Robert Kirkwood from the National Physical Laboratory (NPL) presented the 2018 Christmas Lecture which, as usual, was very well attended. He began by saying that in the last few weeks there had been an international agreement to change the way in which our fundamental units of measurement are defined and so this was a good time to have such a presentation.
He explained how important it was that the same standards were used to define measurements throughout the world, particularly in an age where components can be manufactured many thousand of miles away from the final assembly in which they will be expected to both fit and to function within defined, (by measurement), environments.
A standard length such as the metre began as a carefully constructed rod from which copies were made, which in turn were used to produce further copies. In this way the standard could be shared around the world. Unfortunately each copying stage introduced errors which reduced the precision of any measurement and the use of a physical artifact meant that eventually the original definition could be called into question. Primary copies when compared to each other and the standard might differ in size, casting doubt as to which, if any, had the intended value. That leads to a conflict as, by definition, the original artifact is the standard, yet comparison with copies of the standard and with measurements of items believed to be stable would be evidence that the standard itself had changed.
The units of measurement are arbitrary and to illustrate this a metal bar 'the Michael' was used to define unit length, the unit of mass derived from the weight of Michael's wife and the unit of time as the period of a unit pendulum with a bob of unit mass swinging in the town of Teddington. The audience was quick to appreciate the difficulty of replicating such a system of measures and to point out the many factors that would make measurements using this system impossible to replicate.
What was needed was a measurement system that could be established anywhere, even beyond the Earth, without being tied to a physical artifact no matter how well constructed or preserved. This idea had been proposed by James Clerk Maxwell in the 1860s.
Another requirement, demonstrated by examples of the 'Michael' handed out to the audience, is that the units should be practical, 'human-friendly'. The International System of Units (SI), introduced in 1960, attempts this with units such as the metre, kilogram and the second. (A unit of length, say a kilometre, would be impracticable as it couldn't be represented in an object that could be easily carried in the hand). The set of fundamental SI units was added to over the years and is now the metre, kilogram. second, ampere, kelvin, candela and mole.
In 2007 a decision was taken to work towards a new concept of defining the fundamental units. Rather than using the existing fundamental units to measure the so-called physical constants the relationship would be reversed. In the revised SI four of the SI base units, the kilogram, the ampere, the kelvin and the mole – will be redefined in terms of constants; the new definitions will be based on fixed numerical values of the Planck constant (h), the elementary charge (e), the Boltzmann constant (kB), and the Avogadro constant (NA), respectively. These new definitions will be formally adopted on the 20th May 2019.
The talk was interspersed with brief excursions into the work of the NPL, partly in response to interjections from the audience. We were reminded about the impracticality of early definitions the ampere, with its infinite conductors, and how NPL had pioneered the Kibble balance. There were suggestions that by helping to produce a system of measurement that could be re-created anywhere NPL were putting themselves out of business! In reality the laboratory still had valuable work to do, such as developing compact reference clocks for use in satellites. NPL, in fact, is one of the 'heavy lifters' among the world's standards laboratories when it comes to advancing the technology.
This was an interesting talk at an opportune time and reminded us how vital it is to have precise and stable measurements, especially in the era of high-speed electronic communications where a signal generated in one part of the world might be 'instantly' sent to many places on the far-side of the globe with the sure expectation that it will be correctly interpreted, a true test of measurement being independent of place.
One question from the audience didn't get quite the response that I expected: “How do we know that the physical constants are constant?” Our speaker seemed prepared to entertain the idea that maybe they aren't! Perhaps he just didn't want to be drawn down that route? As it happens I had recently seen a video online asking that question. For what it is worth see the link below.
Links:
The National Physical Laboratory http://www.npl.co.uk/
International System of Units overhauled (NPL) http://www.npl.co.uk/news/international-system-of-units-overhauled-in-historic-vote
The Kibble Balance http://www.npl.co.uk/educate-explore/kibble-balance/
The Science Delusion: Rupert Sheldrake https://www.youtube.com/watch?v=1TerTgDEgUE
James Clerk Maxwell (Wikipedia) https://en.wikipedia.org/wiki/James_Clerk_Maxwell
and finally, an idea for a really practical kilogram! https://www.xkcd.com/2073/