# The Kilogram doesn’t weigh a kilogram any more

The Kilogram doesn’t weigh a kilogram anymore. This sad news was announced during a seminar at CERN on Thursday, October 26 by Professor Klaus von Klitzing, who was awarded the 1985 Nobel Prize in Physics for the discovery of the quantized Hall effect. "We are about to witness a revolutionary change in the way the kilogram is defined," he declared.

Together with six other units – meter, second, ampere, kelvin, mole, and candela – the kilogram, a unit of mass, is part of the International System of Units (SI) that is used as a basis to express every measurable object or phenomenon in nature in numbers. This unit’s current definition is based on a small platinum and iridium cylinder, known as "le grand K", that weighs exactly one kilogram.

The cylinder was crafted in 1889 and, since then, has been kept safe under three glass bell jars in a high-security vault on the outskirts of Paris. There is one problem: the current standard kilogram is losing weight. About 50 micrograms, at the latest check. Enough to be different from its once-identical copies stored in laboratories around the world.

Replica of the national prototype kilogram standard no. K20 kept by the US government National Institute of Standards and Technology (NIST), Bethesda, Maryland. (Image: National Institute of Standards and Technology)

To solve this weight(y) problem, scientists have been looking for a new definition of the kilogram.

At the quadrennial General Conference on Weights and Measures in 2014, the scientific metrology community formally agreed to redefine the kilogram in terms of the Planck constant (h), a quantum-mechanical quantity relating a particle’s energy to its frequency, and, through Einstein’s equation E = mc2, to its mass. Planck’s constant is one of the fundamental numbers of our universe, a quantity fixed universally in nature, such as the speed of light or the electric charge of a proton.

The National Institute of Standards and Technology (NIST)-4 Kibble balance measured Planck's constant to within 13 parts per billion in 2017, accurate enough to assist with the redefinition of the kilogram. Credit: J. L. Lee/NIST

Planck’s constant will be assigned an exact fixed value based on the best measurements obtained worldwide. The kilogram will be redefined through the relationship between Planck’s constant and mass.

"There’s nothing to be worried about," says Klaus von Klitzing. "The new kilogram will be defined in such a way that (nearly) nothing will change in our daily life. It won’t make the kilogram more precise either, it will just make it more stable and more universal."

However, the redefinition process is not that simple. The International Committee for Weights and Measures, the governing body responsible for ensuring international agreement on measurements, has imposed strict requirements on the procedure to follow: three independent experiments measuring the Planck constant must agree on the derived value of the kilogram with uncertainties below 50 parts per billion, and at least one must achieve an uncertainty below 20 parts per billion. Fifty parts per billion, in this case, equals approximately 50 micrograms – about the weight of an eyelash.

Two types of experiment have proved able to link the Planck constant to mass with such extraordinary precision. One method, led by an international team known as the Avogadro Project, entails counting the atoms in a silicon-28 sphere that weighs the same as the reference kilogram. The second method involves a sort of scale known as a watt (or Kibble) balance. Here, electromagnetic forces are counterbalanced by a test mass calibrated according to the reference kilogram.

And that’s where the important discovery made by Klaus von Klitzing in 1980, which earned him the Nobel Prize in Physics, comes into play. In order to get extremely precise measurements of the current and voltage making up the electromagnetic forces in the watt balance, scientists use two different quantum-electrical universal constants. One of these is the von Klitzing constant, which is known with extreme precision and can, in turn, be defined in terms of the Planck constant and the charge of the electron. The von Klitzing constant describes how resistance is quantized in a phenomenon called the "quantum Hall effect", a quantum-mechanical phenomenon observed when electrons are confined in an extra-thin metallic layer subjected to low temperatures and strong magnetic fields.

"This is truly a big revolution," von Klitzing says. "In fact, it has been dubbed the biggest revolution in metrology since the French Revolution, when the first global system of units was introduced by the French Academy of Sciences."

CERN is playing its part in this revolution. The Laboratory participated in a metrology project launched by the Swiss Metrology Office (METAS) to build a watt balance, which will be used to disseminate the definition of the new kilogram through extremely precise measurements of the Planck constant. CERN provided a crucial element of the watt balance: the magnetic circuit, which is needed to generate the electromagnetic forces balanced by the test mass. The magnet needs to be extremely stable during the measurement and provide a very homogenous magnetic field.

Provided by CERN

Featured image credit: The National Institute of Standards and Technology (NIST)-4 Kibble balance measured Planck's constant to within 13 parts per billion in 2017, accurate enough to assist with the redefinition of the kilogram. Credit: J. L. Lee/NIST

Share:

### CERN’s LHCb experiment announces observation of a new particle

Saturday, July 8, 2017

### CERN’s LHCb experiment announces observation of a new particle

Saturday, July 8, 2017

### Exotic particles as the Sun aligns with the center of the Milky Way

Tuesday, December 20, 2016

### CERN scientists developping magnetic shield for astronauts’ protection against cosmic rays

Saturday, August 8, 2015

### “God particle” – Scientists at CERN in Geneva announced their latest results in search for the Higgs boson

Tuesday, December 13, 2011

### Neutrinos are faster than light according to new tests

Saturday, November 19, 2011

### Antiproton ring found around Earth

Tuesday, August 9, 2011

### Supernova-creating particle accelerators will make the biggest bangs we have ever known

Friday, February 25, 2011

### Cerro Blanco eruption among the largest in last 11 700 years, comparable to massive Santorini eruption

Tuesday, May 21, 2019

### Extratropical volcanoes more efficient than tropical in terms of hemispheric cooling

Thursday, January 31, 2019

### When magma prevents volcanic eruptions

Sunday, December 3, 2017

### Decadal variation of relationship between East Asian monsoon and El Nino

Saturday, November 11, 2017

### Detection of Ruthenium 106 in Europe: IRSN investigation results

Friday, November 10, 2017

### Cooling in high and mid-latitudes led to aridification in Northern Africa

Friday, November 10, 2017

### World Risk Report 2017: marked increase in extreme weather events, but people more prepared

Wednesday, November 8, 2017

## Your support makes a difference

We hope that our website has been a valuable resource for you.

The reality is that it takes a lot of time, effort, and resources to maintain and grow this website. We rely on the support of readers like you to keep providing high-quality content.

If you have found our website to be helpful, please consider making a contribution to help us continue to bring you the information you need. Your support means the world to us and helps us to keep doing what we love.

Support us by choosing your support level – Silver, Gold or Platinum. Other support options include Patreon pledges and sending us a one-off payment using PayPal.

Sincerely,
Teo Blašković

## \$5 /month

• Clean user interface and fast browsing
• Direct communication with us via chat and email
• Suggest new features, content and applications

## \$10 /month

• Clean user interface and fast browsing
• Direct communication with us via chat and email
• Suggest new features, content and applications

## \$25 /month

• Clean user interface and fast browsing
• Direct communication with us via chat and email
• Suggest new features, content and applications

## \$200 /year

You can also support us on Patreon

or by sending us a one-off payment using PayPal:

## Commenting rules and guidelines

We value the thoughts and opinions of our readers and welcome healthy discussions on our website. In order to maintain a respectful and positive community, we ask that all commenters follow these rules:

• Treat others with kindness and respect.
• Stay on topic and contribute to the conversation in a meaningful way.
• Do not use abusive or hateful language.
• Do not spam or promote unrelated products or services.
• Do not post any personal information or content that is illegal, obscene, or otherwise inappropriate.

We reserve the right to remove any comments that violate these rules. By commenting on our website, you agree to abide by these guidelines. Thank you for helping to create a positive and welcoming environment for all.

1. Jeffery Barber says:

Not just stupid, but pathetically CIA stupid.

They’re not smart enough to figure out they’re not smart enough.

2. Maritime Maggie says:

Thank God Americans never converted to Satan’s metric system with its horrible nomenclature–KILL-o-gram, KILL-o-meter, KILL-o-pascal, and stuck with God’s measuring system found in the Bible and in the Great Pyramid where it is known as “the Pyramid Inch” based on the laws of universal harmony in units of 8 found on the piano keyboard.

1. Jeffery Barber says:

The biblical standard is a “cubit”. When is the last time you used a cubit.