What is the SI unit of mass? How is it defined?

The SI (International System of Units) unit of mass is the kilogram, symbolized as "kg." The kilogram is defined as the base unit of mass and is currently defined in terms of a fundamental physical constant known as the Planck constant (h).

The definition of the kilogram underwent a significant change in May 2019. Prior to this change, the kilogram was defined based on a physical object known as the International Prototype of the Kilogram (IPK), a platinum-iridium cylinder stored at the International Bureau of Weights and Measures (BIPM) in France.

The new definition of the kilogram is as follows:

"The kilogram is defined by setting the Planck constant (h) to be exactly 6.62607015 × 10^-34 joule-seconds."

This definition ties the mass of the kilogram to a fundamental constant of nature, the Planck constant, which is determined through precision measurements using specialized equipment. This change was made to provide a more stable and universally accessible definition for the kilogram, eliminating the dependence on a physical artifact.

In practical terms, this definition means that the kilogram is now based on fundamental physics and can be reproduced with high precision in laboratories around the world, ensuring consistency and accuracy in scientific measurements of mass.