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Near the heart of Scotland lies a large morass known as Dullatur Bog. Water seeps from these moistened acres and coalesces into the headwaters of a river which meanders through the countryside for nearly 22 miles until its terminus in Glasgow. In the late 19th century this river adorned the landscape just outside of the laboratory of Sir William Thompson, renowned scientist and president of the Royal Society. The river must have made an impression on Thompson⁠—when Queen Victoria granted him the title of Baron in 1892, he opted to adopt the river’s name as his own. Sir William Thompson was thenceforth known as Lord Kelvin.

Kelvin’s contributions to science were vast, but he is perhaps best known today for the temperature scale that bears his name. It is so named in honor of his discovery of the coldest possible temperature in our universe. Thompson had played a major role in developing the Laws of Thermodynamics, and in 1848 he used them to extrapolate that the coldest temperature any matter can become, regardless of the substance, is -273.15°C (-459.67°F). We now know this boundary as zero Kelvin.

Once this absolute zero temperature was decisively identified, prominent Victorian scientists commenced multiple independent efforts to build machines to explore this physical frontier. Their equipment was primitive, and the trappings were treacherous, but they pressed on nonetheless, dangers be damned. There was science to be done.

Prior to this 19th-century cold rush, most European scientists believed that coldness itself was an actual physical substance⁠—made up of atoms of an airborne primordial gas. This explained why water expanded upon freezing⁠—it was taking in a large amount of these cold particles. Physicist Robert Boyle dispelled this notion in 1665 by painstakingly weighing water before and after putting it outdoors on a freezing night, demonstrating that only its volume had changed, not its mass. This helped naturalists to start hypothesizing in the right direction, but in 1783, renowned French chemist Antoine de Lavoisier undid most of this progress by popularizing his own theory that heat is an invisible, weightless, self-repellent vapor called caloric, and that coldness is merely a depletion of the same. This “dark heat” theory was also wrong, but it modeled observations so well that it remained dominant for almost a century.

The Industrial Revolution

At the dawn of the Industrial Revolution, newfangled steam machines began to chuff heat into work, and science into profits. Cracking the true nature of heat would lead to more efficient power plants, so the utmost intellectual and financial assets converged upon the problem. When centuries of “common sense” were finally set aside in favor of the scientific method, theorists and experimenters gradually ascertained that all molecules in nature are restless, agitated things that randomly wiggle and wobble, bumping into neighbors like billiard balls on an overcrowded table. The net effect of these molecular motions is what we observe as heat, and temperature is directly proportional to the speed of these movements. From this, Lord Kelvin inferred that if one were to reduce the heat in a substance sufficiently, one would reach a temperature where the molecules become entirely still⁠—a minimum possible temperature. His calculations correctly indicated -273.15°C as this physical boundary.

This landmark discovery invited even more inquiry than it had quieted. Might it be possible to actually reach absolute zero? What would happen to molecules forced into such stillness? Would they disintegrate? Would they convert to a yet-to-be-observed phase of matter? Who goes there? What is the meaning of this?

One gentleman with the wherewithal to address these questions was Scottish scientist Sir James Dewar, an accomplished inventor and a professor at the Royal Institution in London. His history with cold was the stuff of origin stories⁠—at the age of ten James Dewar was frolicking upon a frozen pond in Scotland when the ice cracked open beneath his feet. Young Dewar was swallowed into the dark, freezing water. His companions managed to extract him, soggy and shivering, but in his compromised condition he became bedridden by rheumatic fever. When he was well enough to walk again he required crutches and struggled with rigid limbs and digits. A kindly violin-maker took him on as an apprentice in order to help him reclaim the fine motor skills in his hands, and when Dewar later chose the path of science, this delicate workmanship experience enabled him to fabricate his own laboratory tools and instruments.

As he advanced in the scientific community, Dewar grew to adore the late Michael Faraday, a brilliant and renowned predecessor at the Royal Institute. In the early 1800s Faraday had discovered that one can persuade practically any gas to liquefy by applying high pressures and ice baths. Faraday’s techniques were capable of reaching temperatures as low as -130°C, but despite his ingenuity, three of the gases he tried⁠—oxygen, nitrogen, and hydrogen⁠—persistently resisted the phase change to liquid. After many failed attempts Faraday tentatively concluded that these three must be “permanent gases.”

Sir James Dewar in his lab, 1902

By James Dewar’s day, scientists with more sophisticated yet still quite quaint equipment were finally liquefying oxygen at -183°C, and nitrogen at an astonishing -196°C, all without the benefit of electricity. They accomplished this by exploiting the Joule-Thomson effect, “Thompson” being the same man who later became Lord Kelvin. The Joule-Thomson effect describes the tendency of most gases to cool when they are allowed to expand through a valve. This offered an opportunity to chill substances in stages using a series of gases that are each more difficult to liquefy than the last. Experimenters employed pumps to compress each gas into separate holding tanks, then they used ice baths and heat exchangers to lower the temperature of each filled tank. Once each compressed gas was sufficiently chilled, the “cascade” process would begin.

To start the process, the valves on the first tank were opened, expanding the gas into a larger enclosure. This rapid expansion would cause the already cold gas to condense into even colder liquid. This liquid would then be used as the coolant to chill the still-compressed gas in the next tank as much as possible, such that when that gas was expanded into its own sealed vessel it would reach even colder extremes. This would then be used to cool the third gas, and so on. As experimenters pressed closer and closer to absolute zero, each degree of heat became more difficult to squeeze out than the last. All that remained to liquefy was hydrogen⁠—an odorless, colorless gas which tends to turn into a universe if left alone for a prolonged period. Scientists at the time expected that hydrogen would not liquefy until -250°C, within spitting distance of the coldest possible temperature in the universe. It was a monumental undertaking for the time, and it would require yet-to-be-invented apparatuses, but he who would be first to liquefy the last remaining “permanent gas” was sure to be showered with scientific acclaim for advancing humanity’s knowledge of the the properties of matter.

James Dewar was fond of referring to this challenge as “Mount Hydrogen.” Only a few facilities in the world had the equipment and aptitude necessary to even hike the foothills of Mount Hydrogen, let alone glimpse its summit, yet a number of chemists were beginning to clamber. Dewar knew that if he could be the first, his name would be whispered in the halls of the Royal Institution alongside the great Michael Faraday’s for decades⁠—perhaps centuries. Thus in the mid-1880s Sir James Dewar decided to direct all of his considerable scientific and engineering resources into cryogenic research for the purposes of liquefying hydrogen.

Kamerlingh Onnes (front center) with his assistants

Unfortunately, one of the resources Dewar lacked was a sense of tact⁠—he smugly ridiculed the missteps of other aspiring liquefiers in his public remarks and essays. But one fellow scientist somehow escaped his condescension⁠—a massively mustachioed Dutchman named Heike Kamerlingh Onnes. Onnes was a brilliant and ambitious younger scientist with a laboratory at the University of Leiden in the Netherlands. He was far behind in the race for liquid hydrogen, but he was making slow, steady advances which he open-sourced for the benefit of all contenders. This was in sharp contrast to Dewar’s tendency to be secretive and hide key components from laboratory visitors. Dewar was also an old-school do-it-yourself scientist, whereas Onnes preferred to employ an assembly line of assistants to fabricate his instruments. Dewar also silently scoffed at his younger colleague’s insistence that experiments should always be preceded by meticulous theorizing and calculations. But despite their differences, in true Victorian-era gentleman-scientist tradition, the two exchanged frequent correspondence across the Channel regarding their research progress and chronic health problems. The Dutchman clearly admired his elder colleague from London, and Dewar clearly shared (rather than reciprocated) this admiration.

Dewar and Onnes were both gradually assembling and rejiggering similar apparatuses based on the cascade method using the best available technologies. This cascade cooling theory was quite sturdy on paper, but in practice its execution was rife with peril. Sometimes liquefied gases would freeze solid in the lines, disrupting the delicate plumbing. The extreme low temperatures caused even the strongest holding tanks, tubing, valves, welds, and connectors to become fearfully brittle. Moreover, many of the most effective cooling gases were highly flammable, as was the hydrogen itself. Not to mention they were being held for long periods at very high pressures. Consequently each ever-evolving multi-ton collection of handmade and scavenged parts was a persnickety and treacherous assembly.

As the experiments progressed, James Dewar conducted a series of public lectures to demonstrate the properties of the coldest liquids he had so far managed to produce. The special “Dewar flasks” he used to handle these liquids were his own invention, a glass vessel with a gap of vacuum trapped between the inner and outer walls. The Scottish scientist dipped ordinarily flexible objects into liquid nitrogen, and then shattered them like glass. He produced a flask of bluish liquid oxygen⁠—which boiled boisterously at room temperature⁠—and used it to flash-freeze a vial of alcohol. Lastly he placed a lit candle in the steam of liquid oxygen vapors, which elicited a dramatic flourish of flame. He concluded these low-temperature demonstrations by explaining to the audience that science was pressing ever closer to the coldest possible temperature, whereupon the molecules would become entirely still, and the “death of matter” was likely to occur. Vigorous applause ensued.

Artist's depiction of a Dewar lecture

In 1886, James Dewar was entertaining guests at his Royal Institution laboratory in London when a lady asked whether it was true that he was able to condense oxygen. To demonstrate, the Scotsman expanded some compressed oxygen into a tube submerged in coolant, and the lady looked on in awe as the air itself condensed into a pale blue fluid. Unfortunately, there was a leak somewhere in the system, and the oxygen was allowed to mingle with the coolant⁠—liquid ethylene⁠—which oxidized violently. “The mixture caught fire and there was a terrible explosion,” Dewar wrote in a later letter to Onnes. “I was nearly killed and as the experiment was being performed before a number of people, several got hurt.”

As James Dewar recuperated from his injuries, Kamerlingh Onnes became a source of serious concern. The younger up-and-comer was rapidly and enthusiastically closing the research gap, imperiling Dewar’s imminent eminence. Onnes offered to come visit the London lab, but Dewar informed him that he if he did so he would not be allowed to set eyes upon the secret cryogenic equipment. Dewar felt he was within sight of the summit, and was reluctant to lend an accidental hand. Despite his ill health and injuries, Dewar resumed his low temperature research as soon as he was able, placing himself back in the midst of what he described as “difficulties and dangers of no ordinary kind.” These dangers again became evident in 1896 when a pressure gauge burst during a low-temperature experiment, flinging glass and shrapnel into the face of Dewar’s loyal laboratory assistant Robert Lennox. The incident cost Lennox an eye, yet he persevered. For science.

In March 1896, just when these laboratory calamities seemed certain to steal Dewar’s scientific prize, he received a letter from Kamerlingh Onnes, wherein the Dutchman confided:

I have not been able to repeat your splendid experiments for since your last letter it was impossible for me to work at low temperatures and that for a reason you will be astonished to hear. The municipality of Leiden has made objections as to my working with condensed gases and has not been content with asking that additional means of precaution are taken, but is gone so far to claim in August last that my cryogenic laboratory be removed from the city! Not withstanding that never any notable accident happened in all the years I have been working there…

A schematic of Dewar's hydrogen liquefier

Dewar replied, decrying this “great disaster to science,” and sent a letter to the Leiden town council complaining of the same. But as Onnes contested the banishment of his gases, Dewar had time to perfect his cascade cooling system. The lab installed a 100-horsepower state-of-the-art gasoline-fueled pump, and on the 10th of May 1898, James Dewar and laboratory assistants Robert Lennox and James Heath prepared their cantankerous contraption for an attempt. The scientists began by opening the valve on their compressed chloromethane, and expanded it to cool a tank of ethylene. This they expanded to liquefy a quantity of oxygen, which in turn was used to chill a canister which held hydrogen gas at an unprecedented 180 atmospheres of pressure. Once the hydrogen tank was at -205°C, the experimenters carefully twisted the ice-encrusted valve.

The gas hissed into the larger expansion tank, with no indication of freeze-ups or clogs. The thermometer steadily fell. Once the reading indicated -252°C, a clear, colorless liquid began to slowly but steadily drip into the glass collector at the bottom of the expansion tank. This continued for five minutes, until a nozzle froze up and forced the experiment to an end. James Dewar took a small vial of the liquid oxygen and submerged it in the new fluid. The pale blue oxygen froze instantly into a pale blue solid. This proved that the twenty cubic centimeters of liquid in the collector was indeed hydrogen. Dewar had done it. Thanks to a decade of effort he had created the coldest and stillest moment in spacetime the Earth had ever seen, just 21 measly kelvin above absolute zero. He had bested the upstart Dutchman, and he’d done what the great Michael Faraday had once deemed impossible. He had liquefied the final available “permanent gas,” and secured his place in history.

Or, so he thought. The anticipated acclaim did not materialize.

His accomplishment, it turned out, had been eclipsed by another discovery: chemists had identified yet another elemental gas whose liquefaction temperature was even lower than that of hydrogen. This new gas, helium, had been hypothesized to exist based on known elements and spectroscopic observations of the sun, but failure to find it in nature had led scientists to believe that the element could only be found in stars and gas giants. Indeed, loose helium is so light that it escapes Earth’s gravity and drifts into space, however intrepid sifters had finally begun to find some trapped in rocks, sands, and cavities. Helium became the new last permanent gas, and consequently its liquefaction became the noblest goal.

In the meantime, across the Channel, Kamerlingh Onnes had taken his laboratory closure complaint all the way to the Supreme Court of the Netherlands, and emerged victorious. The Dutch scientist dusted off his liquefaction machine and began mobilizing his own resources. Adjacent to his laboratory building he established the Society for the Promotion of the Training of Instrument-Makers, and set these “blue boys” to work improving his massive cryogenic apparatus. Using Dewar’s system as a guide, Onnes and his crew constructed their own high-volume hydrogen liquefaction plant with the intent to use its output as a coolant to produce liquid helium. But helium gas proved frightfully difficult to come by. Onnes learned that the great James Dewar had found that the sands surrounding Bath Springs held some trapped helium, and that Dewar had devised a process to extract it. The Dutchman wrote to propose an alliance. He offered to share his data, his calculations, his huge liquid hydrogen supply, and the cost of helium extraction all in exchange for access to the elusive gas. Dewar replied:

The "Blue Boys" in the Leiden machine room

We both want the same material in quantity from the same place at the same time and the supply is not sufficient to meet our great demands. It is a mistake to suppose the Bath supply is so great. I have not been able so far to accumulate sufficient for my liquefaction experiments. If I could make some progress with my own work the time might come when I could give a helping hand which would give me great pleasure.

Sir James Dewar’s supply was not quite as scarce as he suggested, but he had labored alone for far too long to go Dutch on the byline. Aged 62 and suffering from strained health and budget, he supervised the extraction of helium from the sand supply, a slow and tedious process which involved freezing it with liquid hydrogen and exploding it with oxygen. This would liberate a scant few helium atoms which were then trapped by a charcoal filter.

By 1903 he finally had collected enough of the gas for an attempt. The updated cryogenic system was even more complex than before, a massive sprawling cold factory festooned with valves, canisters, vents, and pipes. The scientists connected their precious compressed helium canister to its inlet and used their liquid hydrogen supply to refrigerate the container. When they opened the valve to expand the helium itself, some impurities in the gas froze solid inside the tubing and impeded the flow. An unspecified assistant with quick reflexes reversed the helium valve, but he turned it either the wrong way or too far, because instead of halting the flow of helium, he caused it all to be vented into the laboratory. Dewar’s notes do not indicate whether a high-pitched apology was offered.

Coincidentally, Lord Rayleigh and Sir William Ramsay had a laboratory just next door to Dewar’s, and these gentlemen had access to some helium. However they were also among the many scientists whose egos had been previously dashed upon the rocks of Dewar’s scorn. His deficit of decorum left him few friends. Dewar’s fortunes continued to deteriorate when his London lab was rattled by yet another minor explosion which deprived yet another lab assistant (James Heath) of yet another eye.

In the early dawn on the 10th of July 1908, Kamerlingh Onnes and his assistants gathered at their own low-temperature laboratory in Leiden. Onnes had found his own source of helium-laced sand from North Carolina, and he had patiently spent the intervening years extracting and gathering his own supply of the scarce element. Onnes and his “blue boys” filled every spare space in their vast apparatus with liquid hydrogen for insulation. They had pre-prepared their array of compressed gas canisters the evening prior. As word of the intent to liquefy helium spread through the university campus, a small crowd gathered at the laboratory to observe. Onnes’s wife arrived with sandwiches in the afternoon, but he was too busy to pause for nourishment, so she followed him around the lab thrusting bite-size portions into his mouth as he shouted orders and supervised the preparation of the machine.

Onnes and the blue boys working in the lab

At 4:20 in the afternoon, the critical parts of the assembly were awash in liquid hydrogen, and the men opened the main helium valve. Their lab compressor chugged away noisily, applying a gradually increasing pressure inside the expansion tank to increase the likelihood of helium condensation. Through the remainder of the afternoon and into the evening the scientists continuously replenished the liquid hydrogen coolant, and watched the thermometer as it crept down toward the temperatures where helium was expected to liquefy. As they poured in the last few drops of their liquid hydrogen, Onnes was clearly concerned⁠—although his equipment had achieved the unheard-of low temperature of 5 kelvins, the temperature reading had been stuck there for some time, and no liquid could be seen in the glass collector. Perhaps something was wrong. One of the onlooking professors suggested that the temperature might not be changing because the thermometer might just be submerged in a liquid. Onnes grabbed an electric light and hunkered down next to the glass reservoir. There, with the help of the light, Onnes became the first person on Earth to see liquid helium. The scientists had not anticipated that liquid helium’s index of refraction would be so extraordinarily low that it would be difficult to see in ambient lighting.

Onnes hastened to make observations with the small container of -271°C fluid before it all evaporated away. He found it had a lower surface tension than any previously observed liquid, and just 1/8th the density of water. The modest amount of the stuff he had been able to collect behaved very curiously in general, flowing with strange characteristics and evading easy observation as if enveloped in an SEP field. It was impossible for him to know that he had created a rare and fleeting superfluid, a previously unseen state of matter. The viscosity, or thickness, of a liquid is caused by dissipation of energy due to friction between particles, but since superfluid liquid helium is already in its lowest state it cannot dissipate energy, and it therefore must flow with zero resistance. Quantum mechanics insists.

Kamerlingh Onnes dispatched a telegram to his cryogenic colleague in London to share his exciting news. The telegram he received in reply was heartbreak couched in courtesy:

CONGRATULATIONS GLAD MY ANTICIPATIONS OF THE POSSIBILITY OF THE ACHIEVEMENT BY KNOWN METHODS CONFIRMED MY HELIUM WORK. ARRESTED BY ILL HEALTH BUT HOPE TO CONTINUE LATER ON.

Dewar was less circumspect with his pair of one-eyed laboratory assistants. He tongue-lashed Robert Lennox for failing to find a more abundant helium source, and Lennox stormed out, vowing never to return to the Royal Institution until James Dewar’s bones were cold in the ground. He proved to be a man of his word.

Short video of helium transitioning past the lambda point

It would be 15 years before any other researcher was successful in manufacturing liquid helium. In the meantime Onnes used his helium monopoly to study the effects of near-absolute-zero temperatures on various materials. He discovered that some materials such as mercury were able to conduct electricity with zero resistance at 1-2 kelvin above absolute zero, a phenomenon he dubbed superconductivity. This is the very property that enables some of the most advanced modern technology, such as MRI machines, magnetically levitated trains, and particle colliders.

Despite his considerable contributions to science, Sir James Dewar was never awarded a Nobel Prize, though he received nine nominations. Instead, scientific spoils went to the people who built upon his work. For instance, Lord Rayleigh and Sir William Ramsay used Dewar’s liquid hydrogen as a tool to discover the elements xenon, neon, and krypton, and they won the 1904 prize for Chemistry. Kamerlingh Onnes himself won the prize in Physics in 1913 for improving Dewar’s liquefaction strategy for helium. In his Nobel lecture, Onnes credited Dewar for having been the first to liquefy hydrogen, and for having revolutionized low-temperature research with his ingenious vacuum flask. But even the Dewar flask brought its namesake some vexation⁠—a glassblower he had hired to make some of the first such flasks became so fond of the design that he used the sincerest form of flattery to found the now-famous Thermos company. A lengthy court battle finally decreed that Dewar was entitled to credit for the invention, but not owed any damages. But most scientists today still call these flasks “Dewars” in his honor. The curmudgeonly maverick even filed a suit against the famous Nobel family, claiming that their explosive innovations were based on his earlier chemistry, but the judge dismissed the claims. Dewar never retired, and he maintained the position of Professor of Chemistry at the Royal Institution until his death on 27 March 1923.

Kamerlingh Onnes died in Leiden about three years later on 21 February 1926. He did so with the full awareness that he was among the last of a disappearing breed⁠—the “classical physicists” who had the luxury of simply banging on matter until it did interesting Newtonian things. Science was thenceforth in the capable hands of quantum mechanics. His helium liquefaction apparatus remains on display to this day at Leiden University.

A frost-encrusted liquid helium valve

In 1937, researchers Pyotr Kapitsa and John F. Allen first formally observed and described the strange superfluid state of liquid helium that Onnes had lacked the foreknowledge to identify. They found that when one chills liquid helium below the lambda point⁠—2.17 K⁠—the boiling liquid falls suddenly, eerily still, and it takes on bizarre properties. The individual helium atoms blur into one another and become a single “superatom”, also known as a partial Bose-Einstein Condensation (BEC). This is a demonstration of Heisenberg’s Uncertainty Principle, which states that the more precisely the momentum of a particle is determined, the less precisely its position can be known. Since particles below the lambda point have almost no movement, their momentums are almost entirely “known,” therefore by necessity their positions become so inexact that they begin to overlap one another. In this situation atoms stop behaving like discrete things and become ambiguous smears of quantum probabilities. If one physically scoops up a portion of the superatom, the elevated portion acquires more gravitational potential energy than the rest, and since this is not a sustainable equilibrium for the superfluid, it will flow up and out of its container to pull itself all back into one place. It also flows with zero friction, as Onnes observed, since it has no energy to lose. Matter is indeed strange stuff⁠—it’s just seldom so obvious.

Researchers today are tinkering with temperatures below one kelvin at extremely high pressures in order to freeze helium into helium ice, which may ultimately reveal a never-before-seen theoretical supersolid state. If the theory turns out to be correct, supersolids may make a mockery of the very notion of solidity since they will also be governed by the uncertainty principle. A chunk of helium ice would behave as a single, solid, oversized, and stupefyingly slippery atom, which may be capable of passing ghost-like through certain materials. But that’s another matter altogether.

As of this writing the coldest temperature ever reached on Earth occurred in 2003 when MIT scientists chilled a cloud of sodium atoms to 0.45 nanokelvin⁠—about one half of one billionth of a kelvin above absolute zero⁠—using lasers, evaporative cooling, and “gravito-magnetic traps.” And NASA is planning to routinely reach one ten billionth of a kelvin aboard the International Space Station (ISS) in the Cold Atom Laboratory, first activated in 2018. On Earth, Bose-Einstein condensates fall apart within fractions of a second due to the pull of gravity on the atoms⁠—but in the microgravity on the ISS, these condensates will linger longer, permitting observation for up to six hours at a time.

Modern scientists are confident that absolute zero itself is an absolutely unattainable temperature, as it would require an infinite amount of time and energy to squeeze out the last tiny fraction of heat energy. Nevertheless, there is nothing cooler than seeing science press up against the very boundaries of the physical laws of the universe, and to see how curiously things behave there.