If ever there was a criminally underrated natural resource, it would have to be Helium. Though most commonly associated with party balloons and making one’s voice sound like a cartoon, Helium’s most important application is in cooling the magnets of Magnetic Resonance Imaging or MRI machines. While the finite and ever-dwindling global supply of this vitally important gas is a topic worthy of its own video, perhaps even more fascinating is just how bizarre an element Helium truly is. For if Helium is liquefied and cooled to a low enough temperature, it begins to behave like no other liquid on earth, seemingly violating the laws of gravity, thermodynamics, and even logic itself. This is the story of superfluid Helium II, the weirdest substance known to science.

In order for Helium to be liquefied, it must be cooled to a temperature of -268.8 degrees Celsius or 4.2 Kelvin – that is, only 4.2 degrees above Absolute Zero, the coldest temperature theoretically possible. By contrast, Nitrogen liquefies at a relatively balmy 77 Kelvin, Oxygen at 54 Kelvin, and Hydrogen at 33 Kelvin. The reason Helium is so difficult to liquefy lies in its electron orbitals being completely filled, making it – like the other noble gases Neon, Argon, Krypton, Xenon, and Radon – electrically neutral and chemically inert. This means that the only force which can pull Helium atoms together is the so-called Van de Waals Force, which is caused by electrons shifting from one side of an atom to the other and creating a momentary electrostatic charge. This force is incredibly weak, meaning that Helium must be cooled to extremely low temperatures in order for the Van de Waals forces to overcome the energy of the moving atoms and pull them close enough together for the gas to liquefy. Solidifying Helium is even more difficult – so difficult, in fact, that it cannot be done at regular atmospheric pressures. Only at pressures of 25 atmospheres and above can solid Helium be created.

At temperatures near 4.2 Kelvin, ordinary liquid Helium – known as liquid Helium I – largely behaves like any regular liquid, though it does have certain peculiarities. For example, its density is only 0.126 grams per cubic centimetre – or around 13% that of water – while its refractive index – the degree to which light bends as it passes through – is also 1.025 compared to air, making the surface of liquid Helium very difficult to see. But for Helium to really start showing its wild side, it must be further cooled to below 2.2 Kelvin – a temperature known as the lambda point. Reaching such temperatures, however, is no easy feat. Indeed, just maintaining Helium in liquid form requires the use of a specialized container known as a Dewar flask. Basically a scientific version of the thermos bottles many of us use to store soup or coffee, a Dewar consists of a double-walled vessel with a vacuum between the walls to prevent heat transfer via convection. These walls are often silvered to reflect away infrared radiation, while liquid nitrogen or oxygen is usually circulated around the Dewar, to reduce the temperature rise from the inside to the outside of the flask from 289 to around 50-70 degrees. Further cooling is typically achieved by pumping out the Dewar with a powerful vacuum pump, causing the Helium to boil and heat to be carried away by the resulting gas. However, while Helium’s latent heat of vaporization – that is, the energy needed to vaporize a certain mass of liquid – is around 5 calories per gram, its specific heat – the amount of energy needed to raise the temperature of that mass by one degree Celsius – is around 1 calorie per gram and increases exponentially the colder the Helium gets, making it increasingly difficult to lower the temperature any further.

But then, as the lambda point of 2.2 Kelvin is reached, the liquid suddenly stops boiling and falls eerily still. The Helium has now entered an entirely new phase called Liquid Helium II, whose mind-bendingly exotic properties have fascinated and baffled scientists for nearly a century. To give you a small taste of just how bizarre Helium II is, the reason the liquid stops boiling as it crosses the lambda point is because its thermal conductivity has just increased by a whopping one million times. In fact, Helium II is the most thermally conductive substance known to science, conducting heat energy hundreds of thousands of times faster than the best known solid conductors like copper and silver. The reason normal liquids boil is that they absorb heat before it can be distributed throughout the liquid mass, causing the liquid to vaporize and bubble away. In Helium II, however, the heat is near-instantaneously redistributed throughout the liquid, preventing this from happening. Stranger still, heat does not travel through Helium II via normal convection but rather in wave-like pulses called solitons, a phenomenon known as second sound. In a classic experiment conducted in the late 1930s, Soviet physicist Pyotr Kapitsa immersed two sensitive electrical resistors in Helium II, spaced a short distance apart. One resistor was connected to a signal generator and the other to an oscilloscope, such that the first produced a series of regular thermal pulses while the other detected any variations in the liquid’s temperature. Using this setup, Kapitsa discovered that the second resistor could detect the thermal pulses from the first mere milliseconds after they were generated, revealing that heat travels through Helium II at the speed of sound.

But if you think that’s weird, well brace yourselves, because things are about to get even weirder. Let’s say you place some liquid Helium in a glass beaker with a bottom made of sintered ceramic, the pores of which are only a few micrometers in diameter. At temperatures above the lambda point, the viscosity of liquid Helium I – that is, its resistance to flow – prevents it from flowing through the ceramic. But the moment the Helium is cooled below the lambda point, it suddenly drains through the ceramic like a sieve. This flow is so fast that, for all intents and purposes, Helium II has no measurable viscosity. It is, in other words, a superfluid. This property manifests in even stranger ways, such as the inability of Helium II to be contained in an open-topped vessel. Fill a beaker with Helium II, and it will climb up the inner walls against the force of gravity and leak down the other side, flowing as a liquid film only a couple of atomic diameters or angstroms in thickness. A similar effect occurs when a u-shaped tube is filled with extremely fine powder like jeweller’s rouge and one end is immersed in a vessel of Helium II. The tube immediately acts like a self-priming siphon, causing all the Helium to flow out of the vessel.

Yet this conclusion is completely contradicted by another classic experiment. In this experiment, a metal cylinder is placed in Helium II and spun using an external electromagnet. Placed above this cylinder but not mechanically connected to it is a lightweight paddlewheel. A few seconds after the cylinder is spun up, the paddlewheel begins to turn. This motion is caused by the boundary layer of fluid adhering to the surface of the cylinder entraining other, adjacent fluid particles, causing a circular flow that moves the paddlewheel. However, this is only possible if the fluid has some viscosity. In summary, Helium II displays zero viscosity in certain experiments and a small but finite viscosity in others. This is one of the many apparent contradictions that make superfluid Helium II so utterly baffling.

But by far the most reality-bending property of Helium II is demonstrated using a hollow glass stem with an open-ended bulb packed with jeweller’s rouge. When this apparatus is immersed in Helium II and a beam of light aimed onto the bulb, liquid Helium squirts out the top of the stem. While at first glance this phenomenon – known as the “fountain effect” – may not seem all that strange, a closer examination of the physics involved reveals that it should, in fact, be impossible. For in order for the liquid Helium to flow into the bulb and up the stem, it must spontaneously travel from a colder region to a hotter region – something expressly forbidden by the Second Law of Thermodynamics.

So, what is going on here? How on earth can Helium II break one of the most fundamental and thoroughly-verified laws of the universe? Are these experiments somehow flawed? Are the laws of thermodynamics actually wrong? Or is Helium II so weird it tears a hole in the fabric of reality itself? As it turns out, none of the above. Though the physics of superfluids are still being actively researched, the best theory physicists have been able to come up with to explain their apparently contradictory behaviour is that Helium II is not one fluid, but two. According to this model, one component of the fluid behaves more or less like a normal liquid, while the other, superfluid component has no viscosity and can spontaneously flow from cold to hot regions. And just how is this component able to accomplish this? Simple: it carries no entropy. While commonly defined as the degree of disorder or randomness within a system, entropy is also a measure of energy – specifically energy that cannot be used to perform useful work. The Second Law of Thermodynamics states that in a closed system, entropy always goes up in proportion to the work done to or by the system divided by the ambient temperature. As cooler substances are more ordered and thus contain less entropy than warmer ones, this means that atoms or molecules always flow from warmer regions to cooler ones and not the other way around. However, if Helium II carries no entropy, then it is free to flow in the opposite direction, allowing the creation of a pump that requires no mechanical pumping. Like I said: simple…

But now comes the real question: why does Helium II behave this way? The answer, surprisingly, has to do with quantum mechanics. Describing the behaviour of matter at very small scales, the world of quantum mechanics is more strange and fantastical than anything Alice could ever have found down the rabbit hole or through the looking glass. It is a world where the laws of causality give way to the laws of probability – where particles can be in multiple places at once, communicate with each other instantaneously from opposite sides of the universe, and even change their properties based on whether or not they are being observed. Normally, such strange effects are only observable at the scale of atoms and subatomic particles, but in superfluid Helium II they become visible at much larger scales, explaining why this substance behaves so differently from most ordinary matter. Indeed, the reason Helium II exhibits superfluidity and near-infinite thermal conductivity is because unlike in regular fluids, the atoms in Helium II all occupy the same quantum state, allowing them to act as one large wave function. To explain exactly what this means would take several extra videos, but thankfully physicist Wolfgang Ketterle has a useful analogy:

“Assume that you go to a crowded space and people walk kind of randomly – they run into each other, they bump into each other, there is lots of friction. And if you want to cross the street and there are many, many people in the street, it can take forever. But now imagine all the people march in lockstep. If all the people march in lockstep, there is no friction, there is no elbowing anymore, and all the people can quickly cross the street because they are all walking together.”

Quantum mechanics also explains why some types of Helium are easier to turn into superfluids than others. The most common isotope of Helium is Helium-4, whose nucleus has two protons and two neutrons. This is the type of helium that becomes a superfluid at 2.2 Kelvin. By contrast, the much rarer Helium-3, which has one fewer neutron, only becomes a superfluid at millikelvin temperatures – that is, a few thousandths of a degree above absolute zero. The reason has to do with a property of particles called spin and a law of quantum mechanics called the Pauli Exclusion Principle. Though often described as a particle’s angular momentum, in reality, Spin is a purely quantum mechanical property with no direct counterpart in classical mechanics, the name deriving from an early theory – since disproven – that particles literally spun about their axes. One way to understand spin is as a particle’s rotational symmetry – that is, the number of times it must be rotated to return to its original state. This is given by its spin quantum number, which can be a positive or negative integer or fraction. For example, a spin-0 particle is identical in any orientation, a spin-1 particle must be rotated once or 360 degrees to return to its original state, and a spin-1/2 particle must be rotated twice or 720 degrees. According to the Standard Model of Physics, all matter of the universe is divided into two types of particles: Fermions, which have fractional spins like 1/2, 3/2 and so on; and Bosons, which have integer spins like 0,1,2 etc. Further, according to the Pauli Exclusion Principle, first described by Austrian physicist Wolfgang Pauli in 1925, two Fermions cannot occupy the same quantum state within the same quantum system. Since Helium-4 is a Boson with a spin of 0, it is exempt from this rule and form a superfluid in which all its atoms share the same quantum state. Helium-3, by contrast, is a Fermion with a spin of 1/2, and cannot be made into a superfluid at the same temperatures. At millikelvin temperatures, however, pairs of Helium-3 combine in twos to form what are known as Cooper pairs, which act as bosons and can become superfluid. Clear as mud?

So, what does this all mean? Aside from being a mind-mending, macro-scale demonstration of quantum weirdness, do superfluids like Helium II have any practical applications? Well, no, not yet, but the physics of superfluids are directly applicable to another unusual low-temperature phenomenon: superconductivity. At temperatures below 30 Kelvin, many materials suddenly lose all resistance to electrical flow, meaning an electric current established within said materials would continue to circulate forever without loss so long as the critical temperature is maintained. Superconductors also reject magnetic fields, meaning magnets will levitate above them. This effect is already widely exploited in high-powered superconducting magnets such as those in MRI machines – the primary application of liquid helium. However, more extensive use of superconductors is stymied by the ultra-low temperatures required to achieve this effect, with even so-called “high temperature” superconductors having critical temperatures around 90 Kelvin. This is where the study of superfluids comes in, since according to a 1957 theory by John Bardeen, Leon Cooper, and John Schrieffer, the flow of current through a superconductor can be thought of as a superfluid composed of Cooper pairs of electrons. The study of superfluidity may thus one day unlock the secret to the holy grail of electrical engineering: room-temperature superconductors, which would allow for ultra-efficient electrical transmission and storage and hundreds of other revolutionary technologies. So Helium II is not merely the weirdest substance known to science; it may one day change the course of human civilization.