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Science Technology

The Definition of a Kilogram Just Changed Worldwide (vice.com) 157

Posted by msmash from the PSA dept.
For over a century, the kilogram was defined by a metal cylinder in a French vault. Now, this key unit of mass is defined using the Planck constant, a fundamental figure in physics. From a report: On Monday -- World Metrology Day -- Le Grand K lost its special status as the international prototype kilogram (IPK) and it will no longer represent this base unit of mass to the world. From now on, the kilogram -- along with the ampere, kelvin, mole, and candela -- will be defined by fundamental physical and atomic properties instead of tangible human-made objects. "The Metric System was envisioned to be 'for all people for all time,'" said Barry Inglis, president of the International Committee for Weights and Measures, in a statement. "From its outset it sought to ensure long-term stability by defining the units in terms of an internationally agreed 'constants of nature' instead of an arbitrary reference."

To that end, the "arbitrary" Le Grand K has been deposed by the Planck constant, a fundamental quantity related to the energy of photons, the elementary particles that make up light. Defined as 6.626 x 10-34 joule-seconds, the constant fixes the kilogram to the speed of light and a temporal unit of measurement -- the second. The kilogram is now equal to the weight of 1.4755214 x 1040 photons with frequencies matching a cesium atomic clock. It may sound like a less relatable system of measurement, but what the change loses in familiarity it makes up for in precision. Even though Le Grand K is one of the most carefully protected objects on the planet, it is not immune from physical interactions that can alter its weight.
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The Definition of a Kilogram Just Changed Worldwide More

The Definition of a Kilogram Just Changed Worldwide

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  • 1.4755214 x 1040 photons (Score:4, Funny)

    by ebcdic ( 39948 ) on Monday May 20, 2019 @01:29PM (#58624182)
    Who knew photons were so heavy?

    • Re:1.4755214 x 1040 photons (Score:4, Funny)

      by bugs2squash ( 1132591 ) on Monday May 20, 2019 @01:42PM (#58624264)
      They're not, I think that may overstate their mass by about 37 orders of magnitude.
      • It seems the editor from vice.com hates '^' symbol. It should be 10^40.
      • The kilogram is now defined is "one five hundredth of the weight of Roseanne Barr".

    • Re: (Score:2, Funny)

      by Anonymous Coward

      Now I know why walking out of a dark room into bright sunlight is so annoying. I didn't realize each photon weighed 0.652 grams. One wonders about the origins of the mystical force that counteracts the force of millions of 0.652 gram objects traveling at the speed of light hitting me every second.

      [Goes off to write grant proposal...]

    • Re: (Score:1)

      by Anonymous Coward

      1.4755214 * 10^40

    • So basically... Yoda was talking about weight in photons. [youtu.be]
      Which would mean that naturally, without special adeptness and training, heavier objects are more aligned with the Force.
      Which kinda explains why Jabba is immune to Jedi mind tricks.

      And the weight issues among the Star Wars fans.

    • What was wrong w/ the 'n number of Si atoms' definition? Sure, define an ambient temperature of 300K (kelvins) if one wants
    • Also, why not define it using protons, as opposed to photons? As it is, photons vary as particles or waves depending on their speeds, as per Heisenburg's Uncertainty Principle. Whereas protons are particles, and we know how many in each element. Yeah, we'd have to factor in the weight of the electrons if we are considering neutral atoms, but that's always something that can be mathematically derived

  • Metric nonsense (Score:5, Funny)

    by lgw ( 121541 ) on Monday May 20, 2019 @01:29PM (#58624184) Journal

    It's time we abandoned this outdated "metric system" nonsense. The one true system of measurement is the "F you system", where the fundamental units are: fortnight, firkin, furlong, Faraday, and Fahrenheit (the foot is a derived unit).

    • You forgot Food ball Fields.

    • No fortnight? I was under the impression that the ideal units to express the speed of light is furlongs per fortnight.

  • Actually, it means the opposite. (Score:3, Informative)

    by RyanFenton ( 230700 ) on Monday May 20, 2019 @01:47PM (#58624314)

    Isn't not changing - it means the same amount of matter it used to mean. This is kind of the opposite of change though - the reference has been frozen to match what the object weighted at a fixed point in time, thanks to being able to measure something common in nature enough to get a repeatable figure to stick with going forward.

    No one is going to measure a different number of fractional Kilograms when measuring stuff - they've just taken away the ability of folks to mess with the system by messing physically with the reference object.

    Ryan Fenton

    • Re: (Score:2)

      by skids ( 119237 )

      the reference has been frozen to match what the object weighted at a fixed point in time

      A quibble: the values they chose (for kg and other units) were not what that object used to weigh/measure/etc. Each is a value determined by more than one, methodologically different, repeatable experiment which physicists poured a lot of work into making so precise that the error bars of the experimental measurements were tight enough to fall within the error bars of the old definition, and so will continue to fall within those error bars into the future assuming no changes in the fundamental constants th

  • And the US Pound too, in certain sense (Score:4, Interesting)

    by Midnight Thunder ( 17205 ) on Monday May 20, 2019 @01:51PM (#58624332) Homepage Journal

    From what I understand, at the federal level the metric system is actually the official system and the imperial units are defined in terms of them. Given that, in certain sense, the pound definition has changed as a consequence as well.

    While at the trade level the imperial units are the typical units, a growing number of companies are going metric internally, including the US automobile industry.

    There is a long history:
    https://www.smithsonianmag.com... [smithsonianmag.com]

    • Re: (Score:2)

      by halivar ( 535827 )

      IMXP, Imperial is only used in household, day-to-day stuff. Driving, groceries, recipes, and hanging pictures on the wall. In college, we exclusively used metric in anything math & science-related. That was 20 years ago. When I started grade school 35 years ago in the worst school system in the country, we were still taught both sides of ruler.

      I'd say we're more "dual-system" than imperial.

    • It simply cannot be that those enemy-controlled metric units dictate the sizing of US units! Make America great again - by de-coupling the pound from the kilogram - make it the new Freedom-Pound! Also, those sizes should be put under control by private companies rather than the state. Let the markets decide the mass of a Freedom-Pound!

    • Given that, in certain sense, the pound definition has changed as a consequence as well.

      The international pound [wikipedia.org] is fixed as being 0.45359237 kg. The _definition_ of the pound hasn't changed; it has long been defined by the kilogram. And as the new definition is supposed to produce a result that was consistent with Le Grand K, the actual weight of a pound shouldn't change either.

      How reference pound weights are manufactured might change, depending on how much accuracy you need. But by definition, the definition itself hasn't changed.

      Yaz

  • Can we please get rid of that stupid unit, the tonne

    1000 Kilograms = 1 Megagram

    • Re:Now that that is settled (Score:5, Funny)

      by ChatHuant ( 801522 ) on Monday May 20, 2019 @02:33PM (#58624558)

      Can we please get rid of that stupid unit, the tonne

      1000 Kilograms = 1 Megagram

      Nah, the kilogram is the fundamental unit, not the gram. What we should do is get rid of the stupid gram, and use only proper fundamental units and SI prefixes:

      1 g = 1 millikilogram

      • As a chemist, I readily agree and support your suggestion. The gram needlessly obscures the definition, and is partly why even scientists get confused when calculating things like ppm, as they do not recognize kg as the SI unit.
      • Comment removed based on user account deletion

        • Could we call it a 'milli killy' for short?

          I like that!

          And, of course, the tonne would become the kilokilogram - which could be shortened to killy-killy.

    • Can we please get rid of that stupid unit, the tonne

      1000 Kilograms = 1 Megagram

      This has been one of my pet peeves about SI since I was a kid - they have a perfectly consistent unit (megagram), so why pollute the nomenclature with "metric tons" and such silliness?

    • Megagram, is that the one that had the IPO recently or the one with the fat dude in New Zealand?

    • IMO, one of the few disadvantages of the SI system vs imperial units is that they don't have simple single-syllable aliases for commonly used units. Do you really need to remind people of the exact conversion factor Every Single Damned Time they use it, at the expense of having to use four-syllable phrases?

      There should be *more* words like tonne, micron, klicks in use, not less. Nobody is going to forget confused by a few dozen aliases, and all the convenient 10-based math would be exactly the same.

    • Re: (Score:3)

      by jwhyche ( 6192 )

      I agree, but we should go back to tried and true measurements. Weight is assload, shitload, and my favorite, fuck ton. An data should be measured in libraries of congress just like god intended.

  • Nice, but of limited use (Score:4, Funny)

    by NEDHead ( 1651195 ) on Monday May 20, 2019 @02:08PM (#58624416)

    When they redefine Pi to an even 3.0, that will be useful...

  • I know what you're trying to say, but you're not saying it. Laziest paste job ever.

  • Mass not Weight. (Score:2, Insightful)

    by Anonymous Coward

    it is not immune from physical interactions that can alter its weight.

    You'd think that at least in this instance ./ editors would have been more precise. Mass not weight. Le Grand K is not immune from physical interactions that can alter its mass.

  • Another reason why too much sun is bad for you.

  • The kilogram is now equal to the weight of 1.4755214 x 1040 photons with frequencies matching a cesium atomic clock. It may sound like a less relatable system of measurement, but what the change loses in familiarity it makes up for in precision. Even though Le Grand K is one of the most carefully protected objects on the planet, it is not immune from physical interactions that can alter its weight.

    The kilogram is the SI unit of mass. Weight is a force and is measured in newtons (kgms^-2). I guess it's too much to expect a Slashdot editor to understand the difference, even in a story about the SI unit of mass.

  • Now if we (scientists) could only all agree on the actual speed of light in a vacuum to great precision ... and not recursively define it in terms of a dependent measurement (meter).

    • I thought the speed of light was measured, not defined? The second is defined in terms of a certain transition in a Cesium-133 atom. Light travels a certain distance in a vacuum in one second (a measurement), and the meter is defined in terms of that distance.
      • Not any more, speed of light is defined since 1983, period of cesium clock oscillation is defined, that means you can measure the length of a meter with any tool that previously measured speed of light independent of any external calibration. If c was not defined, you would need a defined meter to measure it, which is how it was done way back when meter was still defined by a metal rod in Paris.
        • So what's the defined value for the speed of light? It can't be "the distance light travels in one second", because that isn't a defined value, it's a definition for which you measure the value.
          • Speed of light is 299792458 m/s precisely, that's by definition. That makes the definition of metre a distance that light in vacuum travels in 1 / 299792458 seconds. Length of metre is what you measure, speed of light is fixed by definition.

            • Speed of light is 299792458 m/s precisely, that's by definition. That makes the definition of metre a distance that light in vacuum travels in 1 / 299792458 seconds. Length of metre is what you measure, speed of light is fixed by definition.

              So if I redefine the meter to be the average circumference of the Earth, the speed of light would change?

              The way it actually works is that the speed of light is measured: light travels X distance in one second. X could be expressed in any units you want. You could say X is some number of feet (i.e. the length of some specific person's foot) or some number times the diameter of a hydrogen atom. Once you have the distance light travels in one second, you define the meter to be 1/299792458 of that distance.

    • Now if we (scientists) could only all agree on the actual speed of light in a vacuum to great precision ... and not recursively define it in terms of a dependent measurement (meter).

      Uh... the speed of light is known to "infinte" precision: it is defined to be 299 792 458m/s

  • Since SI is the total, unquestionable owner of the word "kilo", "mega", etc and now people are using the wonderful "mebibyte" to refer to a proper megabyte, can we request that be unfixed in the case of at least computer technology? Granted, as people always point out, "mega" is wholly controlled by SI on Mount Olympus, and we dare not diverge from what they say, but maybe they can throw us a kibibyte worth of consideration.

    On a side note I love how Chrome spellcheck (UK English) wants to change mebibyte
    • ...but attribution is typically given to SI when you see people giving the official owners of the words "kilo", "mega", etc. Plus they're wrapped up together anyway.

    • Re: (Score:2)

      by Megol ( 3135005 )

      I used to think like you until one day realizing that was just me constructing a reason to keep using what I was used to. Don't be afraid of change.

      • We're not talking about accepting a major life change here, we're talking about what to call a group of a certain amount of bytes. I was being colourful for fun, don't act like you're writing another chapter of Eat, Pray, Love. What possibly benefits me for accepting this change? Will I get instantly far more insightful? Perhaps tons of money, women, cars, or get to sound like a dork talking about accepting "change".

        What change? Putting an i between the M and B? Literally nothing will change except me acc

  • ... about 2 pounds, 3 ounces, right?

    • I'm just hoping that 1 kilogram will still be 1 kilogram. That way we can avoid confusion if we have to convert old kilograms into new kilograms.
      • This is just a scheme to sell scientific equipment to drug dealers.

      • Re: (Score:2)

        by tricorn ( 199664 )

        The "new" kilogram is within the margin of error of weighing the physical "old" kilogram.

        With this change, Planck's constant is now an exact value instead of a measured value with a margin of error. This is the same thing as how the speed of light is (now, and since Oct 21, 1983) an exact value, or the frequency of a specific transition of a cesium atom at 0 Kelvin is an exact value (since 1967). Measuring those physical constants refines the accuracy of the unit (meter, kilogram, second), not the consta

  • It strikes me that there is a certain irony and hypocrisy in this. Those who did this good work tried to pound a square peg of rational new ideas onto the round hole of an older and now dated system, hardly a genuine update or recalibration of the system. The argument goes like this:

    The metric system was devised so that weights and measures had a rational basis in physical objects which in their day in the 19th century seemed like invariant references. It supplanted the various imperial systems that had

    • But, if you are going to make the change, why not pick a logical reference. Instead of 1.4755214 x 10^40 photons at cesium frequency, why not rationalize it some way? Why not 1x10^40? In fact, why 40?

      The Kilogram was originally defined as 1dm^3 of water at standard pressure at the temperature of maximum density. This was (and still is) extremely difficult to correctly validate -- if the temperature or pressure is off by a thousandth, or if you have slightly more or less water than exactly 1L, or if the water has impurities, you're going to get slightly different results. Reproducibility to scientific standards using this definition is nearly impossible, which is why the reference kilograms were create

    • It strikes me that there is a certain irony and hypocrisy in this.

      I completely disagree.

      The metric system was devised so that weights and measures had a rational basis in physical objects which in their day in the 19th century seemed like invariant references.

      The metric measurements were chosen like the imperial ones to be "human scale". They weren't chosen to be as invariant as possible using the technology of the day in order that everyone's meter was as exactly the same as it was possible to get.

      The one

  • Completing the transition to defined constants is a very significant achievements and it's not often properly explained why. The most important thing here is the accessibility to prime standard. Big K could be accessed every few decades, working standards were all several steps removed from it. Now everyone with sufficient amount of money can build themselves a working prime standard, which is a huge deal, highest level of accuracy just got a whole lot more accessible. Think about the level of timekeeping a
  • My gran likes to weigh ingredients (for cakes) using little metal weights. Can you suggest where I might buy photon equivalents?!

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