1427 lines
112 KiB
Text
1427 lines
112 KiB
Text
<!DOCTYPE html>
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<html lang="en" dir="ltr" prefix="content: http://purl.org/rss/1.0/modules/content/ dc: http://purl.org/dc/terms/ foaf: http://xmlns.com/foaf/0.1/ og: http://ogp.me/ns# rdfs: http://www.w3.org/2000/01/rdf-schema# schema: http://schema.org/ sioc: http://rdfs.org/sioc/ns# sioct: http://rdfs.org/sioc/types# skos: http://www.w3.org/2004/02/skos/core# xsd: http://www.w3.org/2001/XMLSchema# ">
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<h1 class="nist-page__title">One of the World’s Roundest Objects Is Helping to Build a Better Mass Measurement</h1>
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<div class="daterange"><time datetime="2024-07-17T12:00:00Z">July 17, 2024</time>
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<span>By: </span>
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<a href="/blogs/taking-measure/authors/megan-king" hreflang="en">Megan King</a>
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<p>Right now, the U.S. and Germany have different approaches to defining, or “realizing” as the researchers call it, the kilogram. Here, NIST researcher Darine Haddad looks on as German researcher Beatrice Rodiek places a sphere on NIST’s highly accurate weighing machine, known as the Kibble balance. The goal of the research is to get closer to the same definition using the different methods.</p>
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<div class="nist-image__credit">
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<span class="nist-image__credit-label">Credit:</span>
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B. Hayes/NIST
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<p>In 2018, the world agreed to redefine the kilogram. Instead of <a>being pegged</a> to the mass of a <a href="https://www.vox.com/science-and-health/2018/11/14/18072368/kilogram-kibble-redefine-weight-science"><span>physical object in a vault in France</span></a>, the kilogram is now defined by a few fundamental constants in nature. </p><p>This means researchers no longer have to worry about a physical object that can decay or change over time, potentially throwing the world’s measurements into chaos. </p><p>Like any unit of measurement, the kilogram needs to be the same for everyone, all the time. Theoretically, it can now be measured in any lab in the world using the <a href="/education/what-planck-constant" data-entity-type="node" data-entity-uuid="4cd05746-a8f5-406c-a393-182eeaa9137c" data-entity-substitution="canonical" title="What Is the Planck Constant?">Planck constant</a> <span>—</span> which defines how small things can be. That measurement can be shared with anyone who needs to find out the mass of an object.</p><p>But in reality, it’s not quite that simple <span>—</span> yet. </p><p>The transition from using a physical object as the kilogram definition to a natural constant has meant differing approaches to defining (or “realizing,” as researchers call it) the kilogram for different countries. </p><p>Countries have their own approaches to the definition, and there’s a slight variation in their measurements. Ideally, everyone would get the same result within a reasonable range, no matter what method they actually use to take the measurement.</p><p>The <a href="https://www.bipm.org/en/"><span>International Bureau of Weights and Measures (BIPM)</span></a><span> currently distributes a consensus value that countries use to define the kilogram</span>. Once we get to the point that various approaches to the definition don’t result in a significant lack of agreement in everyone’s results, BIPM will no longer have to distribute this value of the kilogram. </p><p>So NIST researcher Darine Haddad, <a>along with</a><span> </span> Beatrice Rodiek from Germany’s national measurement science institute, known as <a href="https://www.ptb.de/cms/en.html">the <span>PTB</span></a>, and their colleagues, are working together to figure out why the approaches are leading to different outcomes. </p>
|
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<figure class=" align-center nist-image--lightbox nist-image" role="group">
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<div data-lightbox="https://www.nist.gov/sites/default/files/styles/2800_x_2800_limit/public/images/2024/07/03/DSC_5293_web.jpg?itok=V2lpc6Sw" data-media-id="711226">
|
||
|
||
<img alt="A researcher uses tweezers to adjust a white pad resting on a silvery sphere sitting in a special holder on a lab table. " height="641" loading="lazy" src="/sites/default/files/styles/960_x_960_limit/public/images/2024/07/03/DSC_5293_web.jpg?itok=Zo0tI0PJ" typeof="foaf:Image" width="960">
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<figcaption class="nist-image__caption">German researcher Beatrice Rodiek prepares “the sphere” — one of the roundest objects on Earth — to be measured on a highly accurate weighing machine, known as the Kibble balance, at NIST’s campus in Gaithersburg, Maryland.<div class="nist-image__credit">
|
||
<span class="nist-image__credit-label">Credit:</span>
|
||
|
||
B. Hayes/NIST
|
||
|
||
</div></figcaption>
|
||
</figure>
|
||
|
||
<p>Right now, the U.S. has an exquisitely accurate weighing system, known as a <a href="/si-redefinition/kilogram-kibble-balance" data-entity-type="node" data-entity-uuid="0a3d327a-501f-4e5a-9690-4c81fbb99ec2" data-entity-substitution="canonical" title="Kilogram: The Kibble Balance"><span>Kibble balance</span></a><span>,</span> that uses quantum mechanics to precisely measure mass to get the official definition of the kilogram. Germany, on the other hand, uses a method that means, in simple terms, “counting atoms,” as Rodiek explains. It’s called the XRCD method. <span> </span></p><p>Each approach gets to a slightly different definition of the kilogram. The variations in their results are tiny <span>—</span> about 50 micrograms, which is the approximate mass of a grain of salt. </p><p>So, a 50-microgram difference in measuring a kilogram is a bit like weighing a pineapple on two scales, and your measurements only being off from each other by about a grain of salt! You’d think that’s close enough, but Haddad and Rodiek are trying to close that tiny gap. </p><p>“If you have two completely different approaches measuring the same object, and you get the same results, you can trust them,” Haddad says. “Once we get to that point, everyone can directly measure the unit of mass with the primary definition. These are small gaps we’re talking about here.” </p><h2>A Sphere of Measurement Influence </h2><p>Rodiek recently traveled to NIST’s campus in Gaithersburg, Maryland, to run a series of measurements and tests to see how the two countries’ approaches may be able to get closer to the same results. </p><p>Rodiek brought along one of the <a href="https://www.youtube.com/watch?v=ZMByI4s-D-Y">roundest objects on Earth</a><span>—</span> a perfectly manufactured silicon sphere. <span> </span></p><p><span>First, Rodiek had to follow a special procedure to clean the sphere, so the researchers could avoid dust and other minute deposits on the mass of the sphere affecting the measurements. Then she very carefully walked it to the Kibble balance and placed it on a robot arm. </span></p><figure class=" align-center nist-video--vertical nist-video" role="group">
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<div class="video-embed-field"><div class="video-embed-field-lazy" data-video-embed-field-lazy="
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||
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|
||
"><img alt="Building a Better Mass Measurement with a Sphere and the Kibble Balance" loading="lazy" src="https://cdnsecakmi.kaltura.com/p/684682/sp/68468200/thumbnail/entry_id/1_6kbj1ufl/width/1280" typeof="foaf:Image">
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<button class="video-embed-field-lazy-play"></button>
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<div class="video-embed-field-lazy-title">Building a Better Mass Measurement with a Sphere and the Kibble Balance</div>
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<figcaption class="nist-image__caption">Like any unit of measurement, the kilogram needs to be the same for everyone, all the time. But different countries have their own ways of defining the kilogram (or “realizing,” as researchers call it). NIST researchers and their German counterpart are working on experiments to see how they can bridge the very tiny gap in the two countries’ measurement approaches.</figcaption>
|
||
</figure>
|
||
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||
<p>Haddad tried to run the Kibble balance experiment … but there were challenges along the way. The main issue was that the Kibble balance had not previously measured a spherical-shaped object and had to be adjusted to take that measurement. </p><p>Each time the researchers had to tweak the Kibble balance, the sphere had to be cleaned and gently put back in place. It was a tedious process that took up most of Rodiek’s weeklong visit to NIST in April. </p><p>Rodiek and the sphere have since returned to Germany. The sphere travels in its own protective case with Rodiek as her carry-on, never in checked luggage. The cargo is too precious to jostle. </p><p>“Sometimes it’s a little scary traveling with it in the airplane. You never know if we’re going to hit some turbulence, and it’s just sitting there. Luckily, it’s pretty robust, and we can always clean it,” Rodiek says. </p>
|
||
<figure class=" align-center nist-image--lightbox nist-image" role="group">
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||
<div data-lightbox="https://www.nist.gov/sites/default/files/styles/2800_x_2800_limit/public/images/2024/07/03/DSC_5393_web.jpg?itok=tI4CA-vH" data-media-id="711231">
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<img alt="The faces of two researchers in lab coats are reflected in a silvery sphere that is being held inside a larger scientific device. " height="641" loading="lazy" src="/sites/default/files/styles/960_x_960_limit/public/images/2024/07/03/DSC_5393_web.jpg?itok=aacIx270" typeof="foaf:Image" width="960">
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</div>
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<figcaption class="nist-image__caption">This perfectly measured silicon sphere is one of the roundest objects on Earth — and it’s helping researchers figure out how to best calculate the value of the kilogram.<div class="nist-image__credit">
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<span class="nist-image__credit-label">Credit:</span>
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B. Hayes/NIST
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</div></figcaption>
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</figure>
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<p>Haddad and Rodiek are planning another series of experiments and, hopefully, will be able to make all needed measurements, without the technical difficulties. </p><p>“I’m really excited to come back. I’m planning another trip when we’ll have a bit more time to do these measurements,” Rodiek said. </p><h2>Why More Research on the Kilogram Is Needed</h2><p>You may be wondering why all this effort is required to measure something accurately within a grain of salt. I wondered that, too. I asked Haddad: Why is this even necessary?</p><p>In addition to living up to the spirit of the redefinition of the kilogram <span>—</span> that anyone in any lab should be able to define the kilogram as accurately as possible <span>—</span> Haddad said we have no idea what we might learn from these experiments. </p><p>And as a scientist, that’s exciting for her. Researchers need to understand why this gap in the measurements is occurring, and who knows what we might learn from that.</p><p>“For researchers, science is very interesting for us, and we just want to understand why. Is there a scientific reason?” she said. “The knowledge we get from these experiments, discovering why the errors happen, can help us improve measurement science going forward.”</p>
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<h3 class="nist-teaser__title"><a href="/blogs/taking-measure/authors/megan-king"><span>Megan King</span>
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</a></h3>
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|
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<div class="nist-teaser__content">
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||
<div class="text-with-summary"><p>Megan King is a writer-editor at NIST and edits the Taking Measure blog. After graduating from John Carroll University, she began her career as a newspaper journalist, covering county fairs and school board meetings. Megan has worked in various communications roles in the government for more than 15 years, including as a content manager and strategist. Outside of work, Megan coaches beginner ice skaters, cheers on Pittsburgh sports teams, and knits.</p></div>
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||
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<h3 class="nist-teaser__title"><a href="/blogs/taking-measure/safer-skies-fewer-fires-10-ways-nist-impacts-your-daily-life"><span>From Safer Skies to Fewer Fires: 10 Ways NIST Impacts Your Daily Life</span>
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<div class="daterange"><time datetime="2024-12-19T12:00:00Z">December 19, 2024</time>
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||
<div class="text-with-summary"> If you’ve ever had a medical test, flown on an airplane, or needed to check the time, NIST’s work affects your life every day.
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||
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||
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<span class="nist-comment__author"><span lang="" typeof="schema:Person" property="schema:name" datatype="">Eric Shirley</span></span>
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on
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<span class="nist-comment__date">August 23, 2024 9:25 AM</span>
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<div class="text-long"><p>It is wonderful that this article is keeping up interest in the (formally) 2019 re-definition of the SI. The high point of the story is that labs like NIST and PTB are continuing related work for the realization, and that they are doing so cooperatively and collaboratively! On a technical level, I was especially pleased to see the word "realization" included, for a very specific reason, described below, which is also a reason that we need to keep in mind. </p>
|
||
<p>The SI is defined (see the SI Brochure, 9th Edition) by seven defining constants. The kilogram and the seven other base units now follow from that, and so its definition is unique and independent of measurement. It's realization, in the sense that somebody can hold a mass that they say is some number of kilograms, with some uncertainty, is what has to be done in practice, and which is now a dissemination process producing a slightly different set of challenges than when the mass of the IPK in the BIPM was found to be fluctuating and hence a problematic definition. </p>
|
||
<p>A casual reader of the article might think that the Kibble balance and/or Si sphere will continue to define the kilogram or refine its definition. In a strict sense, they cannot. They can only confirm the accuracy of an artifact, a physical attempt to realize the kg (or part thereof) with a physical mass that one can "hold," so-to-speak. This distinction is quite fundamental.</p>
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