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Science

Kilogram Conflict Resolved At Last (nature.com) 278

An anonymous reader writes: As the capabilities of science and engineering expand, they rely more on the precision of measurements. It's vitally important, then, to make sure the standard units underpinning those measurements don't change. This is a problem for the kilogram. For years, it has remained the only SI unit based on a physical object — a small cylinder of platinum and iridium. Scientists have been arguing about how to replace it for decades, but now it looks like their efforts are finally reaching fruition. They finally have all the data they need to define the kilogram with mathematical constants, which solves the problem of the variability of physical objects. "One method, pioneered by an international team known as the Avogadro Project, involves counting the atoms in two silicon-28 spheres that each weigh the same as the reference kilogram. This allows them to calculate a value for Avogadro's constant, which the researchers convert into a value for Planck's constant. Another method uses a device called a watt balance to produce a value for Planck's constant by weighing a test mass calibrated according to the reference kilogram against an eletromagnetic force." Further research has narrowed down the value of Planck's constant, and experimental data from standards bodies is finally matching up. "If they are proved right, in 2018, Le Grand K will join the meter as a museum piece."
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Kilogram Conflict Resolved At Last

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  • by truck_soccer ( 4286027 ) on Wednesday October 14, 2015 @09:53AM (#50725383)
    I'm sorry.
  • by AndyKron ( 937105 ) on Wednesday October 14, 2015 @10:10AM (#50725511)
    Does this mean the US can now join the rest of the metric world, or are we still in the quagmire?
    • by Chris Mattern ( 191822 ) on Wednesday October 14, 2015 @10:15AM (#50725543)

      Does this mean the US can now join the rest of the metric world, or are we still in the quagmire?

      Giggity giggity...

    • by armanox ( 826486 )

      In all honesty, why would the American people want to change?

      • So the next joint-venture probe doesn't crash?

        • Scientists at NASA already use metric. They have been for decades.
          People in the USA who want to use metric generally use it.
      • In all honesty, why would the American people want to change?

        How about for no other reason than to realign our measurements with the rest of the world? NASA has a document with a few incidents that were caused by using the incorrect units.

    • Re: (Score:3, Funny)

      by Anonymous Coward
      We in the US cannot accept this as settled until we have a way to calibrate our scales based on something that is physical. It is all fine to babble about Silicon atoms and Avocados and the like, but we need something to put onto a balance scale for calibration. I've seen avocados and they aren't constant. And Silicon gets smaller every year - why, they were recently saying that chips are going to 9 nano-whatevers or some such.
    • Does this mean the US can now join the rest of the metric world, or are we still in the quagmire?

      What? Look I LIKE having two set's of tools when I'm working on my cars... SAE/Metric Rules!

      Can you imagine how many tool makers would suffer if we stop using SAE? PLEASE, there is no way they will let this happen...

      (/sarcasm)

  • by creimer ( 824291 ) on Wednesday October 14, 2015 @10:16AM (#50725549) Homepage
    I thought the title read: "Klingon Conflict Resolved At Last"
    • Don't be daft, that'll never happen. It does however raise an interesting point - which will end first, Klingon conflicts or the Metric vs Imperial debate?

    • I thought the title read: "Klingon Conflict Resolved At Last"

      Same here. Even after I shook my head and re-read it.

      Fuck lattes, it's quadruple espresso time and a bag of chocolate doughnuts.

  • My best understanding is: Time is defined as a multiple of time period of oscillation of cesium atom at some pressure and temperature. Length is defined as a multiple of a particular frequency light, frequency depends only on time already defined. Now the change is kilogram is defined as a formula based on Plank's constant, length and time both previously defined. Now we can define charge, temperature etc from these. Thus we claim that we have stable definition of the standards.

    I am not sure *stable* is t

    • by gstoddart ( 321705 ) on Wednesday October 14, 2015 @10:36AM (#50725715) Homepage

      Apparently more stable than a block of something which sheds some matter over time, yes.

      So, your choices are: 1) measure according to a physical object which can change over time, or 2) measure according to a known set of physical properties which can be reproduced.

      And there's nothing to say over time as the science gets better they don't tweak this.

      But, in terms of defining in terms of a measure someone can reproduce, it's gotta be better than "1kg is this artifact we made".

      I mean, this is what we have now [nationalgeographic.com]:

      The origin story of Big K reads like a fairytale. The cylinder-shaped artifact was forged under the guidance of the General Conference on Weights and Measures (CGPM), which stated in 1889, as if by royal decree: "This prototype shall henceforth be considered to be the unit of mass."

      For over a century, the kilogram was sealed within three glass bell jars beneath the International Bureau of Weights and Measures, where it was protected from dust, moisture, fingerprints, and other corruptions of the outside world. Big K could only be retrieved by a gathering of three custodians, each with a different key.
      Gaze into the Crystal Ball

      Forty identical sister copies were shipped abroad to calibrate kilograms worldwide. The cylinders were reunited only three times for comparison. Each time, Big K and its twins were delicately wiped with alcohol and ether, steam-cleaned, and weighed. In 1992, scientists were disturbed to discover that Big K had somehow become lighter than its siblings.

      So, it's gotta be more stable than an artifact.

    • What I learned from this article, was that the metre was the initial arbitrary unit that formed the basis of the metric system (I think). The relationship is that 1kg of water = 1 liter = 1 cubic decimeter. But one of these had to come first, and as a result of an arbitrary unit. If a kilogram was slightly heavier or lighter, then that would have altered our metric measurements of volume and weight by a slight amount too

      Another poster here mentioned that the meter was originally intended to be 1/10,0
      • What I don't get it, shouldn't the kilogram have been defined and redefined then as the weight of a cubic dm of water each of these times then too? Why is there an attempt to base this on Avogadro's or Plank's constant?

        No, because when talking about fundamental units, the gram is a unit of mass, not weight. The weight of a cubic dm of water changes if you move it, since the force exerted by Earth's gravity would be different. For defining the fundamental unit of mass, you'd have to use the mass of a cubic dm of water at some given temperature and pressure. That's just the mass of X water molecules in that volume, though, so now you're back to counting atoms.

      • by j-beda ( 85386 )

        What I don't get it, shouldn't the kilogram have been defined and redefined then as the weight of a cubic dm of water each of these times then too? Why is there an attempt to base this on Avogadro's or Plank's constant?

        The difficulty at various times in the past was that the accuracy available using various deffinitions was different. If we defined the kilogram of mass as being equal to the mass of one litre of water, the uncertainties in that definition would have been much greater than the uncertainties in measuring a standard chunk of metal and replicating that. The uncertainties arries in the purity of water obtainable, the temperature, pressure, humidity effects that would come into play, the uncertainties in creatin

  • The new definition involves counting exactly 60 200 000 000 000 000 000 000 000 000 hydrogen atoms.

    • Re: (Score:3, Funny)

      by donkwich ( 4014861 )
      Sixty octillion one hundred ninety-five septillion three hundred sixty-eight sextillion five hundred forty-one quintillion four hundred seventy-four quadrillion six hundred seventy-three trillion eight hundred twenty-two billion six hundred thirty-one million nine hundred forty-five thousand two hundred ninety-one, sixty octillion one hundred ninety-five septillion three hundred sixty-eight sextillion five hundred forty-one quintillion four hundred seventy-four quadrillion six hundred seventy-three trillion
  • by flink ( 18449 ) on Wednesday October 14, 2015 @10:40AM (#50725749)

    Veritasium did a visit to the facility where they were producing one of the spheres. You can see it here: https://www.youtube.com/watch?... [youtube.com]

  • The relationship between the kilogram and the liter was the most elegant thing in the metric system, so why break it?

    • What's elegant about it? The kilo is "the only SI unit based on a physical object." That's not elegant at all.

      And there isn't a direct relationship between the two, anyway. The litre is a measure of volume, not mass of a particular substance. They were related through the definition of the metre:

      One litre of liquid water has a mass of almost exactly one kilogram, due to the gram being defined in 1795 as one cubic centimetre of water at the temperature of melting ice. Subsequent redefinitions of the metre and kilogram mean that this relationship is no longer exact.

      There. It was inaccurate already.

      • At some point, the definition of the gram was refined to the mass of 1 cubic centimeter of water at water's maximum liquid density.

        Was in some physics text I read in high school. A web search found several confirmations of that definition.

        • by cdrudge ( 68377 )

          In 1975 the gram was defined as 1/100 of a meter cubed of pure water from melted ice. Or in other words, a cubic centimeter of water. Within 4 years, they realized they screwed up and made the water's temp at it's maximum density measured then at 4 degrees C. Also at the same time they realized the impracticality of defining mass with a small size of a easily impure liquid that evaporates so they made the all-platinum prototype that was equivalent to one cubic decimeter of water at 4 degrees C and 1 atmosp

    • by j-beda ( 85386 )

      The relationship between the kilogram and the liter was the most elegant thing in the metric system, so why break it?

      This doesn't break it. Any changes to the precise value of the kilogram are going to be so small that you can safely continue to use the idea that one litre of water has a mass of one kilogram. Changing the definition of the metre from one based on the earth, to the reference piece of metal, to waveleghts of a particular type of light, to a distance light moves in one second did not change the litre-kilogram relationship because the litre-kilogram relationship in practice cannot be examined to the level of

  • The last kilo I bought was off by almost 0.00000000000002%, and I was cranky for days.

    Got a refund though, so it all worked out in the end.

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