- This is liquid helium. It's currently sitting at negative 269 degrees Celsius. That's just four degrees above absolute zero. But watch what happens if you start cooling it down. It starts to boil violently. - That's aggressive. - There are bubbles everywhere. That is until 2.17 Kelvin. At that point, something strange happens. The helium has transformed into a superfluid. - Ooh. It's completely still again. - What we've got is a beaker with a leak tight aerogel base. - This beaker holds the liquid helium well, but the
moment it turns superfluid, it starts to leak through. - Oh, the transition below is happening. Yeah, it's dripping. - Yeah, it's dripping on the right. - And the colder it gets, the faster it pours. - Oh wow. That's gorgeous. So fast. - That's spectacular. Superfluid helium can do things that seem impossible. If you put it in a cup, a thin film climbs up and over the walls, so it empties itself. You can also use superfluid helium to create a fountain with no moving parts. It is
powered solely by light. Scientists spent 30 years looking at this liquid without realizing what they had created. - This is really one of the great phenomenon in 20th century physics, - And it makes you look at matter in a different way. It allows us to explore the boundary of quantum and regular matter. So we teamed up with the gravity laboratory at Nottingham University to recreate the experiments and to find out what really happens when you just keep cooling down. The obsession with cooling things down as
much as possible began in the 1800s. People noticed that when you let a gas expand against a piston, its temperature drops. That's because as the particles hit the piston, they give it a little push and transfer some of their energy to the piston. So the particles slow down and their temperature decreases. Lord Kelvin learned of this and realized it offered a tantalizing possibility, that of an absolute temperature scale. Imagine that you let the gas expand an extreme amount to the point where all the particles effectively
stop. The particles no longer exert any force on the piston and they have no more energy left to give. This would be the absolute coldest you could get, absolute zero. Kelvin quickly computed what temperature it corresponded to and found it was around negative 273 degrees Celsius. People speculated what strange things might happen down there, a place with no thermal motion. Kelvin himself suggested that metals, which are normally great conductors of electricity, might turn into perfect insulators and not conduct any electricity at all. But there was
only one way to find out and that was you had to get there. Fortunately, the race to absolute zero was already well underway. It had started by accident 25 years earlier. In 1823, Michael Faraday was playing around with chlorine hydrate and his scientific mentor suggested that heating it in a sealed tube might lead to some interesting results. So Faraday sealed it up, warmed it in water, and watched as a yellow atmosphere emerged. Then as the pressure increased to around four atmospheres, he noticed that a bright
yellow fluid formed. He had just turned chlorine gas into chlorine liquid. Faraday then went on a bit of a liquefying spree, liquefying six more gases within five weeks. But just as he was hitting his stride, he hit a wall. Some gases like ethylene refused to liquefy no matter how much he squeezed them. You see why you need to zoom into a gas. There you'd find a bunch of particles whizzing around as fast as tiny bullets. And while it might not seem like it, the particles also
attract each other. It's just that they're so spread out that they meet relatively little, and when they do, they're going so fast that they mostly just bounce off each other. In some cases though, if you squeeze the particles together by increasing the pressure, you can get those particles to stick together and slide past each other. In other words, you get a liquid. This is what happened with Faraday's chlorine, but that doesn't work for all gases. If the attraction between particles isn't strong enough and the particles
are too hot, then no amount of squeezing can get them to liquefy. And this is what happened with ethylene. Luckily, there is another way. If you cool the gas down, the particles will slow and eventually they start sticking together and sliding past each other and you get your liquid. So to liquefy more tricky gases, Faraday needed colder temperatures and he got a lucky break. A French inventor had discovered how to make dry ice, which was about negative 79 degrees Celsius. So 20 years after his initial
chlorine success, he tried again. Faraday put ethylene gas in a glass tube with a bend, compressed it using pumps, and then lowered the bend into a bath of dry ice and ether. As the dry ice cooled, the ethylene, a clear colorless liquid formed. Feraday used the same technique and liquefied five more gases pursuing ever colder temperatures in the process and getting all the way down to negative 110 degrees Celsius or 163 Kelvin. But six gases still resisted his attempts, including oxygen, hydrogen and nitrogen. He just
couldn't get cold enough. Over the following decades, scientists all over the world picked up where Faraday had left off. Liquefying a new gas became a matter of national pride and the race to absolute zero was on. In 1883, two Polish scientists liquefied oxygen and nitrogen and the record low temperature fell to 77 Kelvin. 15 years later, James Dewar liquified hydrogen at 20 Kelvin before freezing it at 14. By the turn of the 20th century, there was just one gas left standing, helium, but some thought helium
might be impossible to liquefy. And there was good reason to think that because to turn a gas into a liquid, its particles have to attract each other. But helium is a very simple element. Its most common isotope consists of two protons, two neutrons and two electrons. Electrons form shells around the nucleus and helium, like all noble gases, has a full outer shell. So it isn't really seeking to gain or lose any electrons, and as a result, it doesn't really bind together with anything. That made people
wonder whether helium atoms attract each other at all. And if they didn't, you could cool down helium as much as you wanted, but you would never get a liquid. And even if the atoms did attract each other, well, the attraction might be so small that its liquefaction temperature could be much colder than anyone could practically reach. So when Haiku Kamerlingh Onnes set his sights on liquefying helium, he knew he was in for a challenge. Onnes had joined the race to absolute zero in 1882, and since
then he'd been building an industrial scale laboratory in Leiden, complete with chemists, technicians, glassblowers, and massive liquefiers. By 1906, he too had liquefied hydrogen, and from then on, he dedicated himself fully to liquefying helium. But working with helium came with two major problems. For one thing, just getting it was difficult. Remember, helium doesn't react with anything, so there are no compounds you can get helium from, and the air only contains around five parts per million. So on earth, there's almost no helium, which is at least
a little surprising because in the universe, helium is everywhere. It's the second most abundant element after hydrogen. The issue is just how light helium is. Release any helium and it mixes into the air. Over hundreds of thousands of years, it slowly drifts up to the top of the atmosphere where it gets heated up by the sun and it speeds up. Now, a small fraction ends up going faster than 11 kilometers per second, which is Earth's escape velocity, the velocity required to escape Earth's gravitational well. And
so it escapes into space. A much larger fraction gets ionized by solar radiation and then gets carried away by the polar winds along the Earth's magnetic field before ultimately merging with the solar wind. And either way, the outcome is the same. The helium is lost to space forever. So if I pop this balloon, that helium is effectively gone. As a result, pure helium at the time was hard to come by and the world's total supply was likely less than 50 party balloons worth. So Onnes had
to get his helium from a different source and he turned to sand, specifically two large bags of monazite sand. That's because monazite is a mineral that contains around 5% thorium by weight and thorium is radioactive. When it decays, it spits out an alpha particle, which is two protons and two neutrons. In other words, the nucleus of helium. That nucleus then gets slowed down by the surrounding rock where it steals two electrons and becomes a helium atom. So over millions of years, you get this buildup of
helium atoms trapped inside the rock. In fact, basically all the helium we have on earth today comes from radioactive decay. It's essentially a radioactive fossil, although now we mostly get it from natural gas wells where it accumulates over the eons. But Onnes didn't have that luxury. He had to liberate it from the sand. So he recruited four chemists. They heated up the sand to release the gas, exploded it with oxygen, cooled it with liquid air to freeze out impurities, burned over copper oxide to get rid
of the hydrogen, and last of all, compressed it over charcoal to absorb any remaining impurities. After several months, Onnes had about 25 party balloons of pure helium, an estimated 80% of the world supply. Onnes quickly conducted careful experiments to try and tease out whether helium atoms attract each other at all. - Kamerlingh Onnes did a lot of work and I think he was becoming more and more convinced that helium can in fact be liquefied. - It seemed like the helium atoms did attract each other just
ever so slightly. So how do two neutral atoms that don't react with anything attract each other? Well, a helium atom is just a nucleus with two electrons around it, but in reality you never know exactly where those electrons are. So a more accurate picture looks like this, a nucleus with an electron cloud around it. But that cloud isn't always perfectly evenly distributed. At any particular instant, there's slightly more negative charge on one side than the other. And now if there's another helium atom close enough, well,
this charge imballence can induce a charge imballence on that atom. And now the two exert a force of attraction on each other. This is known as a van der Waals force, and it's why every atom attracts every other if you get it close enough. But helium holds its two electrons very tightly, so the cloud hardly budges and the van der Waals force is tiny, the smallest of any element, but perhaps it's just enough. The second problem with helium is that no one knew exactly how cold
it would have to get to liquefy, only that it had to be colder than liquid hydrogen, possibly much colder. And that's the issue because the rate at which heat transfers in from the environment depends on the temperature difference. The bigger the gap, the more heat flows in and cooling something down to possibly a few degrees above absolute zero while the room it's in is 290 degrees above it, well, that's a humongous gap. Fortunately, Dewar had already invented one key piece of apparatus. Essentially, one glass flask
nested inside another with a vacuum in between. This limited heat conduction to a small connection point at the top. Convection got ruled out entirely due to the vacuum and optionally by covering the insides with the silver coating, you could also reflect radiation. This reduced heat losses by a factor of 30, and it's the basic design behind every thermos flask today. But for Onnes, it still wasn't good enough. So he nested several Dewar flasks inside each other. The helium would go in the inner one, a bath
of liquid hydrogen would sit around that, and a bath of liquid air around that. So each layer acted as a heat shield for the next, and the helium would see a wall of 20 Kelvin instead of 300, cutting the radiated heat coming in by a factor of 50,000. To measure the temperature inside the inner vessel, Onnes inserted a small glass bulb filled with dilute helium gas held at a constant volume. A thin capillary connected this to what was essentially a pressure meter outside the liquefier, because
for a gas at a constant volume, the temperature is directly proportional to the pressure. So if you first measure the baseline pressure at a temperature you know, like the freezing point of water around 273 Kelvin, you could set that to 100%. Then if you later measure a pressure that's only 10% of that amount, then the temperature must also be 10% of the original or around 27 Kelvin. By using this trick, HOnnes could measure the temperature even at a few degrees above absolute zero. Now in their
case, they read off a level of mercury, but for simplicity, we'll animate it as a temperature scale. Onnes finally had everything ready. At Professor Weinfurtner's gravity laboratory, we got to see the setup we'd use. - Is this also what it would have looked like back in the day when Onnes was doing it? - Yeah, so this is actually a really similar setup to Kamerlingh Onnes. - The biggest difference is that we only have two Dewars nested inside each other. The outer one will be filled with
liquid nitrogen and the inner one is where the helium will go. By the 10th of July 1908, Onnes decided to go for it. He arrived at his lab around 5:30 AM where his technicians were already hard at work. They prepared 75 liters of liquid air the day before and Onnes now ordered them to start making liquid hydrogen. By 1:30 PM they had 20 liters, which was enough to launch their attack on helium. They started by filling the outer vessels. - First step, we're going to fill
the outer Dewar with liquid nitrogen. That's going to precool the whole system down to about 77 Kelvin. Once we're full, we'll start filling with helium. So we've used about 25 liters of nitrogen to get to that point. Now consider how big the Dewar is, that probably holds, you know, less than 10 liters. - With the liquid air and hydrogen in place, they started to cool the liquefier. After 30 minutes, it had cooled to 93 Kelvin. Now it was time to start cycling the helium. A pump
compressed it to 100 atmospheres before it went to the liquefier, where it got precooled to around 15 Kelvin. The gas was then rapidly expanded into the inner vessel, greatly increasing the distance between atoms. But remember, helium atoms do slightly attract each other, so moving them apart costs energy and they pay for this with some of their kinetic energy, causing the particles to slow down and their temperature to drop. The gas then got pumped back to the compressor, but the smart thing was that they let this
colder gas cool the incoming helium, slightly lowering its temperature before it expanded. And by doing this over and over, the incoming helium got colder and colder. - It's so faint. - Yeah. - It feels like a ghostly fluid or something. - You know, it's not liquid that's escaping. It's more of kind of a super dense vapor. It's only 10 degrees inside, but it's dropping fast. - A little after 6:30 PM, Onnes and his team had gotten down to six degrees above absolute zero. That is closer
than anyone had ever gotten before, and they kept going. But just below five Kelvin, the temperature stopped falling, so they started to worry that perhaps something was wrong. But then a chemist in the room remarked, "It's so stable, it's almost as if the thermometer is submerged in a bath of liquid." So they rushed to the apparatus, but still saw nothing until Onnes grabbed a torch and shot it from a low angle, and they saw a reflection. After the surface had once been seen, it was no
more lost sight of, Onnes wrote, "It stood out sharply defined like the edge of a knife against a glass wall. They had just made liquid helium." - That's really cool. It's just hitting the light on it while it's yeah. So beautiful. It's a, it's a helium kettle. - Four degrees above absolute zero. - You can see that really faint liquid level on the inside. That's the liquid helium. - Oh wow. Okay. So the, the refractive index of liquid helium is very close to air, is it?
- 1.025. Okay. - So yeah, really, really difficult to see by eye. Wow. - Isn't it crazy to think that that is about four degrees above absolute zero? It doesn't look like. - It doesn't, there's nothing that would tip you off. - Yeah. Back in Leiden, everyone was elated. For the next hour, they ran basic experiments until by 8:30 PM only 10 millilitres were left. But then Onnes thought, "Well, we've made liquid helium. Why not make solid helium?" So he was going to cool it down,
and the way you can cool down any liquid is by pumping on it. - When we pump on it, we're removing all of the hottest atoms from the system, which takes about a third of the fluid away. So we initially take the level way higher than we need it so it can drop down. - So Onnes started his pumps and so did we. - Are we ready? - Three, two, one, go. - Oh, - Wow. Okay. - Oh wow. It's gone down so much. It's gone
down like, what, five centimeters? - Yeah, immediately. - Immediately. - Because we're removing all the hottest atoms from the system. So we're just shy of 3.5K. - This rapidly evaporating helium is quickly cooling down. - So we're about 2.6K. We're below 2.3K. - Okay. - Oh, - It's going super fast, super - Fast. - It keeps boiling until right at 2.17 Kelvin, it abruptly stops and the liquid goes eerily quiet. There it is. - Ooh. It's completely still again. Man, you can hardly see it. It
feels like a mirror surface. It's crazy how smooth that is. - Onnes pumped his helium down to 1.7 Kelvin, colder than outer space, but it didn't freeze. Instead, it just sat there, remarkably still. Onnes concluded it must just freeze at some lower temperature, and since everyone was exhausted, they called it a day. Little did he know what sat there evaporating in his flask. Onnes still wanted to freeze helium, so over the next two years, he tried several more times, getting all the way down to one
Kelvin, but all his attempts failed. This led him to wonder whether helium will not remain perhaps liquid even if it is cooled to the absolute zero. And indeed, scientists later discovered this is true. If you take any substance and keep cooling it down, eventually it always freezes, except for helium. Somehow it's the only substance that never freezes by just cooling it down. So Onnes was wise when he gave up on trying to freeze helium. Instead, he set out to finally test Kelvin's prediction that the electrical
conductivity of metals should stop near absolute zero. In other words, that their electrical resistance should become effectively infinite. To do this, he submerged different metals in liquid helium and measured their resistance. He started with gold and platinum, but their resistance just kept slowly decreasing until finally settling on a steady value. Then on the 8th of April, 1911, he measured mercury and wrote down just three words mercury practically zero. As you cooled down, its resistance slowly decreased until around 4.2 Kelvin, where it suddenly plummets to what
appeared to be zero. Electricity that flowed with no apparent losses, the exact opposite of what Kelvin had hypothesized, Onnes had just discovered superconductivity. - So that's the story most people know. - Okay. - What most people don't know is that later that same day, he also cooled down the helium more and he wrote, "Right before the helium reached the coldest temperature, which was 1.8 Kelvin, the boiling suddenly stopped." - He noticed. - He noticed. - So what did Onnes do after seeing this sudden change? Almost
nothing. He guessed at an ordinary explanation and moved on. But helium kept dropping clues. When they measured its density, they discovered it had a curious peak around 2.17 Kelvin. And when they measured the latent heat, they found that around the same temperature, something happened to the helium, which was perhaps even discontinuous. Yet the biggest smoking gun of all happened when they measured a property known as the specific heat, which is how much energy it takes to heat up one gram by one kelvin. They found that
as the temperature dropped, the specific heat declined until they approached 2.17 Kelvin, where it suddenly shot up. This was as clear a sign as any that something was happening at this point. - But they knew what this was. This was condensation happening, you know, experimental error. So they had these points, but they, they wrote like, "We felt justified in disregarding these irregular points, and they only published the part above 2.61 Kelvin, which neatly fit the formula they had for it." - Onnes brushed off hint after
hint, even while seeing it happen with his own eyes. One day, Onnes put one container of liquid helium inside another, and he noticed that below 2.17 Kelvin, if one level started higher, it always quickly evened out. He even remarked how strikingly fast it happened. He thought it was too fast for distillation, but since he had no better explanation, he stuck with that. If only he had raised his container, he might have discovered the truth. Here, we've got a polymer cup. This is our container. - So
we got the bowl loaded up. It's way at the bottom, and I think what we're going to try and do is lift it up. - Yeah. The colder we get, the better this effect is going to be, and it's a super subtle effect at the best of times. We want to get down to about 1.5, 1.55 K. I'm going to try and raise it just above the fluid surface. - Whoa. - Can you see that? - Yeah, yeah, - Yeah. Yeah. - Yeah, no, you see
like waves almost. - Yeah. - Yeah. Ah, you can really see a pulse down there. - If you look at the bottom, you can see it's dripping. And if we speed it up, you can see it keeps dripping. More and more liquid helium is going to the bottom. What's happening is that somehow the helium is climbing up and over the walls, literally emptying itself out. This was not distillation. Something far stranger was happening. Onnes's successors grew suspicious of the specific heat data, so they remeasured and
they became convinced it wasn't an error. This was real. Something was happening at 2.17 Kelvin. Since the curve resembled the Greek letter Lambda, they called this the Lambda point and they suggested that a phase transition must be happening here, but not an ordinary phase transition. This would be a phase transition to a more exotic kind of matter, from a normal liquid to something else. But what? Figuring out what's really happening when there are so many different clues can be confusing. If not, outright impossible. The same
is still true today. Recently, I came across this story where scientists used AI to design viruses not found in nature, but depending on where you read the story, you might have a completely different reaction. If you read IFLS first, you might think this is an exciting scientific breakthrough, AI designing a virus that can kill bacteria. If you'd seen this first, you'd get a more complete, albeit complex picture, 16 viruses from scratch, but with possible downsides. And if you saw this New York Post article first, you'd
most likely focus on what could go wrong. So three headlines, but three completely different conclusions. Most people just get their news from one source, and as a result, their takeaway is a little bit like the luck of the draw, which outlet did you happen to see first? That's why we use today's sponsor, Ground News. They're a website, an app that brings a data-driven and objective approach to reading the news. They pull thousands of stories daily, and for each, they break down the political leaning, how factual
the reporting is, and who owns the reporting. For example, with the AI virus story, it immediately tells you that over 200 news outlets reported on it and that it got covered fairly evenly across the political spectrum. Below that, you can also see the factuality, ownership, and even who broke the news first, Forbes in this case. The bias comparison feature highlights specific differences in the contents of the articles, but my favorite feature might be their blind spot feed. It shows you stories that are under-reported by one
side of the political spectrum or the other. Trying to figure all these differences out by yourself for every story would be a full-time job. That's what makes Ground News so valuable. Their data-driven approach helps make these stories digestible in seconds. So if you, like us, care about getting to the truth, go to ground.news/ve or scan this QR code. Our link gets you 40% off their vantage plan. I'd like to thank Ground News for sponsoring this part of the video, and now back to what was happening
when you keep cooling down helium. The answer started to take shape throughout the 1930s. - So in 1945, there was this graduate student called Don Misener, and he needed to do some experiments for his coursework to get enough credit. And he was aware of everything that was happening in helium, but he realized that no one had measured its viscosity. So he's like, "Oh, that's weird. I will measure it." - So how do you measure the viscosity? - There are two ways. You have a disc and
you try to see what the resistance of the liquid is. Another way is to have a small capillary, and then you push the liquid through that, and the viscosity you measure between the two methods should be exactly the same. - Misener chose a variation of the first method. He submerged a cylinder that was connected through a torsion fiber in liquid helium. He then rotated the cylinder and let go, causing the cylinder to swing back and forth to oscillate. Now, by watching how long it oscillated, he
could determine the viscosity. The longer it oscillated, the less friction it must experience, so the lower the viscosity. And what he found was that at 4.2 Kelvin, the viscosity was about 14%. That of air, already tiny. Then, as he decreased the temperature, the viscosity slowly increased until at the transition point, it suddenly plummeted, not to zero, but to about 5% that of air. This caught the attention of Jack Allen, a 27-year-old physicist who'd just come to Cambridge. Allen had inherited a helium liquefier from someone who
could no longer use it, and now he resolved to use it to unravel the mysteries of helium. Of course, he could use some help, and Misener seemed like the perfect PhD student, so Allen convinced him, and together, they continued studying helium's viscosity. Only now they turn to the second method of studying it, which works something like this. I've got a tube filled with two layers of glycerol. Now, if I let it flow, what's going to happen? Well, you'd expect the layers to drop, which they do,
but not in a straightforward way. It doesn't maintain its shape. Instead, it stretches out in the middle. What's happening is that if you zoom in, you'd find that the molecules near the wall get stuck, they experience friction, and then the molecules sliding past them experience friction with those molecules. The internal friction is basically what viscosity is. In general, if you make the channel smaller, the flow rate should decrease, and if you push on the liquid, it should increase. Now, by carefully keeping track of how things
change when you alter these parameters, you can determine the viscosity. In November of 1937, Allen and Misener did a version of this experiment with liquid helium at around one kelvin. They let it flow through one wide capillary and one narrow one, and they found that the flow speed through both was almost the same, despite the narrow one having a surface area a thousand times as small. Even weirder was that the speed hardly changed when the pressure increased. They concluded that any known formula cannot, from our
data, give a value of the viscosity, which would have much meaning. As far as they could tell, the viscosity was non-existent. - They know they've got gold on their hands here, right? They know this is a, like a bombshell of a result. - So they spent the next three weeks making sure they hadn't made any mistakes. They repeated all the experiments and did all the other measurements they needed, ready to write it up. - But then on the last day, one of Kapitsa's old students walks
in with a letter from Kapitsa. - "Who's Kapitsa? Well, remember that allen had inherited that liquefier from someone who couldn't use it anymore? Well, that was Soviet physicist Pyotr Kapitsa. He built the liquefier and ran the lab before allen. But then during a trip back home in the summer of 1934, Stalin had his passport confiscated. He was to work and stay in the Soviet Union, where for the past few years, Kapitsa had been rebuilding his lab, telling almost no one exactly what he was working on.
But Allen and Misener were about to find out. - He had done the same sort of experiment, found the viscosity was vanishingly small, and send a note to the editor of Nature that he would be very grateful if they could publish it as soon as possible. Don't bother to send the proofs to me here to Moscow. It takes too much time. He had just scooped this old laugh. - He knew what he had and he raced to publish. Allen and Misener were freaking out, but over
the next four days, they wrote up all their results and also sent it to nature. On the 8th of January, 1938, the two papers appeared side by side, reporting the vanishingly small viscosity, perhaps even zero. And then in analogy to superconductors, Kapitsa called it a superfluid. This was the breakthrough everyone had been waiting for because it finally began to explain some of the strange things they'd been seeing. Above 2.17 Kelvin, helium is just a normal fluid and it has internal friction or viscosity, so when it
flows through tiny cracks, part of it gets stuck to the walls. Now, in the case of the bucket, the bottom is made of a polymer aerogel, which is filled with microscopic channels up to tens of nanometers across. That's so small that the helium gets fully clamped to the walls. It can't flow through. But as you cool down below 2.17 Kelvin, the helium transforms, and now it can suddenly pass through tiny cracks with no friction, so it leaks straight through the bottom. It also explained what was
happening with the little cup. How? Well, remember those van der Waals forces that cause every atom to attract every other? Well, between helium atoms, they're very weak, but the atoms in the walls of the cup have many more electrons in their outer shell, so the charge imballences become larger. And as a result, the helium atoms are much more strongly drawn to the wall than to each other. So they climb up the wall and within seconds wet the entire surface with a layer that's around 30 nanometers
thick. And because the bottom of this thin sheet is below the liquid layer, it now acts as a siphon. Normal fluid can't flow through because it experiences friction, but superfluid can, so it flows up and over the walls until it drips out the bottom. But for all the idea of a superfluid explained, it also raised a much more fundamental question. How do you get a superfluid? - Like if it is actually zero, I want you to think about how crazy that is, right? Because if we
think back to, okay, what is this viscosity? It's internal friction between the molecules. So if you have actually zero viscosity, it means that you have like a billion, billion particles all traveling through this pipe and not a single one is bouncing off of anything and losing some energy or bouncing into each other and losing energy. They all must flow perfectly and somehow not lose a single bit of energy. It seems preposterous. - Within two months of the nature papers, Fritz London proposed a possible mechanism. There
was this forgotten paper from 13 years earlier. It predicted that if you just kept cooling down a certain type of particle, then the atoms would fall into an exotic state. And the author of that forgotten paper was none other than Albert Einstein. To understand what Einstein had predicted, consider this plane of particles. They're not interacting and they're free to move around in every direction with different velocities and different kinetic energies. Now at room temperature, you can just think of them as point particles, but if you
start cooling them down, what Einstein realized is that, well, this is no longer the case because now quantum theory starts to play a role. And according to that theory, particles aren't just particles, they also have a wave-like character. - So-called wave particle duality, the quantum wave function tells you the likelihood of where it would be found if you were to try to measure its location. - The important thing here is that the wavelength of these waves or how spread out they are is equal to planks
constant over their momentum or mass times velocity. - So if you're cooling these particles down, they on average have less and less momentum per particle, their quantum wavelength or the de Broglie wavelength gets longer. And so eventually the sort of quantum state associated with each of those seemingly independent particles starts to smoosh out and overlap. This distinguishability in space starts to melt away. - What happens next depends on which kind of particle you're cooling down. Every particle in the universe is one of two kinds. It's
either a boson or a fermion. Fermions are most of the particles you're familiar with, like protons, neutrons and electrons, and bosons are particles like photons. You can see the difference between bosons and fermions by looking at two indistinguishable particles. Now, one particle just has its own wave function, but if you take two of them together, something special happens. They get a combined wave function that describes both of them together. In the case of bosons, if you could see that wave function, you'd see it's the same
on both sides, that is it's symmetric. But if those particles are fermions, then their combined wave function is the opposite on both sides. It's anti-symmetric. Now I want you to think about what this means if you try to move those particles together. In the case of the bosons, the two sides of the wave function add up and you just get two bosons in the same place or state. But now try doing the same for fermions. You get one peak to overlap with one valley and they'd
cancel out. But that would suggest that all of a sudden there's a 0% probability of finding the particles anywhere, which doesn't make any sense. They have to be somewhere. So what this is really telling us is that two indistinguishable fermions can never occupy the same quantum state. This is known as Pauli's exclusion principle, and it's why particles like electrons can never be in exactly the same state. If the particles you're cooling down are fermions, then they start filling up the energy states like a ladder from
lowest to the higher states, with only one fermion per quantum state. But bosons don't have this restriction. They're very happy to share the same quantum state. When you keep cooling them down, more and more of them start to occupy lower and lower energy levels. And Einstein realized there comes a point where a macroscopic portion suddenly condenses into the lowest energy state or ground state, which in this case is just when they're completely still. - These little particles could actually act in a way where they would
sort of lose, so to speak, their, their individuality, that they could act as a single coherent state of matter, a whole new state of matter that had no analog. - This became known as a bose Einstein condensate after Bose whose work Einstein was building on. But Einstein doubted this could happen in real life. He wrote, "The theory is pretty, but is there also some truth to it?" 13 years later, London believed there might be and that perhaps superfluid helium was a bose Einstein condensate. At first,
that might seem impossible because helium is made up of protons, neutrons, and electrons, and every one of those is a fermion. So you'd expect helium to be a fermion too. And remember, fermions can't share the same state, but helium is not a fermion. bosons and fermions differ in one other fundamental way, their intrinsic angular momentum or spin. For bosons, this is always an integer number, zero, one, two, three, and so on. But fermions always have a half integer spin, like a half. So what happens if
you have multiple fermions together, say in one atom? Well, now you have to add their spins together to get an overall spin for the atom, and then use that to see whether it's a bozon or a fermion. In the case of helium, which has six half spin particles, then no matter how you add them up, you always end up with an integer number. As a result, helium atoms behave as bosons. And as you cool them down below 2.17 Kelvin, a macroscopic fraction of the atoms condense
into the lowest energy state. They lose their individuality and about a trillion trillion atoms all behave as one. Normally, you think of wave functions as these teeny tiny things, but here it's literally the size of the container. It's as if after the transition, the helium has turned into one big atom with all these atoms behaving as one. - This is quantum mechanics with your own eyes. - And it makes you look at matter in a different way. It allows us to explore the boundary of quantum
and regular matter. So it seems like we've solved it. The atoms are in the lowest energy state. They've lost their individuality and act as one, so they can't scatter and they flow with zero internal friction. That's how I first understood superfluids and how I'd explain it to anyone who wanted to know. I mean, it made sense and we're all happy. The only problem is that it's wrong. I mean, the first issue is that in a BEC, all the atoms occupy the state where the particles are
at rest, but in a superfluid, well, they flow. Maybe that issue could be excused because you could imagine moving along with the superfluid and finding a frame in which it effectively stands still, but there's an even bigger problem, which is that in a non-interacting Bows Einstein condensate, even the tiniest push can excite any of the atoms. So if you'd try to let it flow, the smallest disturbance would knock atoms out of the ground state. They would scatter and the flow would lose energy. So clearly a
BEC alone isn't enough. You need something else, but what? Fortunately, Kapitsa knew exactly who could solve the problem. Unfortunately, he had been locked inside Stalin's prison for the last year. - This was right around the time of Stalin's great terror where he was executing or imprisoning all the enemies of state, whether they were real or imaginary. - But in the case of Lev Landau, the physicist Kapitsa was thinking about, it seems like there was some truth to it. Landau was known as a genius theorist who
cared deeply about elevating Soviet physics. He was the head of the physics department and personally trained some of the best physicists of the century. But around the mid 1930s, he became disillusioned with Stalin's regime. He reportedly told one colleague that Soviet officials were "born subhuman." And according to KGB Files, he was accused of heading a counter-revolutionary organization. Landau was arrested on the 28th of April 1938 and taken to the Lubyanka Prison. The first day he was forced to stand for seven hours, followed by six days
of 12-hour persuasion sessions. After two months, Landau broke and wrote a six-page confession. It reads, "We came to the conclusion that the party had degenerated and that the Soviet government no longer acted in the interests of workers, but in the interests of a small ruling group. The interests of the country demanded the overthrow of the existing government." - That's a pretty, pretty strong confession. - This confession at a time where people were being executed for no reason should have meant the end for Landau. But fortunately
for him, he had someone defending him. The morning Landau got arrested, Kapitsa immediately wrote to Stalin. His character, to put it simply, is nasty. He is a bully and a brawler, but I find it very hard to believe that Landau was capable of anything disOnnes. Landau is young and he still has much to accomplish in science. No one else can write about all of this, which is why I'm writing to you. Now, Stalin never replied. So a year later, Kapitsa wrote to one of Stalin's closest
allies. He told him how he had discovered superfluidity and that no theorist other than Landau could explain it. Three weeks later, Kapitsa was summoned to the prison. As he recalled, "I was summoned at 1:00 AM. They escorted me to a big office occupied by two men. Do you understand for whom you are pleading? He's a most dangerous criminal, a spy who confessed to everything." We talked till four in the morning. Towards the end, one of them said, "All right, Kapitsa, if you pledge your word for
Landau, then give us a written guarantee. If anything happens, you will be held responsible." Kapitsa wrote the letter and two days later, Landau turned up at the institute, ready to get back to what he loved doing most, physics. Kapitsa had put his neck on the line. Now it was up to Landau to prove he was up to the task, and superfluid helium had only gotten more interesting. Back in January 1938, just days after the nature papers, allen was studying how superfluid helium conducts heat. To do
this, he packed some Emery powder, basically fine sand, in a glass bulb with an open bottom. To see what was happening, he shone his pocket torch at it. And according to him, as the Emery powder tube came into view, helium spurted out the top of the tube. It continued to spurt as long as the light shone on the powder and ceased when a card cut off the light. allen later showed this fountain with no moving parts to the Royal Society, during which famous writer H.G. Wells
crawled under the table looking for a hidden pump, but found nothing. - Yeah, so we had this kind of specialist made. It's a single piece of glass. There are no seams in it whatsoever. The filtered disc is fused in, and then it's just filled with carbon stones, and we just hit it with a bright light that heats it up, drives the fountain flow. - Amazing. Fingers crossed. You guys ready? Three, two, one. Torch it. Woah! Yeah. Let's go. What? That's absurd. - That's awesome. Woo. Let's
go. - So what was happening? How could light cause the helium to shoot out? - Any theory now had to take into consideration all these phenomena. So for theoreticians was a really heavy task. - Landau spent the next eight years working on it. He started by considering a superfluid flowing at absolute zero, and he realized what London had left out, the forces between atoms. In a Bose Einstein condensate, there are no forces between particles, and as a result, a push can knock a single atom out
of the lowest energy state, resulting in easy friction. But in liquid helium, this is not the case. The atoms interact, so if you push on one, you're pushing on a bunch of them. Landau realized that this rigidity meant that the only way you can create excitations in a superfluid and lose energy in the flow is through the collective motion of the atoms. He argued that these excitations come in two main flavors. The first is that if you push hard enough, you get the atoms to bump
into each other and create ripples throughout the fluid, basically sound waves, just a quantized version of them called phonons. Similar to how photons are particles of light, these are particles of sound. The second kind of excitation is something known as a roton, because originally people thought of them as tiny whirls, but now they're seeing more like a patch of atoms that suddenly orders themselves into a crystal lattice just for a split second. You can think of them almost as tiny ghosts of a solid. And just
like a phonon creating a roton costs both energy and momentum. So Landau argued that these phonons and rotons were the only way that the flow could lose energy. - And you're like, okay, well, it's quite easy, you know, to make a sound wave. Like all I have to do is knock on it, right? That in terms of like how much energy it takes to create a phonon, it can be very little energy. It can be as little energy as you want, but you don't just have
to conserve energy. You also have to conserve momentum. So when you push on it or when the liquid bumps into something, it both needs to satisfy conservation of energy and conservation of momentum. And there is a very nice relationship that tells you exactly how the momentum of a phonon corresponds to the energy of a phonon, and it's connected with the speed of sound in a material. And so the only way you can satisfy both constraints is if the flow is going at least the speed of
sound, which in helium two is very fast. It's about, I think, 220 or 240 meters per second. Rotons, he realizes, can get created a little bit easier. I think the modern value is about 60 meters per second, but it's still a lot. - So in Landau's theory, below about 60 meters per second, the flow can't create a single phonon or roton. The atoms all stay in the ground state and no energy is lost. So it's not that the friction is vanishingly small in a superfluid. Landau
proved that it's strictly zero. Friction is forbidden. In practice, other excitations are created at lower speeds and therefore the actual flow rate at which superfluidity breaks down is lower. But if the friction is actually zero, that implies something else. You should be able to get a superfluid flowing in a loop, and once it's going, it should just keep going like a perpetual motion machine. In 1964, two physicists at Yale tested this. They got superfluid helium circling in a stack of thin rings and waited. And what
they found was that no matter whether they waited five minutes or more than 12 hours, the flow was just as strong. - But then, you know, if the friction is zero, how is it possible that Misener, you know, when he did his like torsion experiment, he didn't measure zero. - The liquid wasn't just passing over liquid, like you had like a cylinder of something in there? - Yep. - Yep. I don't know. Is it coming from the wire? Is it coming from the cylinder? Like the
different materials? What is it coming from? - Your instinct is that Misener must have done something wrong. He didn't. Both experiments are true. How can both be true? - You're measuring different things. I don't know, but how are you measuring different things? This is where the second part of Landau's work comes in. So far, Landau had only looked at a fluid at absolute zero, but every real experiment happened at higher temperatures. So next he considered what if it flows at some finite temperature? To do this,
he built on the work of Tisza, who suggested that helium below 2.17 Kelvin acts as a mixture of two fluids. One is the superfluid. It's effectively at absolute zero. It carries no heat and flows without friction. You can think of this as the background fluid. And the second is the normal fluid, which is made up of the phonons and rotons. It carries all the heat and has viscosity. At absolute zero, the whole fluid is superfluid, but as it heats up, part of the superfluid turns into
normal fluid. The superfluid fraction decreases and the normal fraction increases until at 2.17 Kelvin, the superfluid component is zero. - Now, if you were to look at one atom in the fluid and say, "Is that superfluid or normal fluid?" Well, it's hard to tell because the superfluid atoms are constantly exchanging energy with the normal fluid atoms. - Sure. - So any one atom is constantly bouncing up and down between these two modes. - Now, only the superfluid can flow through tiny channels. So when Allen and
Misener pushed helium through their capillaries, they only measured the super fluid part and they found a zero viscosity. But Misener's torsion experiment sat in a mixture of both fluids. So the cylinder felt the viscosity from the normal fluid. Misener hadn't made a mistake, they were just measuring different things. And once you know what you're looking for, you can also physically see the effect. Just below the transition temperature, say at 2.15 Kelvin, only around 13% of the helium is superfluid. So only that 13% can leak through
the bottom and you get a drip. But if you cool it down to 1.5 Kelvin, well now around 89% is superfluid. That's almost seven times as much helium that can flow through, so it just pours out the bottom. It even explains how light can power a fountain. When the light hits the rocks, the rocks absorb some of the photons and heat up a little - Bit. - This causes part of the superfluid to turn into normal fluid, but now the superfluid density is lower inside the
fountain, so more superfluid immediately rushes in through the bottom to restore the proportions. That bottom, however, is just like the bottom of the bucket. It only allows superfluid through. So the normal fluid can't go out that way. The only way the excess helium can go is up and out. And so you end up with this, a fountain powered solely by light. The heat from the light gets turned into the mechanical energy of the spouting helium. But the two fluids do more than make fountains. They change
how helium transfers heat. Take a bath of liquid helium below the lambda point with a heater on one side. - The helium atoms that are in contact with that hot face - - Yeah. - Are going to gain energy. They're going to jump up this ladder. - Yeah. - So they're going to be part of the normal fluid. - Right. - So my normal fluid is going to flow away from the heater, carrying all of the heat and the entropy that's imparted into the fluid away.
- Yeah. - But as the normal fluid flows away - Yeah. The superfluid flows back to replace it. Right. And the overall density of the fluid remains constant. - Sure. - So this is a two-fluid thermomechanical counterflow. They basically flow through each other like they weren't even there. - But you can take this one step further. Imagine changing the temperature of the heat source periodically. What is going to happen? Well, the amount of normal fluid created will also change periodically. So you get these fluctuations in
the normal fluid and superfluid densities. They spread out and travel away, almost like a sort of sound. - When we think of first sound, regular sound - Yeah. That's density waves in the air driven by tiny pressure gradients. - But with a superfluid, you can get density waves in the normal and superfluid components. And since the normal fluid carries all the heat, that means you get temperature waves. - And they can spread out like sound waves. - Yeah. - So we call temperature waves in helium
second sound. - The crazy thing about it is it's a completely new mode of heat transport, unlike anything else in the uni - right? Yeah. When I heard that, I was like, "That's crazy." So you always think of conduction, convection, and radiation. And now helium gets a fourth. - This is a combination of second sound and thermal counterflow, and it is effective, around a thousand times more conductive than copper, which is one of the best thermal conductors we know of. It finally helps explain one last
thing you've been seeing this whole time, that is why does the boiling suddenly stop? Well, above 2.17 Kelvin, helium boils just like a normal liquid. - Say, for example, I've got a bucket of water and I take a red hot poker and I dump it in. The bubbles aren't going to form anywhere in the bucket. They're going to form around that poker because you've got this high temperature region where you've dumped in enough energy locally to vaporize the water and then that's where the bubbles are
going to form. The amount of energy it takes to do this is the latent heat of vaporization. In water, that is about 2,200 joules per gram. In helium, it's about 23 joules per gram. So if you've got helium in its liquid state, a tiny thermal fluctuation is enough to locally vaporize a small pocket, which then forms a bubble. And that's why boiling is so constant, because tiny thermal fluctuations will form bubbles very, very easily. - But as you cool helium down below 2.17 kelvin, thermal counterflows
take over. They transfer heat so effectively that the entire liquid almost immediately reaches the same temperature. So there are no hotspots and no places for bubbles to form, the boiling suddenly stops. This lack of boiling paired with its unparalleled heat transfer also makes superfluid helium the ideal coolant at one or two kelvin, and that's why CERN uses around 700,000 liters of it to cool the LHC's 9,000 superconducting magnets. - There is still one thing that bothers me though, Chris, which is that, you know, if you
take any substance and you keep cooling it down, it always freezes. But helium never does, yet it does liquefy, which is the annoying part. So because it liquefies, it means that there is some attraction between the atoms. And so you'd expect, you know, I can just keep cooling it down more and more and more until eventually the kinetic energy becomes arbitrarily small and it must, like the attractive energy must be much larger and so it must form a solid. - Yes. So this is the kind
of intuitive picture that you would expect. Well, you can't answer that in an intuitive way. You have to bring in quantum mechanics. - Because according to quantum mechanics, the more precisely you know a particle's position, the less you know about its momentum and vice versa. This is known as Heisenberg's uncertainty principle, and mathematically it tells you that the uncertainty in position delta x times the uncertainty in momentum delta p is greater than or equal to h-bar over two. For simplicity, let's just say it's around h-bar.
- Let's say I've got an atom in a liquid and it has some freedom. It can move around, but you know, it's very closely packs in with all of its neighboring atoms. Let's say that it occupies some spherical volume with a radius r. Well, we can say that that r is The uncertainty in its position. So if the uncertainty in the momentum is going to look like h-bar, planks constant, over R. - But if it's got an uncertainty in momentum, it means it must be jiggling
around inside that sphere. - We know if it's jiggling, we can say it's got an energy. Well, we know what it's momentum is. It's planks constant over, over R. So how do we get the energy? Well, it's easy. We say it's the momentum squared over twice the mass. Now, helium being such a light atom, that mass turn is very small. So that means that the energy that it has, because it's confined, is very, very large. Now, this is what we call the zero point energy. This
is the energy it cannot get rid of. It's nothing to do with thermal energy. It's not because there's a temperature in the liquid. In fact, when you're in the liquid state, that zero point energy accounts for more than 98% of the total energy in the fluid. - Wait, what? - It is, it is enormous. - That is crazy. I had no clue. - This zero point energy is too large for the atoms ever to lock down in place. So helium never freezes just by cooling it
down. - So when you're thinking of this kind of ladder of states, this kind of BEC picture, like, you know, everything's macroscopically occupying this bottom state, we think we, the, the intuition is to think of it as like ladder where each rung is nice and evenly spaced. But the way to think of it in terms of this zero point energy is that that first rung is 10 stories off the floor. And then the thermal states stack up on top of that. - Oh, that's so good.
- But there is one way in which you can freeze helium, which was discovered by Onnes's successor. That is by cooling helium down to about one Kelvin and by increasing the pressure to 25 atmospheres. If you do that, helium finally freezes. As for Onnes, he died in 1926 at the age of 72. He had been making super fluid helium for 18 years, looking at it, seeing hint after hint, and yet he never realized what he'd been staring at the entire time. Now, you might look at
this and think HOnnes was a little stupid or ignorant to miss it and to not put the pieces together, but I don't think that's the case. Instead, I think it reveals a fundamental feature of being human. I think it goes to show a lot of people think you got to see it to believe it. And I think often you have to believe it in order to see it. It makes me think a lot about Kuhns, the structure of scientific revolutions, because it focuses a lot on,
you know, you have the existing science, you have the existing paradigms, you have the existing formulas that these things are supposed to follow. And how often would the data point going off the line just be some error, someone messed up, condensation, something happening? It's easy to write those things off. So I have a lot of sympathy and I don't think, "Oh, how could they miss it?" I kind of think like, "No, that's actually how we operate." Especially, I don't know if scientists are supposed to be
a little bit more observant or a little bit more skeptical, but at the same time, you're up against so much chaos in the world that you do kind of sometimes just see, "Yeah, yeah, that fits that. Yeah, that fits the data." And they sort of ignore things that don't fit because you're looking for something. I don't know. - Yeah. But I think probably especially if you are a scientist, it's these unexpected things that can end up having the biggest breakthrough because those are the things that
disconfirm your view of reality. - And the thing is, often all the signs for the breakthrough are there. It's just that we're not ready to see them. So it makes you wonder what signs are we missing right now? I used to think that the reason someone didn't make the breakthrough was because maybe they weren't smart enough or they didn't work hard enough or didn't get lucky enough. But now I feel like I've always missed something. Just look at Onnes. He had all of those things. He
was an experimental genius, had an immense work ethic and even discovered super connectivity, one of the biggest breakthroughs of the 20th century. He got three out of three. So perhaps sometimes making the breakthrough doesn't require any extra intelligence, hard work or even luck. Maybe all it takes is for us to let go of the way we believe the world to be and instead, just for a moment, try to see it as it really is. What if that is all it takes? And Onnes, well, he may
have missed superfluidity, but he changed cold temperature physics forever. And in 1913, he received the Nobel Prize in physics for his achievements at finding out what happens when you just keep cooling down. Hey, two quick final things. First, this video would not have been possible without the help of Professor Weinfurtner's Gravity Laboratory at Nottingham University and the funding provided to them to run their superfluid experiments. So I want to give a big thank you to all of them for helping us recreate these experiments. The second
thing is that we didn't even get to show you their research because while it might seem like we've solved all the mysteries of superfluids, it turns out we're just getting started because really weird things happen when you start rotating them. And this rotation or the physics of it can play a role in everything from neutron stars to simulating rotating black holes to a black hole bomb. And because if you look closely enough, you can kind of see the entire universe in a drop of superfluid. So
let us know if you want us to make a part two on that.