It’s one of those things where if you think about it too much, your head might explode. We know there are 1,000 grams in a kilogram, and 1,000 kilograms in a metric ton, but how was it ever decided what any of these units actually physically *weighed*? Well... the modern metric system is part of the Système International d'Unités (International System of Units) or SI. It states that a kilogram is the weight of one specific 130 year-old platinum-iridium cylinder, which is kept in a vault at the International Bureau of Weights and Measures in France... and no, don’t ask how they knew if they’d got *its* weight right, when they were making it. The problem is, that cylinder’s mass changes slightly over time. Now, a worldwide effort is under way to change the definitive weight of a kilogram to something more permanent.

The kilogram is the only “base unit” in the SI that is still defined by a physical artifact – the other base units are the second (time), the meter (length), the ampere (electric current), the kelvin (thermodynamic temperature), the mole (amount of substance) and the candela (luminous intensity). That said, the ampere, mole and candela are all defined by their relationship to the kilogram – for instance, a mole is defined as the number of carbon-12 atoms whose total mass equals 12 grams. Therefore, these units are also ultimately linked back to the ever-so-slightly-changing French block of metal.

Researchers at the National Institute of Standards and Technology (NIST) are proposing that the kilogram now be defined in terms of the Planck value *h*, which is a constant in quantum physics. In order to establish an accurate value for *h*, they have been performing experiments with a watt balance (an electromechanical instrument that measures weight by the strength of an electric current) and trying to determine the mass of one mole of silicon atoms.

A resolution for the new SI will be submitted for consideration at the General Conference on Weights and Measures, which takes place next October. If it passes, the new system could be in use later in the decade.