Should The Standard Kilogram Be Redefined?

Although the platinum-iridium alloy standard kilogram had served us very accurately since 1879, is there a need for a more accurate kilogram standard? 

By: Ringo Bones 

The egg-sized platinum-iridium alloy cylinder that has officially defined the mass of a kilogram may soon be set aside in favor of a measurement that not only is more accurate – but one that is actually defined by fundamental constants of nature in order to make it several magnitudes more accurate than anything that is used before. The platinum-iridium alloy defining the standard kilogram known as “Le Grand K”, has sat inside a hermetically sealed room in Paris since 1879 and has served as the benchmark against which all other kilograms are compared. 

Sadly, the metric system’s “Le Grand K” has its failings. For one, it must be housed inside three glass bell jars in a climate-controlled room, under multiple locks and keys. The slightest fleck of dust or smudge of sweat or residue could alter its weight or corrode its surface, changing its mass. The hunk of metal is only taken out once every 40 years to be compared against similar replicas from around the world. Stephan Schlamminger, a physicist at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland says:”The problem with the kilogram in Paris is that it’s so precious that people don’t want to use it.” 

So for years, physicists have chased an elusive dream: replacing the standard physical kilogram with a standard inherent in properties of nature such as the speed of light, the wavelength of photons and the Planck Constant (also called h-bar), which links the energy a wave carries with its frequency of oscillation. Well, physicists had managed to replace the x-shaped platinum- iridium bar which has served as the standard meter since 1890 with the 1,650,763.73 wavelengths of radiation from the krypton-86 atom back in 1960.

Scientists could use the Planck Constant to compare the energy of a wave with Albert Einstein’s iconic E=mc² equation; in that way, they would determine mass solely through the physical constants. Unfortunately, no one has yet been able to measure the Planck Constant to a level of precision that could rival what has been achieved using the Le Grand K as the benchmark. 

But researchers are making strides, and at the current pace, believe they can redefine the standard kilogram as soon as 2018. In the new study published in the journal Review of Scientific Instruments, NIST physicist Stephan Schlamminger and his colleagues managed to measure the Planck Constant to a high level of precision using the NIST-4 watt balance, a sophisticated scale that measures a weight by the electromagnetic force that counterbalances it. The electromagnetic force can then be used to calculate the Planck Constant. With this method, the NIST team calculated the Planck Constant down to an uncertainty of 34 parts per billion. That result lines up well with what other teams have calculated. A separate experiment measuring the atoms in a silicon sphere has calculated the Planck Constant down to an uncertainty of 20 parts per billion, while the best watt measurement has achieved an uncertainty of just 19 parts per billion. All of this spells good news to increase the accuracy of the standard kilogram and could eventually be used to determine the exact number of platinum and iridium atoms that have rubbed off the Le Grand K since we started using it in 1879.