back to article Holy moley! The amp, kelvin and kilogram will never be the same again

As incredible as it may seem, until this week the definitive measurement of a kilogram was a cylinder made of an alloy comprising 90 per cent platinum and 10 per cent iridium sat under a glass dome in a room in Paris. The cylinder is one of six official exact masses of one kilogram, and it has been that way since 1889: The …

  1. malle-herbert Silver badge
    Joke

    And as usual...

    There's always an appropriate xkcd...

  2. The Man Who Fell To Earth Silver badge
    Go

    "...using methods that can be replicated anywhere on the planet."

    Er, no.

    "...using methods that can be replicated anywhere in this Universe."

    FIFY

  3. Joe W

    Re: "...using methods that can be replicated anywhere on the planet."

    We-ell... Not anywhere, not anytime. There are measurements that suggest those constants are in fact not. Constant, that is (cannot remember which ones, though) . And I'm not taking about the first moments (big bang, inflation, etc).

  4. Charles 9 Silver badge

    Re: "...using methods that can be replicated anywhere on the planet."

    Incorrect fix. The second is still defined terestrially, and there's currently no way to change this because of the lack of any kind of absolute point of reference. Put it this way. The measurement of cesium oscillation can change depending on how high up one is on the planet (due to time dilation effects). That's why the qualification of using mean sea level on a theoretical geoid.

  5. DonL

    Re: And as usual...

    "There's always an appropriate xkcd..."

    Perhaps it was part of the point of the joke, but the definition of the pound is actually linked to the kilogram:

    "Various definitions have been used; the most common today is the international avoirdupois pound, which is legally defined as exactly 0.45359237 kilograms"

    https://en.m.wikipedia.org/wiki/Pound_(mass)

  6. The Man Who Fell To Earth Silver badge

    @Charles 9 Re: "...using methods that can be replicated anywhere on the planet."

    Wrong. Relativity, both General and Special, are the way they are precisely because of all clocks being local. Apparently Physics wasn't your "thing" in school.

  7. diodesign (Written by Reg staff) Silver badge

    "...using methods that can be replicated anywhere in this Universe."

    Well, yeeeeeah... our planet is in this universe, at least it was the last time I checked ;-)

    I kinda thought it would be presumptuous to say the constants would work everywhere in the universe. I expected a physics PhD to pop up and yell at us if we said "anywhere in the universe" because, I dunno, at least one of the constants used by the SI base units may not apply or change near black holes or other weird crap out there.

    I don't know if people realize how much of a tightrope it is writing for a fair number of readers, most of them experts in a technical field. We try to get everything right within a reasonable time frame.

    So anyway, that's why we thought "planet" would be a sensible non-offensive, non-triggering bounding box for boffins, seeing as humans aren't going much further into space for a while.

    C.

  8. Ken Moorhouse Silver badge

    Re: "...using methods that can be replicated anywhere in this Universe."

    The Kibble balancer wouldn't work in a weightless environment, for starters. The target kilogram, though anchored, would be floating around the lab.

  9. Spamfast

    Re: "...using methods that can be replicated anywhere on the planet."

    The measurement of cesium oscillation can change depending on how high up one is on the planet (due to time dilation effects).

    There is no absolute reference frame.

    But if we both use the same definition then we can agree that we're measuring the same thing.

    When we get different numbers for the same measurement, that gives us data about the difference between our reference frames, itself useful information.

  10. Tomato42 Silver badge

    Re: "...using methods that can be replicated anywhere in this Universe."

    > The target kilogram, though anchored, would be floating around the lab.

    it can also work in a mode in which the acceleration imparted on the object by the electric field is used to derive its mass

  11. RobThBay

    Re: And as usual...

    I was just about to post the same link. :) :)

  12. Spazturtle Silver badge

    Re: "...using methods that can be replicated anywhere on the planet."

    "...using methods that can be replicated anywhere in this Universe."

    If you have proof that the laws of physics are consistent throughout the universe please come and collect your Nobel prize.

  13. Charlie Clark Silver badge

    Re: "...using methods that can be replicated anywhere on the planet."

    There are measurements that suggest those constants are in fact not. Constant, that is (cannot remember which ones, though)

    There are some observations that suggest that Planck's constant ("h") might not be, er, constant in space and time. Time is sort of understandable, at least in the very early phases of the universe during expansion but I think the observations are of much more recent times. Came across this a few years ago so it could have been debunked or theorised away but it would not suprise me in the least if the universe didn't have quite a few more surprises for us.

  14. cosmogoblin

    Re: "...using methods that can be replicated anywhere on the planet."

    Not so. You simply define your local second, the second in your frame of reference, and adjust it appropriately for other frames of reference (inertial/gravitational).

    As long as your cesium atom and your experimental equipment are in the same frame of reference, you'll get the same value for the second as anybody else in the known Universe.

    There is a slight uncertainty due to the variation in gravitational field strength at different parts of the apparatus, but this in fact makes your measurement more precise and reliable if your experiment is in space, where the gravitational field gradient is minimal.

  15. cosmogoblin

    Re: "...using methods that can be replicated anywhere in this Universe."

    True enough, but there are other methods (currently, I think, even more ludicrously expensive).

    A lot of news outlets have been reporting that the kilogram is now defined in terms of the Kibble balance, which is completely missing the point. The kilogram is defined independently of any measuring equipment; you just need appropriate measuring equipment to effectively use that definition, and the Kibble balance is our best current equipment.

    The Kibble balance compares the weight of an object with the electrical power needed to lift that object, and therefore needs a known gravitational field strength. It would be possible to make a related device that instead of lifting an object, accelerated it in zero g; thus comparing the acceleration with electrical power, and determining its mass that way. Doing away with the need for gravity, I imagine this could be made more precise than the Kibble balance.

  16. DropBear Silver badge

    Re: "...using methods that can be replicated anywhere in this Universe."

    "The Kibble balancer wouldn't work in a weightless environment"

    That is not wrong, but it needs to be noted that on the other hand the balance would work just fine on the Moon or Mars - for any of those, including Earth, you also need a precise measurement of the local "g", measured using meters and seconds.

    The other thing to note is that while the Kibble balance was used to pin down a value as precise as possible for the Planck constant so that it results in our legacy kg being as close to the new definition as possible, once that is done one doesn't necessarily need to use a Kibble balance specifically to derive a reference kg from the Planck constant again - any apparatus linking the two units would suffice - weight may be gravity-dependent but mass isn't.

    And indeed, while the Kibble balance may be the most famous, other approached to define the kg starting from the same constant do exist - most well known probably being the Avogadro Project silicon sphere atom count.

  17. Adrian 4 Silver badge

    Re: And as usual...

    Given the way almost every unit is defined by its relationship to others, I think maybe this one is appropriate.

    I do hope they didn't just cut off the branch they were sitting on.

  18. rajivdx

    Re: "...using methods that can be replicated anywhere on the planet."

    Not true. The speed of light is fixed for all observers. This means that time dilation effects are not detectable to you if you are in the same frame of reference. This means that you will still measure 1 second correctly for your frame of reference using a Cesium clock whether you are in total void or on the surface or the earth or on the event horizon of a black hole.

  19. Anonymous Coward
    Anonymous Coward

    Le Grand K's fate

    So what's going to happen to Le Grand K? Used as a paperweight? Melted down for souvenirs? Or put on show in a museum?

  20. Voyna i Mor Silver badge

    Re: Le Grand K's fate

    I think maybe I'll buy it. It's accurate enough for normal use and will save me my annual trip to Paris to recalibrate my kitchen scales.

    Incidentally the article is pretty damn good but I think it might need explaining that there is one arbitrary factor in all of this - the caesium atom wavelength which was chosen for stability and lack of fine structure, IIRC. The original metric system relied on two arbitrary numbers - the period of the Earth's rotation and its circumference - neither of which were in fact constant.

  21. Will Godfrey Silver badge
    Unhappy

    Re: Le Grand K's fate

    It will be put in a museum with great fanfare, then as interest in it wanes, will be moved by degrees to less auspicious locations, until it is almost forgotten. There will be a brief resurgence of interest when it is stolen to order for some obscenely rich person's private collection.

    In tens of thousands of years time, archeologists in special protective gear will find it while searching though the highly radioactive remains of what appears to have been a fairly advanced civilisation. Recognising that it contains important metals that are in short supply it will be immediately taken to the Whole Earth Technology centre to be put to use.

  22. Lars Silver badge
    Happy

    Re: Le Grand K's fate

    "the period of the Earth's rotation and its circumference - neither of which were in fact constant.".

    The period of the Earth's rotation, was a very funny thing to write and has nothing to do with the metre.

    The "assumed" distance from the equator to the pole was divided by 10.000.000.

    (the circumference 40.000 km).

    A glimpse into the British soul we find in this "On 20 May 1875 an international treaty known as the Convention du Mètre (Metre Convention) was signed by 17 states.".

    Britain was not among those 17 states.

    There used to be an absolutely marvellous account of Britain's indecisive journey towards the metric system, but I cannot find it any more, perhaps it was too revealing.

    https://en.wikipedia.org/wiki/History_of_the_metric_system

  23. Anonymous Coward
    Anonymous Coward

    Re: Le Grand K's fate

    "So what's going to happen to Le Grand K?"

    SAme as teh Berlin Wall when that became surplus to requirements(*) ... you'll be able to buy "chips from Le Grand kg" from a wide range of didgy street vendors all over Paris

    ---

    (*) also cf "splitners from the one true cross"

  24. Voyna i Mor Silver badge

    Re: Le Grand K's fate

    @Lars, I'm making allowance for the fact that English is obviously your second language, but this article is about the redefinition of units, not the redefinition of the metre.

    The old MKS system was based on several non-universal numbers but for everyday use the most important were [spoiler alert] the metre, kilogramme and second.

    As I noted, the metre was supposed to be a fraction (actually 1/40 000 000) of the Earth's circumference, which can't be stated with a very high accuracy.

    The second is based on the rotation period of the Earth, which also varies slightly.

    The kilogramme is 1/1000 the mass of a cubic metre of pure water at a particular temperature - highly impractical and so soon replaced by standard masses.

    So there was nothing "funny" about mentioning the period of the Earth's rotation because it defined one of the three basic units.

    In fact, using the way the metre is defined nowadays, you could derive the "old" metre from the "old" second by saying that the metre is the distance that a point on the equator at mean sea level moves in a certain time - to be exact 86400/40 000 000 seconds.

  25. Ken Moorhouse Silver badge

    Re: "So what's going to happen to Le Grand K?"

    There will be a slimmed-down version of it which they will call "Special K".

  26. This post has been deleted by its author

  27. Lars Silver badge
    Pint

    Re: Le Grand K's fate

    @Voyna i Mor

    Sorry for misunderstanding you. They did use the 10.000 km as the distance from the equator to the pole by simply saying it's 10.000 and thus 1m is what it is. The exact distance to the pole doesn't change that, it would be like pointing out that each feet is not exactly one foot.

    English is my third of four, but that had nothing to do with my mistake.

    Have beer on me.

  28. Michael Habel Silver badge

    Re: Le Grand K's fate

    In tens of thousands of years time, archeologists in special protective gear will find it while searching though the highly radioactive remains of what appears to have been a fairly advanced civilisation. Recognising that it contains important metals that are in short supply it will be immediately taken to the Whole Earth Technology centre to be put to use.

    I think you forgot to pin the blame of all this on both Brexit, and the Orange Man. Otherwise +1

  29. DropBear Silver badge

    Re: Le Grand K's fate

    So what's going to happen to Le Grand K?

    Nothing much. Greengrocers won't suddenly sprout Kibble balances in the back rooms of their shops; you still need practical artefacts to calibrate stuff against, with the Grand K (and its copies around the world) continuing to sit at the top of that pyramid - it's just that instead of being exactly 1kg by definition it will now simply embody a reference kg with a measured and documented (tiny) error.

  30. Alan Brown Silver badge

    Re: Le Grand K's fate

    "The old MKS system was based on several non-universal numbers but for everyday use the most important were [spoiler alert] the metre, kilogramme and second."

    Anyone who's had to calculate anything in rods, poles perches and chains (never mind gills, barrels, hogsheads, bushels and pecks) will have heaved a sigh of relief that most of the sane world switched to something that used common multipliers AND UNITS across the board.

  31. Alan Brown Silver badge

    Re: Le Grand K's fate

    "They did use the 10.000 km as the distance from the equator to the pole by simply saying it's 10.000 and thus 1m is what it is."

    For some reason the metre was defined as 1/10,000 of 1/4 of the circumferential arc a few times in history.

    The French innovation seemed to be defining it from pole to equator (of course that's going to be different from north pole vs south pole, oblate spheroid etc). Previous definitions were either at the equator or sliced at the latitude of the city of the civilisation which defined it - so the egyptian metre was slightly longer than the greek metre, etc (the ancients were perfectly well aware that the earth was round and had a pretty accurate idea of its size. Colombus' fallacy was that it was much smaller than it is and everyone thought he'd starve to death on the voyage, not fall off the edge. He nearly did starve too - and falsified ship's log records to make it seem as if the distance travelled was considerably less than reality in order to deceive his crew, because by his stated original calculations they should have reached Asia long before they hit the Carribean.)

    The roman mile was slightly different, being "1000 marching paces" and defined to allow for planning of rest breaks, etc.

  32. HelpfulJohn

    Re: Le Grand K's fate

    Hmm, it won't work out that way.

    After the fall of the Wizards, when the wars of lightning and magics kill the Great Cities, the remnant species that were once Humans will eke out short, crude, barbaric lives in the rubble, for a while, until they evolve into other species.. Eventually, erosion and tectonics will delete all trace of Man from the planet, whether Life exists on it or not.

    There will never be any archeologists because no second wave of City Cultures can ever arise without cheap, easy to reach oil and coal to kick-start it and we've eaten all of those.

    The Fall of The Cities is the end of technology, the end of the Dream of Stars, the end of any hope of a Human Galaxy.

    There will be no one left to hear our songs or mourn our kings.

  33. Anonymous Coward
    Anonymous Coward

    Re: Le Grand K's fate

    By "Orange Man" do you mean Mr. I. Paisley, or the equally damaging Tangerine Trumpet?

    Both, along with their followers and cults, have done severe harm to our cultures. The legacy of one is still a drag on the Brexit process and all it contains.

  34. HelpfulJohn

    Re: Le Grand K's fate

    "Greengrocers won't suddenly sprout Kibble balances in the back rooms of their shops;"

    No, not suddenly, but with a little more technological development it may be that carrying such a thing around in a pocket will be cheap and easy in ten years or so. Probably as part of a mobile computing and sensing device. We might even have a classy name for it ... "mobile phone", perhaps?

  35. Anonymous Coward
    Anonymous Coward

    Is there such a thing as a complete physical vacuum even in observable space?

  36. Def Silver badge

    No, but as far as the speed of light is concerned, you only need space to be devoid of anything that would interact with light passing through any given point. As long as nothing interacts with it, it's full speed around (because space is curved). ;)

  37. Voyna i Mor Silver badge

    "Is there such a thing as a complete physical vacuum even in observable space?"

    Yup, just go somewhere where the density is around 1 hydrogen atom per cc, and select a region of about half a cc. Fifty percent of the time you'll have a complete vacuum.

  38. ridley

    No, Heisenberg tells people off if they tell you where something is or isnt.

  39. ridley

    No, no no.

    If you went down that path you would eventually be able to say where that Hydrogen atom actually is and that just isnt allowed...

  40. asdf Silver badge

    Yep quantum tunneling makes sure there is no such thing as a truly closed system (sorry if not using official technical term right as layman).

  41. asdf Silver badge

    Oh yeah forgot virtual particles as well even if there is no real particles in your vacuum. Quantum stuff makes a mess of classical world at smallest scales.

  42. Baldrickk Silver badge

    uncertainty

    If you went down that path you would eventually be able to say where that Hydrogen atom actually is and that just isnt allowed...

    Of course it is.

    Only problem being that you would have no idea where it was going.

  43. Schultz Silver badge

    As with most scientific concepts, you extrapolate.

    Nuff said.

  44. Anonymous Coward
    Anonymous Coward

    Yeah, but when you get the dilution that low the vacuum *remembers* that it once had hydrogen in it...

  45. John Brown (no body) Silver badge

    "No, Heisenberg tells people off if they tell you where something is or isnt."

    A meeting between Heisenberg and Yoda might be fun.

  46. Alan Brown Silver badge

    "virtual particles as well even if there is no real particles in your vacuum"

    Which gets even messier when you start apparently extracting energy from those virtual particles. Evidence of extra dimensions that we simply can't perceive?

  47. Voyna i Mor Silver badge

    "If you went down that path you would eventually be able to say where that Hydrogen atom actually is and that just isnt allowed"

    I carefully avoided precisely that issue by saying that 50% of the time you would have a pure vacuum in one half cc. I did not say you could identify which that one was. Several people seem to have failed to notice that I was referencing Erwin's feline.

  48. #define INFINITY -1

    "c"

    "you can derive the distance wherever you are without relying on a single artificial reference object."

    Is there an app for that?

    (There's also the question not answered here, as to how the Ampere will be accurately measured--whether the process of measuring a single "e" is stable yet)

  49. Anonymous Coward
    Anonymous Coward

    Sad case of science ignoring the evidence

    What seems to being ignored here is that somehow the various copies of the kilogram made over the years, with the best equipment of the time, ended up being different to each other.

    At one time this would have been enough to warrent an investigation but now it seems easier for science to just ignore the anomaly and redefine the kilogram as something else.

  50. TRT Silver badge

    Re: Sad case of science ignoring the evidence

    Blame the weight loss on weapons of mass destruction. And when it gains, weapons of mass creation.

    So it's either the bomb or God. Take your pick.

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