SciBar: How Low Can You Go?

A history of temperature physics with Holly Middleton-Spencer.

Words by Sarah de Launey, edited by Nicola Simcock

For the last SciBar of the Summer, Palace of Science readies us for the changing of the seasons with a talk on the cold. Holly Middleton-Spencer, PhD student in Mathematics at Newcastle University, takes us through the story of the original cold war: the race to absolute zero.

Our story begins in the mid 1600s: England had just entered a new era of Cromwellian austerity, France was ruled by its youngest king and Germany dominated the Holy Roman Empire. Sound familiar?

Up until this time, scientists did not have accurate instruments to measure temperature. The best they had was a wine powered thermometer (which as it turns out, is not very accurate) and so were only able to describe the effects of temperature anecdotally, at best. The arrival of Parisian Ismael Boulliau’s mercury thermometer in 1659, with its unprecedented reliability, quickly became a hot favourite among scientists, engineers, cooks and doctors (if you don’t like temperature puns, stop reading now). Despite this huge technological development, there were just as many forms of notation and scales as there were groups who used them. The Réaumur scale, popular in France and Russia, took 0Ré as the freezing of water but used boiling wine as its upper benchmark of 80Ré. Meanwhile the Romer scale set its benchmark points at 0Ro for the freezing of water and 60Ro for the boiling of water. Free-thinking Newton chose 0 as the heat of air in winter and 17 as the temperature of sensible bath water. Celsius decided on 0C for the boiling of water and 100C for freezing (swapped coincidently after his death by Swedish botanist Carl Linnaeus).

Not having a common scale or system of notation meant that scientists were effectively working alone, without any means of sharing experimental results and advances.

Robert Boyle’s frustrations boiled over into an exasperated declaration that “the common instruments show us no more than the relative coldness of the air; whence we cannot communicate the idea to another person”. He had a point! It would be impossible for temperature physics to develop into a real discipline without a measurement standard. In 1665 the Royal Society endorsed the Boyle and Hooke Standard Thermometer and the Celsius scale, and that was that: temperature physics became a thing.

Equipped with accurate instruments and common notation, research in temperature physics started to heat up. Although for the next 50 years, scientists and engineers focused on the upper limits of heat and its applications (work trip to the Caribbean?). It wasn’t until the 17th century that scientists began to investigate the lower limits of temperature.

The first to explore the lower limits was French physicist Guillaume Amontons. Despite having never been to Newcastle in Winter, he was the first to talk about the concept of absolute zero. Amazingly, in 1702 he predicted that zero was equivalent to about −240° C — not far off what we now understand as absolute zero. As he believed that absolute zero could never be reached, he never bothered to compute it explicitly, instead he went on to casually invent the optical telegraph and water clock.

Fast-forward to the 19th century, we now know a lot of fundamental things about heating and cooling things: water can remain liquid below 0°C, the boiling point of liquids varies with pressure, we even have a scale of absolute temperature (the Kelvin scale). However, we still don’t actually know what temperature is. The major theory at the time was the caloric theory: where heat is a weightless fluid gas called caloric that flows from hotter bodies to colder bodies.

Enter James Joule: a brewer who did science as a hobby. In 1843, he noticed that when he agitated liquids (…beer) in his laboratory (…brewery), its temperature increased. Now, this seems obvious to us, rubbing your arms in winter warms you up and when you trip over you get a carpet burn. But this was revolutionary stuff! Joule had discovered that heat is a type of energy.

Meanwhile, scientists were frantically inventing new instruments and experimenting to be the first to liquify (cool down) all the known gases. Just like a game of limbo at a fresher week party, the game of “how low can you go?” was one fueled by competition and ego (…and alcohol).

Master liquefier, Faraday, cooled most of the known gases down to their liquid form except 6: oxygen, hydrogen (which almost killed him), nitrogen, carbon monoxide, methane and nitric oxide. No matter how hard he tried, they just wouldn’t liquify. So, in 1845, he labelled them “permanent gases” and retired to Middlesex.

The heat was on for physicists to liquify those last 6 gases! The main contenders: best friends Cailletet and Pictet, sworn enemies Dewar and Ramsey, and silver-tongued Onnes.

Louis Cailletet and Raoul Pictet won oxygen and nitrogen – liquifying them with the “nozzle and porous plug” trick then left the race. Hotblooded Dewar, who had almost liquified nitrogen, was furious… when he found out Ramsay was making progress, he broke into his lab to sabotage his experiments. Ramsay didn’t end up on the charts for liquifying gases but was happy to leave the race having discovered noble gases instead.

With Ramsay out of the picture, Dewar turned his attention to Onnes in a challenge to liquify the remaining gases: hydrogen and helium. Dewar was first by a nose to liquefy hydrogen, reaching a new low temperature record of −252 °C in 1898. However, Onnes got the last laugh, winning the Nobel prize in 1913 for liquifying Helium and reaching the coldest temperature achieved on earth: −269 °C (1.5 Kelvin). In doing so, he noticed that as mercury approached 0 Kelvin (absolute zero), the resistance of the metal dropped to almost 0. Suddenly, liquifying things didn’t seem very important… he had discovered superconductivity!

A loop of superconducting wire can sustain an electric current indefinitely with no power source (youtube superconductor levitation, it’s awesome!). Just like Joule’s discovery that heat is energy, this was the next big revolution in temperature physics.

This cool new state was later described as the Bose–Einstein condensate (BEC). When low density gases get to a fraction of a Kelvin from absolute zero, all bosons (one of the two classes of quantum particles) present go into the same state. This means that a BEC made up of millions of particles behaves as a single quantum particle. Just like if, all at once, a giant rave party fell into a military march. Trippy.

Meanwhile in the East, in a lab that a groveling Stalin had personally paid for, Piotr Kapitsa was observing a weird phenomenon. He noticed that as helium approaches zero, it goes through a bubbly stage and then climbs up the chamber walls and spreads out. This became known as a superfluid, a type of quantum fluid. Unlike classic fluids (like whatever you’re drinking right now), it is totally unpredictable and still not fully understood. Holly Middleton-Spencer and her team at Newcastle University are currently working on mathematical models to understand this strange fluid.

Until we know more, when it gets close to 0 in Newcastle this Winter, don’t go outside. It will be chaos.

Want to keep reading? Holly Middleton-Spencer recommends “Absolute zero and the conquest of cold”, by Tom Shachtman

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