Do My Grams Look Fat In The New Kilo? – Hope 103.2

Do My Grams Look Fat In The New Kilo?

By Anne RinaudoWednesday 19 Dec 2018Open House Interviews

Listen: Petr Lebedev in conversation with Stephen O’Doherty.

If you thought that the kilogram was one of the certainties of life, you are in for a shock. The world is about to make the biggest change in measurement for over a century.

The world’s scientists and arbiters of measurement met in Versailles in France and took a vote in November 2018. They agreed that from May 2019 there will be a metric makeover.

So hold on to your hats, people! Changing from next year are the definition of the kilogram and three other base units in the International System of Units (SI) (i.e. the metric system) – the ampere, kelvin and mole; and all units derived from them, such as the volt, ohm and joule.

How can mass be a number?

The kilo will no longer be measured against a cylinder of metal held under high security in a vacuum vault outside Paris for the last 130 years. The kilo will now be defined by a fundamental natural constant – Planck’s Constant – set as 6.62607015 x 10-34 kg·m2 per second.

So how can a number define mass? It is, as they say, complicated. Petr Lebedev, science communicator and a doctoral candidate at the School of Physics at the University of Sydney spoke to Open House and ran us through the change.

Natural constant is dependable

“Redefining the kilogram in relation to Planck’s constant is a very good thing because it is fundamental and does not change.

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“Linking the definition of International System of Units (SI) to unchanging, fundamental constants is not new – for example, the metre is linked to the speed of light in a vacuum which is a constant.” says Mr Lebedev

“Before that change in the definition in 1983, the metre was defined by the length of a particular platinum rod, just as the kilo is defined, at the moment, by the metal cylinder in France.”  he explained.

Metric Makeover

The new definitions impact four of the seven base units of the SI (or metric system): the kilogram, ampere, kelvin and mole; and all units derived from them, such as the volt, ohm and joule.

The International Standard Units of Measurement and the natural constants that now define them. Image Credit BIMP.

  • The kilogram (mass) will be defined by the Planck constant (h)
  • The ampere (electric current) will be defined by the elementary electrical charge (e)
  • The kelvin (temperature) will be defined by the Boltzmann constant (k)
  • The mole (amount of substance) will be defined by the Avogadro constant (NA)

Yes a kilo is still a kilo

Although the size of these units will not change (a kilogram will still be a kilogram), the four redefined units will join the second, the metre and the candela (luminous intensity) to ensure that the set of SI base units will continue to be both stable and useful.

Biggest thing since Treaty of the Metre

This is the largest single shift in international measurement since the Treaty of the Metre was signed in Paris in 1875 by representatives of 17 countries. Scientists expect the change will spur technological innovation and lower the cost of many high-tech manufacturing processes. The anniversary of the signing, 20 May, is now known as World Metrology Day.

The promotion of Metrology (the science of measurement) has come a long way since the the Woodcut illustrating the new decimal units which became the legal norm across all France on 4 November 1800. Image Credits Wikipedia and World Metrology Day

Constants of the natural world

The Metre Convention created the International Bureau of Weights and Measures (The Bureau International des Poids et Mesures or BIPM). Representatives from 54 of the BIPM’s Member States were the ones who voted in Versailles on 16 November 2018 to revise the International System of Units (SI).

The vote comes after decades of groundbreaking laboratory work; for the first time all of the worlds measurement units will be accurately defined by natures fundamental laws based units of weight and measurement on other earthly objects.

The metre was originally defined in 1791 as being one ten millionth of the distance from the North Pole to the Equator through Paris. Image credit: Wikipedia.  Credit for figuring that out in 1791? I don’t know but wow, just WOW!

Keeping up with science and technology

The vote was an agreement for the world’s scientific and technical community to redefine four of the seven base units for the International System of Units (SI).It means the kilogram (mass), kelvin (temperature), ampere (electric current) and mole (amount of substance) will now be determined by fundamental constants that describe the natural world rather than by physical objects.

This will assure the future stability of the SI and open the opportunity for the use of new technologies, including quantum technologies, to implement the definitions.

The platinum-iridium cylinder (right) was the primary standard kilogram for all metric measurements in the U. S. Known as Kilogram No. 20, it was a copy of the International Prototype Kilogram. Kilogram No. 4 (left, under double bell jar), a duplicate of No. 20, was used as a secondary standard. Image Credit US National Institute of Standards and Technology. Public Domain.

Retiring ‘Le Grand K’

The changes, will come into force on – of course – World Metrology Day, 20 May 2019. It will bring an end to the use of physical objects to define measurement units.That means the International Prototype of the Kilogram (IPK), the last physical artefact to define a unit of measurement (mass in this case) in SI will be retired after 130 years.

The IPK is a cylinder of 90% platinum and 10% iridium alloy stored in a vacuum vault at the BIPM, in Sevres, just outside Paris. Le Grand K, the IPK will be replaced by a definition based on the Planck constant – the fundamental constant of quantum physics.

The head of the School of Physics at the University of Sydney, Professor Celine Boehm, said: “This is not going to change physical measurements but relating the SI units to our fundamental constants brings a more robust foundation to these measurements.”

Tiny weight changes equal a big problem

The stability of the IPK is unreliable and could only be confirmed by comparisons with other official copies. In fact, when the IPK has been officially verified about every 40 years by doing those comparisons there have been tiny variations in comparative weight – not a good thing for an international standard of measurement.

The verifications only measured difference in mass and it is not certain if the IPK was losing weight or the copies were gaining. For example re-comparison of Kilogram No. 20 (pictured above) with the international standard in 1937 showed that the United States standard had changed by one part in 50 million during approximately 50 years. That is not very much for most purposes but modern science needs very precise measurement. For instance, the DNA helix width is 2 nanometre – not roughly, not about, not more or less, but exactly.

This link from the BIPM answers every imaginable question about the IPK and the possible reasons for the changes.

The IPK “had just one job”

The one job of the IPK was to be a reliable measurement of a kilo. It held up well for over a century but the precision demanded by science and technology called for something better. And who knew in 1888 that mercury in the atmosphere, gravity or a number of other suggested possibilities could mean the IPK would need to retire?

Using a universal constant (it is literally totally constant or unchanging) like Planck’s constant to define the kilo means we know there will be no longer be the inconvenient discrepancies that have been found to occur with a physical object like the IPK.

The kilogram will be redefined by setting Planck’s constant as 6.62607015 x 10-34 kg·m2 per second.The Planck constant is ready for use “For all times, for all peoples”, and its invariability can be relied on. This video is a great explanation of those tiny physical differences in the IPK and official copies and how scientists have worked out how the Planck constant will fix the problem.

A random candle not accurate

The problem of using physical objects to measure is well  illustrated by the canadela. The only SI base unit based on human perception, the candela converts physical watts to perceived luminance, originally defined as one candlepower (with the candle made of sperm whale wax).

The candela is the SI base unit for luminous intensity. It is the brightness of a light source over a given solid angle. The candela is used to measure the brightness of light sources, like light bulbs or the bulbs in torches. It is the only

In the past, a ‘standard’ candle defined the candela. However, this standard varied from country to country, and from candle to candle, meaning it lacked sufficient accuracy. The candela, or ‘the new candle’, replaced these definitions in 1948 and it measures luminous intensity. The current definition of the candela was made in 1979, in terms of the watt at only one frequency of light.

The candela will not be redefined in 2019, as it is already defined using the Planck constant. It is defined as the luminous power emitted by a point light source over a given solid angle.

Landmark moment

“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.”

“Today marks the culmination of decades of work by measurement scientists around the world, the significance of which is immense,” said Barry Inglis, President of the International Committee for Weights and Measures (CIPM).

Greater accuracy

“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.”

Impact on many areas

The revised SI will maintain its relevance by facilitating technical innovations. Just as the redefinition of the second in 1967 and the metre in 1983 provided the basis for technology that has transformed how we communicate across the globe, through GPS and the internet, the new changes will have wide-reaching impact in science, technology, trade, health and the environment, among many other sectors.

“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.”

Years of effort

“This marks the culmination of decades of work by measurement scientists around the world, the significance of which is immense,” said Barry Inglis, President of the International Committee for Weights and Measures (CIPM).

“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.”

Some nations still not metric

There are only three countries in the world that don’t use the metric system as their national system of measurement; Liberia, Myanmar and the United States of America. However they do use International System of Units in scientific and technical applications.

Ironically, America was one of the seventeen original parties to the 1875 Metre Convention but has never adopted the metric system preferring to measure the daily temperatures in Fahrenheit, worry how much weight they put on over christmas in pounds and they measure the distance they need to jog to lose those pounds in feet and inches.

The three countries not using the metric system are the USA, Liberia and Myanmar

To listen to the podcast of this conversation click the red play button at the top of the page, or you can subscribe to Open House podcasts in iTunes and they will appear in your feed.

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