How low can our temperatures go?

As cold as cold gets...

Over the last month we have been experiencing some of the coldest weather in living memory, with temperatures in many parts of the UK dropping as low as -20C.

However, on a global scale these temperatures are positively balmy . . . the lowest temperature ever recorded on planet Earth is a chilly -89C measured at Vostok Station in Antarctica in 1982.

This raises a very interesting question — is there a limit to how low a temperature we could reach?

To answer this question we first have to consider what we really mean by “temperature.”

Scientifically, the temperature of an object is related directly to the energy of the atoms or molecules that make up the object.

The faster the atoms and molecules move or vibrate the hotter the object seems, the slower they are moving the colder the object feels.

This implies that if we were able to continue cooling the object then eventually all of its constituent atoms and molecules would stop moving, and it would be impossible to cool any further.

In other words we would have reached the absolute zero of temperature.

The concept of an absolute zero was first proposed as early as 1702 when Guillaume Amontons observed the change in volume of air as temperature decreases.

Amontons calculated absolute zero — the temperature at which the volume of air should reach zero — to be -240C.

This is remarkably close to the value of -273C estimated almost 150 years later by Lord Kelvin, and the value of -273.15C we use today.

In 1848, Lord Kelvin proposed a new absolute temperature scale in which absolute zero naturally took the value of zero, and the unit of temperature, later named a Kelvin (or K for short) in honour of his work, was given the same value as the degree Centigrade.

The Kelvin temperature scale, which is now universally used in all scientific work, has the ice point of water at a temperature of 273.15K and the boiling point of water at 373.15K. Correspondingly, the coldest recorded temperature on Earth is 184K.

Interestingly, carbon dioxide transforms directly from a gas to a solid at 194.65K, so on that very cold day in Antarctica in 1982, carbon dioxide would have been dropping like snowflakes from the atmosphere.

In comparison, oxygen is liquid below 90K, nitrogen, the main constituent of our atmosphere, is liquid below77K while hydrogen boils at 20K.

Liquid gases are very useful in maintaining and exploiting low temperatures in so called cryogenic technologies.

For example, liquid nitrogen is used to freeze-dry food, or human tissue for medical purposes, liquid hydrogen is used as rocket fuel for the space shuttle, while the low-lying mist that singers often walk through on TV shows like the “The X Factor” is produced simply by allowing carbon dioxide to change from its solid to gaseous form.

Perhaps the safest and most useful cryogenic fluid is liquid helium, with a boiling point of 4.2K, first produced by Kamerlingh Onnes in Leiden in 1908.

Liquid helium is the coolant for the powerful superconducting magnets which are so necessary for medical imaging and particle accelerators.

Indeed, it was Kamerlingh Onnes’ quest for even lower temperatures that led him to discover superconductivity itself, the property of a material to carry an electrical current without any resistance, precisely 100 years ago.

Over the last hundred years science has developed ever more intricate processes for reducing temperature.

The current record, held since 2003 by Nobel Laureate Wolfgang Ketterle and colleagues at the Massachusetts Institute of Technology, stands at 450picoKelvin, or less than one half of a billionth of a degree Kelvin above absolute zero!

Will we ever achieve absolute zero in the laboratory? Quantum physics tells us that we probably can’t, because we can never stop all motion of atoms and electrons.

On the other hand, it seems ironic that in the UK we only have to reach -10C to stop all motion of cars, trains and planes!




Professor Cywinski is Dean of Applied Sciences at the University of Huddersfield