The Kilogramme Will Cease To Be A Physical Entity

One of the most illuminating high school courses no doubt for many readers as much as for your scribe, was the series of physics lessons during which the SI units were explained. That glorious sense of having the order of the universe unlocked into an interlocking series of units whose definitions could all be derived in terms of a series of base units was mind-blowing in those early teen years, and even though the explanations might have been at a for-the-children level that has been blown out of the water by later tiers of learning it’s still a bedrock that will serve an engineer or scientist life-long.

The definitions of the SI base units have evolved with scientific advancement to the point at which they are no longer tied to their original physical entity definitions. Of all the base units though there is still one that has resisted the urge to move away from the physical: the kilogramme (giving it its French spelling to preserve context) is still defined in terms of a metal cylinder in a laboratory just outside Paris. Kg diehards have not much time left to cling onto their platinum-iridium alloy though, for a new definition has been adopted in which it is derived from Planck’s Constant. From next May this will become the official kilogram, at which point concerns over microscopic erosion of the metal standard become irrelevant, and an SI kilogram can be replicated by any laboratory with the means to do so.

The piece of apparatus that makes this definition possible is the Kibble balance, a balance in which the force required to overcome the effect of gravitational force on a given mass is measured in terms of the electrical power required to do so. The gravitational force at a given point can be measured accurately and is defined in terms of the other SI units, while the electrical power can be derived from a Josephson junction, a superconducting junction whose current is defined in terms of Planck’s constant. As a result, the kilogram can be measured solely in terms of the constant and other SI units, consigning the metal cylinder to history.

This high-end metrology and physics make for interesting reading, but it’s fairly obvious that the de facto kilogram we all use will not change. Our everyday measures of everything from sugar to PLA filament will be the same today as they will be next May. But that’s not the point, everyday measurements do not need the extreme accuracy and reproducibility of a laboratory. The point of it all comes in as yet unforseen applications, as an example would the ability to synchronise timing to create GPS or digital radio have been possible were the second to be still defined in terms of astronomical movements rather than atomic states?

Standard kilogramme replica picture: Japs 88 [CC BY-SA 3.0]

72 thoughts on “The Kilogramme Will Cease To Be A Physical Entity

  1. doesn’t this require that gravity be constant? and we know that gravity is not, because we have satellites up designed to measure the difference in gravity as they orbit the earth. If it’s detectable at orbital distances, how can you have it be consistent enough for a standard at ground level?

    I’d be surprised if I was the first person to think of this, but it does seem like a hole is the scheme.

    1. From Wikipedia:
      The weight of the kilogram is then used to compute the mass of the kilogram by accurately determining the local gravitational acceleration. (See gravimeter.) This will define the mass of a kilogram in terms of a current and a voltage, as described below.

      1. thats’ right, they measure gravity across and around the testing location so they can have error analysis on every variable involved in the experiment. In setups so precise there are very few assumptions.

        1. For a very short time the _yard_ was defined as being exactly the length of a seconds pendulum. However this has two problems. 1) The gravitational field varies slightly as you travel around the earth and 2) Any future human colonies in other star systems would need to take a reference earth-sized mass with them to calibrate their yard sticks.

        2. – friction in the pivot
          – air resistance
          – coriolis force
          – measuring the exact center of mass for the pendulum
          – local gravity

          Those are the practical fudges for the pendulum. Dropping a mass in a vacuum chamber eliminates a whole bunch of those variables.

      1. Wikipedia: “The principle that is used in the Kibble balance was proposed by Bryan Kibble of the UK National Physical Laboratory (NPL) in 1975 for measurement of the gyromagnetic ratio.[9]”
        If you are right, you should edit Wikipedia.

          1. gram used to be the base unit with the CGS system, but then we changed to the MKS system in the 70’s.

            Kilograms are just too big to be a practical base unit. If we changed the name to get rid of the “k” then what would be the new gram would need a prefix m- in front of it, and you’d just push the problem into having to measure every day objects in milli-somethings.

  2. From the May 2019. not only the kilogramme will be changed but a lot more units (amper is one of them). I had a pleasure to speak about this with one German scientist who was a part of “working group” in charge of this for the past several years.
    These changes will not have a significant impact to everyday measuremets, but will make some work to those who are working in laboratories on some precise equipment (volt, as disseminated unit, will be changed by 0.1 ppm or something like that if I remember correctly).

    1. The value of planck’s constant is now set by definition – they picked a value close to the measured value and fixed it. That means 1kg doesn’t change appreciably, but the error is now moved to the physical objects that the constant is used to measure, rather than the other way around.

        1. That depends on your unit system.
          You can make a consistent system where the pound is a unit of force, and one such system uses the “slug” as the unit of mass.
          Or, alternatively, the pound can be a unit of mass, and one such system uses the “poundal” as the unit of force.

      1. The fractions are well known and averaged into molar masses. If you were really concerned you could set up a still to only get the precise isotope blend you want. Natural fractionation from the water cycle is fairly well mapped out but (inter)national standards bodies could easily make water of arbitrary isotope blends. I’m sure those exist already; at commensurate expense.

    1. Ultimately this boils down to the mass of a certain number of electrons, protons, and neutrons. From what I heard, and alternative method for redefining the kilogram was accurately determining Avagadro’s number. Then a kilogram could be defined as the mass of x neutrons/protons. It seems however, that the kibble balance won out.

        1. But then you are tied to a physical item that can’t be duplicated. It would be impossible to make an identical sphere with the same number of atoms in it. So it really needs to be based on some other non-physical constant.

          1. You wouldn’t need an identical item, just one that can be measured exactly. The point of the silicon sphere was to define how many atoms of silicon there are in the volume of the sphere and how much they weigh.

            After that, any sphere of silicon would do as long as you could measure its exact volume. The physical kilogram standards aren’t exactly a kilogram either – they’re offset by some amount, which is known and accounted for.

    1. Not at all, just like how you can measure 1 second by counting oscillations in a caesium gas, now you can measure a kg by comparing to a voltage source. This means that we could explain a kg to aliens over a radio and they could make exactly the same mass.

      1. I am sure that aliens are using their own number system based on odd numbers (like base 13 or base 17) and would have pragmatic solution of 13kg=1odd kg and 7 odd kg= 1 secodd kg and 19 secodd kg = pragma kg. 2.37 pragma kg is roughly…

      1. Except how are *those* values defined? I’m not sure I want to armchair scientist this when literally the most experienced scientists in the field, the ones who *actually* have to use the definition practically, have just voted on a different approach.

        1. By the quantity of electrons and/or the voltage- as I understand it that is the purpose of the new method to measure a mass (and weight) using electromagnetism- which is perhaps thought to be more accurate than gravity-only scales. I have no problem with the method, or majority rule, but just using Planck’s constant which to me seems arbitrary. The value is in the measuring tool, not some platinum brick sitting in a case in Iceland used as a theoretical reference for the love. Personally, submission to a small group of “experts” is foolish- experts have been wrong many many times- in particular in this bizarro single-bullet/19 hijacker conspiracy theory age- there simply is a lot of corruption in what little windows we see in front of us. The method of many corrupted insiders is to carry on a ceaseless effort to snow the blind outsider public with holier than though arrogant know-it-all snow-them-with abstract math and hope that they never invent a nanocamera or device that hears thought-audio- and pretty much surrenders to authority- just sayin’

          1. we should really measure mass in light particles, hydrogen atoms, and I see no problem with “grams” or “drams”- they are all fine- like the rise of Christianity and FitzGerald’s theory of space contraction that grew into the theories of relativity- newer is not always better! How many other times have we seen not “new and improved” but “new and worsened”?

        1. Not in this case.
          This is making light of the uncertainty in the measurement / definition. Essentially the nanogram is below the ‘noise floor’ of the definition of Planck’s constant and by extension this definition of the kilogram.

    1. Is that uncertainty in the constant, or uncertainty in the standard used to measure it? :D

      Anyway, there’s zero uncertainty in the value now. PI IS EXACTLY THREE!

  3. The beginning of the end of humanity as we know it …

    How humanity will, after a catastrophic massive event that has wiped 3/4 of civilization, will “measure” and agree in 1 kilogramme???
    With a physical weight is easy … with Planks is not that easy.

    Is absurd but it is a possibility nonetheless.

    1. why would people in those conditions care about a universal ‘Kg’? they will probably develop new measurements that are more directly applicable and conversions to metric or imperial measurements will matter only to the extent that they are interacting with ‘pre disaster’ measuring artifacts or books.

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