The official definition of a kilogram has been changing for well over a century, but this week it may be redefined for the final time. After years of debate and discussion, scientists from around the world are meeting in Paris on Friday to vote on whether the kilogram, the mole, the ampere and the kelvin should be changed to more stable and reliable definitions.
Currently, the kilogram is the only unit of measurement to still be based on a physical object – specifically, a lump of metal in a vault in France. This International Prototype of the Kilogram (IPK) has been the official standard since 1879, but it isn't as unchanging as you might think.
Naturally, the IPK has been gathering microscopic contaminants during the past 140-odd years, meaning the official definition of a kilogram has to keep being updated to match the new mass, while the artefact itself needs to undergo regular cleaning. Complicating things further, 40 "exact" copies of the IPK were made and distributed to institutions around the world, but their own masses are also changing slowly at different rates, meaning their definitions are drifting out of sync.
Obviously, it's far easier to base an official definition on something that doesn't change, and that's the goal of this week's General Conference on Weights and Measures. It's not just the kilogram either – four of the seven SI units are due for a tune-up, to bring them in line with unchanging laws of nature that can be measured anywhere at any time, without pulling an old metal block out of storage.
"The SI redefinition is a landmark moment in scientific measurement," says Dr JT Janssen, Director of Research at the National Physical Laboratory in the UK. "Once implemented, all the SI units will be based on fundamental constants of nature whose value will be fixed for ever. This will pave the way for far more accurate measurements and lays a more stable foundation for science."
If the vote is successful, going forward the kilogram will be defined by the Planck constant. This is calculated in an instrument known as a Kibble balance, which suspends a 1-kg weight using electromagnetic forces. The constant is the amount of energy it takes to balance the weight, and after years of experiments and measurements, that value has been determined to a precise degree.
The other units will have similarly stable new definitions. Measuring constant current, the ampere will be defined by the elementary electrical charge – the charge carried by a single proton. The kelvin, measuring thermodynamic temperature, will be defined by the Boltzmann constant, and the mole, measuring the amount of a substance, will be defined by the Avogadro constant.
So what does this change mean? To most of us, not much really. It's not like a kilogram is about to be redefined to the mass of two kilograms or anything – the update will be too minuscule for the general public to ever really notice the difference. It may affect scientific, industrial and technological sectors, but ultimately, the change will be for the better.
If the vote goes ahead, the redefinitions are set to officially come into effect on May 20, 2019, which is World Metrology Day.
Source: National Physical Laboratory
Randy