The peculiar electromagnetic properties
0:37:45 > 0:37:47of supercooled materials
0:37:47 > 0:37:51have given us a powerful new tool in engineering and medicine.
0:37:55 > 0:37:58Some countries already use a super-sized version
0:37:58 > 0:38:03of this magnetic levitation effect in their high-speed rail systems.
0:38:03 > 0:38:06Having no contact with the track,
0:38:06 > 0:38:09trains run faster and more smoothly and efficiently.
0:38:11 > 0:38:13And, inside MRI scanners,
0:38:13 > 0:38:18liquid helium supercools massive coils of copper wire
0:38:18 > 0:38:23to a temperature of minus 269 degrees Celsius.
0:38:23 > 0:38:25At this extreme cold,
0:38:25 > 0:38:29an electric current can flow with almost zero resistance,
0:38:29 > 0:38:33which helps generate the powerful and stable magnetic field
0:38:33 > 0:38:35that the MRI machine needs.
0:38:38 > 0:38:40The extraordinary discoveries we've made
0:38:40 > 0:38:43at extremely low temperatures are now driving
0:38:43 > 0:38:48one of the biggest scientific quests of the modern age.
0:38:48 > 0:38:51How cold is it possible to go?
0:38:51 > 0:38:53How do we get there?
0:38:53 > 0:38:57And what new properties of matter might we uncover?
0:39:02 > 0:39:06The first step on that journey is to understand how things cool down.
0:39:11 > 0:39:14Take this humble cup of tea.
0:39:16 > 0:39:18I always drink my tea far too quickly
0:39:18 > 0:39:21because the experience of a lifetime tells me that, if I don't,
0:39:21 > 0:39:23it will cool down.
0:39:23 > 0:39:25Heat will flow out of the tea, which is warm,
0:39:25 > 0:39:28into its surroundings, which are cooler.
0:39:28 > 0:39:30If I look at this glass of iced water here,
0:39:30 > 0:39:32this is cooler than the surroundings,
0:39:32 > 0:39:34and, if I leave that alone, it will heat up
0:39:34 > 0:39:37until it matches the temperature of everything around it.
0:39:41 > 0:39:45This is a demonstration of a fundamental principle of physics -
0:39:45 > 0:39:48the second law of thermodynamics.
0:39:48 > 0:39:55Heat flows from hot to cold until equilibrium is reached.
0:39:55 > 0:39:58We can see this in action through the thermal imaging camera.
0:39:58 > 0:40:03The hot tea is cooling and the chilled water is warming
0:40:03 > 0:40:07until both are the same temperature as their surroundings.
0:40:07 > 0:40:13It's a law that can't be broken, but it also raises a question.
0:40:13 > 0:40:17How can you ever make something colder than its surroundings,
0:40:17 > 0:40:19like an ice cube?
0:40:20 > 0:40:21Here's the problem.
0:40:21 > 0:40:24At some point, this ice was liquid water,
0:40:24 > 0:40:26and, to cool it down, to freeze it,
0:40:26 > 0:40:30heat had to flow out of it to make it colder,
0:40:30 > 0:40:33but that seems to go against this fundamental law.
0:40:33 > 0:40:35So, how is this possible?
0:40:39 > 0:40:42The answer to that question can be found here.
0:40:45 > 0:40:48This 33,000-square-metre
0:40:48 > 0:40:50food distribution centre in Warwickshire
0:40:50 > 0:40:56handles almost 200,000 home grocery deliveries every day,
0:40:56 > 0:41:01and much of it is chilled well below ambient temperature.
0:41:01 > 0:41:03The invention of refrigeration
0:41:03 > 0:41:05made an enormous difference to our society
0:41:05 > 0:41:09because it allowed us to control our food supply.
0:41:09 > 0:41:12So, something like this - frozen carrots -
0:41:12 > 0:41:16was probably frozen just after it left the field, and, since then,
0:41:16 > 0:41:19it's passed through an unbroken chain of cold -
0:41:19 > 0:41:21refrigerated lorries, refrigerated warehouses,
0:41:21 > 0:41:23all the way to us,
0:41:23 > 0:41:27and places like this gigantic freezer are part of that.
0:41:27 > 0:41:29And, after being in here,
0:41:29 > 0:41:32I will never take frozen food for granted ever again
0:41:32 > 0:41:35because it's so cold. It's minus 22!
0:41:40 > 0:41:44And when you stop to think about that, it's strange.
0:41:44 > 0:41:47How DO you make this building so much colder
0:41:47 > 0:41:51than the ambient temperature here in balmy Warwickshire?
0:41:52 > 0:41:55The secret to this place is the same as the hidden workings
0:41:55 > 0:41:57of the fridge-freezer in your kitchen...
0:41:58 > 0:42:01..and it begins with something counterintuitive.
0:42:03 > 0:42:06The odd thing about the process of making something cold
0:42:06 > 0:42:09is that it starts with a huge input of energy,
0:42:09 > 0:42:11and that happens here.
0:42:11 > 0:42:13These are compressors.
0:42:13 > 0:42:17They're taking ammonia gas, and all the energy is being used
0:42:17 > 0:42:20to squeeze the gas to a high pressure.
0:42:20 > 0:42:23And, at the same time, that heats it up,
0:42:23 > 0:42:28so what leaves here is both at high pressure and high temperature.
0:42:32 > 0:42:36Next, this pressurised ammonia needs to be cooled down.
0:42:38 > 0:42:41Ammonia gas comes out of the plant room downstairs
0:42:41 > 0:42:46at 100 degrees C, and this is where it's cooled down.
0:42:46 > 0:42:49It's flowing through all these pipes in the inside of here.
0:42:49 > 0:42:53And the water falling down is cooling it down
0:42:53 > 0:42:55much closer to room temperature.
0:42:55 > 0:42:57This is where the energy is lost
0:42:57 > 0:43:00from the refrigerant fluid - the ammonia.
0:43:00 > 0:43:04By the time it leaves here, it's much cooler and it's a liquid.
0:43:07 > 0:43:10Crucially, even though the ammonia is now cooler,
0:43:10 > 0:43:12it's still under pressure.
0:43:14 > 0:43:16Releasing this pressure is the secret
0:43:16 > 0:43:19of how this vast warehouse space is cooled,
0:43:19 > 0:43:22and I can show you how using something very familiar.
0:43:24 > 0:43:28What happens is the same as when you have an aerosol spray
0:43:28 > 0:43:31and you spray it and you can see it. I've got a thermometer here.
0:43:32 > 0:43:33If I spray the bottom of the thermometer,
0:43:33 > 0:43:36the temperature goes right down.
0:43:36 > 0:43:40This process is called adiabatic cooling.
0:43:40 > 0:43:43As the high-pressure gas leaves the can,
0:43:43 > 0:43:47it pushes outwards on the air around it and expands.
0:43:47 > 0:43:51But that push uses energy that can only come
0:43:51 > 0:43:53from the movement of the atoms,
0:43:53 > 0:43:56and so the expanding gas cools down.
0:43:58 > 0:44:00The exact same process happens
0:44:00 > 0:44:03as liquid ammonia is pumped into the warehouse.
0:44:03 > 0:44:06The high-pressure liquid passes through a valve,
0:44:06 > 0:44:08and, still contained within the pipes,
0:44:08 > 0:44:11it rapidly expands into a gas
0:44:11 > 0:44:14causing its temperature to instantly drop.
0:44:15 > 0:44:18Heat then flows from the warm air inside the warehouse
0:44:18 > 0:44:23to the much colder ammonia until they reach equilibrium,
0:44:23 > 0:44:26just as the second law of thermodynamics
0:44:26 > 0:44:28dictate they should.
0:44:28 > 0:44:30And, since the equilibrium temperature
0:44:30 > 0:44:33is far colder than that at which the air began,
0:44:33 > 0:44:37this vast space cools down.
0:44:37 > 0:44:41Refrigeration works because you keep moving the goalposts,
0:44:41 > 0:44:44so that heat only ever flows from hot to cold.
0:44:44 > 0:44:46No laws of physics are broken.
0:44:46 > 0:44:50As long as you keep pumping a bit of energy in at the compressors,
0:44:50 > 0:44:52the refrigerators will keep working.
0:44:57 > 0:45:02As we saw, the process of lowering temperature requires energy...
0:45:03 > 0:45:06..and this raises an interesting question.
0:45:06 > 0:45:09How much colder is it possible to go?
0:45:13 > 0:45:15We know that, as you cool materials down,
0:45:15 > 0:45:18they tend to turn to liquid and then solids, but, actually,
0:45:18 > 0:45:21the question of how cold you could make something
0:45:21 > 0:45:23started with gases,
0:45:23 > 0:45:25and this was the kind of experiment that was used.
0:45:25 > 0:45:28What I've got here are four beakers,
0:45:28 > 0:45:30each of which is at a different temperature.
0:45:32 > 0:45:36They range from minus five to 50 degrees Celsius.
0:45:36 > 0:45:39Into each, I'm placing a syringe
0:45:39 > 0:45:43containing 15ml of air at room temperature.
0:45:43 > 0:45:46This air will heat up or cool down
0:45:46 > 0:45:49until it's at the same temperature as what's in the beaker.
0:45:51 > 0:45:53So much science is about waiting,
0:45:53 > 0:45:55and this is one of those experiments.
0:45:57 > 0:46:00But it's not the change in temperature I'm interested in here.
0:46:00 > 0:46:02It's something else.
0:46:04 > 0:46:07After five minutes, the air that's heated to 50 degrees
0:46:07 > 0:46:11has expanded from 15ml to 16ml,
0:46:11 > 0:46:14while the air that's cooled to minus five
0:46:14 > 0:46:17has reduced to 14ml.
0:46:17 > 0:46:20In other words, there's a direct relationship
0:46:20 > 0:46:23between the temperature of a gas and its volume.
0:46:24 > 0:46:27So, the first scientists who saw this kind of relationship
0:46:27 > 0:46:29did something very straightforward -
0:46:29 > 0:46:33they plotted a graph that showed temperature against volume.
0:46:33 > 0:46:35And, at the higher temperatures, the volume was higher,
0:46:35 > 0:46:39and, as you go down to the lower and lower and lower temperatures,
0:46:39 > 0:46:40the volume decreases.
0:46:40 > 0:46:43And then there's a question because, at some point,
0:46:43 > 0:46:48even though they couldn't see it, if that line kept going,
0:46:48 > 0:46:51it was going to pass through zero volume,
0:46:51 > 0:46:53and at that point and past that point,
0:46:53 > 0:46:56what happens to the temperature? What does it mean?
0:46:56 > 0:46:59And that was the first hint that there might be a limit
0:46:59 > 0:47:01on just how cold you can go.
0:47:03 > 0:47:08This observation led to a concept known as absolute zero -
0:47:08 > 0:47:11the theoretical limit of cold.
0:47:14 > 0:47:16And now we know exactly what it is.
0:47:16 > 0:47:20On the Celsius scale, it's minus 273.15 -
0:47:20 > 0:47:23a fantastically low temperature.
0:47:23 > 0:47:25But, below that, there's nowhere to go.
0:47:25 > 0:47:26That's the coldest you can get.
0:47:30 > 0:47:34And it remains a theoretical point on the temperature scale.
0:47:36 > 0:47:40The Boomerang Nebula 5,000 light years away from Earth
0:47:40 > 0:47:44is the coldest place we know of in nature.
0:47:45 > 0:47:49It's a star in the late stages of its life
0:47:49 > 0:47:51that's shedding huge plumes of gas.
0:47:51 > 0:47:56As this gas expands rapidly into the void of interstellar space,
0:47:56 > 0:47:58it loses energy quickly,
0:47:58 > 0:48:01resulting in its unusually low temperature
0:48:01 > 0:48:06of minus 272 degrees Celsius.
0:48:06 > 0:48:09But even this is one whole degree warmer
0:48:09 > 0:48:11than absolute zero.
0:48:19 > 0:48:21Though we've yet to find absolute zero
0:48:21 > 0:48:24in the far reaches of the universe,
0:48:24 > 0:48:28we're trying to create it ourselves much closer to home.
0:48:28 > 0:48:32At Imperial College London, Professor Ed Hinds and his team
0:48:32 > 0:48:35are working at the very limits of the ultracold,
0:48:35 > 0:48:39within fractions of a degree of absolute zero.
0:48:42 > 0:48:45It promises to open up a whole new world of physics,
0:48:45 > 0:48:48which could revolutionise our future.
0:48:50 > 0:48:55The stuff they're cooling here is tiny clouds of molecules.
0:48:55 > 0:48:59Chilling them to absolute zero requires two phases of cooling.
0:49:00 > 0:49:03First, using liquid helium,
0:49:03 > 0:49:07they take them down to within four degrees of absolute zero.
0:49:07 > 0:49:11But it's these last few degrees that pose the problem.
0:49:13 > 0:49:15There are ways to make helium a bit colder,
0:49:15 > 0:49:18but to get to the millionths of a degree,
0:49:18 > 0:49:21there is no fluid that you can use,
0:49:21 > 0:49:24so, instead, we use light.
0:49:26 > 0:49:31By scattering the light, the molecules will get colder.
0:49:36 > 0:49:39Even at this temperature,
0:49:39 > 0:49:43the molecules still have some movement.
0:49:43 > 0:49:48Photons in the laser light collide with the slowly moving molecules,
0:49:48 > 0:49:49and, in that instant,
0:49:49 > 0:49:54what little momentum they have is transferred to the photons.
0:49:54 > 0:49:57The photons are scattered,
0:49:57 > 0:50:01but the molecules slow down and so get even colder.
0:50:08 > 0:50:11By using an array of different colours of laser light
0:50:11 > 0:50:13in just the right order,
0:50:13 > 0:50:15Ed and his team can reach temperatures
0:50:15 > 0:50:19within a few millionths of a degree of absolute zero.
0:50:20 > 0:50:23At these incredibly low temperatures,
0:50:23 > 0:50:26materials begin to behave differently
0:50:26 > 0:50:28at the subatomic or quantum level.
0:50:28 > 0:50:33In this quantum state, they exhibit strange properties,
0:50:33 > 0:50:36which might lead to a new type of computer.
0:50:36 > 0:50:41A normal computer bit can only represent a zero or a one,
0:50:41 > 0:50:46but these quantum materials can be zero and one at the same time.
0:50:47 > 0:50:50Link these multitasking bits together,
0:50:50 > 0:50:54and they can do vast numbers of calculations simultaneously
0:50:54 > 0:50:58far faster than any conventional computer chip.
0:51:04 > 0:51:07This opens up the possibility of quantum computing,
0:51:07 > 0:51:10quantum sensing, quantum cryptography.
0:51:10 > 0:51:13These are all ways of doing useful things,
0:51:13 > 0:51:18but much better than can be done with conventional techniques.
0:51:23 > 0:51:27The world of absolute zero is a strange new realm of physics
0:51:27 > 0:51:31and one that we're only just beginning to get to grips with.
0:51:31 > 0:51:35But there is something ironic about the vast effort required
0:51:35 > 0:51:39to push things extremely close to absolute zero.
0:51:41 > 0:51:45Because wait long enough - billions of years -
0:51:45 > 0:51:47and everything will get there.
0:51:47 > 0:51:51The universe itself is cold and it's getting colder.
0:52:10 > 0:52:12Humans have always looked up at the sky
0:52:12 > 0:52:15and asked questions about the stars and the structure of galaxies -
0:52:15 > 0:52:16everything they could see.
0:52:16 > 0:52:21But in the 1940s and '50s, a new type of question emerged
0:52:21 > 0:52:25about what was between the stars and about what dark really was,
0:52:25 > 0:52:30and this question opened the door on the nature of the universe.
0:52:30 > 0:52:34And, then, in 1964, it was answered by accident.
0:52:37 > 0:52:39In a small laboratory in New Jersey,
0:52:39 > 0:52:43astrophysicists Robert Wilson and Arno Penzias
0:52:43 > 0:52:45stumbled on a discovery
0:52:45 > 0:52:48that changed our understanding of the universe forever,
0:52:48 > 0:52:51revealing something profound about its temperature.
0:52:55 > 0:52:58I'm meeting Professor Tim O'Brien,
0:52:58 > 0:53:01an astrophysicist at the University of Manchester
0:53:01 > 0:53:04and director of the Jodrell Bank Observatory.
0:53:06 > 0:53:09So, at some point during every undergraduate physicist's degree,
0:53:09 > 0:53:11they hear the names Penzias and Wilson.
0:53:11 > 0:53:13Tell me what they did.
0:53:13 > 0:53:15So, these were these two great characters
0:53:15 > 0:53:19that were working in the USA in the 1960s,
0:53:19 > 0:53:22and they built themselves a remarkable telescope.
0:53:22 > 0:53:24It was incredibly well-built
0:53:24 > 0:53:27to try and study the outer regions of the Milky Way,
0:53:27 > 0:53:31and they were measuring very weak signals coming from space.
0:53:31 > 0:53:33But there was this last bit of noise
0:53:33 > 0:53:35that they had no idea where it came from.
0:53:35 > 0:53:38They could not get rid of it.
0:53:38 > 0:53:39It was a faint hiss,
0:53:39 > 0:53:42and that faint hiss came from everywhere on the sky.
0:53:42 > 0:53:45It had the same sort of strength, the same brightness
0:53:45 > 0:53:46of the radio signal everywhere on the sky.
0:53:46 > 0:53:48And they tried everything.
0:53:48 > 0:53:49They tried all kinds of things, didn't they?
0:53:49 > 0:53:51They did try everything. At one point,
0:53:51 > 0:53:54they thought it might be coming from pigeon droppings in the telescope.
0:53:54 > 0:53:57So, big telescope that the pigeons were sitting in.
0:53:57 > 0:54:00Washed it all out - no, this stuff was still there.
0:54:02 > 0:54:06There remained only one possible explanation for this noise,
0:54:06 > 0:54:10and it had enormous implications for our view of the universe.
0:54:12 > 0:54:18The strange hissing was coming from beyond our own galaxy.
0:54:18 > 0:54:20It's what we now know and they didn't know at the time...
0:54:20 > 0:54:22It's what we call the cosmic microwave background -
0:54:22 > 0:54:24the fading glow of the Big Bang.
0:54:24 > 0:54:28- Where was this coming from? - It's coming from the whole sky. So, it's coming from everywhere.
0:54:28 > 0:54:31And it's actually the light that was emitted by the universe
0:54:31 > 0:54:34about 380,000 years after the Big Bang.
0:54:39 > 0:54:42The cosmic microwave background radiation
0:54:42 > 0:54:46is invisible to the naked eye, but it fills the universe.
0:54:49 > 0:54:52If we could see it, the entire sky would glow
0:54:52 > 0:54:54with a brightness that's astonishingly uniform
0:54:54 > 0:54:56in every direction.
0:54:58 > 0:55:02What's remarkable is that these microwaves carry information.
0:55:02 > 0:55:05They allow us to take an accurate temperature
0:55:05 > 0:55:09of the entire universe without the use of a thermometer.
0:55:11 > 0:55:13A thermometer has a fundamental limitation,
0:55:13 > 0:55:16which is that it has to be touching the thing that it's measuring.
0:55:16 > 0:55:19And that's not much use if you're looking at the rest of the world
0:55:19 > 0:55:21or even the rest of the universe.
0:55:21 > 0:55:24But the laws of physics themselves offer another route
0:55:24 > 0:55:28because every single object in the universe with a temperature
0:55:28 > 0:55:30is radiating some of that energy away as light,
0:55:30 > 0:55:33and every single object has a temperature.
0:55:33 > 0:55:36The reason you can see me now on the infrared camera
0:55:36 > 0:55:39is that I have a temperature, and so I'm glowing in the infrared -
0:55:39 > 0:55:41effectively, a human infrared light bulb.
0:55:44 > 0:55:48The temperature of an object determines the exact wavelengths
0:55:48 > 0:55:50of the light that it radiates,
0:55:50 > 0:55:53and this means that there's a precise relationship
0:55:53 > 0:55:56between temperature and colour.
0:55:56 > 0:55:59So, when an astronomer sees the star of a certain colour,
0:55:59 > 0:56:02they know it has a certain temperature.
0:56:04 > 0:56:08The reddest star visible to the naked eye is Mu Cephei.
0:56:08 > 0:56:11The wavelength of red light that it radiates
0:56:11 > 0:56:13tells us that this star has a temperature
0:56:13 > 0:56:17of around 3,200 degrees Celsius.
0:56:18 > 0:56:20And this is Spica,
0:56:20 > 0:56:24a star that glows a brilliant blueish-white.
0:56:24 > 0:56:28The shorter wavelength light is indicative of a young, hot star
0:56:28 > 0:56:33that's burning at a temperature of around 22,000 degrees Celsius.
0:56:35 > 0:56:37And then you can go back the other way -
0:56:37 > 0:56:40cooling down through all those very hot temperatures,
0:56:40 > 0:56:42down through our everyday temperatures,
0:56:42 > 0:56:44and keep going and keep going.
0:56:44 > 0:56:46The wavelengths get longer and longer.
0:56:50 > 0:56:53Eventually, you reach the very long wavelengths
0:56:53 > 0:56:56of the cosmic microwave background.
0:56:56 > 0:56:58They're not part of the visible spectrum,
0:56:58 > 0:57:01but the wavelengths of these microwaves
0:57:01 > 0:57:03reveal its temperature,
0:57:03 > 0:57:06and that temperature is cold.
0:57:08 > 0:57:10Today, the cosmic microwave
0:57:10 > 0:57:11background radiation glows
0:57:11 > 0:57:17at a temperature of minus 270 degrees Celsius -
0:57:17 > 0:57:22only 2.7 degrees warmer than absolute zero.
0:57:22 > 0:57:25We live in a nice, warm bubble on planet Earth,
0:57:25 > 0:57:29but out there in the universe, it isn't just very empty.
0:57:29 > 0:57:32It's very, very cold.
0:57:34 > 0:57:38But that's not the end of our story of temperature...
0:57:40 > 0:57:45..because amidst the vast swathes of cold and nothingness,
0:57:45 > 0:57:48we're starting to find other bubbles of warmth
0:57:48 > 0:57:50out there in the universe -
0:57:50 > 0:57:53planets with a temperature similar to our own,
0:57:53 > 0:57:56which means they may have the right conditions
0:57:56 > 0:57:59for liquid water and complex chemistry.
0:57:59 > 0:58:04These discoveries are causing huge excitement among scientists
0:58:04 > 0:58:07because they offer up the tantalising possibility
0:58:07 > 0:58:10that maybe - just maybe -
0:58:10 > 0:58:14we might not be alone in this vast universe.
0:58:15 > 0:58:18Next time, the temperatures of life.
0:58:18 > 0:58:21This is an awesome force of nature.
0:58:21 > 0:58:25I'll discover how the slenderest knife-edge of temperature
0:58:25 > 0:58:30here on Earth provided the catalyst for life to flourish.