In Our Time - Absolute Zero

Episode Date: March 7, 2013

In a programme first broadcast in 2013, Melvyn Bragg and his guests discuss absolute zero, the lowest conceivable temperature.  In the early eighteenth century the French physicist Guillaume Amontons suggested that temperature had a lower limit.  The subject of low temperature became a fertile field of research in the nineteenth century, and today we know that this limit - known as absolute zero - is approximately minus 273 degrees Celsius.  It is impossible to produce a temperature exactly equal to absolute zero, but today scientists have come to within a billionth of a degree.  At such low temperatures physicists have discovered a number of strange new phenomena including superfluids, liquids capable of climbing a vertical surface.With:Simon Schaffer Professor of the History of Science at the University of CambridgeStephen Blundell Professor of Physics at the University of OxfordNicola Wilkin Lecturer in Theoretical Physics at the University of BirminghamProducer: Thomas Morris

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get your podcasts. 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.uk. Slash Radio 4. I hope you enjoy the program. Hello, the coldest natural temperature ever known on Earth was recorded 30 years ago at a Soviet research base in the Antarctic. At a quarter to three in the morning,

the thermometer registered minus 89.2 degrees Celsius. Beyond our atmosphere can be dramatically colder even than this. Astronomers believe that interstellar space is a temperature of around minus 270 degrees. But the coldest temperatures yet known, colder even than space, have been created artificially in the laboratory. Scientists have been creeping ever closest to the lowest possible temperature known as absolute zero. The idea that temperature had a lower limit was first suggested in the 17th century. The race for ever colder temperatures began too,

200 years later and has resulted in some of the strangest, most important and useful discoveries of modern science. But although physicists can get within a billions of a degree of absolute zero, they'll never quite get there. Why not? And why does temperature have a minimum? With me to discuss Absolute Zero are Simon Schaffer, Professor of the History of Science at the University of Cambridge, Stephen Blundell, Professor of Physics at the University of Oxford,

and Nicola Wilkin, lecturer in theoretical physics at the University of Birmingham. Let's start with the idea of temperature. What did early scientists think temperature was, and how did they investigate it? Are we going back to the Greeks? We might want to go back to the Greeks. They were certainly fascinated by problems of heat and cold. They lived in a culture in which all energy sources were either human, mechanical or thermal,

and understanding heat and temperature was fundamental to their cosmology. temperature is one of those concepts that seems very straightforward until you start thinking about it. You can have bodies which have a lot of heat but very low temperature. You can have bodies which have very high temperatures but really not very much heat. For Aristotle, for example, temperature in the sense of possessing heat was an absolutely fundamental quality of all bodies. and in his natural philosophy, he'd proposed the possibility of what came to be called the primum frigidum,

the fundamental cold. It was even supposed by many scholastics, followers of Aristotle, that cold was, as it were, a substance equivalent to heat. When substances became colder, it was because they were absorbing cold, and when they became hotter, it was because they were absorbing heat. In the 17th century... That's quite a leap. Yeah.

Criticism of those views emerges. I think this is characteristic of the story of cold and of temperature, along with the development of new technologies, new instruments. In the 17th century, that meant the thermoscope. A rather elegant device involving a sealed bulb full of air in the first place who's plunged into water, and the height of the water would measure in some sense the temperature of the body surrounding the bulb. Using devices like that,

experimenters such as Robert Boyle were able to show, at least to their own satisfaction, that there is no such thing as the primum frigidum. There is no fundamentally cold substance. Rather, according to Robert Boyle, and here he was following his master and influence, Francis Bacon, cold has to be understood as a deprivation

of motion, heat is a form of motion, there's no such thing as a fundamentally cold substance, but the possibility of an absolute limit to cold, a fundamentally cold temperature, became possible. And this was uncovered more specifically by Frenchman, Guillum, Amonton. Amonton is a fascinating character in the history of 17th century sciences, congenitally deaf, correspondent of the Royal Academy of Sciences in Paris, an entrepreneur and businessman as well as an experimenter, keen to build a machine, which he called rather wonderfully a fire engine,

which would have driven a pump by the expansive force of air. So he concentrated his attention on intensifying air's expansive force, and I think this is decisive, working out ways of measuring what he thought of here following Boyle, as the spring of the air. And by designing a really pretty accurate thermometer with a scale marked along its side, he was able to work out that as temperatures drop,

the springiness, what we might think of as the pressure of the air drops. And he projected the idea that there must be a point at which the air loses all its spring. And that would be the limit of temperature, the absolute zero in that sense of temperature. He didn't pursue the speculation, but after its work,

that idea of an absolute limit became thinkable. Stephen Blondell, in the 1830s, one of this country's greatest scientists, Michael Faraday, started to explore low temperatures. Why did he do with that, and what did his work involve? Well, it really started because he had managed to get a job at the Royal Institution. He'd been a bookbinder,

and he'd attended some of Sir Humphrey Davy's lectures at the Royal Institution, and then presented Humphrey Davy with a bound set of his notes. Davy was so impressed he took Faraday on. And so then Faraday finds himself as David's research assistant and doing some of the more dangerous experiments. In a formal sense, barely educated. Barely educated. but doing these extraordinary experiments

and a whole range of different things, of course we know him from his work in electricity and magnetism, but what's relevant for today's topic is that he started to do some experiments which took on from one of Davies's great achievements which was to show that chlorine was an element. And so Faraday starts playing with crystals of what was called at the time chlorine hydrate.

This is basically ice with chlorine dissolved. inside it. And Faraday did some experiments where he put chlorine hydrate in a sealed glass tube and heated it up. Now the problem was that's quite a dangerous thing to do because the chlorine is released and goes to very high pressure

and the glass tubes frequently exploded. So after many of these experiments Faraday found himself with glass all over his face and had to pick them out of his eyes, the shards. So there was no health and safety of course in those days so he was allowed to do these experiments. Surely shards have

glass in his eyes? Yes, absolutely. He then wrote in his diary after two or three days, his eyesight returned to nearly normal. So you can see, you know, it didn't seem to bother him at all. What was important was the breakthroughs. Can you just remind us what he's doing this for? Well, he was really doing it because what he wanted to do was to raise the pressure inside the glass tube by heating them up. And by putting chlorine at very high pressure, what he actually achieved was he made a little oily liquid that appeared on the inside of the glass and this oily liquid was in fact liquid chlorine. He'd liquefied chlorine. The way this works, it's the same effect that you get when you go to the top of a mountain and you try and make tea and

the tea is awful. And this is because on the top of a mountain, the air pressure is lower. He was doing the reverse effect. So the tea is so awful because you boil the water at a lower temperature when the pressure is reduced. Conversely, if you raise the pressure, the boiling temperature increases. Now chlorine would normally go from liquid to gas at about minus 30 degrees centigrade. By going to high pressure, he could raise that up to the temperature of the Royal Institution in February. So he was able to liquefy chlorine gas

the first element that had ever been liquefied. And then when he reduced the pressure, he had some very, very cold liquid chlorine in his laboratory. I mean, while you're talking, which is to listen to it, it's wonderful to think that three or four streets were, this was happening in the smallest laboratory. not much bigger than this small studio and the barrage which is still there.

Absolutely. With what we've now regarded as Neanderthal set of instruments and away he goes. Anyway, never mind, that's another programme. I said, well, we've done that programme. So he does this and then what? Well, having liquefied one material, chlorine, in fact

ammonia had been liquefied before by Van Maureram elsewhere, Faraday using this technique was then to go on and liquefy a number of other substances which were normally thought of as gases. So he does the same with hydrogen sulfide, nitrogen dioxide, sulfur dioxide, later on somebody else does it with carbon dioxide

and actually turns that into a solid using exactly the same technique. So he's taken what becomes the search for absolute zero, quite a long way forward. Absolutely. And enter another Brit, Elsterman, who went to Glasgow University and then trained in France, Kelvin, became Lord Kelvin. He has a big influence here. I'm sorry, but can you bring that to the table?

Yes, so William Thompson is a completely different kind of person. His father is a professor. He's educated at Glasgow University and then at Cambridge. So he's a person of learning and of scholarship. And unlike Faraday, who was an experimentalist, Kelvin is a theoretical physicist. And he approaches the concept of temperature and takes further the work that Simon was describing with Amonton.

Kelvin was very much concerned by the issue of thermometer because if you're trying to measure temperature, one of the problems is that any material that you might use to make a thermometer, whether it's mercury or liquid alcohol, looking for its expansion in a tube, the problem then that you have is that you're trying to measure temperature by the expansion of a liquid, assuming that that gives you a well-defined temperature.

But how do you know? But we're at this time talking at the time when Mr. Farranite and Mr. Celsius have put two thermometers into circulation. They have... So we've got these two thermometers and there within Calvins. All that Farnoite and Celsius had done is basically to say, well, if we have some liquid which expands in a tube, we have a particular scale. So Celsius, for example, sets zero degrees at the boiling point of water and 100 degrees at the freezing point. later inverted to give the more familiar Celsius scale. But essentially what they were looking for is fixed points on a scale,

but they were always reduced to thinking about how some material property behaves, whether it's mercury or alcohol or water or gas. And Kelvin wants to take us away from thinking about a material substance. Is there an absolute definition of temperature? And that's what he tries to do with his scale? That's what he tries to do with his scale. He was essentially, his contribution was particularly to relate the work on heat engines from Kano in France,

which had essentially been rediscovered, and to try and define some kind of absolute temperature scale. And when he does this, the net result of his work is to see that absolute zero, the absolute zero of Amonton minus 273 degrees centigrade, which we now call zero Kelvin, is the right zero of the scale. So we're going towards absolute zero.

Nicola Wilkin. We have this way of measuring temperature, zero Kelvin's entered into the discussion. But what do we actually mean when we talk about temperature? Can we revisit that? Okay, so everyone's quite happy with the concept of something being hot and another thing being hotter. And what this temperature scale that we've just alluded to, the Kelvin scale, which is the thing that's scientifically used now, is it enables us to be able to measure a temperature of one cup of coffee and then measure another one separately and determine whether they're actually the same. But what is this actual temperature measuring? And what it's picking up is essentially it's a bit like a speedometer.

It's looking at the average speed of the atoms in your cup of coffee. So your cup of coffee looks like it's completely still. But inside it, all the atoms that they're moving around, they're bouncing off each other, they're bouncing off the walls, they're vibrating, they're rotating. And that average speed is what's giving you. you the temperature. So as your coffee cools and it'll look pretty much the same as it did before, what's actually happened is that the atoms have slowed down very slightly. And that's what

you're picking up on the temperature scale. So there's a relationship between temperature and energy here. So energy comes into the equation now. So we would think about the energy, the average energy that's in the system as being basically how it's related to the temperature. That's what we're measuring. And so temperature becomes much more complicated, doesn't it? It's a different concept. So we're no longer just thinking about a linear scale. But what it means is the knock-on effect is that as things cool down

and as we hit this absolute zero that Kelvin had conjectured, we end up thinking about something that's becoming gradually slower and slower. So you've got the idea of the absolute zero and the idea of energy slowing down. What would absolute zero have meant to scientists at the end of the 19th century? Okay, so it's very important that you put the time in there. So this is pre-quantium mechanics.

At that point, everything is just slowing down and slowing down. There's no other source of energy in there. That means that as we hit absolute zero, we're left with the scenario that everything must be rigidly pinned down. That includes not just the atoms, but also things, the electrons, for instance. So that has consequences, obviously.

One thing might be if you were to look at the electrical resistance, one conjecture was that that would then become infinite because nothing was able to move. And that would be important. And that's important. So at the end of the 19th century, the situation is, can you just re-describe exactly what most scientists, people in this field, there aren't many in this field,

but those in this field are thinking. They've come a long way in a short time, haven't they? They've come an extremely long way. and they're just thinking that if they can call things even further they'll be able to get to this goal of things being stationary. But the main thing is this slowing down? The main thing is the slowing down, yes. Simon Schaffer, they developed what could be called a race,

almost like the first person to the South Pole, wasn't it? It was the first person to the lowest temperature. I think we should say here that it's a wonderful example of pure research. they did it because they wanted to do it they did it because they wanted to know what was there as it turned out it has often happens it had wonderful benefits for the whole many of us and in society

well all of us in society so but they're hammering away at that and it throws up people in competition therefore it throws up characters almost by definition because of competition two fascinating figures are Dewa and Ramsey they sound like a pair of solicitors but in fact they were competing physicists

I'm sorry over to you yes I think the analogy with the race for the pole is a very, very good one, and in fact it was explicitly made at the time. This is the 1880s and after the heroic triumphs of

European imperial geography are on everybody's minds. James Dewar is one of the British protagonists of this and he was working in a redesigned version of Faraday's Lab in the basement

of the Royal Institution where he held the chair of chemistry, not very far from where we are now. His rival, William Ramsey, was also extremely near where we are now at University College London on Gower Street, and Dewar and Ramsey loathed each other. This was, in a certain sense, tragic for the development of London physics, London chemistry, the science of cold in London. since what these two men had, if they'd been put together,

was an absolutely unrivaled combination of experimental technique, theoretical understanding, and high-powered engineering equipment. They were both Scots, that's presumably coincidental. Duer, the senior man, he'd actually beaten Ramsey to the Royal Institution job in the 1870s. Dure was a professional melancholic, described by his. students as deeply pessimistic. As is common, that went on with a deal of showmanship

and a lot of aggression. Dewar saw the move towards absolute zero as his destiny. And by assembling in the basement of the raw institution, an almost unrivaled group of lab technicians. None of this is possible.

On your own, you need a team of skilled workers. And kind of Heath Robinson-esque, assemblage of of air pumps, pressure devices, gauges, variably safe bits of glassware, he was able by the end of the 1890s to liquefy hydrogen. And that was an extraordinary achievement

because, as we heard, Faraday had managed to liquefy a range of gases, but there'd been another range of gases, notably oxygen and hydrogen, which Faraday called the permanent gases, because apparently they could not be liquefied, Duo managed to liquefy them. Ramsey, on the other hand, and his allies, notably his brilliant lab technician and assistant Maurice Travers, were producing in ever more dramatic ways a range of hitherto unknown gases called the rare or noble gases,

argon, neon, and ultimately helium, which also seemed to challenge liquefaction and which also seemed to offer the possibility of moving towards absolute zero. Why did this personal deadlock turn into a professional deadlock and mean really the end of any serious development of that in this country which was leading

the field at the time and it passed over to other countries? Can you briefly tell us why they screwed it up? There were two reasons that mattered. One was that Ramsey Travers and their allies persistently accused Duer of claiming originality when he had none.

There were other researchers in Europe, notably Carol Olshevsky, great Polish scientist based in Krakow, whom the UCL crew reckoned had anticipated Dua in almost all his discoveries. They were in large measure wrong, but it didn't make the atmosphere, if you'll pardon the expression, any karma. The other reason, perhaps much more due. decisively was the uneven availability of the machinery to produce very low temperatures and the gas samples to produce very low temperatures.

So that, for example, Ramsey had control over most of the helium in Britain, which happened to be in Bath, and that made it extraordinarily difficult for Dewar, his enemy, to move on from hydrogen to working with helium. gas and turning it liquid. So we have kind of helium water. I think en passant it's worth saying for poor El pessimistic duo that he invented an object and he tried to patent it to make money out of it. It completely failed

and it was called the thermos flask. It was called the duer flask. Maybe it was what's in the name. Could be that. Anyway, Stephen Blondell helium, this becomes, as Simon has said, that comes into the story quite fiercely. Ramsey denies helium to duer.

The British thing, this is a summary if it still crunches to halt. But what is important about helium? Well, it's ironic really because when the permanent gases were identified in the mid-19th century, these were the ones that couldn't ordinarily be liquefied. Helium wasn't one of them because it hadn't been discovered. And helium is unusual because it was first discovered on the sun. It was seen in solar spectra.

And so it's very odd that it was discovered on another heavenly body before it was actually discovered on planet Earth. so that's why it's called helium, Helios from Sun. And shortly after it had been seen in spectra from the sun, it was found actually dissolved in certain minerals. So helium doesn't, if you have helium on planet Earth, it tends to leave the planet. It's such a light molecule that it doesn't hang around in our atmosphere.

But it's actually also part of alpha particles from radioactivity. That's essentially helium. So the Earth is actually full of many different radioactive, compounds, helium is being produced all the time and is therefore found in minute traces in certain minerals. And so the race for helium was partly, as Simon was explaining, the fact that helium was very difficult to obtain and only certain places had it.

Now why was it so special? It was so special because once liquid nitrogen, liquid hydrogen, liquid oxygen, who had all been made, it was discovered that when you put helium gas in contact with those liquids, it wouldn't itself liquefy, which means that helium had to liquefy closer to absolute zero than anything else. So it was going to be the gas

that gave you the closest approach to absolute zero, and therefore it was the final part in the race for the pole. Would it be fair to say that the transition in these materials to get to absolute zero was from a vaporous gas to liquid, too solid, and you track those through, and then you had a... You track those through, exactly. Yes. Nicola Wilkin, can we continue with the liquefying of helium, please,

with this man, Oneez, I think, is that how he pronounced it, who he also discovered a strange phenomenon. And while doing that, he discovered this phenomenon of superconductivity, which is very important indeed, an influential in various areas and modern. Can you just talk about Ones and helium and superconductivity? It's a bit tall order, but we're not rushed for time. So Honest succeeded in getting superfluid helium, and that in itself was an extraordinary achievement.

What did you do that others hadn't done? I don't know if you're... Well, I mean, I think his particular strength was that he had a huge array of technicians. We were hearing before that Dewa had a few, and he'd fallen out with Ramsey, who had somebody very good. But Conning Onis had trained an entire army of technicians to an incredibly high level of precision. And so they had the most equipped

and the best organised laboratory in the world. Factory science began at that time period. It did, yes. So it was more efficiency rather than anything particularly special at that point. But having generated the liquid helium, he rather than that seemed the lowest that they could go, he started investigating what might possibly happen to other materials if they were subjected to these extraordinarily low temperatures.

And you're still driving to all. finding if it can reach the lowest temperature? And seeing what would happen. So curiosity, what happens at these temperatures apart from things getting very cold? And so he thought that he better have something that was very pure in order that he'd be sure that it was an effect associated with that particular material.

And he was very lucky. He had a choice of two things in his lab. He had mercury and gold. And fortunately for him, he actually picked mercury. So he stuck it in his system. He cooled it down, and the anticipation from what we were discussing earlier was that the electrical resistance in this material would possibly head off to some sort of constant or would become sort of infinite and nothing would flow.

So imagine the surprise in the laboratory when as he cooled his mercury, he found that neither of these things. Instead, it just very abruptly just fell off a cliff. That was it. No electrical resistance. of course What did he conclude from that? The first thing he concluded was

probably there was a difficulty with the experiment. Surprising results should first of all rule out experimental error. So they spent quite a while cycling through and indeed they could not make this effect disappear. When you say fell off a cliff

can you tell us precisely on me it stopped, things stopped happening that he could register. On the equipment that he had, he could no longer see that there was a resistance. But temperature is going down and down and down as he's heading for the lowest for absolute zero. So he was reducing the temperature in the system, but at that point he was not searching to look for absolute zero. He was concentrating on this property of the electrical resistance of the system.

So after he's tested his equipment, what was his conclusion? His conclusion was possibly it was true. So fortunately in those days, one could then do an extended experiment. So he managed to set up within a ring of mercury a current flowing. And he let it stay there. In fact, he let it stay there for a year. So he set this current flowing without any batteries, left it there, undisturbed for a year, current still flowing.

So because there was no resistance, he had, this current, this persistent current, and hence it became a superconductor. Was this an entirely new observed phenomenon? Completely new. So he was actually very quick off the mark in realising that there were extraordinary technological possibilities for this,

that if one had something that could conduct without resistance, then one could perhaps build huge magnets. And that is something that, It didn't happen in his lifetime, but certainly are around us everywhere today. From the superconduct, that we only arrived at. Now, why is it that we could only arrive? We, wee, you. People like you, not like me.

Only arrived at this when they drove this far down. Will you tell me? Well, I think one of the important things to realise about superconductivity, just to take things back a little bit. The ancients had realised that the heavens had a different kind of motion to Earth. Everything seemed to be pure and ethereal. the planets went round without any friction, whereas motion on Earth seemed to have friction and decay.

Of course, we now realise that that's just because space is largely vacuum and that the same dynamical Newtonian laws work in space as they do on Earth. But in fact, when quantum mechanics began in the 20th century, we were able to see that electrons would go forever around an atom. They don't need a battery. They just carry on in their orbits forever. That's very much like the kind of ethereal, ancient phenomenon of the heavens. Now, in fact, what had happened in superconductivity

is you have something very similar to an atom. You have a current of electricity that will go around a loop of wire and it will carry on forever. And so, in fact, it was a scientist called Fritz London working in Oxford in the 1930s who made the connection that superconductivity was essentially like a giant atom. A phenomenon would normally only be operative in the microscopic world of atoms,

that's electrons going around and around forever, is now seen in a macroscopic way, seen in large objects. And so somehow with superconductivity and some of the other low-temperature phenomena, you're seeing something about quantum mechanics that is manifest on a huge length scale. And this was the crucial observation. It only works at low temperature because you need those low temperatures to stop the disruption that occurs due to thermal fluctuations.

Because so little is going on. Because so little is going on. The basic stuff comes through. It turns out that these kind of macroscopic phenomena, are disrupted by the thermal vibrations that we heard about earlier, that jiggling around that occurs at high temperatures. So it becomes a low-temperature phenomenon. Briefly, Simon, could you tell us about the techniques that they were being employed now?

Yes, I mean, as has been said, Cameling Ones was an extraordinary technician as well as one of, I think, the greatest laboratory managers that physics has ever known. And he and his colleagues and rivals, like Duer, like Ramsey, were drawing on state-of-the-art engineering. Here's an example where, on the one hand, these are inquiries driven by curiosity. On the other, they're absolutely working hand-in-glove with and helping develop extraordinarily important machines like fridges, without which modern civilization might be said to be impossible. The two key techniques that Duer, that Camille Ganes principally drew on.

On the one hand, what was called at the time, the Jewel Thompson, that's to say the dual Kelvin effect, when a gas escapes from a containment at very high pressure through a nozzle or siphon into an area of very low pressure, you get an enormous dramatic and very sudden cooling.

Engineers like one of my heroes, William Hampson, working here in London with Ramsey and with Travers, turned that process into an automatic air. liquefying machine relatively cheap leading directly to the establishment of a very important enterprise, the British Oxygen Company. The second process was called the cascade technique, which is to

liquefy a gas and then use that liquid to liquefy a second gas and so on and so on. So that you have a kind of regenerative feedback loop in which you can drive down the ambient temperature of the gas and then the liquid pretty fast and very efficiently. Combine the two, that's to say the dual Kelvin effect and the cascade effect as Dewa to a certain extent did, and Camelangling Ones did

amazingly, brilliantly. And you can reach the temperatures of liquid and, excuse me, of liquid helium and below. That's to say 4 degrees K. That's 4 degrees above Kelvin, 5,000 minus 273 degrees. Right.

talking about absolute zero, and we're going through very interesting jungles to get there. Nicola Wilkin, this is a compound question again, questions I'm afraid. How did the breakthrough of quantum mechanics change perceptions of absolute zero? And can you tell us about another strange phenomenon that arose, which is super fluid? But can you tell us, first of all, how did the breakthroughs of quantum mechanics in the other 20th century change? I'm repeating my question. I must be getting old.

Change, absolutely. knowledge of absolute zero. So, as ever when studying physics, just as you think you've understood it and you head into the regime which is going to absolutely prove what's going on, nature throws something in your way. In this case, it's quantum mechanics and obviously that's a rather large field in itself, so I'm just going to pick out a couple of things that are pertinent to what we're discussing today. So the first thing that happens as far as quantum mechanics is concerned is that there's something called the Heisenberg Uncertainty Principle. and what that tells you is that you are physically unable,

so it's precluded, from knowing precisely where a particle is and how fast it's going. So you can immediately see that there's a problem with the physics that we had of how we were describing absolute zero earlier, where we were going to have particles nailed down and absolutely no speed associated with them, because quantum mechanics says we can't do that. And so what that results in is that there has to be some sort of energy still left at absolute zero. And that, of course, has a special name.

You may have come across it at zero point energy. So the quantum mechanics means that we still have some sort of energy in the system. So you can't get absolute zero? You... That's a tricky question. Exactly. I'll come back to it. If it's tricky, that's tricky.

We can come back to you. Sorry, I interrupted you. Where you go. Sorry. So there are other reasons why you can't get absolutely zero. But there are energies. So the energy in addition, quantum mechanics tells us that we can't just pick out any old energy that we might want our atoms to have. They're actually quantized and there are specific levels that are dictated by the particular system that we're looking at.

So those two things are what will ultimately be able to explain superconductivity. But you... I asked you about... Another strange new phenomenon, as new as superconductivity, is another super-superfluid. Can you take us into the secret of superfluid? Okay, so history doesn't always follow in the order that you might logically think that the physics could have happened in.

a long time after helium had actually been liquefied. So the first thing that was actually discovered was superconductors by using the cold temperature that you had from the helium. But in fact, by investigating helium much more thoroughly, and it took, so it was about 1938, they discovered that helium itself, as you called it down further, had some very interesting properties of itself, as well as being a very cold liquid.

and that property is now known as being a superfluid. So what might we mean by that? Well, let me sort of give you an example. If you were to wade through very deep water, you would be very aware that you were having to work against that water to get through it. If that were a superfluid and you could stain the cold temperatures, you would not be aware of it. There would be no viscosity associated with it.

So it's a very strange liquid. It's fantastic because again like the superconductor instead of having a super atom we now have quantum mechanics that we can see again on a macroscopic scale. So it's actually possible to see this superfluid in a laboratory if you put it for instance in a beaker

it can climb the edges of that beaker. It can climb the edges I think I sort of did a back of the envelope calculation It's something like, depending on how accurately you do this, about 30 kilometres it could keep climbing up before it went out over the edge. It gets a sort of weird feeling. Strange feeling went through the studio, then it's...

That's the world, isn't it? Stephen Blondell, let's go back to the... We're getting closer or are we closer, can and get close to absolute zero. Where are we now? Where are they? Well, the... People like you, I mean.

Well, the race for absolute zero, we now realize in some senses, is not one we can ever win. What is that? Any cooling process that you can imagine will essentially reduce the temperature by a certain fraction. Now, you can do that process in several stages. So, for example, you might be able to imagine a process in which the temperature is reduced by half, and then you can do it again, and then you can do it again. So you go to a quarter, 1-8th, 1-16th. but you can never get there in a finite number of steps.

You need an infinite number of steps. So why do you call it absolute zero then? Well, because you can describe it in the limit. It is there, but you can never reach it. And this sort of idea is familiar in mathematics as the limit of a series. So this is something that we completely understand. In fact, in practice, it's even worse than that, because when you get down to very, very low temperatures,

you find that if you're trying to cool something, there will always be some environment that is warmer, and that will leak heat in. So the closer you get, you then start finding the thing slightly warms up a bit. And so it's like travelling on an escalator that's going against you. You can never get to the top. But nevertheless, Kelvin is minus 273 Kelvin is called absolute zero. And as you said earlier, Nicola, things in maybe a different context,

things fall off a cliff when you get there. So it's still something that is a lowest temperature, whereas there is no highest temperature. It's down there, even though you're billions away from it. And with advanced cooling techniques, since the Second World War or so, we've been able to get to a millionth of one Kelvin, that's a millionth of a degree above absolute zero. Further techniques have been evolved to cool materials down to one micro-calvin,

and then that's a millionth of a Kelvin. You can cool the nuclei of atoms, little less than one. billionth of a Kelvin, and recent techniques have evolved to cool small collections of gas atoms in a trap to even lower temperatures around a billionth of a Kelvin. Okay, time for two meaty questions, really. I'd like to know, let me starting with you, Simon, is can we move between you and Nicola to the practical use of these low-temperature research? Start with you and then move over to Nicola with that. Well, these are the experts on just how practical and just how exciting those uses are.

On the one hand, working at these ultra-low temperatures, especially below the temperature of liquid helium and then down to a few billionths of a degree, generates new kinds of substances and new kinds of behaviours that turn out to have the most extraordinary uses. On the other hand, working at those very low temperatures, think about all. the uses of something that has no longer any or a ridiculously tiny electrical resistance, promises the possibility of power systems and magnetic systems of the most extraordinary significance. Nicola. Okay, so these cold gases that were just mentioned,

so these things that can go down to a billionth of a degree, from an experimental perspective, normally you would say the fact that they're very sensitive, to their environment is not a positive feature. That's not normally a good thing about an experiment. But in fact, these gases, because they are so sensitive, are now becoming sensors. So one thing that they're

sensitive to, for instance, is variations in gravity. Now, normally walking around, your gravity has got a fixed value. So what practical use is this having? I'm sorry to Russia. Practical uses, so proposed practical uses as opposed to what's

there already. People are investigating using it for searching for oil, underground water supplies, archaeology. And on they go, in that way. Stephen, can you say a word about that? Then I've got to rub it out of the hat question to finish with. Okay, I just wanted to say that when you take a visit to a new land, this essentially has been a story of exploration to a new land. One of the questions is what can you bring back from it,

much in the same way that the potato was brought back from Peru by the Spanish, so this is a foreign thing that is brought back and it becomes useful. And really one of the most exciting areas of research, I think, is to try and get superconductivity, which is a purely low temperature phenomenon. We have to cool down all the magnetic resonance imaging magnets to get them to work. How can you get that to work at room temperature? Okay, rub it out of the hat. We've been talking about absolute zero.

You can't get to absolute zero. Recent research in Germany, we've got to be fast at, and now, they've created a temperature below absolute zero. Now, what do you make of that? It's just the fact that we've defined temperature in a way that is inconvenient for understanding this. It's actually hotter than infinite temperature. You've completely lost me. Is it below minus 273 Kelvin or not?

I mean, the producer is crossing his arm saying I've got to get off the air. Is it below? It's lower, but we haven't gone through T-4-0 to get there. Right. Well, we're all on the Cliff-Age. How very nice. Thank you very much. Nicola Wilkins, Simon Schaffer, Stephen Bundell. And next week, where's next week?

Yes, we'll be talking about Anton Chekhov. Thank you for listening. There are many more Radio 4 arts and discussion programmes to download for free. Find these on the website at BBC.co.uk. Okay.com.com.com.