The fundamental mass (this is not about the Higgs)

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

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. “Over the years, several official copies have been produced and distributed to various national metrology offices,” says Ali Eichenberger, a physicist at the Swiss Metrology Office (METAS). “Although it is not yet possible to define the kilogram mass in an absolute way, modern technology makes it possible to compare different masses with very high precision, up to 1 microgram. Looking at the different official copies there seems to be a significant variation in masses.” Moreover, not knowing the kilogram with the appropriate precision has an impact on other units, such as the ampère.

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.

“One of the crucial elements of the watt balance is the magnetic circuit, which needs to be extremely stable during the measurement,” explains Davide Tommasini, a magnet expert from the Magnets and Superconductors group in CERN’s Technology Department, who is directly involved in the METAS watt balance project. “By using a correctly dimensioned ‘magnet shunt’ with a low Curie temperature, it is possible to drastically reduce the effects of temperature variation. The circuit must also provide a very homogenous magnetic field in the whole volume involved in the measurement.” The magnet circuit will be assembled at CERN. “We are expecting the permanent magnet and the shunting cylinder to arrive soon. We will then work on testing the performance of the circuit,” says Davide Tommasini.

Over the past century, significant variation is seen between the masses of the official kilogram copies. (Courtesy of METAS).

“The requirements associated with the magnets are very strict and we are very happy that CERN agreed to take part in the project in the framework of its knowledge transfer activities,” says Henri Baumann, a physicist at METAS who launched the project together with Ali Eichenberger. “This measurement will also lead to a significant improvement in the determination of the Planck constant. The CERN theorists will be happy to know that!”

“This project is a clear indication of the impact that the skills and the expertise needed in particle physics have on other research domains and on society, says Hartmut Hillemanns from the KT group, who has fostered the project with the scientific team at CERN and led the negotiations with the other project partners.

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!

The principle of the watt balance

The watt balance is an electromechanical instrument that measures the weight of a test mass very precisely. In the watt balance a coil is suspended on one arm and is immersed in a horizontal magnetic flux. During a first measurement phase, the current in the coil exerts a vertical force on the conductor that is balanced against the weight of the test mass. In the second phase, the coil is moved at a constant velocity through the magnetic field, and the voltage induced across the coil is measured. By combining the equations and performing various subsequent calculations one arrives at the equation:

where C is a calibration constant, fj and f'j are the Josephson frequencies used during the static and the dynamic phase and h is the Planck constant. The watt balance experiment therefore allows the unit of the mass to be related to the metre, the second and the Planck constant.

Several watt balances are currently in operation around the world and are being used for metrology purposes.


Another way of fixing the problem

The most important alternative for defining the kilogram, known as the “X-ray crystal density” method or the Avogadro project, consists in accurately measuring the density of a very pure crystal silicon sphere.


by Antonella Del Rosso