One of the goals of science is to provide the most accurate description of reality as possible, so it’s always fun telling people how the unit of mass is defined. The kilogram, for example, is defined as the mass of a platinum-iridium cylinder in Sèvres, France.
Yes, every mass and every scale in the world has to be compared with the French block, which is aptly called the International prototype kilogram (IPK). When the kilogram was cast in 1889, 40 official physical copies were made and sent around the world. The specific measurement of the kilogram was taken to be equivalent to 1,000 cubic centimeters of water.
Obviously, having a physical object to quantify a unit is not great. The actual mass of the IPK and the sister copies have not stayed the same. Measurements in 1948 and 1989 have indicated that the masses are all diverging, with some losing mass and some gaining it, for a variety of reasons including simply getting dirty.
This issue has been troubling people working in metrology for quite some time. If things need to be measured precisely, the definition of the kilogram needs to be more accurate.
“The degree of instability is acceptable, but scientifically is a bit of an anomaly," Dr Stuart Davidson from the National Physical Laboratory told IFLScience. "While we can live with a few microgram changes over a few years, what we are looking for is something that is going to fundamentally be constant forever."
The meter is defined as the distance light travels in 1/299,792,458 seconds, and the second is defined as 9,192,631,770 oscillations of a certain frequency of radiation from the cesium atom. With the right apparatus, everyone in the universe could measure distances and times according to the international system.
To achieve this universality for the unit of mass as well, scientists are hoping to redefine the kilogram in terms of Planck’s constant, a fundamental constant that connects the frequency of a particle to its energy, found in many physical formulas.
Replica of the fundamental kilogram at Cité des Sciences et de l'Industrie. Jasp 88 via Wikimedia Common CC BY-SA 3.0
Mass, at the moment, is earthbound. Researchers have been looking for ways to free the mass definition from a physical object, with the Committee for Weights and Measures (CIPM) deciding in 2011 that a new definition was needed.
Since the uncertainty is in the realm of micrograms, the importance of stability is actually seen when we try to make very precise measurements of either tiny or large quantities. For example, pharmaceutical companies measure very small quantities of active drugs, and since the discrepancies in the fundamental kilogram are around the size of the quantities measured, this could create problems.
In a similar way, large masses are affected. If you aim to precisely measure the mass of an aircraft, a 0.01 percent uncertainty could have a large effect on both cost and fuel efficiency.