If ever there was a criminally underrated pure useful resource, it must be Helium. Although mostly related to social gathering balloons and making one’s voice sound like a cartoon, Helium’s most necessary utility is in cooling the magnets of Magnetic Resonance Imaging or MRI machines. Whereas the finite and ever-dwindling world provide of this vitally necessary gasoline is a subject worthy of its personal video, maybe much more fascinating is simply how weird a component Helium really is. For if Helium is liquefied and cooled to a low sufficient temperature, it begins to behave like no different liquid on earth, seemingly violating the legal guidelines of gravity, thermodynamics, and even logic itself. That is the story of superfluid Helium II, the weirdest substance recognized to science.
To ensure that Helium to be liquefied, it should be cooled to a temperature of -268.8 levels Celsius or 4.2 Kelvin – that’s, solely 4.2 levels above Absolute Zero, the coldest temperature theoretically attainable. In contrast, Nitrogen liquefies at a comparatively balmy 77 Kelvin, Oxygen at 54 Kelvin, and Hydrogen at 33 Kelvin. The rationale Helium is so tough to liquefy lies in its electron orbitals being fully crammed, making it – like the opposite noble gases Neon, Argon, Krypton, Xenon, and Radon – electrically impartial and chemically inert. Which means the one pressure which may pull Helium atoms collectively is the so-called Van de Waals Drive, which is brought on by electrons shifting from one aspect of an atom to the opposite and making a momentary electrostatic cost. This pressure is extremely weak, that means that Helium should be cooled to extraordinarily low temperatures to ensure that the Van de Waals forces to beat the vitality of the shifting atoms and pull them shut sufficient collectively for the gasoline to liquefy. Solidifying Helium is much more tough – so tough, actually, that it can’t be performed at common atmospheric pressures. Solely at pressures of 25 atmospheres and above can stable Helium be created.
At temperatures close to 4.2 Kelvin, bizarre liquid Helium – often called liquid Helium I – largely behaves like every common liquid, although it does have sure peculiarities. For instance, its density is barely 0.126 grams per cubic centimetre – or round 13% that of water – whereas its refractive index – the diploma to which mild bends because it passes by way of – can also be 1.025 in comparison with air, making the floor of liquid Helium very tough to see. However for Helium to actually begin exhibiting its wild aspect, it should be additional cooled to under 2.2 Kelvin – a temperature often called the lambda level. Reaching such temperatures, nonetheless, is not any straightforward feat. Certainly, simply sustaining Helium in liquid type requires using a specialised container often called a Dewar flask. Mainly a scientific model of the thermos bottles many people use to retailer soup or espresso, a Dewar consists of a double-walled vessel with a vacuum between the partitions to forestall warmth switch through convection. These partitions are sometimes silvered to mirror away infrared radiation, whereas liquid nitrogen or oxygen is often circulated across the Dewar, to cut back the temperature rise from the within to the surface of the flask from 289 to round 50-70 levels. Additional cooling is often achieved by pumping out the Dewar with a robust vacuum pump, inflicting the Helium to boil and warmth to be carried away by the ensuing gasoline. Nonetheless, whereas Helium’s latent warmth of vaporization – that’s, the vitality wanted to vaporize a sure mass of liquid – is round 5 energy per gram, its particular warmth – the quantity of vitality wanted to lift the temperature of that mass by one diploma Celsius – is round 1 calorie per gram and will increase exponentially the colder the Helium will get, making it more and more tough to decrease the temperature any additional.
However then, because the lambda level of two.2 Kelvin is reached, the liquid abruptly stops boiling and falls eerily nonetheless. The Helium has now entered a wholly new section referred to as Liquid Helium II, whose mind-bendingly unique properties have fascinated and baffled scientists for practically a century. To provide you a small style of simply how weird Helium II is, the explanation the liquid stops boiling because it crosses the lambda level is as a result of its thermal conductivity has simply elevated by a whopping a million instances. Actually, Helium II is essentially the most thermally conductive substance recognized to science, conducting warmth vitality a whole lot of 1000’s of instances sooner than the perfect recognized stable conductors like copper and silver. The rationale regular liquids boil is that they soak up warmth earlier than it may be distributed all through the liquid mass, inflicting the liquid to vaporize and bubble away. In Helium II, nonetheless, the warmth is near-instantaneously redistributed all through the liquid, stopping this from occurring. Stranger nonetheless, warmth doesn’t journey by way of Helium II through regular convection however slightly in wave-like pulses referred to as solitons, a phenomenon often called second sound. In a basic experiment carried out within the late Nineteen Thirties, Soviet physicist Pyotr Kapitsa immersed two delicate electrical resistors in Helium II, spaced a brief distance aside. One resistor was related to a sign generator and the opposite to an oscilloscope, such that the primary produced a collection of standard thermal pulses whereas the opposite detected any variations in the liquid’s temperature. Utilizing this setup, Kapitsa found that the second resistor might detect the thermal pulses from the primary mere milliseconds after they have been generated, revealing that warmth travels by way of Helium II on the velocity of sound.
However in case you assume that’s bizarre, effectively brace yourselves, as a result of issues are about to get even weirder. Let’s say you place some liquid Helium in a glass beaker with a backside manufactured from sintered ceramic, the pores of that are only some micrometers in diameter. At temperatures above the lambda level, the viscosity of liquid Helium I – that’s, its resistance to circulation – prevents it from flowing by way of the ceramic. However the second the Helium is cooled under the lambda level, it abruptly drains by way of the ceramic like a sieve. This circulation is so quick that, for all intents and functions, Helium II has no measurable viscosity. It’s, in different phrases, a superfluid. This property manifests in even stranger methods, akin to the lack of Helium II to be contained in an open-topped vessel. Fill a beaker with Helium II, and it’ll climb up the inside partitions in opposition to the pressure of gravity and leak down the opposite aspect, flowing as a liquid movie solely a few atomic diameters or angstroms in thickness. An analogous impact happens when a u-shaped tube is stuffed with extraordinarily positive powder like jeweller’s rouge and one finish is immersed in a vessel of Helium II. The tube instantly acts like a self-priming siphon, inflicting all of the Helium to circulation out of the vessel.
But this conclusion is totally contradicted by one other basic experiment. On this experiment, a metallic cylinder is positioned in Helium II and spun utilizing an exterior electromagnet. Positioned above this cylinder however not mechanically related to it’s a light-weight paddlewheel. Just a few seconds after the cylinder is spun up, the paddlewheel begins to show. This movement is brought on by the boundary layer of fluid adhering to the floor of the cylinder entraining different, adjoining fluid particles, inflicting a round circulation that strikes the paddlewheel. Nonetheless, that is solely attainable if the fluid has some viscosity. In abstract, Helium II shows zero viscosity in sure experiments and a small however finite viscosity in others. This is without doubt one of the many obvious contradictions that make superfluid Helium II so totally baffling.
However by far essentially the most reality-bending property of Helium II is demonstrated utilizing a hole glass stem with an open-ended bulb full of jeweller’s rouge. When this equipment is immersed in Helium II and a beam of sunshine aimed onto the bulb, liquid Helium squirts out the highest of the stem. Whereas at first look this phenomenon – often called the “fountain impact” – could not appear all that unusual, a more in-depth examination of the physics concerned reveals that it ought to, actually, be unattainable. For to ensure that the liquid Helium to circulation into the bulb and up the stem, it should spontaneously journey from a colder area to a warmer area – one thing expressly forbidden by the Second Regulation of Thermodynamics.
So, what’s going on right here? How on earth can Helium II break one of the vital elementary and thoroughly-verified legal guidelines of the universe? Are these experiments in some way flawed? Are the legal guidelines of thermodynamics really flawed? Or is Helium II so bizarre it tears a gap within the cloth of actuality itself? Because it seems, not one of the above. Although the physics of superfluids are nonetheless being actively researched, the perfect idea physicists have been capable of come up with to clarify their apparently contradictory behaviour is that Helium II just isn’t one fluid, however two. Based on this mannequin, one part of the fluid behaves kind of like a traditional liquid, whereas the opposite, superfluid part has no viscosity and might spontaneously circulation from chilly to sizzling areas. And simply how is that this part capable of accomplish this? Easy: it carries no entropy. Whereas generally outlined because the diploma of dysfunction or randomness inside a system, entropy can also be a measure of vitality – particularly vitality that can’t be used to carry out helpful work. The Second Regulation of Thermodynamics states that in a closed system, entropy at all times goes up in proportion to the work performed to or by the system divided by the ambient temperature. As cooler substances are extra ordered and thus include much less entropy than hotter ones, because of this atoms or molecules at all times circulation from hotter areas to cooler ones and never the opposite manner round. Nonetheless, if Helium II carries no entropy, then it’s free to circulation in the wrong way, permitting the creation of a pump that requires no mechanical pumping. Like I stated: easy…
However now comes the actual query: why does Helium II behave this fashion? The reply, surprisingly, has to do with quantum mechanics. Describing the behaviour of matter at very small scales, the world of quantum mechanics is more odd and fantastical than something Alice might ever have discovered down the rabbit gap or by way of the wanting glass. It’s a world the place the legal guidelines of causality give option to the legal guidelines of chance – the place particles might be in a number of locations directly, talk with one another instantaneously from reverse sides of the universe, and even change their properties primarily based on whether or not or not they’re being noticed. Usually, such unusual results are solely observable on the scale of atoms and subatomic particles, however in superfluid Helium II they change into seen at a lot bigger scales, explaining why this substance behaves so in another way from most bizarre matter. Certainly, the explanation Helium II reveals superfluidity and near-infinite thermal conductivity is as a result of in contrast to in common fluids, the atoms in Helium II all occupy the identical quantum state, permitting them to behave as one massive wave perform. To clarify precisely what this implies would take a number of additional movies, however fortunately physicist Wolfgang Ketterle has a helpful analogy:
“Assume that you simply go to a crowded house and folks stroll form of randomly – they run into one another, they stumble upon one another, there may be plenty of friction. And if you wish to cross the road and there are numerous, many individuals on the street, it could take perpetually. However now think about all of the individuals march in lockstep. If all of the individuals march in lockstep, there is no such thing as a friction, there is no such thing as a elbowing anymore, and all of the individuals can rapidly cross the road as a result of they’re all strolling collectively.”
Quantum mechanics additionally explains why some varieties of Helium are simpler to show into superfluids than others. The most typical isotope of Helium is Helium-4, whose nucleus has two protons and two neutrons. That is the kind of helium that turns into a superfluid at 2.2 Kelvin. In contrast, the a lot rarer Helium-3, which has one fewer neutron, solely turns into a superfluid at millikelvin temperatures – that’s, just a few thousandths of a level above absolute zero. The rationale has to do with a property of particles referred to as spin and a regulation of quantum mechanics referred to as the Pauli Exclusion Precept. Although usually described as a particle’s angular momentum, in actuality, Spin is a purely quantum mechanical property with no direct counterpart in classical mechanics, the identify deriving from an early idea – since disproven – that particles actually spun about their axes. One option to perceive spin is as a particle’s rotational symmetry – that’s, the variety of instances it should be rotated to return to its authentic state. That is given by its spin quantum quantity, which could be a constructive or destructive integer or fraction. For instance, a spin-0 particle is an identical in any orientation, a spin-1 particle should be rotated as soon as or 360 levels to return to its authentic state, and a spin-1/2 particle should be rotated twice or 720 levels. Based on the Commonplace Mannequin of Physics, all matter of the universe is split into two varieties of particles: Fermions, which have fractional spins like 1/2, 3/2 and so forth; and Bosons, which have integer spins like 0,1,2 and so forth. Additional, in accordance with the Pauli Exclusion Precept, first described by Austrian physicist Wolfgang Pauli in 1925, two Fermions can not occupy the identical quantum state throughout the identical quantum system. Since Helium-4 is a Boson with a spin of 0, it’s exempt from this rule and type a superfluid wherein all its atoms share the identical quantum state. Helium-3, in contrast, is a Fermion with a spin of 1/2, and can’t be made right into a superfluid on the identical temperatures. At millikelvin temperatures, nonetheless, pairs of Helium-3 mix in twos to type what are often called Cooper pairs, which act as bosons and might change into superfluid. Clear as mud?
So, what does this all imply? Other than being a mind-mending, macro-scale demonstration of quantum weirdness, do superfluids like Helium II have any sensible functions? Effectively, no, not but, however the physics of superfluids are immediately relevant to a different uncommon low-temperature phenomenon: superconductivity. At temperatures under 30 Kelvin, many supplies abruptly lose all resistance to electrical circulation, that means an electrical present established inside stated supplies would proceed to flow into perpetually with out loss as long as the crucial temperature is maintained. Superconductors additionally reject magnetic fields, that means magnets will levitate above them. This impact is already broadly exploited in high-powered superconducting magnets akin to these in MRI machines – the first utility of liquid helium. Nonetheless, extra intensive use of superconductors is stymied by the ultra-low temperatures required to attain this impact, with even so-called “excessive temperature” superconductors having crucial temperatures round 90 Kelvin. That is the place the examine of superfluids comes in, since in accordance with a 1957 idea by John Bardeen, Leon Cooper, and John Schrieffer, the circulation of present by way of a superconductor might be considered a superfluid composed of Cooper pairs of electrons. The examine of superfluidity could thus sooner or later unlock the key to the holy grail {of electrical} engineering: room-temperature superconductors, which might enable for ultra-efficient electrical transmission and storage and a whole lot of different revolutionary applied sciences. So Helium II just isn’t merely the weirdest substance recognized to science; it might sooner or later change the course of human civilization.
