On May 20th, the 26th General Conference on Weights and Measures instituted a new standard for the kilogram mass. A metal block kept in a hermetically sealed vault in Paris had been the mass standard for 130 years. New standards were also introduced for the unit of current (Ampere), temperature (Kelvin), and the mole.

It used to be the case that standards for the basic units used in science had been defined in terms of macroscopic objects like this and thus could be easily understood. But the need for increasingly precise and unvarying standards meant that these were no longer suitable and standards have increasingly shifted to using the fundamental constants of nature and getting from those to the familiar quantities involves quite a long chain of reasoning. The kilogram is the last remaining physical object to be so displaced.

This article recounts the history of those gradual changes in the standards.

Starting Monday, the kilogram will be redefined not by another object, but by a fundamental property of nature known as Planck’s constant. Like the speed of light, the value of Planck’s constant cannot fluctuate — it is built with exquisite precision into the very fabric of the universe.

“Unlike a physical object, a fundamental constant doesn’t change,” said Stephan Schlamminger, a physicist at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md. “Now a kilogram will have the same mass whether you are on Earth, on Mars or in the Andromeda galaxy.”

…A similar philosophy of using fixed constants underlies the new definitions of the mole, the kelvin and the ampere. After Monday, the mole will be defined by the value of Avogadro’s constant, the kelvin by the value of the Boltzmann constant (which relates temperature to energy), and the ampere by the value of the elementary charge, the smallest observable charge in the universe.

Understanding how the new mass standard is used is not easy as can be seen from this article. As another article explains, the new kilogram standard depends upon the Planck constant because the value of that constant influences the value of Avogadro’s number that in turn determines the number of atoms in a given mass of an element. Arriving at agreement on the standard required a convergence of values obtained by different methods. This article gives a more detailed explanation. For yet more details, see here.

Here’s a video explainer.

The figure from this article summarizes the state of affairs of the standards just before these new standards were decided upon.

Reginald Selkirk says

I am relieved they didn’t base anything on the Hubble “constant”

Sunday Afternoon says

@#1, Reginald Selkirk,

I love the units of the Hubble constant: km/s/Mpc (kilometers per second per megaparsec)!

A quick dimensional analysis cancels out the 2 lengths, and you are left with 1/T -- the Hubble constant is a measure of frequency.

Pierce R. Butler says

Does changing the Kelvin degree entail changing the Celsius degree, or do they diverge now?

Mano Singham says

Pierce @#3,

The Celsius is now defined to be equal to the Kelvin so when that changes, it does to. This will apply to all the secondary units.

Peter Butler says

Avogadro’s number changed in 1961. Does anybody remember why?

I don’t particularly like picking an exact value for Avogadro’s number. All 26 decimal digits? The unit of mass is defined. So why not continue using the number of carbon-12 atoms in a gram? If the number of significant digits of accuracy continues to improve we might have the chemists’ gram and the physicists’ gram. That would be silly.

Rob Grigjanis says

Sunday Afternoon@2:But “frequency” implies oscillation, and that’s not what’s happening here. Generally, three kinds of simple exponential behaviour occur in physics;

Oscillating, in which H would be a frequency:

exp[iHt] or exp[-iHt]

Decaying, in which H would be a decay rate:

exp[-Ht]

Expanding, in which H is an expansion parameter:

exp[Ht]

The universe is expanding

approximatelyas the last type.Mano Singham says

Peter Butler @#5,

Avogadro’s number is now called Avogadro’s constant and is fixed to be

exactly6.02214076×10^{23}mol^{−1}.I am guessing that the physics and chemistry communities are not going to allow a difference in the values.

Pierce R. Butler says

Mano @ # 4:

The Celsius is now defined to be equal to the Kelvin…Pending Brexit negotiations may “fix” that too.

alanuk says

I was worried when I read that the ampere was now redefined. In the series of posts on ‘Electrical Machinery’ in my blog:

https://agrumpyoldphysicstechnician.wordpress.com/

I used the the previous version of the definition to show how the basic equations could easily be derived. When I read about the change in definition, I was seriously concerned about the validity of my argument -- I was lying awake worrying about it. Fortunately, I had shown how the ‘arbitrary’ constant in the definition could be replaced by the magnetic constant (permeability of free space) and it turns out that this ‘constant’ is subject to change under the new definition -- thus all is well.

On a less serious note (if one can joke about Brexit) we will be returning to the Fahrenheit scale -- on second thoughts, he was a foreigner, and a ‘European’ one at that; perhaps we could go back to Newton and his ‘Scala graduum Caloris’ (see blog).

BTW the written form of the unit names should start with a lower case letter when derived from a person’s name: thus ‘ampere’, ‘kelvin’ but ‘degree Celsius’ -- unless the rules of grammar require otherwise.