How much does a kilogram weigh? Depends on your 'Planck constant'

By Luke Baker PARIS (Reuters) - It may not change how you buy bananas, but scientists have voted to redefine the value of a kilogram, in what they called a landmark decision that will boost the accuracy of scientific measurements.

Reuters November 17, 2018 00:07:33 IST
How much does a kilogram weigh? Depends on your 'Planck constant'

How much does a kilogram weigh Depends on your Planck constant

By Luke Baker

PARIS (Reuters) - It may not change how you buy bananas, but scientists have voted to redefine the value of a kilogram, in what they called a landmark decision that will boost the accuracy of scientific measurements.

Since 1889, a kilogram has been defined by a shiny lump of platinum-iridium kept in a special glass case and known as the International Prototype of the Kilogram. It is housed at the headquarters of the International Bureau of Weights and Measures (whose French acronym is BIPM), just outside Paris.

Members of the BIPM, which groups some 60 nations, agreed on Friday after a week-long meeting at the nearby Palace of Versailles to redefine a kilogram in terms of a tiny but unchanging value called the "Planck constant".

They also voted to update definitions for the ampere (electrical current), the kelvin (thermodynamic temperature) and the mole (amount of a substance).

All modern mass measurements are derived from the kilogram, whether micrograms of pharmaceutical medicine or gold dust, kilos of fruit or fish, or tonnes of steel.

The problem is the prototype doesn't always weigh the same. Even inside its three glass bell jars it picks up microparticles of dirt and is affected by the atmosphere. Sometimes it needs cleaning, which can affect its mass.

That can have profound implications. If the prototype were to lose mass, atoms would in theory weigh more since the base kilogram must by definition always weigh a kilogram.

Scientists have been trying for decades to define a constant value for the kilogram that is derived from immutable physics, in the same way they have done for other standard units (SI units) overseen by the BIPM.

For example, a metre isn't 100 centimetres, it's actually "the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second".

The "Planck constant", which derives from quantum physics, can be used along with a Kibble balance, an exquisitely accurate weighing machine, to calculate the mass of an object using a precisely measured electromagnetic force.

"The SI redefinition is a landmark moment in scientific progress," said Martin Milton, director of the BIPM.

"Using the fundamental constants we observe in nature as a foundation for important concepts such as mass and time means that we have a stable foundation from which to advance our scientific understanding, develop new technologies and address some of society's greatest challenges."

Barry Inglis, who heads the committee for weights and measures, said the implications were immense.

"We will now no longer be bound by the limitations of objects in our measurement of the world, but have universally accessible units that can pave the way to even greater accuracy, and even accelerate scientific advancement," he said.

It is arguably the most significant redefinition of an SI unit since the second was recalculated in 1967, a decision that helped ease communication across the world via technologies like GPS and the internet.

The new definitions agreed by the BIPM will come into force on May 20, 2019.

(Writing by Luke Baker; additional reporting by Kate Kelland and Emilie Delwarde; Editing by Gareth Jones)

This story has not been edited by Firstpost staff and is generated by auto-feed.

Updated Date:

TAGS:

also read

France, Germany to agree to NATO role against Islamic State - sources
| Reuters
World

France, Germany to agree to NATO role against Islamic State - sources | Reuters

By Robin Emmott and John Irish | BRUSSELS/PARIS BRUSSELS/PARIS France and Germany will agree to a U.S. plan for NATO to take a bigger role in the fight against Islamic militants at a meeting with President Donald Trump on Thursday, but insist the move is purely symbolic, four senior European diplomats said.The decision to allow the North Atlantic Treaty Organization to join the coalition against Islamic State in Syria and Iraq follows weeks of pressure on the two allies, who are wary of NATO confronting Russia in Syria and of alienating Arab countries who see NATO as pushing a pro-Western agenda."NATO as an institution will join the coalition," said one senior diplomat involved in the discussions. "The question is whether this just a symbolic gesture to the United States

China's Xi says navy should become world class
| Reuters
World

China's Xi says navy should become world class | Reuters

BEIJING Chinese President Xi Jinping on Wednesday called for greater efforts to make the country's navy a world class one, strong in operations on, below and above the surface, as it steps up its ability to project power far from its shores.China's navy has taken an increasingly prominent role in recent months, with a rising star admiral taking command, its first aircraft carrier sailing around self-ruled Taiwan and a new aircraft carrier launched last month.With President Donald Trump promising a US shipbuilding spree and unnerving Beijing with his unpredictable approach on hot button issues including Taiwan and the South and East China Seas, China is pushing to narrow the gap with the U.S. Navy.Inspecting navy headquarters, Xi said the navy should "aim for the top ranks in the world", the Defence Ministry said in a statement about his visit."Building a strong and modern navy is an important mark of a top ranking global military," the ministry paraphrased Xi as saying.