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United States Science

US Objects To the Kilogram 538

Velcroman1 writes "For 130 years, the kilogram has weighed precisely one kilogram. Hasn't it? The US government isn't so sure. The precise weight of the kilogram is based on a platinum-iridium cylinder manufactured 130 years ago; it's kept in a vault in France at the International Bureau of Weights and Measures. Forty of the units were manufactured at the time, to standardize the measure of weight. But due to material degradation and the effects of quantum physics, the weight of those blocks has changed over time. That's right, the kilogram no longer weighs 1 kilogram, according to the National Institute of Standards and Technology. And it's time to move to a different standard anyway. A proposed revision would remove the final connection to that physical bit of matter, said Ambler Thompson, a NIST scientist involved in the international effort. 'We get rid of the last artifact.'"
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US Objects To the Kilogram

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  • by XanC ( 644172 ) on Friday October 29, 2010 @03:44PM (#34066696)

    Last I heard, nobody had come up with a way to define mass without referring to an artifact. It seems easy but they all turn out to be circular.

  • by XanC ( 644172 ) on Friday October 29, 2010 @03:47PM (#34066742)

    Because this prototypical kilogram is what the definition of the pound is currently based on.

  • by Imabug ( 2259 ) on Friday October 29, 2010 @03:48PM (#34066764) Homepage Journal

    seriously, this is pretty old. physicists working in metrology have been working to redefine the kilogram for at least the last few decades

  • by sexconker ( 1179573 ) on Friday October 29, 2010 @03:48PM (#34066766)

    The US cares that much why? Its only a trade matter, as we still use primitave imperial measurements. Maybe if we had switched to metric like they had told us we were going to every year in grade school this would be a big deal, but right now, who cares?

    Because prices, taxes, tariffs, etc. care about pounds and kilograms. We still have a department of weights and measures, and they still do extremely important work. The fact that you don't ever notice any problems means they're doing their jobs.

  • by Max Romantschuk ( 132276 ) <> on Friday October 29, 2010 @03:49PM (#34066776) Homepage

    Funnily enough I never ever think of a kilogram as the weight of some standard weight in a vault somewhere. The only way I ever think about the kilogram is the weight of one liter of water. Also comes in handy when I'm calculating how much liquids I can afford to buy when shopping groceries, given that I often go to the store on foot for the exercise and have to make sure I can manage the haul back.

    So, um, does this all really matter? In practice, that is.

  • by jdgeorge ( 18767 ) on Friday October 29, 2010 @03:50PM (#34066786)

    Aren't they just proposing removing the dependence on the 1 kilogram cylinders?

    From the article:

    Physicists may scoff at the thought people allowed to walk among the living who don't know what a Planck value is. But all you need to know is, they're using it to determine the mass of one mole of silicon atoms.

    From there on, they'll theoretically be able to deduce a perfect kilogram and it won't have anything to do with lumps of metal ever again. /quote

  • by Anonymous Coward on Friday October 29, 2010 @03:51PM (#34066808)

    Nah, it's actually pretty easy. You say something like "one kilogram is the mass equivalent of the energy of 3.40812408 gazillion photons with a wavelength of 550.9466543 nanometers." The meter is already defined in terms of speed of light and the second, and the second is defined in terms of the natural frequency of the caesium-133 atom. So in the end, everything is defined in terms of the speed of light and the caesium atom, with no artifacts needed.

  • by Wyatt Earp ( 1029 ) on Friday October 29, 2010 @04:02PM (#34066980)

    The US was one of the original signatories to the treaty that defined the meter and started the BIPM which lead to the SI.

    All US weights and measures, no matter what standard they are on, come from the National Bureau of Standards which standardized on the metric system, as has the USGS (since the early 19th century).

    THe NBS has standard meter and kilograms that are copies of the originals kept in Paris, so the US has a valid reason to wonder about the new kilogram definition.

  • by kenj0418 ( 230916 ) on Friday October 29, 2010 @04:06PM (#34067070)

    Last I heard, nobody had come up with a way to define mass without referring to an artifact. It seems easy but they all turn out to be circular.

    kilogram: the amount of mass required to deflect a proton by X degrees at a distance of Y meters.

    I'm guessing X and/or Y would have to be quite small.

  • by Abcd1234 ( 188840 ) on Friday October 29, 2010 @04:10PM (#34067140) Homepage

    No, the new "lump of metal" will be a physical representation of the underlying definition, that being that a kg = the weight of X silicon atoms. No such precise definition exists for the current standard.

  • by goodmanj ( 234846 ) on Friday October 29, 2010 @04:10PM (#34067144)

    Very clever, Mr. Wittgenstein. Unfortunately shortly after you died we defined the meter in terms of the speed light travels in a certain amount of time, and abandoned the Paris standard meter. So one thing can be said for sure: the Paris standard meter is definitely *NOT* one meter long."

  • by Sique ( 173459 ) on Friday October 29, 2010 @04:16PM (#34067292) Homepage

    But you know that it depends on the actual structure of the silicon crystal how much X silicon atoms weigh?

  • Re:Whoosh! (Score:3, Interesting)

    by John Hasler ( 414242 ) on Friday October 29, 2010 @04:20PM (#34067362) Homepage

    > ...we still insist on using the pound over the kilogram.

    But we don't. The metric system has been legal for trade in the USA since 1866 and the official customary units have been based on it for almost as long. In 1975 it was official adopted by the Federal government for its use and in 1985 it was identified as the "preferred" system for trade. Most goods are labeled in both metric and customary units. It's just that, unlike other countries, the USA has not outlawed the use of customary units as we tend to prefer freedom of choice.

  • by Abcd1234 ( 188840 ) on Friday October 29, 2010 @04:22PM (#34067398) Homepage

    But you know that it depends on the actual structure of the silicon crystal how much X silicon atoms weigh?

    No, I don't know that at all. Please to explain.

  • by Abcd1234 ( 188840 ) on Friday October 29, 2010 @04:45PM (#34067764) Homepage

    Ahhh, thank you, very interesting!

    That said, all that means is that the reference mass will have a very slight divergence from the theoretical definition. That doesn't change the fact that the kilogram will *have* such a definition, which it currently doesn't.

  • by david.given ( 6740 ) <dg AT cowlark DOT com> on Friday October 29, 2010 @05:34PM (#34068360) Homepage Journal

    Oddly enough, back in about 1780, the US was desperate to switch to the new metric standard that was being developed by France.

    The reason why the US didn't go for it was the definition of the metre. Benjamin Franklin, who was a pretty good scientist when he wasn't being distracted by all this political nonsense, was unhappy with the French definition, which was a certain ratio of the Earth's circumference. The trouble with this is that not only is it practically unmeasurable, but it's not even a knowable value, as it changes depending on what you consider to be the Earth's surface. Franklin was aware that industry can always use as much precision as it can get. Events bore him out as the first metre artifact made turned out to be out by 0.2mm.

    Instead he advocated an alternate definition based on the swing of a pendulum of a fixed period. This was a knowable value; it could be theoretically calculated to as much precision as your definition of the second. As the second was at the time was based on the length of the average solar day it could be determined as precisely as you could build your telescopes, it was a much more useful definition.

    Unfortunately for complicated political reasons France was unwilling to go with this (possibly because their arch enemies, the British, were also considering a pendulum-based definition), so Franklin decided to stay with home-grown units rather than adopting the new metric system.

    So if Franklin had been just a little bit more convincing when addressing the committees in Paris, the US might have been one of the driving forces of metricisation, and maybe my web browser would have the word 'metre' in its spellchecker dictionary.

  • by anwaya ( 574190 ) on Friday October 29, 2010 @05:41PM (#34068448)

    And it depends on their respective type of forming a solid object how much they actually weigh. A crystal of n atoms is a little lighter than n atoms of the same isotope as a fluid or a gas.

    Lighter or has less mass? Fluid lead pretty much weighs nothing when it's in orbit.

    Less mass.

    Consider a sample of n particles as a crystal, and a sample of n similar particles as a fluid or gas. The atoms in the crystal have very little motion relative to each other, while the particles in the fluid (or gas) have a lot more motion. More motion means more mass, by a factor of 1/sqrt(1 - v^2 / c^2) times the rest mass (IIRC) for each particle. The sum over the sample for the crystal will be less than the sum over the sample for the fluid (or gas).

  • by Teancum ( 67324 ) <robert_horning AT netzero DOT net> on Friday October 29, 2010 @07:27PM (#34069468) Homepage Journal

    The problem has always been trying to decide upon what new standard might be the replacement, not necessarily that it needed to be replaced. There apparently were some objections to the Silicon sphere approach and there were some suggestions to try instead to use an electrical standard (so many watts & volts with a certain applied current that would exert a certain amount of force from which a mass standard could be derived) and some other approaches were considered. I'm not sure how many different ideas were presented, but it was more than a few and the Silicon sphere certainly isn't the only possible solution.

    The pound and the inch are defined as a matter of law in the USA in terms of metric units anyway (the inch is 2.54 cm exactly to as many decimal places as you want to make it) so accepting a new standard isn't going to be a problem. The question is if the larger international standards bodies are going to accept any new definition for the kilogram, of which the NIST is a part of those bodies... and which standard will apply.

  • by MaXintosh ( 159753 ) on Friday October 29, 2010 @07:44PM (#34069604)
    Before I went into science, I did automation engineering (think factories with robots). A lot of that is fluid power. And any person working with hydraulics worth their salt would tell you that depending on factors, most hydraulic system fluids will compress between .5 and 1 percent per thousand PSI.
    So, when I took physics, and they told me that fluids aren't compressible, I objected. The instructor told me that sure, fluids change volume, but only in weird pressures like near vacuums and absurd pressures. To which I pointed out most of the universe is a very low pressure, dotted with spots of exceptionally high pressures. STP ain't standard for anyone but people on the top of the crust of moderate mass rocky planets (and even then, not all of them).
  • by Anonymous Coward on Friday October 29, 2010 @08:06PM (#34069746)

    Are you saying a NIST article is more accurate than a Fox article? Who would have thunk it

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