Want to read Slashdot from your mobile device? Point it at m.slashdot.org and keep reading!


Forgot your password?
What's the story with these ads on Slashdot? Check out our new blog post to find out. ×
United Kingdom Science

Kilogram Gets Controversial; Why Not Split the Difference? 520

gbrumfiel writes "As Slashdot has noted, the kilogram has a problem. The SI unit is officially defined as the weight of a 130-year-old platinum-iridium cylinder in France. But the physical object appears to be getting lighter. Scientists want to replace the cylinder with a new standard based on Planck's constant, but two experiments designed to facilitate the switch keep coming up with different results. Now one researcher is proposing a solution: just average the two diverging experiments and use that value as the official definition. Not everyone thinks that averaging the two amounts to sound research: 'Deciding to just average these two results would be perfectly proper mathematics, but it would not be science,' says Michael Hart, a physicist at the University of Manchester, UK."
This discussion has been archived. No new comments can be posted.

Kilogram Gets Controversial; Why Not Split the Difference?

Comments Filter:
  • Impossible (Score:5, Funny)

    by camperdave (969942) on Sunday January 30, 2011 @07:11PM (#35051532) Journal
    The physical object cannot get lighter (less massive). By definition is 1kg no matter how much mass it has. The obvious conclusion is that the rest of the universe is getting heavier.
    • ...leaving traces. Over time, changes accumulate.

      And when you are measuring something at 9 digits behind the point - a little can be a lot.
      http://en.wikipedia.org/wiki/Kilogram#Stability_of_the_International_Prototype_Kilogram [wikipedia.org]

    • by SuperKendall (25149) on Sunday January 30, 2011 @07:21PM (#35051634)

      It turns out that France imposed a Mass Tax in the last few years which means the cylinder has to cough it up for the good of the state.

      On the plus (or more like the non-plus) side, the people of France are now looking fit & trim.

    • Re:Impossible (Score:4, Insightful)

      by ehrichweiss (706417) * on Sunday January 30, 2011 @07:25PM (#35051678)

      What I *still* don't get is why we moved away from the ORIGINAL definition of a gram which used to be the mass of 1 cubic centimeter of water. I've heard all the "because this type of measurement was more accurate", etc. explanations but it seems that now they have no idea how to get to where they were whereas(AFAIK) the mass of 1 cubic centimeter of water hasn't really varied. Anyone able to break this down into something that actually makes sense beyond the typical responses?

      • Re:Impossible (Score:5, Informative)

        by XanC (644172) on Sunday January 30, 2011 @07:27PM (#35051700)

        Such a definition is ultimately circular. The volume of water depends on pressure, which itself has a mass component.

        • Not dependent on pressure, temperature perhaps?

          https://secure.wikimedia.org/wikipedia/en/wiki/Properties_of_water#Compressibility [wikimedia.org]

          Regardless, the temp and pressure could be standardized...

          • by XanC (644172)

            The first paragraph of your link describes how water is compressible, and not only that, how the compressibility changes with pressure.

            You can't standardize pressure because to even define pressure you first have to define a kilogram. Circular.

          • Better yet, from that same page: The triple point. All you need is pure H20 and you have a reference point for temperature and pressure. You could work backwards from there to the definition of mass.

          • by tyrione (134248)
            Temperature and Pressure are directly dependent upon one another. Ideal Gas Law.


        • Re: (Score:3, Interesting)

          by o'reor (581921)
          There is such a thing as "standard conditions of temperature and pressure" (293.15 K, 101.325 kPa by the NIST) so it is possible to perform those measurements in similar conditions. And I guess my point on the hydrogen isotopes is moot too sincethere is such a thing as Vienna Standard Mean Ocean Water [wikipedia.org]. Duh.
          • by sjames (1099)

            Pressure is measured in terms of force/area. In turn, force is a measure of mass and acceleration. Now we're back where we started and still haven't a clue.

          • by Kjella (173770)

            1 pascal (Pa) = 1 N/m^2 = 1 kg/(m*s^2). So you can't define 101.325 kPa without first defining the kilogram.

            • by blueg3 (192743)

              Circular definitions aren't really a problem for anyone well-versed in algebra.

        • by MightyYar (622222) on Sunday January 30, 2011 @10:58PM (#35052946)

          Then they should base it on the pound, which of course is 96 Roman drams, which of course is 96 * 32 / 25 Greek drachma, which is of course 96* 32 / 25 * 6 obols, which is of course 96 * 32 / 25 * 6 * 12 grains of barley.

          Or maybe, since measurements were originally based on important items of trade, we should modernize that a bit and standardize based on a dozen iPhones.

      • by o'reor (581921)
        Well, it all depends on your water -- how much deuterium or tritium would you like with your regular hydrogen ?
      • It's supposed to be repeatably measurable. The best way of doing it is not just any random one that's exactly defined, but one that's easy to replicate.
      • by JAlexoi (1085785)
        Ironically the meter prototype got shorter, so when you calculate the mass 1 cm3 of water and multiply it by 1000 you still get the mass of the lighter kg block.
      • by blueg3 (192743)

        The mass of a cubic centimeter of water spans a relatively huge range, depending on the composition of the water and the circumstances under which it's measured. The composition of even distilled water varies, since both hydrogen and oxygen have a variety of isotopes, the ratios of which vary from one source of water to another.

        If you boil this down to a theoretical, idealized system (e.g., using a composition of water that's impossible to reproduce), you might as well base it on something more stable, like

    • Re:Impossible (Score:4, Informative)

      by mysidia (191772) on Sunday January 30, 2011 @07:32PM (#35051732)

      The physical object cannot get lighter (less massive). By definition is 1kg no matter how much mass it has.

      Actually... it can get lighter. Earth's gravitational field can get weaker as matter from earth is ejected or evaporates into space.

      It can also get lighter as Earth's atmosphere gets heavier, making it more buoyant in earth's atmosphere.

      That has nothing to do with how much mass the cylinder has, because MASS is not a measure of weight.

      Mass and weight are independent. Weight is due to forces applied to mass inside a gravitational field; if the field weakens or other forces are applied to the mass inside the field, the weight will decrease or increase without any change of mass.

      Earth's gravitational field and atmosphere is also not uniform, so there are places (or altitudes) you can bring the same object to, and it will be lighter or heavier, with its amount of mass being the same.

      • Earth's gravitational field and atmosphere is also not uniform, so there are places (or altitudes) you can bring the same object to, and it will be lighter or heavier, with its amount of mass being the same.

        A Local G [xkcd.com] effect. Pole vaulters be aware.

    • by MarkRose (820682)
      And here I thought it was the beer and donuts causing me to gain weight!
  • by paiute (550198) on Sunday January 30, 2011 @07:13PM (#35051564)

    A physicist, engineer and a statistician are out hunting. Suddenly, a deer appears 50 yards away.

    The physicist does some basic ballistic calculations, assuming a vacuum, lifts his rifle to a specific angle, and shoots. The bullet lands 5 yards short.

    The engineer adds a fudge factor for air resistance, lifts his rifle slightly higher, and shoots. The bullet lands 5 yards long.

    The statistician yells "We got him!"

    • by 140Mandak262Jamuna (970587) on Sunday January 30, 2011 @07:47PM (#35051842) Journal
      The statistician is right. Because if the deer has not moved between the first and the second shot, it is already dead. QED.
      • by ghmh (73679) on Sunday January 30, 2011 @10:13PM (#35052754)

        Not necessarily - everything is relative. For example, you have to also look at it from the deers frame of reference:

        A deer is wandering through the forest. Suddenly, a physicist, engineer and a statistician appears 50 yards away holding guns.

        The deer looks at them carefully and thinks - a physicist, an engineer and a statistician: I'd best just stand still.

        • by Xyrus (755017) on Monday January 31, 2011 @02:16AM (#35053912) Journal

          A deer is wandering through the forest. Suddenly, a physicist, engineer and a statistician appears 50 yards away holding guns.

          The deer thinks about this carefully for a moment. The likelihood that a physicist, engineer and statistician being able to form a cohesive group is unlikely. Forming a cohesive group in a forest, even less likely, and forming a cohesive group with guns involved practically improbable, as they most likely would have killed each other arguing over some pointless aspect or theory that the deer had worked out ages ago.

          The deer comes to the realization that the only way that this situation could be real is if some other being had concocted it as a piece of fiction in some alternate universe. The deer sighs and holds perfectly still as a shot goes long, and another goes short, and the third guy never shoots his gun, claiming the dear has been shot. The deer shakes its head sadly for the poor being who created this temporary reality, and for itself as he will have to endure this ritual for as many times as other beings invoke it. Eventually the deer will be allowed to return the ethereal pool of creation, where perhaps its next incarnation will be something more interesting, perhaps as the man from Nantucket or the woman from New Zealand.

    • A mathematican, an astrophysicist and a statistician were walking along a road in Scotland. They saw a black cow. The astrophysicist said, "All the cows in Scotland are black". The statistician said, "No, there is at least one black cow in Scotland". The mathematician said, "All we now know is, this side of that cow is black."
    • Re: (Score:2, Funny)

      by Phil06 (877749)
      A physicist believes that it takes extremely high pressure to produce diamonds. An engineer knows it just takes a little suction.
    • by Kral_Blbec (1201285) on Sunday January 30, 2011 @08:20PM (#35052036)
      Since a bullet's trajectory isn't very parabolic, landing 5 yards long would mean it passed through the deer.
      • Since a bullet's trajectory isn't very parabolic, landing 5 yards long would mean it passed through the deer.

        If a bullet hit the ground five yards past a deer, it went under his legs, not through him.

        • What if it was a very fat deer? : )

          (emoticon used to prevent pesky "insightful" mods)
          • by Chrisq (894406)

            What if it was a very fat deer? : )

            The physicist would be worried. If the deer were fat and made of neutron the bullet could take a hyperbolic course and come back and hit one of the party. The engineer would be worried - if they shot it how on earth would they drag it back to the car. The statistician would be looking around to see if he could find the very skinny deer that maintained the statistical average.

        • Or through one of his legs.
  • Does it matter? (Score:3, Interesting)

    by crow (16139) on Sunday January 30, 2011 @07:14PM (#35051578) Homepage Journal

    The differences are so minimal that I can hardly believe it matters. The only issue is if the difference between the new definition and previous measurements is statistically significant. If you can't show that that would be the case, then pick whatever number between the two measurements that is easiest to work with mathematically, perhaps one with the most zeros (in decimal, since the metric system is designed to work well with powers of 10).

    • Re:Does it matter? (Score:5, Interesting)

      by drolli (522659) on Sunday January 30, 2011 @07:45PM (#35051832) Journal

      Speaking as an experimental physiscist

      ahem. 175parts per billion is 1.75e-7. For metrology that is a huge discrepancy. What is worse is that the measurements themself are a factor of 5 better, leaving no room for error.

      For experiments where the physicists believe they understand them this is unacceptable, because it actually means the pysics of at least one method of both is not well enough understood, i.e. you have a systematic error. If the physics is not well understood then you don't know if the systematic error will be constant.

      If the measurement will not be constant then the average will also not be constant. So an metrology institute where a reference weight should be define will need both methods and still not get any stable definition.

      If you already need to afford both methods, then you can create reference weights and at the same time check if the difference between both methods is the right one and constant at your place.

      Important rule in experimental physics: NEVER average over systematic mistakes. Average over random results. On systematic mistakes, the word average makes no sense

      • by DCFusor (1763438)
        Hear, Hear! Yeah, it matters - a tiny error in m becomes a crazy error in e= mc^2. As well as everything else the parent mentions. It's not even good math to average if you have the least hint there's something missing other than purely random noise creating the differences -- that averages out, systemic errors don't.

        Besides, if someone learns why, well, that's more knowledge in our bag o' tricks, eh? And that is what science is really all about.

      • Re: (Score:3, Interesting)

        For experiments where the physicists believe they understand them this is unacceptable, because it actually means the pysics of at least one method of both is not well enough understood, i.e. you have a systematic error. If the physics is not well understood then you don't know if the systematic error will be constant.

        And that's the crux of the issue. Both results should be the same within the margin of error. The fact that they're not either indicates that the methodology is off or we simply don't unders

      • by Mashiki (184564)

        Maybe there's something more to it. We already know that depending gravity isn't a constant in the universe and can be effected by outside forces. It may be that the cylinder's weight is a true constant, but do to whatever be it something is causing mass to weigh less compared to 130yrs ago or even 10yrs ago.

        Personally I'd say it warrants more investigation beyond 'well lets weigh the average'.

  • by Sobieski (1032500) on Sunday January 30, 2011 @07:16PM (#35051588)

    Let them eat pounds!

  • The measure of length called a foot that we use for practical commerce was established in pretty much that way. See the story of the international foot [wikipedia.org] as differed from the different foots which were already in widespread use.
    • by sznupi (719324)
      Ultimately, a foot was established by defining it in meters / I don't think they aim for self-reference in this case ;p (or that there was much of it in the past - while exact value comes from the object in France, you can get to something damn close for most purposes from the size of this planet or properties of probably the most common chemical compound in the Universe)
  • Well, duh. (Score:5, Funny)

    by Black Parrot (19622) on Sunday January 30, 2011 @07:23PM (#35051652)

    Why don't they just take the weight of a gram and multiply it by 1024?

  • by PJ6 (1151747)
    I floated this idea years ago to a few physicists and they hated it for reasons I can't fathom. The whole idea of basing a unit on a single, random object instead of something universal seemed silly to me.
  • Is a kilogram in terms of fractions of an elephant please?

  • by fermion (181285) on Sunday January 30, 2011 @07:36PM (#35051766) Homepage Journal
    Science, and teaching science, is hard because it is often difficult to determine which are the truly salient facts, and what background is necessary.

    In this case the background is that the standard for mass, unlike time or distance, cannot independently be constructed in the lab. This means that science and industry are susceptible to two issues. The first is degradation of a physical standard, in this case a hunk of metal in France. The second is that one is dependent on other to create proxies of the standard, and as a result have no true assurance of the accuracy of the standard. A suitable lab with suitable personal can masure time and distance without the need of a proxy manufactured by others, and no dependence on a fixed physical object.. There is a desire for the same to be true for mass.

    Second, no one knows if the hunk of metal is shrinking, and if it is how much it is shrinking by. If the experts knew it was shrinking, then they could figure out how to at least partially correct it. The hunk of metal might not be charging at all, or it could be accreating matter. Without an independent standard, which does not apparently exists, as everything is based on the hunk of metal, all there is is guesswork.

    The third is the idea that Planck's Constant is being used to create the standard. In fact Planck's constant is one two approaches. The other is to create a sphere from a silicon and use Avagadro's Constant to define the mass. The problem is that these two approaches do no lead to consistant results, with an error about an order of magnitude large than the expected error.

    The issue with averaging is that while one does average within a result, and even results that are taken from similar procedures, it is unclear that averages in this case is suitable. It seems to me that the results point to an interesting area of research, and rather than just averaging, more work should be done understanding the inconsistency. If it is not random error, and not an artifact, then something really fascinating might be going on.

    • by tompaulco (629533)
      Well, they could put it on a scale and see how much mass it has. That would tell them if it is increasing or decreasing. Of course, that is somewhat faceticious, but really, do they actually use this sacred piece of metal for actually calibrating something else outside? My supposition is that they may have done so at some point in the past and now the other scales are now the practical standard used.
  • How do they determine the mass of their 1Kg reference?

    Is it simply by measuring the force it exerts when influenced by a gravitational force of 1G?

    If so, how do they measure to ensure that 1G is still the same acceleration that it was when the standard was introduced?

    Do they also allow for the fact that it is displacing a certain amount of air -- and therefore is subject to the forces of buoyancy that will tend to make it lighter, depending on air density, humidity, etc?

    While the predominant factor is the m

    • by sznupi (719324)
      Oh I'm sure they figured a thing or two after Newton... ;)
    • How do they determine the mass of their 1Kg reference?
      Is it simply by measuring the force it exerts when influenced by a gravitational force of 1G?

      Each national bureau of standards carts its standard kilogram over to France and compares it directly to the international standard kilogram with a balance. G is not involved.

      Unfortunately, each time they do this they get slightly different results. The difference between the international standard and the average of the national standards is increasing.

  • What's with all these science stories with country icons? First the "Atomic Disguise Makes Helium Look Like Hydrogen" is tagged as Canada, and now this is tagged as UK. Slashdot, make your story icons relate to the more relevant tags, like science.
  • So you are defining a somewhat universal (ok,at least, global) constant as something that should be there, but isnt?
  • Why don't they just count how many atoms are in it, and define the kilogram as the sum of the counts of each of the types of atoms making up the alloy?

    Time is defined in a similar way, and don't tell me we don't have the technology, IBM has been shoving individual atoms around for decades now. They could do it again, and this time it would be for a cause more useful than making tiny IBM logos.

    I'm not sure what the composition is, but I don't think there can be more than 6 * 10^24 atoms in it. Should be to

  • Here's an easy reference: Planck units.

    Just define everything in terms of Planck units and nothing will ever change.

  • The kg should nod be taken light-heartedly. Many other units depend on the kg. I say "keep sciencing" until a true solution emerges!
  • I've always said this whole metric system has been a farce from the get go. First the unit of length is based on a fraction of the circumference of the earth, only measured WRONG, and now we get that the reference mass has been changing with time putting the amount of kippers in a kilogram in doubt.

    I say we just scrap the whole thing and go to a more humanistic system based on things like the length of a man's stride etc. since obviously getting something accurate is just right out.

    • Re:Metric System (Score:5, Informative)

      by Michael Woodhams (112247) on Monday January 31, 2011 @12:00AM (#35053262) Journal

      I'm not at all sure you are serious, but enough people seriously hold this opinion that it is worth responding.

      A good system of units needs:
      1) Base units which are well defined and independently reconstructible (i.e. a suitably equipped lab can calibrate their equipment purely from the definition of the units.)
      2) Logically constructed compound units (e.g. units of force are derived from the units of mass, time and distance.)
      3) Logically constructed convenience units (e.g. kilometres for use for distances which would be an inconveniently large number of metres.)
      4) To be widely used.

      The initial choice of your base units is largely arbitrary - whether it was a from a not-very-accurate measure of a king's foot size or from a not-very-accurate measure of the Earth's circumference. Item (1) can be satisfied equally well (or, in the case of mass, badly) by the metric or imperial systems. The definition of the metre has long since changed from the size of the Earth to quantities measurable in a lab (as has the definition of the foot.)

      The SI system (based on metric measures) beats the imperial system hands down on items 2 and 3, and because of this now has a large advantage also on item 4.

      Item 2: In Imperial you might measure (heat) energy in BTU and mechanical energy in some mixture of foot-pounds-seconds, but then you need a conversion factor to compare the two. Such conversion factors are never needed in SI.

      Item 3: Imperial also messes up the convenience units by having lots of weird conversion factors (e.g. an acre is (I think) a furlong by a chain. How many square feet is that? How many ounces in a ton?*) Metric uses convenience units constructed from base units via consistently named factors of 10 or 1000.

      You can't use the current problems with the kilogram as a reason to prefer imperial to metric, as imperial will be just as prone to exactly the same problems. The (UK) Imperial pound is similarly defined by the mass of a unique artifact. In the US, it is defined relative to the kilogram. Mass is the last base unit which doesn't satisfy requirement (1), and the efforts to fix this are what has triggered this entire debate.

      One could go a step further, and define your fundamental units in terms of fundamental physical constants (i.e. the Plank mass, Plank time and Plank distance, charge on an electron, etc.) In such a system of units, the speed of light is 1, the formula for the energy of a photon doesn't need a constant in it etc. In practice, we can't use such a system, because we can't measure (in particular) the universal gravitational constant G with sufficient accuracy. Every time we got a better measure of G, our entire system of units would need to be updated. (I.e. with current technology, this system can't satisfy requirement (1) above.)

      * And how many different sorts of ounces and tons are there? It is quite a few.

  • Spacetime (Score:4, Interesting)

    by GrahamCox (741991) on Sunday January 30, 2011 @09:13PM (#35052360) Homepage
    Mass bends space-time, right? So why not define it as a certain amount of curvature - say the mass needed to bend a light beam in vacuo by some measurable amount, divided by a chosen constant to give 1kg according to the theory.
    • by jmv (93421)

      The whole point of changing a definition is to base it on something that's easier to measure to very high accuracy. We can't even measure the curvature caused by the Earth that accurately and even if we could, we don't know the mass of the Earth that accurately either.

    • You're aware of how impractical that is, right? The first experiment to prove that had to be on the astronomical scale. Einstein's experiment was to examine how the light from stars was bent when viewed close to the Sun, and was only possible to perform during a total eclipse. To define something that requires those kinds of distances and masses to observe when the precision required is so high seems unwise.

      The quality of instruments you'd need to determine how far a 1kg weight would bend a beam of photo

  • by martin-boundary (547041) on Sunday January 30, 2011 @09:37PM (#35052518)
    Pff, that's not math.

    Math is: When there's this room... with only one person in it... and then two people leave that room... now you have to wait until another person goes back in before it's actually empty.

  • by Sanat (702) on Sunday January 30, 2011 @10:23PM (#35052784)

    Mathematically I was under the impression that one kilogram is what exactly one liter of water weighs.

    Do not believe that the French developed the metric system for it is based on an ancient system of weights and measures based upon the time for Venus to move (transit) a particular distance across the sky. In those days a circle was divided into 366 degrees rather than 360 which matches the number of days in a year. The ancient clock system used then was more accurate than what we use today as well as the calender. Their system avoided the "leap year"

    This technique developed thousands of years ago combines both the avoirdupois pound and the metric system and is based on what is referred to as a "Megalithic inch".

    There is much substantiated already that ancient monuments such as Stonehenge were measured with an accuracy of 1/10000 of a millimetre.

    For further information check Amazon for "Civilization One" by Christopher Knight and Alan Butler. A very interesting book, I am about half through this very enlightening book.

    See what a pint, gallon, or bushel really is and how it was developed.

Related Links Top of the: day, week, month.

"I have not the slightest confidence in 'spiritual manifestations.'" -- Robert G. Ingersoll