New Boltzmann Constant may Redefine the Definition of Temperature

A refined and more-accurate-than-ever measurement of the Boltzmann constant - the physical constant relating temperature with energy at the individual particle level - may revolutionize the way we define temperature, according to scientists at the UK's National Physical Laboratory.

Current definitions of the kelvin and the degree Celsius are founded on a temperature known as the "triple point" of water, where liquid water, solid ice and water vapor can all exist in equilibrium. This standard temperature has been defined as exactly 273.16 K. Every temperature assessment we make, whether it's reading a thermometer or feeling an object that's cold to the touch is a comparison to how much hotter or colder an object is compared to the triple point.

"The further away one measures from the temperature of the triple point of water, the harder it gets to precisely determine the ratio of exactly how much hotter or colder the temperature is than the standard temperature. This adds uncertainty to temperature measurements on top of the normal practical difficulties," said lead study author Michael de Podesta.

By redefining the kelvin using a fixed constant of nature - similar to how the definition of a meter was redefined from a physical piece of metal to the length of the path travelled by light in vacuum over a specified number of nanoseconds - researchers believe they have a solution that will allow for the greatest accuracy possible. Their solution is to use the latest definition of the Boltzmann constant as a base for defining temperature.

The new measurement for the Boltzmann constant is 1.380 651 56 (98) × 10^-23J K^-1, where the (98) shows the uncertainty in the last two digits, which amounts to an uncertainty of 0.7 parts per million -- almost half the previous lowest uncertainty. To calculate the new Boltzmann constant, researchers built an acoustic resonator and cooled it to the triple point. They then injected the resonator with isotropically pure argon gas and measured the speed of its sound to calculate the average speed of the argon molecules, hence the average amount of kinetic energy present. From that data, the researchers were able to calculate the Boltzmann constant with extremely high accuracy.

"It is fascinating that we worked out how to measure temperature long before we knew what temperature actually was. Now we understand that the temperature of an object is related to the energy of motion of its constituent atoms and molecules. When you touch an object and it feels 'hot' you are literally sensing the 'buzzing' of the atomic vibrations. The new definition directly links the unit of temperature to this basic physical reality," de Podesta said.

"This experiment has been exhilarating, and after six years we are exhausted. Every aspect of the experiment has required pushing science and engineering to the limit," continued de Podesta.

"In this kind of work we need to worry constantly about all the things which might go wrong, and how they might affect the results. We are looking forward to worrying a little less and getting on with exploiting some of the new technology we have invented in the course of the project,"

De Podesta and his colleagues' work is published in the journal Metrologia.