Kilogram has lost the weight of a fingerprint

The platonic ideal of a kilogram is represented by a lump of metal in a vault outside Paris. This lump of metal appears to be losing weight, however, having shed the "weight of a fingerprint" since it was stored. No one knows why.
Physicist Richard Davis of the International Bureau of Weights and Measures in Sevres, southwest of Paris, says the reference kilo appears to have lost 50 micrograms compared with the average of dozens of copies.

"The mystery is that they were all made of the same material, and many were made at the same time and kept under the same conditions, and yet the masses among them are slowly drifting apart," he said. "We don't really have a good hypothesis for it."

The kilogram's uncertainty could affect even countries that don't use the metric system -- it is the ultimate weight standard for the U.S. customary system, where it equals 2.2 pounds. For scientists, the inconstant metric constant is a nuisance, threatening calculation of things like electricity generation.

Link (Thanks, RickB!)

(Photo credit: Fair use is made here of a reduced-size crop from a larger image attributed to AP Photo/Jacques Brinon)


  1. The reference kilo cannot, by definition, have lost mass. It still masses exactly 1kg, there being no other definition of a kilogram. So this story is actually that all the copies of the reference kilo have gained a few micrograms, and it is believed to be due to physical changes in the reference kilo.

  2. Mike: Riiiiiiight. When you start coming up with positions like that, you should start to re-think you basis. I mean, you’re correct in the sense that the reference kilogram is the kilogram, but once this sort of thing occurs my reaction would be to change the reference, not warp everyone’s perspective.

  3. This is eerily like the start of Isaac Asimov’s “The Gods Themselves,” in which a scientist notices small changes in a sealed laboratory sample of metal. Turns out that aliens from a parallel universe are replacing the metal in our universe with radioactive plutonium, creating unlimited free energy for earth. Of course, this being SF, this energy supply also threatens to destroy the entire universe.

    Should we be worried? Is Asimov promoting his book from beyond the grave?

  4. Has oxidation of the “dozens of copies” been considered as a possible cause? Seeing how hermetically sealed the kilogram is being kept, this might explain a relative loss of weight with respect to its copies…

  5. The article doesn’t mention how long it’s been since the last comparison was made. ie: how fast is the reference kilogram losing weight?

    My first hunch is radioactive decay. Maybe the reference Kilo contains some trace amounts of an Iridium isotope?

  6. No, Mike is totally correct. When you say “my reaction would be to change the reference”, change the reference to what exactly?

  7. Actually, there is absolutely no problem.

    A gram is defined as: One millilitre of water is 1 g at 4 °C. A kilogram would be a thousand times as heavy as that.

    You can see that this definition then depends on the defintion of the metre, which has been set to be: The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

    For many decades no one has depended on these original metal measures in Paris, and it would be absurd to do so!

  8. Mike makes a clever point, but his logic is off.
    To see this intuitively, consider his final claim
    that the change in measure of the copies “is believed to be due to physical changes in the reference kilo.”

    What physical changes in the reference kilo could this be? A change in color wouldn’t have this effect, nor would a mere change in shape. It would have to be a change in its mass. The error is in mistaking the unit of measure for the property that it measures.

    Let’s call the reference kilogram R, and let’s suppose that Mike is right that the definition of a kilogram is ‘The mass of R.’

    Already this definition is ambiguous between two readings.
    D1: ‘The mass of R at the time of its selection in 1889’
    D2: ‘The mass of R right now’ (Where ‘right now’ serves as an indexical, indicating the timeframe of the relevant writing, speaking, or interpreting of the term ‘kilogram.’

    Obviously under D1 there is no problem with saying that R has lost mass. Both Mike and the scientists quoted in the article, of course, seem to be using D2.

    However, under D2 the following is true when uttered now: ‘The original mass of R in 1889 was greater than one kilogram. However, at that time people used the term ‘kilogram’ to refer to that larger quantity of mass.’

    Thus, even on D2 we can perfectly intelligibly compare the mass of R in 1889 to its mass now using the term ‘kilogram.’

  9. Mike K is not correct – the kilogram is not defined as the mass of a volume of water at 4°C. That definition would be a lot more unstable than the platinum-iridium object in the vault in France, because water likes to dissolve things. To nail it down you’d have to specify the precise composition of the water sample, the container, and the air above the water — not to mention the pressure of the air. By the time you’d done all this work it would be easier to just use a lump of platinum-iridium. Which is what we do.

    There are people working on redefining the kilogram to not rely on an artifact, the same way all the other basic SI units have been redefined, but the alternatives they are looking at tend to be things like “one kilogram is the mass of exactly 2.1507e+25 atoms of pure silicon 28”. And we need to be able to write that number down to a lot more decimal places before it will do a better job than what we’ve got. is a good overview of the present state of play, the problems, and what people are trying to do about it.

  10. Like most modern mysteries, the simplest explanation is most likely true. The original reference kilo has obviously been swapped by a criminal mastermind bent on slowly throwing off the world’s measurements, resulting in widespread chaos and eventually world domination.

  11. How radioactive would the reference kg have to be in order to lose 50 micrograms in X years?
    Maybe all the other ones oxidized adding on average 50 micrograms of oxygen (due to impurities in the metal)? I’m sure the scientists involved have taken these things into account, right?
    1 kg != 2.2 lbs btw.
    1 kilogram = 2.20462262 pounds

  12. The article states that 1kg = 2.2lbs, which is only true in a 1-gee gravitational field. Kilograms are a unit of mass, and pounds are a unit of weight.

    The English analog to kg is the slug. Pounds, of course, are analogous to newtons.

    Interestingly, Google Calculator will incorrectly assert that 1kg = 2.2lbs. It should be 1kg = 2.2lbs/32fpss (feet per second squared), or about 0.069 slugs.

  13. A group of Australian scientists at CSIRO are leading an international project to replace the Paris Standard:

    “Redefining the kilogram through the Avogadro constant

    The kilogram is the only remaining fundamental unit within the SI system that is defined in terms of a material artefact (a PtIr cylinder kept in Paris). It is proposed in the medium term to redefine the kilogram in terms of the Avogadro constant NA. By definition an Avogadro number of carbon-12 atoms weigh exactly 12 g, so the kilogram could be defined as the mass of 1000/12 x NA carbon-12 atoms if the Avogadro constant is known with an uncertainty of 0.01 ppm. The Avogadro constant is obtained from the ratio of the molar mass to the mass of an atom. For a crystal, the atomic volume is obtained from the lattice parameter and the number of atoms per unit cell. The atomic mass is then the product of the volume and density.

    The preferred method for determination of the Avogadro constant is to use a highly polished 1 kg single crystal silicon sphere manufactured with out of roundness < 40 nm. Silicon is used because of its well known crystal structure, its stability and its relative ease of use. The volume is determined from the diameter and roundness measurements. Accurate measurement of the mass then allows the density to be measured. [...]"

  14. These problems are avoided by ‘natural’ units. Look up Planck units (in Wikipedia) based on the most precisely & accurately measured constants.

  15. Er, how is this being measured, perhaps a distortion in the local gravity may account for the perceived loss of “weight”?

    I am pretty sure they aren’t counting all the atoms….

  16. Zack Weinberg, I don’t know whether we can; but for you, and for a thread like this, I’ll ask.

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