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New kilogram proves to be a heavy challenge for scientists

Every kilogram in the world can be traced back to the international prototype kilogram, which is stored in Paris. Image:BIPM
Every kilogram in the world can be traced back to the international prototype kilogram, which is stored in Paris. Image:BIPM

Scientists working on a new definition of the kilogram that will fundamentally overhaul the international measurement system are finding the problem harder than expected due to the difficulty of accurately measuring natural constants in the lab.

For the last 135 years, every kilogram in the world could be traced back to a single reference kilogram known as the international prototype. This metal cylinder was adopted in 1889 as the agreed standard for mass, and has been stored under a series of glass jars in a vault near Paris ever since.

The problem is that in the early 1990s, scientists found measurable differences in mass between the original prototype kilogram and copies that were made at the same time. It turns out that the mass of the prototype is changing slowly over time, although researchers can’t say whether it is getting heavier or lighter, or by how much, because there is no other reference kilogram to benchmark it against.

The solution is to redefine the kilogram so that it is no longer based on a physical object but on a naturally occurring number – a fundamental constant – with a known value. This will mean the kilogram’s value remains stable over time and, with the right instruments, will be reproducible anywhere in the world. Defining measurement units in terms of fundamental constants also helps scientists to produce more accurate and precise measurements.

The Planck constant

Professor Hendrik Emons, head of unit for reference materials at the Joint Research Centre, the EU’s in-house science service, which has been involved in the transition process, explained that scientists have agreed to peg the value of the kilogram to the Planck constant, a naturally occurring value in quantum mechanics.

‘The measurements are tricky and time-consuming. It is also a consensus-finding process.’

Professor Hendrik Emons, head of reference materials at the Joint Research Centre
The difficulty – and the delay – has arisen from working out how to measure the Planck constant most precisely and agree a fixed value that can be used in all future calculations.

‘The issue is that moving from a macroscopic measurement – a piece of metal – to a microscopic measurement requires also that you have ways to realise these units,’ said Prof. Emons. ‘It turned out that the measurement science was not ready.’

There are currently two competing methods to measure the Planck constant and the target is that the value produced by each of these methods should be identical to within two billionths of a gram. Originally it had been hoped that this would happen in 2012 but it has now been delayed until 2015.

‘The measurements are tricky and time-consuming,’ said Prof. Emons. ‘It is also a consensus-finding process. This takes time.’

  • Delegates to the Second CGPM pictured in front of the Pavillon de Breteuil, Paris, France. Image courtesy of the BIPM.

    Seventeen nations sign the Metre Convention in Paris, which establishes an international standard for length and mass. Three intergovernmental organisations are set up to oversee the new metric system, the most senior of which is the General Conference on Weights and Measures (CGPM), an intergovernmental body that still coordinates metrology and design of the metric system.
  • The international metre and kilogram prototypes.

    The international prototype kilogram and metre rule are constructed, containing 90 % platinum and 10 % iridium. Forty copies are distributed to National Metrology Institutes throughout the world while six are kept with the original in Paris.
  • The original Metre Convention, from 1875, was updated in 1921. Image: BIPM

    The Metre Convention is revised to make the CGPM responsible for all physical measurements.
  • The first caesium atomic clock, built in 1955. Image courtesy of Science Museum/Science & Society Picture Library, London.

    The first atomic clock is constructed, which is less precise than existing clocks but shows the potential for future models. The first accurate atomic clock is built in 1955 and keeps time to an accuracy one second within 300 years. This leads in 1967 to a change in the SI standard for time, with the definition of a second now based on the changing state of a caesium 133 atom. 
  • The seven base units of the SI system.

    The Système International d’Unités, or the SI system, is published. For the first time this defines a coherent measurement system based on six units: metre (length), kilogram (mass), second (time), kelvin (temperature), ampere (electric current) and candela (luminosity). In 1971, the mole (substance) is added to the SI system, completing the seven measurements that we know today.  
  • The international prototype metre became redundant in 1960. Image: BIPM

    The international prototype metre becomes redundant when the CGPM agrees to redefine the metre in terms of the wavelength in a vacuum of a krypton 86 atom. In 1983 this definition is updated to the length of the path travelled by light in a vacuum in a specific fraction of a second.
  • The International Prototype Kilogram, which is stored in Paris, France. Image courtesy of the BIPM.

    Scientists complete a series of comparisons between the international prototype kilogram and its copies and discover the international prototype kilogram is changing mass. Work begins on a method of defining the kilogram in terms of a natural constant, with a formal resolution to do so being agreed by the CGPM in 1995.  
  • The redefinition of four SI units will mean all seven will be based on naturally occurring fundamental constants.

    The 26th meeting of the CGPM is expected to adopt the new definition of the kilogram, ampere, kelvin and mole, with the result that for the first time all seven SI units will be based on naturally occurring fundamental constants.

Fundamental overhaul

The kilogram is the only current basic unit of measurement to still refer to a physical object and its redefinition will mean that each one of the seven international measurement units – known as SI units – is derived from a natural constant.

The change will constitute a fundamental overhaul of our systems of weights and measures, which is reflected in the international nature of the work. Researchers at national metrology institutes and laboratories around the world are working on the redefinition, which will be agreed by international committee, written into an intergovernmental treaty and signed by heads of state.

The world body responsible for measurements – the General Conference on Weights and Measures – should adopt the new definition in 2018. ‘It is important that people know that these things are established in a responsible manner, with a global approach and agreement and based on the best science available,’ said Prof. Emons.

Scientists are also taking the opportunity to increase the precision and reliability of three other units of the SI system – the kelvin (which measures temperature), the mole (which measures the amount of substance) and the ampere (which measures electric current).

‘It has implications for the achievable precision of other units such as the ampere,’ said Prof. Emons. ‘The new ampere will be related to the charge of one single electron. In technology we are moving towards building up from elementary particles; for instance in nanotechnology and nanoelectronics.’

Pushing the limits

As our fields of investigation become smaller and smaller, the limits of what we can measure are being expanded by quantum metrology, an area that uses quantum physics to enhance the sensitivity and precision of measurements.

Professor Morgan W Mitchell, a quantum optics expert at The Institute of Photonic Sciences in Barcelona, Spain, said the redefinition of the kilogram may become important in quantum enhanced sensing – an area that tries to improve the sensitivity of measurements using quantum effects – and could lead to instruments such as nano-scale sensors that measure extremely tiny weights.

Atomic measuring instruments such as atomic clocks and atomic magnetometers are already used in applications from space science to biomedicine. The EU-funded AQUMET project, which is led by Prof. Mitchell, is working to improve the sensitivity of these atomic instruments.

‘Several real-world applications are starting to reach sensitivity limits that can only be beaten with quantum metrology techniques,’ said Prof. Mitchell. ‘We are working on new tools for super-precise measurements with atoms.’

Improved atomic magnetometers could provide more accurate measurement of magnetic fields from the brain and help in our understanding of brain disorders and language processing.

Further afield, increasing the sensitivity of gravitational wave detectors, which measure waves produced by cosmic events such as the collision of two black holes, could mean that these events are detectable from earth. ‘If these instruments detect gravitational waves, it will be using quantum metrology techniques,’ said Prof. Mitchell.

The European Metrology Research and Innovation Programme

The European Metrology Research and Innovation Programme (EMPIR) is a ten-year programme for European research on metrology. It brings together national metrology institutes, industry and academia across Europe in order to study industrial measurements and standards with important practical and legislative applications.

EMPIR is implemented by the European Association of National Metrology Institutes (EURAMET) and coordinated by 23 national metrology institutes. It follows on from its predecessor, the European Metrology Research Programme (EMPR), and aims to target industry’s needs, address grand challenges in the fields of energy, environment and health, and open up further opportunities for collaborative research.

EMPIR is jointly-funded by the EU, Member States and Associated Countries and has a total budget of around EUR 600 million. The EU contribution comes from Horizon 2020, the EU's biggest framework programme for research and innovation, which runs from 2014 until 2020.

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