From Cern Bulletin: “The fundamental mass (this is not about the Higgs)” – Consider the Kilogram


Antonella Del Rosso
For Monday 2 April 2012

“At a time when we are about to shed light on the fundamental question of the creation of mass after the Big Bang, we are also close to solving another basic mass-related problem. The kilogram is the only base unit of the International System of Units (SI) whose official definition is still based on a material artefact rather than on invariant quantities. If you are now thinking that this concerns you less than the glamorous Higgs boson, think again: your scales could give you a different value when you use them tomorrow.

The international prototype of the kilogram is a cylinder of platinum-iridium alloy whose height (39 mm) is equal to its diameter. It was machined in 1878 and is kept at the Bureau International des Poids et Mesures (BIPM) in Sèvres, near Paris. To date, while all the other units in the SI system have been redefined to be based on fundamental constants or atomic properties, the kilogram continues to be defined according to this piece of matter.

A piece of matter that people, or at least one person, must clean, and there is a risk that atoms – that is, fractions of mass – might be lost in the process.

A metrology project launched by METAS in which CERN is participating should be able to fix the problem. The idea is to build an ultra-precise watt balance – an instrument that compares mechanical and electrical power (see box). Using the watt balance and its equations, it is possible to relate the unit of mass to the metre, the second and the Planck constant, i.e. to all fundamental units and constants.

The watt balance built by METAS to perform previous measurements of the Planck constant. A new balance is currently under development. (Courtesy of METAS).

The new definition of the mass unit should be available in a couple of years from now. Chances are that by then we will have also understood how mass is created at the most fundamental level… yes, we are talking about the Higgs this time!

Meet CERN in a variety of places:

Cern Courier








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