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Experts Suggest Replacing Definition of Kilogram 844

Posted by Zonk
from the still-won't-be-used-in-the-states dept.
fenimor writes "The kilogram is the only one of the seven basic units of the international measurement system defined by a physical artifact rather than a natural phenomenon. International team of scientists suggest replacing the kilogram artifact -- a cylinder of platinum-iridium alloy about the size of a plum --with a definition based on one of two unchanging natural phenomena, either a quantity of light or the mass of a fixed number of atoms. They propose to adopt either one of two definitions for the kilogram by selecting a specific value for either the Planck constant or the Avogadro number."
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Experts Suggest Replacing Definition of Kilogram

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  • I suggest (Score:5, Funny)

    by Anonymous Coward on Saturday February 26, 2005 @07:53PM (#11790192)
    They set it to 1000 grams.
    • Re:I suggest (Score:5, Interesting)

      by tibike77 (611880) <tibikegamez@ y a h o o . com> on Saturday February 26, 2005 @08:23PM (#11790477) Journal
      Not only that, but it is also the only "basic measuring unit" that's not even a measuring unit, but a multiple of another one that's not considered "basic". At least in name. Kilo-gram. Get it ?

      Actually, if they happend to re-define it based on Avogadro's number, they might as well just say the GRAM is the new "basic unit" and the kilogram is just 10^3 grams.
      Why ?
      Because Avogadro's number is JUST an artifact of the definition of the (kilo)gram, not a fundamental constant - it's (been originally) defined as the number of atoms in 12 grams (or, whatever, 0.012 kilogram) of Carbon-12.
      Talk about circular references then...

      Now, basing the definition of the kilogram (might I suggest they also change that basic to gram instead of kilogram... please) on Planck's constant somehow would be a MUCH better ideea. However, the value of that constant [i.e. 6.6260693111111 * 10^-34 and so on] makes it pretty wierd to work with unless you multiply it with 9 [to get exactly 5.96346238 * 10^-33 which makes more sense somehow]. And even then it won't satisfy some people, as I'll bet you'll hear that 0.111111 and so on *9 does not equal 1 :p
      Not only that, but Planck's costant was ALSO measured "accurately" using the kilogram unit as reference.

      Ok, this actually does give me a headache.
      • Re:I suggest (Score:5, Informative)

        by bcrowell (177657) on Saturday February 26, 2005 @09:13PM (#11790904) Homepage
        Not only that, but it is also the only "basic measuring unit" that's not even a measuring unit, but a multiple of another one that's not considered "basic". At least in name. Kilo-gram. Get it ?
        There are two common systems of units, mks (meter-kilogam-second) and cgs (centimeter-gram-second). The mks system is now more often referred to as the SI. In the cgs system, the gram is a base unit. In any case, what you're referring to is utterly trivial and/or irrelevant when it comes to the real work of defining the units. Any definition of the gram suffices to define the kilogram, and vice-versa.

        Because Avogadro's number is JUST an artifact of the definition of the (kilo)gram, not a fundamental constant - it's (been originally) defined as the number of atoms in 12 grams (or, whatever, 0.012 kilogram) of Carbon-12.
        It's happened before that they've changed things around so that something different was considered to be the more fundamental quantity: the speed of light used to be a measured quantity, but now it has a defined value. The whole issue is that as techniques change, you want to base your system of units on the things that can be most accurately measured (and reproduced) with the latest techniques.

        Now, basing the definition of the kilogram (might I suggest they also change that basic to gram instead of kilogram... please) on Planck's constant somehow would be a MUCH better ideea. However, the value of that constant [i.e. 6.6260693111111 * 10^-34 and so on] makes it pretty wierd to work with unless you multiply it with 9 [to get exactly 5.96346238 * 10^-33 which makes more sense somehow].
        I'm not sure where the <joke> tags belong here. Anyhow, giving h a defined value would be very much like the step they took when they gave c a defined value -- they did it because when techniques changed to the point where c was one of the most accurately measurable things in nature.

        • by hawk (1151) <hawk@eyry.org> on Saturday February 26, 2005 @10:33PM (#11791494) Journal
          Either way, as long as we do it quickly, before it's too late.

          After all, an ounce of prevention is worth a pound of cure.

          hawk
      • by lgw (121541) on Saturday February 26, 2005 @11:36PM (#11791865) Journal
        The furlong-firkin-fortnight system is the one true system. All other systems are silly.

        Any fool can see what faction of an acre is a rectangle bounded by a furlong and a chain, or measure speed intuitively in millifurlongs per microfortnigt. This metric system is just unintuitive.

        I know just how many furlngs per firkin my car gets, what the heck is that in litres per meter?
    • Better yet, make it 1024 grams. Make it consistent with the kilobyte.
  • I wonder... (Score:4, Funny)

    by elid (672471) <[eli.ipod] [at] [gmail.com]> on Saturday February 26, 2005 @07:54PM (#11790202)
    ...if the change it, what would happen if they would auction off the cylinder on eBay?
  • by zerkon (838861) on Saturday February 26, 2005 @07:55PM (#11790205)
    I'm going to finally lose some weight?
  • How about ... (Score:3, Interesting)

    by canwaf (240401) on Saturday February 26, 2005 @07:56PM (#11790212) Homepage Journal
    1 litre of H2O at ATP?
    • Pressure (Score:5, Insightful)

      by XanC (644172) on Saturday February 26, 2005 @08:02PM (#11790290)
      That would work fine, and I believe was the original definition. Unfortunately, pressure has a mass component, so your definition is circular.
    • Re:How about ... (Score:5, Interesting)

      by InfiniteWisdom (530090) on Saturday February 26, 2005 @08:18PM (#11790418) Homepage
      The point is to have something that you can define just by counting some phenomenon or natural objects. For example a second is defined as:
      "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom."

      and a metre is defines as:
      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.

      Unfortunately, there hasn't been a good way to count the number of atoms with any kind of precision, so that has precluded a good definition of the kilogram so far. Maybe now the physicists can actually count atoms accurately enough.

      One could define it as the mass of some number of H2O molecules, but maybe its easier a measure a quantity of light or to count some larger atoms.
      • Re:How about ... (Score:3, Insightful)

        by Dizzle (781717)
        Now, I know those definitions are techincally correct, but who thinks these ideas are easily applicable? I mean, the point of having a definition is to be able to calibrate everything else, right? So how on earth is a watch manufacturer or repair person going to say "alright, the cesium atom vibrated 9,192,631,769... 9,192,631,770 times. That's a second."

        Is there actually a method of directly using these definitions?
        • Mod Parent Up (Score:3, Insightful)

          by Gabrill (556503)
          Someone smart famous once said "Any technology, sufficiently advanced, is indistinguishable from magic." These fundamental definitions are following the same path. Superbly and unarguably accurate, but also completely incomprehensable for anyone that doesn't have half a million dollars worth of sophisticated technology.
        • by Anonymous Brave Guy (457657) on Saturday February 26, 2005 @10:27PM (#11791463)
          Is there actually a method of directly using these definitions?

          Where you actually need to use them directly, sure.

          To give a real world example of how the standards work in practice... I used to write software for a company in the metrology (high precision measurement) business. They made machines that are used, for example, in quality control at the end of production lines. The gauges on the most popular machines gave accurate readings with resolutions of say 1-10m.

          Those machines were calibrated from reference artifacts. These were themselves checked for accuracy on still higher precision equipment. (How they actually manufacture something so close to physical perfection is an interesting area in itself...)

          Ultimately, there were white room areas with very careful decontamination procedures in place that were used almost exclusively for calibrating the company's most precise equipment and checking their reference artifacts.

          From there, you were one step removed from the national standards laboratories. At that level the formal scientific definitions are just fine.

          In other words, you work from major standards labs that can use the precise definitions effectively, and propagate the information (with some less, but little enough to be acceptable for the application in question) to more widely distributed testing facilities. A more trendy application of the same basic idea is the use of Internet-based real time clock services.

        • Re:How about ... (Score:3, Insightful)

          by bleckywelcky (518520)
          Well, your watch manufacturer is simply producing watches for the public. To an individual person it really doesn't matter if they are ahead or behind in their day by 15 or 20 seconds ... maybe even +/- 5 or 10 minutes. So to them, a watch that can hold time to within 5 or 10 seconds over the course of a month or so is just fine. I'm not sure how accurate a quartz watch can get, maybe it's even more like 5 or 10 seconds over a year or so?

          Anyways, it's one thing for a watch manufacturer to achieve a certain
      • by gotr00t (563828) on Saturday February 26, 2005 @09:59PM (#11791266) Journal
        Before laser technology, the meter was defined as the distance between two markings on a bar of platinum-iridium kept in Paris. It was after Michelson invented his interferometer that the meter was redefined as 1,650,763.73 wavelengths of a certain orange line in the spectrum of krypton-86. This was later redefined in the 80's to be in terms of C, the speed of light.

        As technology to measure substances to great precision increases, its about time the kilogram got a redefinition as well, one not based on a single object.

    • Re:How about ... (Score:4, Interesting)

      by Geoffreyerffoeg (729040) on Saturday February 26, 2005 @11:19PM (#11791772)
      Don't you mean 1 L H2O at STP?

      Adenosine triphosphate doesn't have much bearing on the mass of a quantity of water, even though it does provide biochemical energy to the physicists who're measuring it. :-)
  • Just wait. (Score:5, Funny)

    by jwcorder (776512) on Saturday February 26, 2005 @07:56PM (#11790215)
    The next thing you know they will be trying to get the US to switch from imperial units to the metric system....
    • by bobscealy (830639) on Saturday February 26, 2005 @08:29PM (#11790519)
      I have always found it intriguing that the US both celebrates the day of its independance from the British empire and continues to use old British units of measurement.
      • Re:Just wait. (Score:3, Informative)

        by Guppy06 (410832)
        Depends how you look at it. We call the things we use "pounds" and "feet," but we abandoned the flawed British standards in the 1890's (their yard shrank, their pound leaked), instead basing them on the SI standards. A pound is defined as 0.45359237 kg and a foot is 0.3048 m.*

        Also, we were the only ones sane enough to base our unit of volume/capacity on the cube of our linear standard (1 gal US = 231 in^3, as it's been since the 1800's or so). Both the British gallon and the SI liter both had ugly/cumbe
  • by WormholeFiend (674934) on Saturday February 26, 2005 @07:57PM (#11790222)
    I'm going back to pounds and stones.
  • by rollingrock (653505) on Saturday February 26, 2005 @07:58PM (#11790235)
    Pi is exactly equal to 3!
  • by tepples (727027) <tepples&gmail,com> on Saturday February 26, 2005 @07:58PM (#11790237) Homepage Journal

    You might find some additional background information about this effort in an earlier Slashdot article about this topic [slashdot.org], posted in May 2003.

  • Replacing the second while your at it, and the meter! Units based off of the earth.
    • Re:How about (Score:5, Informative)

      by be-fan (61476) on Saturday February 26, 2005 @08:07PM (#11790330)
      The second and the meter have long since been based off of more fundemental measures. The second is defined as how long it takes for 9,192,631,770 cycles of microwave light to be emitted by the hyperfne transition of cesium-133 atoms. The meter is defined as the distance traveled by light in a vacuum in 1/299,792,458 of a second.
  • by Resound (673207) on Saturday February 26, 2005 @07:59PM (#11790243)
    I thought one cc of water weighs one gram. Thus one litre of water weighs one kg. Am I wrong? This would certainly satisfy the criteria of natural phenomena vs. artifact, although I suppose that definition gets a trifle fuzzy when we start talking about measurements like picograms.
    • Sure, a gram is defined by a volume of water at a certain pressure and temperature. However, this is impracticable in many settings. Water changes density very readily. It is much simpler to define a gram in other terms that is close enough to the 1.0 g/1.0 ml H20 yet still is stable enough to use in experiments. From the article:

      For instance, it would improve the precision of certain electrical measurements 50-fold and would enable physicists to make more precise calculations in studying the fundamental

  • Hmm... (Score:4, Interesting)

    by ProudClod (752352) on Saturday February 26, 2005 @08:00PM (#11790258)
    Planck's constant would be a very elegant solution - it being the smallest possible quantity of energy, and of course, energy == mass * c^2
    • Re:Hmm... (Score:3, Informative)

      by rangek (16645)
      Planck's constant would be a very elegant solution - it being the smallest possible quantity of energy

      Huh? The units of Planck's constant are energy times time (eg., J s).

  • by pboyum (139114) on Saturday February 26, 2005 @08:01PM (#11790264) Homepage
    Picture of the International prototype kilogram:

    http://www1.bipm.org/utils/common/img/mass/prototy pe.jpg [bipm.org]
  • by shiafu (220820) on Saturday February 26, 2005 @08:03PM (#11790298)

    Lisa: Principal Skinner, how's your transportation project coming?

    Skinner: Not only are the trains now running on time, they're running on metric time! Remember this time people, 80 past 2 on April 47th. It's the dawn of a new enlightenment!
  • by physicsphairy (720718) on Saturday February 26, 2005 @08:03PM (#11790300) Homepage
    Well, count the number of atoms in the platinum-iridium alloy, and voila! You have your new definition! (without having to fuss with the traditionalists)

    Why the motivation for the change? The mass of subatomic particles have been given in kg for over a century. What exactly needs a more precisely reference of measurement? Physicists use their own units when it's convenient anyway. . . .

  • Bah (Score:3, Funny)

    by jlechem (613317) on Saturday February 26, 2005 @08:05PM (#11790317) Homepage Journal
    "My car gets forty rods to the hogshead, and that's the way I likes it!"

    A little offtopic but still revelant ;-)
  • Finally... (Score:3, Funny)

    by Jon Abbott (723) on Saturday February 26, 2005 @08:07PM (#11790334) Homepage
    I can rest at night, not thinking about plum-sized cylinders of platinum-iridium alloy.
  • by alw53 (702722) on Saturday February 26, 2005 @08:15PM (#11790400)
    What happens when the speed of light changes? [newscientist.com].
  • by hot_Karls_bad_cavern (759797) on Saturday February 26, 2005 @08:18PM (#11790420) Journal
    Meh, how about changing the size of a pint!? Huh? Yeah, who's with me on this one? i could certainly go for pints being larger ... 'specially around lunch time :)
  • My thoughts (Score:3, Insightful)

    by Raul654 (453029) on Saturday February 26, 2005 @08:42PM (#11790617) Homepage
    This actually came up in my high school physics class a few years back. Since then, I've given it some thought, and my best guess was to define a kilogram in terms of the deflection of a beam of light under the influence of gravity over a given distance. In other words, define it in terms of the deflect of a beam of light passing a kilogram point charge at a certain distance.
  • by tod_miller (792541) on Saturday February 26, 2005 @08:52PM (#11790708) Journal
    They are cleaning the definition, not the value.

    A new kilogram with equal an old kilogram. This will only make a difference to the history books and those who actually want to make thier own 'kilogram'.

    I can imagine how many 'net savvy drug runners are looking at this and thinking, 'shit, I have snorted too much coke, does this affect my business? whats a planck? oh man, Avocado constant? [sic]

    I say since the kilogram was an arbitrary measurement (in any definition) then why try and make it more formalised? I realised that celcius fit nicely with pure water at sea level freezing and boiling, and other measures have thier own basis (has the definitions have changed). Take my friend the meter. I always use the old skool definitions for rules of thumb.

    Year Definition
    1793 1 / 10 000 000 of the distance from the pole to the equator.
    1795 Provisional meter bar constructed in brass.
    1799 Definitive prototype meter bars constructed in platinum.
    1889 International prototype meter bar in platinum-iridium, cross-section X.
    1906 1 000 000 / 0.643 846 96 wavelengths in air of the red line of the cadmium spectrum.
    1960 1 650 763.73 wavelengths in vacuum of the radiation corresponding to the transition between levels 2p10 and 5d5 of the krypton-86 atom.
    1983 Length traveled by light in vacuum during 1 / 299 792 458 of a second.

    So you see, a meter was the same in all these cases, but they just wanted to act clever.

    The thing is, after world war 3, which measure will be easiest to revert to for a meter? trying to find scientist who can measure "Length traveled by light in vacuum during 1 / 299 792 458 of a second." or just comparing a brass stick with a length of wood while trying to build something using pre-existing specs (that you are relying on to build a post WW3 bridge). ;-)
    • The definitions are done for scientific accuracy as the ability to measure improves.

      Celcius is trickier than just temperatures of boiling and melting, because I think it must also declare the pressure too. The temperatures that water boils and freezes depends on air pressure. Kelvins are defined as divisions such that the range from absolute zero to the triple point of water is 273.16 kelvin. At least that doesn't depend on a the standard for pressure.

      The problem with a meter standard depending on a ph
  • Why not gravity? (Score:3, Interesting)

    by clambake (37702) on Saturday February 26, 2005 @11:17PM (#11791756) Homepage
    Why not define it in terms of gravity? i.e. 1kg of mass is equal to the mass of a perfect sphere of platinum that can accelerate from rest another equally sized perfect sphere of platinium placed 1 metere away by X m/s?

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