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Kilogram Gets Controversial; Why Not Split the Difference? 520

Posted by timothy
from the try-a-different-scale dept.
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."
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Kilogram Gets Controversial; Why Not Split the Difference?

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  • 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:Impossible (Score:5, Interesting)

    by mike260 (224212) on Sunday January 30, 2011 @07:28PM (#35051712)

    I think the GP's point was that even if you chopped a sizeable chunk off it, it would still weigh precisely 1kg. It logically follows that the universe's weight, expressed in kg, would suddenly jump upwards by a very large amount.

  • 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.

  • Re:Impossible (Score:3, Interesting)

    by o'reor (581921) on Sunday January 30, 2011 @07:43PM (#35051818) Journal
    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.
  • 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

  • Stupid definition (Score:1, Interesting)

    by Anonymous Coward on Sunday January 30, 2011 @08:10PM (#35051978)

    The SI unit is officially defined as the weight of a 130-year-old platinum-iridium cylinder in France.

    Where will we get a 130-year-old platinum-iridium cylinder next year? And the year after that? I wonder how they got hold of a 130-year-old platinum iridium cylinder 130 years ago?

  • Re:Does it matter? (Score:3, Interesting)

    by 10101001 10101001 (732688) on Sunday January 30, 2011 @08:53PM (#35052242) Journal

    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 understand the underlying physics well enough. I'd imagine it might be a bit of both (that a lack of understanding on the underlying physics results in incorrect methodology).

    To that end, the fact that a scientist is trying to sort of whitewash the discussion to get out a set definition of a kilogram is disturbing to me. It'd be, in my opinion, possibly in the same vein as redefining light to avoid the clear particle/wave duality. If anything, I would hope that this discrepancy of results would spark even more research because it opens up a great opportunity to better understand our universe. Worrying about setting the definition of a kilogram seems a bit more moot, especially if it turns out that such a thing is impossible using the tools being suggested (ie, that one of the metrics isn't cosmologically fixed).

  • 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 TheoMurpse (729043) on Monday January 31, 2011 @01:41AM (#35053766) Homepage

    the French

    Ceci n'est pas une kg.

Nothing succeeds like success. -- Alexandre Dumas

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