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.'"
Re:BASE16 (Score:2, Informative)
Computers use base 2, humans use base 10.
Did the OP even read the NIST doc? (Score:5, Informative)
Re:BASE16 (Score:2, Informative)
You're almost right. Base 16 (hexadecimal) happens to be a convenient way for humans to do base 2 math. Any programmer worth his salt can do hex math in his head. ;)
Re:Get rid of the artifact? (Score:3, Informative)
The goal is to use a single atom as the artifact. Atoms (of a specific isotope) are always *exactly* the same, so there's no concern about variations in the weight of the artifact over time.
So all you've got to do is build an object with a mass as close to 1 kilogram as possible, precisely count the number of atoms it contains, and then make a definition like:
"A Kilogram is defined as the mass of 5.018451 x 10^22 atoms of Carbon 12".
The difficulty is precisely counting the number of atoms in a macroscopic object: the Avogadro Project has been working on this for years.
This is similar to how 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."
Kilogram is a mass not a weight (Score:5, Informative)
Newton is a weight. The summary (and the Fox article) are incorrect, while the NIST article correctly refers to the reference mass.
MJC
Headline is sensationalist (Score:5, Informative)
and misses the point. The variability of the kilogram standard is a scientific and engineering concern, not a political one.
Wikipedia discusses the issue: http://en.wikipedia.org/wiki/Kilogram#Proposed_future_definitions [wikipedia.org]
In a nutshell - in order to create 1 kilogram physical standard masses, you have to first know what a kilogram IS. The physical standards referred to in the article do not appear to have retained constant mass over time. You can't define a constant based on something that is variable, so the current masses are (as I understand it) acknowledged to be an inadequate basis for the definition of the unit. The problem arises when you try to pick something to define it with that is both stable (i.e. a fundamental property of the natural laws of the universe) and practical (can actually create one to use as a practical mass standard against which you can prepare working standards.)
From articles that have popped up about this over the years, my guess is they will have to pick something as a basis and then work on various practical techniques to get as close to that ideal as possible - the question is what specifically to pick. N Carbon atoms? N Si atoms? What are the pros and cons when trying to physically create something that represents those numbers? How stable will a standard created according to a chosen standard be over time? (I.e., how often to we have to make new master standards? It's an important question - obviously the existing masses were not chosen with the expectation that their mass would vary with time, so how do we know to trust a given solution?)
So it's not the US objecting to the kilogram as a unit, but rather concern over the methods used to DEFINE the unit. That's something quite rational, not specific to the USA, and of scientific interest. Editors, how about changing the title to "US to Propose New Method of Defining a Standard Kilogram" instead?
Re:Did the OP even read the NIST doc? (Score:3, Informative)
Re:Get rid of the artifact? (Score:5, Informative)
Ain't no such thing, or else something as basic as an electronic weigh scale wouldn't work. To rephrase: solid metals are compressible enough to measure the effect (strain) due to very reasonable external loads -- you'd think that liquids would be, too. And yes, they are.
Bulk modulus of steel, commonly strain gaged in weigh scales: ~160 GPa
Bulk modulus of water: ~2.2 GPa.
Water is on the order of 100 times more compressible than steel. Yet steel's and similar metals' compressibility (modulus) is routinely used in measurement applications!
Now to give you an idea of how compressible metals are: a soft iron sphere with a single strain gage bonded to it will give you, IIRC, 1m depth resolution if you hook it up to reasonable digital strain meter. I did the math once on Yahoo Answers somewhere, don't have the link handy.
Don't believe all they tell you in grade school for a lot of it is bullshit.
Re:Get rid of the artifact? (Score:2, Informative)
Re:Get rid of the artifact? (Score:5, Informative)
The entire point of redefining the kilogram would be to allow any sufficiently-technical laboratory to make their own mass. Right now, there are forty artifacts that must be kept safe. If you do not have one of these artifacts, you in fact have no way to determine what your kilogram actually is. Hell, the artifacts probably do not even have the same mass as each other. So they are proposing to replace a few sets of metal with an instruction manual on how anyone with the right technology can make their own reference weight. That's a huge difference.
Re:Speaking as a metric man (Score:3, Informative)
http://en.wikipedia.org/wiki/Properties_of_water#Density_of_water_and_ice [wikipedia.org]
good luck measuring that to any degree of accuracy beyond three or four decimal places. just because you know how much of something you SHOULD have, does not mean you know how much you DO have.
Re:Question... (Score:3, Informative)
international avoirdupois pound [wikipedia.org] is set to exactly 0.45359237 kilograms... or is it? I guess that's why the US cares if the kilogram loses weight.
Re:Get rid of the artifact? (Score:5, Informative)
Re:Get rid of the artifact? (Score:2, Informative)
So here are natural units [wikipedia.com]
Planck units do not use any prototypes at all, not even elementary particles, but instead rely exclusively on fundamental constants. There are also several other similar systems, all of them non-circular, some are even in use.
Re:Get rid of the artifact? (Score:3, Informative)
A liter is now officially defined as 1 cubic decimeter, which makes the comparison to water only approximate.
Re:Get rid of the artifact? (Score:5, Informative)
Essentially a sphere will be created of a specific isotope of silicon and a specific diameter. This sphere will have a known number of atoms. This is superior not only because of degradation of a physical standard, but also because it will be easier to create a standard from basic principles using appropriate lab equipment.
The US is quite late in it's objection as the problem has been known and accepted for many years. TIme and distance is essentially measured with light, and only the kilogram still has a physical representation.
It is probably a simple matter for the US to accept the new standard.
Re:Ben Franklin & precision (Score:3, Informative)
we were very close to converting. Reagan killed that, because he need to make it look like he was cutting spending. Fucking douche.
Re:Get rid of the artifact? (Score:5, Informative)
The Avogadro project (the thing in your link) has been going on since 2007.
The NIST (the U.S. measurements standards body) provided an implementation of another possible solution to the problem in April of 2007.
To say that the U.S. is just now objecting is inaccurate.
To say that the U.S. is late in its objection ignores the fact that the U.S. has been working on the problem with international standards bodies for many years.
What (unsurprisingly) the Fox News article gets wrong is that the NIST is not submitting a formal objection.
The Consultative Committee for Units (one of the advisory groups for CIPM), of which the NIST is a member, has submitted a formal resolution to change the definition to the CIPM. The CIPM is about to submit that resolution to the CGPM, which is the international body that regulates these definitions.
Re:Get rid of the artifact? (Score:3, Informative)
Hell, the artifacts probably do not even have the same mass as each other.
They don't. There's variances between all the official copies which have changed over time: https://secure.wikimedia.org/wikipedia/en/wiki/File:Prototype_mass_drifts.jpg [wikimedia.org] Though technically there is only one official kilogram so even if all the others trend one direction the official answer will be that they are all wrong.
Re:Get rid of the artifact? (Score:3, Informative)
The Poisson ratio of water is 0.499975, which is not nearly close enough to 0.5 to be acceptable.
Re:Get rid of the artifact? (Score:3, Informative)
The meter has already been defined in terms of the speed of light. The circle stops there.