Talking about the ‘New’ SI

I was asked to give a talk about the SI to some visitors tomorrow morning, and so I have prepared some PowerPoint slides

If you are interested, you can download them using this link (.pptx 13 Mb!): please credit me and NPL if you use them.

But I also experimentally narrated my way through the talk and recorded the result as a movie.

The result is… well, a bit dull. But if you’re interested you can view the results below.

I have split the talk into three parts, which I have called Part 1, Part 2 and Part 3.

Part 1: My System of Units

This 14 minute section is the fun part. It describes a hypothetical system of units which is a bit like the SI, but in which all the units are named after my family and friends.

The idea is to show the structure of any system of units and to highlight some potential shortcomings.

It also emphasises the fact that systems of units are not ‘natural’. They have been created by people to meet our needs.

Part 2: The International System of Units

This 22 minute section – the dullest and most rambling part of the talk – explains the subtle rationale for the changes in the SI upon which we have embarked.

There are two key ideas in this part of the talk:

  • Firstly there is a description of the separation of the concepts of the definition of a unit from the way in which copies of the unit are ‘realised‘.
  • And secondly, there is a description of the role of natural constants in the new definitions of the units of the SI.

Part 3: The Kilogram Problem

This 11 minute section is a description of one of the two ways of solving the kilogram problem: the Kibble balance. It has three highlights!

  • It features a description of the balance by none other than Bryan Kibble himself.
  • There is an animation of a Kibble balance which takes just seconds to play but which took hours to create!
  • And there are also some nice pictures of the Mark II Kibble Balance installed in its new home in Canada, including a short movie of the coil going up and down.


This is all a bit dull, and I apologise. It’s an experiment and please don’t feel obliged to listen to all or any of it.

When I talk to a live audience I hope it will all be a little punchier – and that the 2800 seconds it took to record this will be reduced to something nearer to its target 2100 seconds.




9 Responses to “Talking about the ‘New’ SI”

  1. rosssmason Says:

    You are very brave. I don’t think I could get away with calling my mass scale after the “weight” of my wifey thing.

  2. protonsforbreakfast Says:

    Perhaps I didn’t emphasise just how light one Stephanie is. It’s much less than a kilogram….

  3. edhui Says:

    Channeling Sheldon in the Big Bang theory and his summer evenings in ancient Greece, yet again you take me back to those summer evenings at NPL with the wagonwheels and the glowing gherkins. I suggest to keep to time, you should just cut all that rubbish about the Boltzmann Constant, and add a few songs.

    But seriously, I’m following everything but left with one question. You’ve spent all this time measuring the Boltzmann Constant, but you and all your colleagues around the world must agree that you have no certainty about what the number actually is, however accurately you measure it. As I understand it, you are taking an estimate and SI is declaring that to be the Boltzmann constant for all time, and it will be one of the cornerstones of the SI. So just for clarity, imagine in the future that the Boltzmann constant is known to be, with some certainty, not quite what you measure it to be now. Is the plan to just carry on, knowing that all the units will just be a little bit out when realised, or will there be a redefinition of the constant, and therefore of all the units?

    I think the talk is great. Seriously- it’s ultra clear and feels a bit like Feynman talking to the BBC. Perhaps you should deliver it from an armchair. I don’t think you have a hope in hell of 2100s; but I don’t think the audience will care. Time is, after all, relative- see the Chronophage clock in Cambridge…


  4. protonsforbreakfast Says:


    I think you are possibly the kindest most positive person on this Earth. Thank you. Reading your words is balm to my troubled soul.

    Every best wish


  5. edhui Says:

    You’re welcome. But I’m still wondering what happens if the Boltzmann Constant eventually gets measured with even greater accuracy and is known to be different to the consensus view now. Will they accept a kelvin that is known to be a little wrong, or will they redefine the constant so that the kelvin is still based on the most accurate estimate of Boltzmann?
    In short I am in awe of the idea of _defining_ a constant as its human generated estimate of 2019, for all time. It is an agreed fudge, but a fudge all the same.

    • protonsforbreakfast Says:


      Our standard temperature is the temperature of the triple point of water which is considered to 273.160000000…000 K above absolute zero. We measure the Boltzmann constant in terms of the energy per molecule of a substance at that temperature.

      After the redefinition, we will consider the Boltzmann constant to be fixed.

      If I then carry out a new experiment at the temperature of the triple point of water and find a different amount of energy per molecule, hen this would indicate that the temperature was not exactly 273.16000000000… exactly. We might say it was 273.160 028 K for example.

      So it isn’t a fudge, its a question of what we think is really constant. If we fix the triple point of water as an exact temperature, then the Boltzmann ‘constant’ would change as our knowledge improves. If we fix the Boltzmann constant, then our knowledge of the amount of energy per molecule – i.e. the temperature – as our knowledge improves.

      All the best. Michael

  6. edhui Says:

    The penny has dropped further. You could have defined the Boltzmann Constant as some easy round number, and all that would have happened would have been a bigger recalibration of the triple point of water. All that time that everybody spent getting the best possible estimate of the Boltzmann constant was to reduce or negate any need to touch the triple point of water in kelvin, and so minimize the effect on the real world of bringing in the new SI. Is that right?

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