Understanding the Microscopic Change in Kilogram Weight
In a significant leap for scientific measurement, the kilogram, one of the seven base units of the International System of Units (SI), underwent a historic redefinition in 2019. Unlike its predecessor, the new definition is based on the Planck constant rather than a physical object. This change has garnered widespread interest, particularly in relation to the weight difference between the old and new definitions.
The Shift from a Physical Prototype to a Constant-Based Definition
Until 2019, the kilogram was defined by the International Prototype of the Kilogram (IPK), a platinum-iridium cylinder stored in France. However, this physical reference was subject to variations due to factors such as wear, contamination, and environmental conditions. In response, the International Committee for Weights and Measures (ICWM) adopted a new definition based on the Planck constant, a fundamental physical constant known to great precision. This change was necessary to ensure that the kilogram remains stable and unaffected by physical imperfections over time.
Theoretical vs. Practical Implications
From a theoretical standpoint, the redefinition aims to align the new kilogram with the old one as closely as possible to maintain continuity in measurements. However, in practical terms, the difference in weight between the new and old definitions is negligible. The ICWM sought to ensure that the new definition reflects the mass of the IPK as accurately as possible, with any observed variations being within the realm of measurement uncertainties.
The ICWM's primary goal was to change the way the kilogram is defined, not to alter its fundamental mass. Even though the IPK's mass was exactly 1 kilogram by definition, it will now be less relevant as the new definition focuses on a constant rather than a physical object. Any measurable difference between the two definitions would have been immediately addressed.
Empirical Evidence and Precision
According to the ICWM, as of December 2018, the International Prototype Kilogram had a mass of 1 kilogram by definition. Post-20 May 2019, this will no longer be the case, as the definition has shifted to a constant-based standard. The redefinition ensures that the kilogram is defined based on a physical constant, which we understand with great precision, to 9 significant digits.
The actual difference between the currently defined mass of the IPK and a measured mass using the new definition should be within a standard deviation of previous comparative measurements. Uncertainties should be within a few micrograms per kilogram, emphasizing the precision and stability of the new definition.
In conclusion, while there is a microscopic change in the weight of the kilogram, the primary objective is to redefine the unit in a way that is less prone to variation and more aligned with fundamental physical constants. This shift marks a significant advancement in the field of metrology, ensuring that our measurements of mass remain as accurate and stable as possible.