Revolutionizing Measurement: The Latest Definition of the Kilogram and Its Implications
Goodbye to the old days when the kilogram was defined by a physical prototype in Paris! Today, we introduce the latest and most precise definition of the kilogram, which pivots on the Planck constant, a fundamental physical quantity.
From Prototype to Constant
The historical definition of the kilogram has been based on a physical prototype stored in France since 1889. This prototype, known as the International Prototype Kilogram or IPK, served as the standard mass for the world. However, due to its inaccuracy and the potential for change in its mass over time, new methods were needed. This led to the adoption of a new definition, marking a significant shift in the way we define units of mass.
The Old Definition
In 1889, one kilogram was defined as the mass of a specific cylindrical piece of platinum-iridium alloy, kept in a vault in Sevres, France. The prototype was initially defined as the mass of one liter (10^-3 cubic meter) of pure water. On Earth, one kilogram roughly equates to 2.20 pounds (lb).
The New Definition
With advancements in precision measurement, the kilogram is now defined in terms of the Planck constant (h). This constant has been measured with extraordinary precision in recent years. The value of the Planck constant is set at 6.62607015 × 10^-34 kilogram meters squared per second (kg m^2 s^-1) as of the General Conference on Weights and Measures (CGPM) in 2018.
This new definition allows scientists to make precise mass measurements using equipment such as the Kibble balance, which recreates the historical definition of the kilogram using fundamental physical constants. This new standard promises unparalleled accuracy and stability, ensuring that the mass unit remains consistent over time.
The Latest SI Definition
The CGPM officially adopted the following definition in 2018:
The kilogram (symbol kg) is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10-34 when expressed in the unit J·s, which is equal to kg·m2·s-1, where the meter and the second are defined in terms of c and nu;Cs.
In other words, the kilogram is defined as h/m^2s^-1, where h is assigned the given value, and s (the second) and m (the meter) are defined as per the 1st and 2nd paragraphs of Appendix 3.
The Photon-based Definition
From a practical standpoint, one kilogram is now defined as the mass of 1.47552138 × 10^40 photons at the frequency of the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom (νCs). This definition ensures that the kilogram can be defined and measured using fundamental physical constants, eliminating the reliance on a physical prototype.
Historical Background
The concept of the kilogram has its roots in the French Revolution, where it was first defined as the mass of one liter of water at the temperature of melting ice. This was later replaced by a platinum prototype in 1799. In 1879, a cylinder of platinum-iridium, the International Prototype of Kilogram (IPK), became the standard for the metric system. This prototype has remained the standard until the adoption of the new definition on May 20, 2019.
Implications and Future
The new definition of the kilogram revolutionizes metrology and ensures the continued accuracy and reliability of the mass unit, regardless of physical prototypes. As more countries and industries adopt this new standard, we can expect a cleaner, more precise, and consistent measurement system. This will have significant implications across various fields, including physics, engineering, and everyday applications.