New kilogram proves to be a heavy challenge for scientists

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.

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

‘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.’

1. 2018

   Adoption of new definitions

   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.

   The redefinition of four SI units will mean all seven 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.