In Our Time - States of Matter

Episode Date: April 3, 2014

Melvyn Bragg and his guests discuss the science of matter and the states in which it can exist. Most people are familiar with the idea that a substance like water can exist in solid, liquid and gaseous forms. But as much as 99% of the matter in the universe is now believed to exist in a fourth state, plasma. Today scientists recognise a number of other exotic states or phases, such as glasses, gels and liquid crystals - many of them with useful properties that can be exploited.With:Andrea Sella Professor of Chemistry at University College LondonAthene Donald Professor of Experimental Physics at the University of CambridgeJustin Wark Professor of Physics and Fellow of Trinity College at the University of OxfordProducer: Thomas Morris.

This BBC podcast is supported by ads outside the UK. Thank you for downloading this episode of In Our Time. For more details about In Our Time and for our terms of use, please go to BBC.co.com. UK slash Radio 4. I hope you enjoy the programme. Hello, most of the matter we encounter in everyday life appears in one of three states, solid, liquid or gas. Water is one of the few substances that we regularly see in all three forms, as ice or water, or as vapour. But it turns out there's much more to matter than solid,

liquid and gas. Ninety-nine percent of the visible universe is believed to consist of matter in a fourth state plasma. And there are plenty of common materials such as glass that don't fit easily into these categories. Today, scientists are discovering a variety of new and exotic states of matter, and many of them, such as liquid crystals, have properties that make them enormously useful to us. But what are the differences between these states or phases of matter and what is current research telling us about them? With me, to discuss the states of matter are Andrea Seller, Professor of Chemistry at University of College London. Atheney Donald, Professor of Experimental Physics at the University of Cambridge

and Justin Walk, Professor of Physics and Fellow of Trinity College at the University of Oxford. Andrew Seller, I'm just given a very rough outline, but would you give us a more nuanced idea of what's meant by the phase, states of matter? Well, when we think about the states of matter, as you've just said, of course, we immediately start thinking of water because this idea that you can go from the solids to the liquid to the gas, you know, it's something that is drummed into us from childhood. But when we start thinking about chemistry and physics, we actually now start thinking a little bit more deeply about this, and we tend to use the word phase rather than state.

And this is really to distinguish the possibility that you might have sort of structurally different sort of arrangements of the matter. So, you know, of course, solids normally will have orderly arranged atoms. In liquids, things are more mobile, and in gas, the particles are further apart. But in order to describe this, the way we tend to do it is to use a map. And this is called a phase diagram, and it's actually a picture, which amongst chemists anyway, it sets the fear of God amongst undergraduates. But if you were to think, for example, of a map of Africa,

where you have a series of countries, and there are borders between them. And you can really think of these states or phases of matter as being sort of regions within this map in which these arrangements are stable. Now, if you think about the temperature, for example, as being increasing, moving towards the east and pressure as increasing towards the north. What this really means is that we're mapping out a kind of energy landscape, a landscape of the world thinking about, for example, ice, under different conditions, sorry, ice, water, steam, and so on,

under different conditions of temperature and pressure. And each time we go across one of these borderlines, then that's what we call, for example, melting or we call boiling. So all of our understanding, of states of matter is really about exploring this map of matter under conditions of different energy. What's the significance of your word of energy in this process? Well, energy is absolutely fundamental in this whole thing, because as you put energy into a system, then what you find is that that energy is distributed in different ways, and that results in us sort of moving from one,

one state, let's call it, to another. And so energy really underpins all of these ideas about the states or phases of matter. Can you give us an example of how that might be realized? Well, one of the key things that happens, for example, with ice, is that in order to go from the ice lands, let's say, to the liquid ocean region, one of the things you have to do is to put energy in. And we often call this latent heat or the enthalpy of fusion. if you want to be really, really technical. But what this is saying is that in order to be able to transform our material from one state to the other, you've got to put energy in.

And that will happen. But what you're putting energy in as if you've got a bucket with a spoon and you ladled it in. In one sense, it's a little bit like that. I mean, for example, you might have a beaker, which is filled with ice. And what you do is you heat it up from the outside. So you put a Bunsen burner underneath, and what that does is it transfers energy into the beaker, and the ice will gradually be transformed across into the liquid. Now, the interesting point here is that although we are putting energy in, the temperature doesn't actually change.

And so this is one of the key things about these boundaries that we have in our map, is that those are places where you actually have quite a lot of energy change, without, however, a movement in terms of temperature, for example. Adini, can we have some examples of the molecular level of how solids and liquids differ from each other? Okay, so Andrew has been talking about ice, and in a crystal structure like ice, the atoms or molecules in that case are sitting on a well-defined position. So you can identify the crystal structure. There will be a repeat between the different molecules.

and although the molecules may be vibrating a bit, they're basically stuck on this periodic structure, this lattice. And as the heat enters the system, as it's warmed up, could be just a puddle warming up on a sunny day, then the increase in energy that Andre was referring to will cause the molecules to move around a bit more, and they'll start jiggling around. And at a certain point, the motion becomes so great

that individual molecules can slight past each other, and that's the fluidity we would associate with the melting of water. So again, we can see how this energy input is absolutely crucial in distinguishing between the solid and the liquid. And if we heat it up further, then the motion of the molecules get greater again. And ultimately, in the vapour, in the gas state, then they are moving all over the place, and you can't identify a particular position around which the molecules are just vibrating.

So can you tell us, has there a bigger part, an essential part that molecules play? Can you just explain that a bit further for people? Well, water is made up of water molecules. So the molecules are what the water is. And so the point is how much they are moving. And in the solid state, they are not moving very much. So that if one has a lump of ice, it just sits there on the table. But as it melts into water, the molecules can move around rather more. And they're They will slide past each other, and so water will change shape to fill the container. So the speaker of water on the table, you know, I can pour it out and put it in another container, and it will take up a different shape. If it was a lump of ice, you couldn't do that. And of course, in the gas phase, in the vapour phase, that the distance between the molecules is so great that, you know,

you don't really see it as a condensed though at all in that case. Is what you, I think it was in your paper that you said that great research, liquids is the most mysterious of these three? Well, personally, I think it is. Why is that? Because a solid, a crystal, you can define where the molecules or the atoms all sit. You can do a lot of analysis on them, and that's sort of all well defined. And in the gas state, the molecules are all far apart. You can make various approximations. You can characterize them perfectly well. But in the liquid state, it's somewhere in between. And it's actually quite hard to identify. what's special about it, or indeed, I think the theories of precisely what temperature

a given material will melt are not that well developed. There are trends you can explain in terms of the size of the molecule, the strength of the bonds between them, all that. You can explain trends, but actually to come up a priori with a prediction of exactly what temperature a given material will melt at is quite hard. So it is this kind of, we don't quite understand. it perfectly. We can describe it, but there are still mysteries about it. Can I just ask you once more, not once more, push-fire, why is it so hard? Why is it so hard? Because I suppose the liquid is this

intermediate state. So in the crystal, we know exactly the spacing between the atoms and molecules, and the gas, they're way apart, and you can characterize them in different ways. And it's this, at what point would you expect to see this fluidity? We can do computer simulations, and there's a lot of work done on that to try and understand. But I think that a sort of really neat simple explanation does not exist in a way that I personally find satisfying. Just in Wark, most of us are familiar with solids, liquids and gases. Can you tell us a bit more about plasma? I'll say for the last time 99% of the visible universe, if that is correct. Well, it's probably more than 99% of the visible universe, but let's qualify that by saying that probably only 5% of the universe is in the

visible state. So the universe is a very strange place, but whenever you're going out and looking at the night sky, or even if you're going out this morning and looking at the sun, you're looking at an example of the plasma. And so most of the universe that we can see is in the plasma state. And I think to explain the plasma state, we can move on from what Andrea and Atheney have been saying and develop those ideas a bit further. Because when we're talking about states, or as Andreas said, phases of matter, energy, as Andrea said, is key. And what's key here is a competition between different forms of energy. So the form of energy that keeps matter together are the binding forces between atoms. So that's why we have solids and why we have liquids is that, if you like,

there's a spring binding this material together. When a solid melts or then when a liquid becomes a gas, it's the thermal energy, the energy due to motion that is becoming large enough to overcome those bonds. That finally, as we heat things up, as Andrea said, will take us into the gas phase. So what happens if we give energy, more energy to the system than when it's in the gas phase, and we heat it even further? Now, if we think about a gas, it can be a gas of molecules like water vapor, and I think he's just been talking about water, or a gas of individual atoms, like a neon gas or xenon gases are just indium. individual atoms. If we give even more energy, something else is going to break. And the point is that if we think about an atom, that is now normally in the gas phase a neutral object. That's to say it's got a positive nucleus, a very heavy positive nucleus made up usually of protons and neutrons or just a proton in the case of a hydrogen atom. And it's surrounded by these very light negatively charged electrons are moving around a little bit like planets moving around. the star and a solar system. Now, when we add even more energy to the system so that these

gas particles are moving very, very fast, they can bump into each other and eventually they're moving so rapidly they can knock the electrons away from individual atoms. Then you reach a state, which is what we call a plasma, where you have these negative, very light electrons moving around, if you like, a gas in their own right, in some sense, within another gas of heavy positively charged atoms and they're positive because they've lost an electron. So it's going way beyond the normal state of matter where we think of things as being electrically neutral

and this system now is made up of positive and negative charges. And again, it's this idea of putting in energy and you've now put in so much thermal energy that you're overcoming the binding forces that really hold electrons to atoms themselves. And that's why the sun is such a great place, a source of example of plasma? Well, yes, and of course

that the sun is very important because the plasma physics there in the centre is what is producing nuclear fusion, if you like. You need to get to very, very high temperatures in order to produce nuclear fusion, which is going on in the centre of the sun, which eventually gives us light, of course, our source of

energy. But when you're getting to those extremely high temperatures, by definition, you've ripped the electrons away from the constituent nuclei, all of the hydrogen and helium and so forth within the sun and so the fusion reaction takes place at these very high temperatures. I know this is obvious

but so we all know what solids, liquids and gas do or we have a working idea. We don't know as you three know, but we know enough to sort of go through the door. What does plasma do? When you say what does it do? You mean what are its properties compared to other sorts of matter? Okay. I wondered whether you said

what a plasma has ever done for us. Which of course the sun is the best thing they have done for us. We wouldn't be here without it. I didn't ask that. But they, first of all, they are conducting electricity when a normal gas would not. And this is, so this is why, if you like, the common examples we have on Earth of plasmas would be fluorescent and high efficiency light bulbs. They also, because they conduct electricity, they are very good at forming filamentary structures and storing magnetic energy. Whenever a current flows, which is what electricity is,

and the electrons in a plasma can move, then it creates a magnetic field. And this causes lots of filamentary structures to occur as the magnetic energy gets stored and twisted. And this is what we see, for example, on the surface of the sun in solar ejections and so forth. But I think from a physicist point of view, one of the most remarkable things about plasmas

is they have a property which we call collective behavior. and they have properties that we call collisionless properties. And let me try and explain that. The way that my voice is getting to your ears at the moment is via collisions. My voice box is pushing against air molecules, and those air molecules bump into the next set, which bump into the next set and so forth until they reach your ear.

And so we think of vibrations in the air as coming about due to collisions. The amazing thing about plasmas is that Vibrations can occur, very important vibrations, without collisions. And the way this works is if you imagine I have this positively charged gas and negatively charged electrons within it, it's possible for me to pull all of the electrons back to one side a little bit, all of them at once. And if I do so, that will set up a large electric field

between all of the positive ions that are left on one side and those negative electrons. And so all of the electrons will want to rush back towards the positive ions. And then they rush back, they overshoot, like in every wave, and then oscillate backwards and forwards. And now they're oscillating backwards and forwards, producing a vibration without actually the electrons colliding with the ions.

They're moving so fast they have a little small deflection, like perhaps a very fast spacecraft going past the planet. But they're not really colliding for a long time while these oscillations are taking place. So for physicists, what we find, therefore, is plasmas have a vast, rich array of possible vibrating states, as we call them, which are very, very different from almost all other states for matter. Andrew Seller, there's a special kind of phase transition known as sublimation. Can you tell us about that?

Well, sublimation is something that we often think of as rather unusual because we only really encounter it regularly in one place. And I've actually brought into the studio a little bit of dry ice. and which I've actually got here in a flask. And what I'm going to do is I'm going to press a little piece of this beautiful chunk of ice against a piece of metal, and I want you to listen. And you can hear that hissing, squealing sound. And what that is, is the solid, which is actually transforming spontaneously into the gas.

Now, if we return to my original map analogy, what we're saying, really, is that we're moving across that boundary, across that transition from the solid state to the gas state directly. And the reason this is possible is because, in fact, the binding energies, the energies that hold the carbon dioxide molecules one to the other are actually relatively weak. But you're missing out of phase, that's the point, isn't it? Well, the interesting thing is that we are... I'm going from solid to gas without going through a liquid. Indeed.

I mean, we are in a sense, jumping through, but I think this is actually a misconception. The reason it's a misconception is because we are accustomed to living in a world where we see solids, ice, which are then warmed up, which melt, and then turn to the gas phase. And that's simply a coincidence. And the crucial second parameter that we have to consider when we're imagining this map is this idea of the pressure. So at atmospheric pressure, ice melts and then water boils. Now, if you look in your freezer, one of the most infuriating things is that you decide you're going to make yourself a genitonic in the evening, you open the freezer door, you pull out the ice cube tray, and to your total rage, the thing is half empty. And you say, where's my ice gone?

Well, the reason is, because in fact, what's happening is that the ice is subliming away and it's actually evaporating effectively. and ending up in a colder place in your fridge, and therefore your freezer tray is emptying out. So there it's perfectly possible for water to do this, and we do this routinely, industrially, to make freeze-dried food. So instant soups and instant coffee, those kinds of things, are made precisely by that process. The key thing is that the relationship between the solid, the liquid, and the gas

is not actually a sequence. It's not solid liquid gas in the way that we learn in primary school. We actually think of it in a much richer way as being a series of borders that we can go across, and these are what we call the phase transitions. Any different substances melt or boil at widely different temperatures. What determines this range of behavior?

There are various things that feed into it, so that one of them is the strength of the bonding we heard about the springiness between the atoms and the molecules. So if you take something like neon that was mentioned earlier, which is it's a noble gas, the interactions between the atoms there are very weak. So neon actually is solid at absolute zero, and as you warm it up,

it transforms into a liquid at only about 25 degrees above absolute zero, because the interactions are very weak. So it's quite easy to separate. the atoms and the crystal to make a liquid state and likewise it then boils only a few degrees higher than that. So we think of neon as a gas because that's what it is at room temperature and that's because the intra-atomic interactions are relatively weak. Neon is used as a model hard sphere material

when physicists try and describe these things because you can really practically ignore those attractive interactions. But in other cases like ice, they're very strong hydrogen bonds, or relatively strong compared with neon, anyhow, hydrogen bonds, which you have to break in order to melt the structure. And so you go to much higher temperatures, or everyday world kind of temperatures, in order to get that breakage occurring. The size of the molecule is also important because clearly if you're putting heat in to cause the atoms to jiggle around, the heavier that object is that you've got to cause

to jiggle, the more thermal energy you need to put in. So size of the molecule is very important. And if you think about water and heavy water, which is where the hydrogen atom has been replaced by Deuterium, which has an extra neutron in each atom. So instead of having H2O, you have D2O, that actually melts at a slightly higher temperature because the strength of the bonds are slightly stronger and the molecules are slightly heavier. So it melts at about three degrees instead of the 0 degrees centigrade that we're familiar with for ice, conventional ice. So one of the objects of our research is to find out precisely where this temperature is relevant? Well, as I say, it's very hard to predict, but it's quite easy to measure where these melting processes occur.

But I think it comes back to the fact that... If it's easy to measure, why is it hard to predict? Because the theory for the melting transition is still kind of... I mean, the simplest picture is that the melting will occur when the average size, the average displacement of the vibrations is some proportion of the so-called lattice spacing, which is the mean distance between the atoms or the molecules in the crystal structure. But what proportion that is actually depends on the details of the crystal structure, for instance.

Andrew, you want to say something. One of the really interesting things I think which has come out, of studies of, for example, melting, is that melting is not this sort of on-off, almost binary process where one moment, you know, everything's locked in step, and the next moment, you know, everything starts to move past everything else. And one of the places this has come out is the really intriguing question about why is ice slippery. And what has really emerged is this idea that actually as you approach the phase transition, as you approach the transition temperature, that there is an initial onset of what sometimes called pre-melting.

There's almost a kind of slushy, disordered layer of atoms, which happens on the surface of the material. And so, in fact, this melting process, which looks so simple and smooth to us when we're staring at ice and a glass, is in fact an unbelievably complex process. Because I think one of the reasons why it's very difficult to predict exactly where something is going to,

melt and lots of other properties of matter. It's because really those springs that we've been talking about that are holding matter together, in order to compute what is going on there is extremely difficult because those bonds are made from electrons which are quantum mechanical particles. They're acting in a quantum mechanical way. And the overlap of the way functions that causes these bonds to take place is, a tough problem. I would say it's really only in the last 10 or 20 years that we've had enough computing power to start from first principles on a computer and try to predict the states

of matter and where the phase boundaries occur. And for some materials, we can do that very well already. But the sheer computing power you need to solve the equations of motion in a quantum mechanical way to really work out where all the springs are are extremely difficult. There are some spectacular successes and I think as we go forward that theory is getting better developed but it is a tough thing to do from first principles. Can I say with you

Justin Walker a second can you, there are exotic states but can we talk about one called the Bose Einstein condensate? Would you explain what that is and why it's exotic? Well it comes back to this idea of quantum

mechanics that we've just been talking about. most of what we've been discussing so far you've been hearing us talk about particles or atoms on atomic sites or moving around in a gas but we also have the concept in physics the very important concept that particles can also have a wave-like property

and this is one of the things that quantum mechanics tells us and what we've it all goes back a lot of it to the early part of the 20th century and DeBroy in his 1924 thesis told us that the wavelength of matter, if you like, in its wave-like particle, the wave-like nature of it is such that the wavelength is inversely proportional to the velocity. So if particles slow down, they seem to have a longer and longer quantum mechanical wavelength. Up until now, we've been talking as though a gas was like a set of billiard balls moving

around on a snooker table. But imagine slowing those particles down, slowing them down, and as you slow them down, rather than looking like this particle, this hard bit of stuff, it's as though the billiard ball starts to become fuzzy and has a wavelength and a spreading out, and you're not sure quite where it is. Now if you slow down far enough, you'll get to a point where the wave-like properties,

it's spreading out so much, of one of the ball, if you like, starts to overlap that of its neighbour and all of the others. And eventually you get to such a point of low energy that the material becomes one quantum object. It's no longer possible to think in terms of individual particles. That's partly what a Bose-Einstein condensate is. It's when all of the atoms go down into the lowest possible energy state, and Bose said that they're all allowed in that energy.

state if they are these particles called bosons. And this therefore has to occur at very, very low temperatures. It was predicted by Einstein in 1925, but it was only 70 years later, in 1995 that it was first achieved in the lab by Eric Cornell and Carl Weimand. They received the Nobel Prize in 2001 for this. And so in their first experiment, they managed to get about 1,000 atoms right down into this state where the wavelength spread out so much that it became one quantum blob of matter. And now we can do that with about a million atoms or more.

Briefly, Andrea, we think of solids being a distinct phase, but even solid metal can exist in several phases. Can you give us the example of tin? Well, this is a really important point. And when at the very beginning I talked about the difference between states and phases, one of the interesting things is that you can have solids which have distinct structures and you can switch from one to the other. Now, in 1850, a German physicist by the name of Erdmann was called in to investigate the organ pipes of a church. And he took a look at it and the organ pipes were in a very strange state. They were pockmarked and they seemed to be crumbling away.

And they were wondering what this was. In fact, it was referred to then as tin pest. In other words, it looked like the pox, but for tin. Erdman did some very careful analyses and he realized that the, these postules that appeared on the surface were in fact tin. And the weird thing was that you could actually have tin in two different forms. A beautiful metallic form, and I've got a little strip of tin foil, and you can see that is malleable and ductile.

Of course, you can see our audience can't, but our audience can actually take a look on the website because we have photographs of one of my research samples of infected tin. And so I have an ingot here, which you can hear clinking, and the surface of it is completely shattered and broken. And this is the result of moving across this boundary of switching from the room temperature form of tin, which is metallic, to the low temperature form, which on the other hand is much darker.

What's the importance of this? Well, the importance of this is that, in fact, all solids exist in a series of phases. If you were to think of carbon, you have diamond and grass. and other forms. But what we've discovered is that actually this initial kind of gut feeling that the world is made up of solids, liquids and gases, the world is much richer than that.

Anthony, one interesting, another exotic state of matter was discovered quite recently, and that's liquid crystal. How is that distinct from what you've been talking about? Okay, so liquid crystals were first identified back in the 19th century in 1888 by a German called Friedrich Reinitzer. And what he noticed was when he took,

he was working on a cholesterol derivative, when he heated it up, it melted into a cloudy liquid, and then at a higher temperature, it melted into a clear liquid. So this material had two melting points. And this intermediate phase is now known as liquid crystal. And it's easiest to think about,

I've talked about crystals where you know exactly where each atom molecule is sitting. and then in the fluid, the liquid, that's not the case. But if you take something like a bunch of pencils, say, the liquid crystal phase happens when you have a molecule that's got some anisotropic shape, not spherical. So if you take a bunch of pencils on a table, when they are spread out far apart,

they can orient and end it every which way. But if you start pushing them together, they have to align. and in a liquid crystal, one of the phases, and there are many, but one of the possible phases is that you have rod-like molecules, and they all line up. And so you can go from a state in the crystal where you know exactly where each molecule is sitting to one in the liquid crystal state where you know which direction the molecules are pointing, but they are not actually any longer on a lattice. They have liquid-like order in position, but they have this orientational order.

And then if you heat up further, you will go back into the standard, liquid phase where the molecules will be pointing all over the place and they are also not on a lattice. So a liquid crystal is, as the name suggests, intermediate between this ordered crystalline structure and the fluid liquid structure. Why is this important? It's important. We all see liquid crystal displays around us every day. You've got a computer screen there, our TVs.

So the point is that liquid crystals orient spontaneously at a certain packing density, but they also can be easily oriented by an electric field. So you can easily move from a state where they are disordered to where they are lined up and how they transmit light will depend on whether they're ordered or not. So that's why you can address each point on a TV screen by switching with an electric field and that's why they're useful in displays. Justin Walk, you've discovered, scientists have discovered, that for some substances it's possible to be both a liquid and a solid at the same time, is it possible for you to encapsulate that phenomenon succinctly? Well, this can happen in a very strange sort of structure, which has a name that's quite

appropriate to what we're doing now on in our time. It's called a host guest structure. So here we are as your guest and you're the host, Melvin. So what's a host guest structure? Well, first of all, if we think of a crystal as Athene has described it to us, it looks, as she said, like an atom sitting in a regular arrangement, a bit like soldiers on parade. Everything's lined up. And imagine taking a crystal where all of the atoms are in this very, very regular lattice

like the soldiers on parade. Then if you take a big block of this crystal and drill holes from the top to the bottom, like a well hole, and again you make those holes regularly spaced. So now we have a picture, if you like, of a cube of crystal matter, somebody's drilled a series of holes down it. Now it gets a little bit more complicated,

if you can hold this in your mind's eye. Take some more atoms and a string of them, like an unclasped necklace, and let those dangle down the holes. Now, the amazing thing about this structure, and we call it a host-guess structure, so the host is the crystal, and the guest is the string of pearls,

if you like, going down the holes, is that if you look at the spacing of the atmosphere, atoms in the crystal, and the spacing of the atoms on this string, they can be completely incommensurate. What that means is the ratio between those spacings can be an irrational number. No relationship by ratio that we know on. And already this is a very, very strange state of matter, only recently discovered, but it happens, what's even more unusual is it happens in very simple materials. It's been discovered in sodium, in potassium, things that we would teach our our undergraduate students are the simplest metals possible.

Then take it a step further, if you heat up these materials, in certain cases, it's possible for the guest, for the string of pearls, if you like going down the holes, to melt while the host remains a solid. So what do I call this? Do I call this a solid? Do I call it a liquid? It seems to be both at the same time, yet made from a single element. I mean, it's utterly remarkable how nature creates these incredibly complex structures from very simple atoms. And we used to think that this was extremely unusual, but computer calculations are indicating that this might happen in a whole variety of materials. Another interesting example, Andrew Seller, is glass.

How does that fit into this discussion? Well, glass is very important because although it looks like a solid, And so if you think about the window in your house, it certainly has that solidity, that lack of, you know, it doesn't change shape. If you start looking at it at an atomic level, then what you find is that it's not regular. And so whereas with ordinary crystalline solids, you have the atoms arranged in a very regular way. And when you melt them, they sort of move around higgledy-piggledy, although staying together, what you find is that what a glass consists of is essentially a kind of snapshot of the liquid. In other words, it's arranged a bit like a liquid pretty well randomly, but nothing can move.

And so perhaps the best example, in a sense, the best analogy, is to think about an old plate of spaghetti. It's cooled down on your plate. Everything's locked into place. You know that you can pick them all up and the whole thing will retain its shape. But on the other hand, there's no order. There's no sort of regularity about it. And for this reason, we would call it an amorphous material, one which really doesn't have a defined structure.

Can I come to you now, you know, it's possible to influence the phase of a material, I'm told, by changing its composition. Would you give us an example of that and why it's useful? Well, you can get very complicated phase diagrams. Andrea was talking about phase diagrams earlier. And one of the ones historically that was very interesting and very important was understanding steel. So that in steel, which is basically iron but with impurities, you can start changing the crystal structure.

You will start changing the properties. And the Industrial Revolution was largely based on people being able to change the crystalline structure in ways that enable them to get much stronger materials. and addition of carbon was very important to that. So these are much more complicated phase diagrams where you may have many components present. So you change the composition and you will get a malleable material or a material

which is very brittle, for instance. And you can also change these by changing the thermal processing as well. So these are complex and crucial things. If I could come back to what Andrea was saying about the glass as well, at the beginning he very carefully distinguished in phase and state. One of the interesting things about a glass is it's a non-equilibrium structure. It's a structure which is a snapshot in time.

It's frozen and it's stuck in that shape. It's a frozen liquid rather than a thermodynamically equilibrium structure. And these are also really important materials. His analogy with the spaghetti, I like to think of that more as the case of a gel. So we're all familiar with jelly, making jelly from jelly tablets, dissolves, dissolving this, which is basically gelatin in boiling water and then letting it cool down and you get a soft solid forming. And that's very much like the spaghetti. You can think of the spaghetti as being the gelatin molecules, which are long chain molecules. And gels are another very important non-equilibrium structure, which have lots of applications in the modern world, which are very important, not just table jelly, but much more subtle applications than that.

And so there are these, they're not exotic phases because they're not phases, but they are states of matter which are different from our conventional solid liquid and gas classification. Just in York, there's a link between what we've been talking about and the discovery of planets outside the solar system. What's the connection? Well, the connection there is that we're very interested in what happens to matter under high pressure. So if we first of all stay in our own solar system for the moment, the pressure at the center of our own planet is about 3.5 million atmospheres.

At the center of Jupiter is about 70 million atmospheres. Now, what's gone on in recent years with the Kepler mission and so forth is we now know that there are planets around many, many other stars in our galaxy. Over close to 2,000 planets have been officially discovered. There's many more waiting to be confirmed. You can download a little app for your iPhone, show you where they all are. The furthest one away is about 27,000 light years away,

but there are planets that have been discovered just a few light years away. Now, many of the planets that have been discovered are much larger, for example, than Jupiter. And that's what made them a little bit easy to discover, if you like. They were so large. And what we as physicists would love to find out is what are these planets made of? We can work out their radius. We can work out how big they are, because in fact, to discover them, you watch them move across their sun, if you like, across their star,

and you watch the light diminish, and you can work out how big they are. And then by looking at the star wobble, you can tell their mass. And so with these two things together, you can work out how dense, on average, are planet is. So if we have that information, if we knew how squashable matter was, we could, at very, very high pressures, we could make a stab at telling you what the planet was made of. And already physicists are doing that. For example, they've got an idea that there's a planet out there

composed almost entirely of water. It's a really water world in reality. But we'd love to know what the... But in order to confirm these sorts of impressions, we need to discover in the lab how to make matter extremely high pressures and test our theories of how squashable or how compressible it is. Some of the pressures you come to in the labs are phenomenal.

They can't get your... That's true. As I said, the centre of the Earth is three and a half million atmospheres. You can do that almost relatively easily in the laboratory. And the way that that's done is by taking two diamonds and making anvils from them, making them into sharp points with a flat on, and putting your material between these diamonds and squashing it, you can get close to centre of Earth-type conditions.

My own research is to try and go way beyond that to centre of Jupiter-type conditions. Finally and briefly, I'm afraid of the evening, What was research going most strongly at the moment in this area? Oh, I don't think you could identify a single area. I mean, the three of us are each tackling this kind of problem in many different ways. So for me, as a soft matter physicist, things like the gels, the liquids, crystal state, are what interests me. And there are lots of applications, biomedical applications for gels, for instance. So that's what excites me, but it will be different for the others.

And we'll love to come back for another program for that. Thank you, Atheney, Donald, Andrew Seller, and Justin. walk next week we'll be talking about the scholar Strabo and his great work at Geographica 2,000 years ago. Thanks for listening. And the In Our Time podcast gets some extra time now with a few minutes of bonus material from Melvin

and his guests. So there certainly is plenty more we could have discussed. Yeah, so what's the plenty more? Everybody says, I wouldn't mind a 10 or 5, or thomps would do, but there's plenty more. But I think what I find interesting is you come up with this very simple idea that you have a competition, a competition between matter wanting to bind together,

and then you give it energy of motion, what we call thermal energy, that makes it want to fly apart. And those things are in competition. But the way that nature, if you like, solves that problem, she does it in so many different ways that astounds you at each turn. And I just find it so fascinating

how we get amazing complexity, even in the physical world, even talking about the biological world, coming from very, very simple equations. Yeah, and I think the interesting thing is that much of the discussion was about sort of pure materials, elements, water, and so on. And Athene brought out this point of changing the composition slightly and starting to make alloys. Now, for me, as a cyclist, one of the really exciting things is the fact that there are new phases, for example, of steel, which are being found, which gives you kind of extraordinary stiffnesses, extraordinary likenesses. And this is really

by playing around on this, on this energy landscape that you get, but where you're, you're tinkering in this case with a combination of the composition. And then, of course, the history, you know, what you've, the thermal treatment of the material. Processing, we didn't get into at all. But in the case of steel, I mean, undoubtedly, the Industrial Revolution and steel could have been a topic in itself. And it's not one that I'm hugely expert. but the control, the thermal processing, the making sure you've got the crystal size that you want so that it doesn't just fracture

if you go over a bump in the road in the case of the bicycle. It did make the industrial revolution what it is because of... So you see a direct relationship between the signs there and the industrial... A direct relationship. Well, a lot of it was empirical at the time. No, the metallurgy, the understanding to a large extent came later, but they knew what they were doing.

and I mean people like Wedgwood to move away from steel the things that he did it was very scientific in a way even if they didn't have the language that we are using it was extremely it was extremely systematic and I think that was the thing it was the experimental method even if they didn't have the models he had an extraordinary analytical mind and he was able to to kind of map things out

within ceramics and what's interesting is the extent to which you were able to progress in this area without necessarily having a theoretical framework to support it and that would come later. And when it comes to steel and the industrial revolution, it's interesting to note that that comes about by learning, well even in

the Iron Age way before, learning how without any theory whatsoever how to put impurities into material. And in a controlled way. And of course we've had exactly the same revolution that you might call the silicon revolution that's led to the information age. And that's also come about by learning how to put impurities

into silicon in a controlled way. That's how you make semi-conduble do exactly what you want them to do. And it's also learning how to make the silicon pure enough in the first place. It's to be able to make it in pure. Absolutely. I agree. And that's back to the processing. But there's many other sorts of things we could have discussed, for example, this concept of meta-stability. So, which is, Andrea is talking about an energy landscape. And so one of the things you can have in that landscape is something like a mountain lake where you have a dip up in the mountain where you can get water pooled. And so it's not at the bottom of the hill. It's halfway up in a pool,

and that's what we call metastability. And I think it would have been very interesting to have been able to point out to people, for example, that diamond is not the stable form. I wrote diamond because I trailed diamonds and we didn't get around to diamonds and we could be sued under the act of not doing what I said we would do. Diamonds are very important in all this, aren't they?

I've kept scribbling diamonds. First of all, when you're trying to map, out where you know what the most stable form of carbon is at a particular condition of temperature and pressure then you find that there's a region where you have the diamond you have a region where you have graphite and so on under our conditions of room temperature and pressure it's not diamond and so diamonds at least theoretically are not forever because they should be converting but they have a lot barrier to go over to get back this mountain range the real problem is

that stepping across that turns out to be very, very difficult. But of course, very soft. Tom Marys has come in, our producer, and Victoria, our other, is over there. I would like, because we like our own little chat. So, chat on, but we're going to be joined. Would you like a cup of tea or coffee? I'd love a cup of tea.

I'd love a cup of tea or coffee. There are many more Radio 4 arts and discussion programmes to download for free. Find these on the website at BBC.com.com.com. slash radio four