Editorial: 鈥�For ultra-precise measures, go for nature鈥檚 constants鈥�
A NOISY ball of gas is now the most accurate thermometer ever. That removes one hurdle to redefining the scientific unit of temperature, the kelvin, in terms of a fundamental constant.
At the moment, the kelvin is defined in terms of the temperature at which ice, liquid water and water vapour can coexist in equilibrium 鈥� 273.16 K or 0.01 掳C. Other temperatures are measured by comparison with this 鈥渢riple point鈥�, but the further away you get from it, the more inaccurate the result. 鈥淲hen you鈥檙e measuring at 1500 掳C, it is actually slightly bonkers to have to compare back to the triple point of water,鈥� says of the National Physical Laboratory in Teddington, UK.
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The International Committee for Weights and Measures (CIPM) in Paris, France, suggests defining the kelvin in terms of the Boltzmann constant, which relates the energy of particles to their temperature. That would be accurate even at extremes. The tricky part is setting an accurate value for the constant.
De Podesta and his colleagues filled a copper vessel with a known volume of argon gas. A gas鈥檚 energy is related to the average speed of its atoms, which is linked to the speed of sound. By playing notes through the vessel at many frequencies, the team determined the speed of sound in the argon, and the energy of its atoms.
The team knew the temperature of the argon, which was at the triple point of water, so could convert the energy measurement into a value for the Boltzmann constant.
of the German National Metrology Institute in Berlin, who heads the CIPM task group on redefining the kelvin, says the margin of error in the measurement, a record-breaking 0.71 parts per million, is potentially low enough to officially redefine the kelvin.
Some details need to be ironed out first. A team at the LNE, the French National Laboratory of Metrology and Testing in Paris, calculated the constant via the same method but came up with a different value. Their error is slightly higher, at 1.24 ppm, but the kelvin can鈥檛 be redefined until the discrepancy is understood.
Redefinition also won鈥檛 happen until a slew of other units that are due to be pinned to fundamental constants are agreed 鈥� and there is argument over the kilogram. 鈥淭he game is not quite over,鈥� says Michael Moldover at the US National Institute of Standards and Technology in Gaithersburg, Maryland, who created the argon method in 1988.
For now, de Podesta needs a rest. Smashing the Boltzmann record was painstaking, including sculpting the argon vessel to a precise volume. 鈥淭his has been occupying my mind for six years and I鈥檓 absolutely exhausted,鈥� he says.
This article appeared in print under the headline 鈥淪weet argon music helps redefine temperature鈥�
