{"71878":{"#nid":"71878","#data":{"type":"news","title":"A Better Definition for the Kilogram?","body":[{"value":"\u003Cp\u003EHow much is a kilogram?  It turns out that nobody can say for sure, at least not in a way that won\u0027t change ever so slightly over time.  The official kilogram - a cylinder cast 118 years ago from platinum and iridium and known as the International Prototype Kilogram or \u0022Le Gran K\u0022 - has been losing mass, about 50 micrograms at last check.  The change is occurring despite careful storage at a facility near Paris.\n\u003C\/p\u003E\n\u003Cp\u003EThat\u0027s not so good for a standard the world depends on to define mass.\n\u003C\/p\u003E\n\u003Cp\u003ENow, two U.S. professors \u0027 a physicist and mathematician \u0027 say it\u0027s time to define the kilogram in a new and more elegant way that will be the same today, tomorrow and 118 years from now.  They\u0027ve launched a campaign aimed at redefining the kilogram as the mass of a very large - but precisely-specified - number of carbon-12 atoms.\n\u003C\/p\u003E\n\u003Cp\u003E\u0022Our standard would eliminate the need for a physical artifact to define what a kilogram is,\u0022 said Ronald F. Fox, a Regents\u0027 Professor Emeritus in the School of Physics at the Georgia Institute of Technology.  \u0022We want something that is logically very simple to understand.\u0022\n\u003C\/p\u003E\n\u003Cp\u003ETheir proposal is that the gram - 1\/1000th of a kilogram - would henceforth be defined as the mass of exactly 18 x 14074481\u003Csup\u003E3\u003C\/sup\u003E carbon-12 atoms.\n\u003C\/p\u003E\n\u003Cp\u003EThe proposal, made by Fox and Theodore P. Hill - a Professor Emeritus in the Georgia Tech School of Mathematics - first assigns a specific value to Avogadro\u0027s constant.  Proposed in the 1800s by Italian scientist Amedeo Avogadro, the constant represents the number of atoms or molecules in one mole of a pure material - for instance, the number of carbon-12 atoms in 12 grams of the element.  However, Avogadro\u0027s constant isn\u0027t a specific number; it\u0027s a range of values that can be determined experimentally, but not with enough precision to be a single number.\n\u003C\/p\u003E\n\u003Cp\u003ESpurred by Hill\u0027s half-serious question about whether Avogadro\u0027s constant was an even or odd number, in the fall of 2006 Fox and Hill submitted a paper to \u003Cem\u003EPhysics Archives \u003C\/em\u003Ein which they proposed assigning a specific number to the constant - one of about 10 possible values within the experimental range.  The authors pointed out that a precise Avogadro\u0027s constant could also precisely redefine the measure of mass, the kilogram.\n\u003C\/p\u003E\n\u003Cp\u003ETheir proposal drew attention from the editors of \u003Cem\u003EAmerican Scientist\u003C\/em\u003E, who asked for a longer article that was published in March 2007.  The proposal has so far drawn five letters, including one from Paul J. Karol, chair of the Committee on Nomenclature, Terminology and Symbols of the American Chemical Society.  Karol added his endorsement to the proposal and suggested making the number divisible by 12 - which Fox and Hill did in an addendum by changing their number\u0027s final digit from 8 to 6.  So the new proposal for Avogadro\u0027s constant became 84446886\u003Csup\u003E3\u003C\/sup\u003E, still within the range of accepted values.\n\u003C\/p\u003E\n\u003Cp\u003EFast-forward to September 2007, when Fox read an Associated Press article on the CNN.com Web site about the mass disappearing from the International Prototype Kilogram.  While the AP said the missing mass amounted to no more than \u0027the weight of a fingerprint,\u0027 Fox argues that the amount could be significant in a world that is measuring time in ultra-sub-nanoseconds and length in ultra-sub-nanometers.\n\u003C\/p\u003E\n\u003Cp\u003ESo Fox and Hill fired off another article to \u003Cem\u003EPhysics Archive\u003C\/em\u003E, this one proposing to redefine the gram as 1\/12th the mass of a mole of carbon 12 - a mole long being defined as Avogrado\u0027s number of atoms.  They now hope to generate more interest in their idea for what may turn out to be a competition of standards proposals leading up to a 2011 meeting of the International Committee for Weights and Measures.\n\u003C\/p\u003E\n\u003Cp\u003EAt least two other proposals for redefining the kilogram are under discussion.  They include replacing the platinum-iridium cylinder with a sphere of pure silicon atoms, and using a device known as the \u0027watt balance\u0027 to define the kilogram using electromagnetic energy.  Both would offer an improvement over the existing standard - but not be as simple as what Fox and Hill have proposed, nor be exact, they say.\n\u003C\/p\u003E\n\u003Cp\u003E\u0022Using a perfect numerical cube to define these constants yields the same level of significance - eight or nine digits - as in those integers that define the second and the speed of light,\u0022 Hill said.  \u0022A purely mathematical definition of the kilogram is experimentally neutral - researchers may then use any laboratory method they want to approximate exact masses.\u0022\u003C\/p\u003E\n\u003Cp\u003EThe kilogram is the last major standard defined by a physical artifact rather than a fundamental physical property.  In 1983, for instance, the distance represented by a meter was redefined by how far light travels in 1\/299,792,458 seconds - replacing a metal stick with two marks on it.\n\u003C\/p\u003E\n\u003Cp\u003E\u0022We suspect that there will be some public debate about this issue,\u0022 Fox said.  \u0022We want scientists and science teachers and others to think about this problem because we think they can have an impact.  Public discussion may play an important role in determining how one of the world\u0027s basic physical constants is defined.\u0022\n\u003C\/p\u003E\n\u003Cp\u003EHow important is this issue to the world\u0027s future technological development?  \n\u003C\/p\u003E\n\u003Cp\u003E\u0022When you make physical and chemical measurements, it\u0027s important to have as high a precision as possible, and these standards really define the limits of precision,\u0022 Fox said.  \u0022The lack of an accurate standard leaves some inconsistency in how you state results.  Having a unique standard could eliminate that.\u0022\n\u003C\/p\u003E\n\u003Cp\u003EWhile the new definition would do away with the need for a physical representation of mass, Fox says people who want a physical artifact could still have one - though carbon can\u0027t actually form a perfect cube with the right number of atoms.\n\u003C\/p\u003E\n\u003Cp\u003EAnd building one might take some time.\n\u003C\/p\u003E\n\u003Cp\u003E\u0022You could imagine having a lump of matter that actually had exactly the right number of atoms in it,\u0022 Fox noted.  \u0022If you could build it by some kind of self-assembly process - as opposed to building it atom-by-atom, which would take a few billion years - you could have new kilogram artifact made of carbon.  But there\u0027s really no need for that. Even if you built a perfect kilogram, it would immediately be inaccurate as soon as a single atom was sloughed off or absorbed.\u0022\n\u003C\/p\u003E\n\u003Cp\u003E\u003Cstrong\u003EResearch News \u0026amp; Publications Office\u003Cbr \/\u003E\nGeorgia Institute of Technology\u003Cbr \/\u003E\n75 Fifth Street, N.W., Suite 100\u003Cbr \/\u003E\nAtlanta, Georgia  30308  USA\u003C\/strong\u003E\n\u003C\/p\u003E\n\u003Cp\u003E\u003Cstrong\u003EMedia Relations Contact\u003C\/strong\u003E: John Toon (404-894-6986); E-mail: (\u003Ca href=\u0022mailto:jtoon@gatech.edu\u0022\u003Ejtoon@gatech.edu\u003C\/a\u003E).\n\u003C\/p\u003E\n\u003Cp\u003E\u003Cstrong\u003ETechnical contacts\u003C\/strong\u003E: Ron Fox (\u003Ca href=\u0022mailto:ron.fox@physics.gatech.edu\u0022\u003Eron.fox@physics.gatech.edu\u003C\/a\u003E) or Ted Hill (\u003Ca href=\u0022mailto:hill@math.gatech.edu\u0022\u003Ehill@math.gatech.edu\u003C\/a\u003E)\n\u003C\/p\u003E\n\u003Cp\u003E\u003Cstrong\u003EWriter\u003C\/strong\u003E: John Toon\n\u003C\/p\u003E","summary":null,"format":"limited_html"}],"field_subtitle":[{"value":"Scientists propose a precise integer number of carbon atoms"}],"field_summary":[{"value":"Two Georgia Tech emeritus professors - a physicist and mathematician - say it\u0027s time to define the kilogram in a new and more elegant way. They\u0027ve made a proposal to redefine the kilogram as the mass of a very large - but precisely-specified - number of carbon-12 atoms.","format":"limited_html"}],"field_summary_sentence":[{"value":"Georgia Tech profs offer new kilogram definition"}],"uid":"27303","created_gmt":"2007-09-21 00:00:00","changed_gmt":"2016-10-08 03:03:24","author":"John Toon","boilerplate_text":"","field_publication":"","field_article_url":"","dateline":{"date":"2007-09-21T00:00:00-04:00","iso_date":"2007-09-21T00:00:00-04:00","tz":"America\/New_York"},"extras":[],"hg_media":{"71879":{"id":"71879","type":"image","title":"Ronald F. Fox","body":null,"created":"1449177414","gmt_created":"2015-12-03 21:16:54","changed":"1475894644","gmt_changed":"2016-10-08 02:44:04"},"71880":{"id":"71880","type":"image","title":"Theodore P. Hill","body":null,"created":"1449177414","gmt_created":"2015-12-03 21:16:54","changed":"1475894644","gmt_changed":"2016-10-08 02:44:04"},"71881":{"id":"71881","type":"image","title":"Computer-generated International Prototype Kilogra","body":null,"created":"1449177414","gmt_created":"2015-12-03 21:16:54","changed":"1475894644","gmt_changed":"2016-10-08 02:44:04"}},"media_ids":["71879","71880","71881"],"related_links":[{"url":"http:\/\/www.americanscientist.org\/template\/AssetDetail\/assetid\/54773","title":"American Scientist article"},{"url":"http:\/\/www.math.gatech.edu\/~hill\/","title":"Theodore P. Hill"},{"url":"http:\/\/www.physics.gatech.edu\/people\/faculty\/rfox.html","title":"Ronald F. Fox"},{"url":"http:\/\/arxiv.org\/abs\/0709.2576v1","title":"Physics Archive article"},{"url":"http:\/\/www.cnn.com\/2007\/TECH\/science\/09\/12\/shrinking.kilogram.ap\/index.html","title":"CNN.com article on kilogram"}],"groups":[{"id":"1188","name":"Research Horizons"}],"categories":[{"id":"145","name":"Engineering"},{"id":"135","name":"Research"},{"id":"150","name":"Physics and Physical Sciences"}],"keywords":[{"id":"7541","name":"kilogram"},{"id":"5340","name":"mass"},{"id":"169415","name":"Standard"}],"core_research_areas":[],"news_room_topics":[],"event_categories":[],"invited_audience":[],"affiliations":[],"classification":[],"areas_of_expertise":[],"news_and_recent_appearances":[],"phone":[],"contact":[{"value":"\u003Cstrong\u003EJohn Toon\u003C\/strong\u003E\u003Cbr \/\u003EResearch News \u0026amp; Publications Office\u003Cbr \/\u003E\u003Ca href=\u0022http:\/\/www.gatech.edu\/contact\/index.html?id=jt7\u0022\u003EContact John Toon\u003C\/a\u003E\u003Cbr \/\u003E\u003Cstrong\u003E404-894-6986\u003C\/strong\u003E","format":"limited_html"}],"email":["jtoon@gatech.edu"],"slides":[],"orientation":[],"userdata":""}}}