- 2184 - Measurements - Redefine the kilogram. Rather than basing the unit on this physical object, the measure is based on a fundamental factor in physics known as Planck's constant. This infinitesimally small number, which starts with 33 zeros after its decimal point, describes the behavior of elementary packets of light known as photons.
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------------------ 2184 - Measurements - Redefine the kilogram
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- From bathroom scales to medical lab balances, the mass standard is now based on a value that is “woven into the fabric of the universe.” Sealed under a trio of nested glass bell jars, a gleaming metal cylinder sits in a temperature-controlled vault in France. Dubbed “Big K“, this lonely hunk of platinum and iridium has defined mass around the globe for more than a century, from bathroom scales to medical lab balances.
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- Representatives from more than 60 countries voted to redefine the kilogram. Rather than basing the unit on this physical object, the measure will be based on a fundamental factor in physics known as Planck's constant. This infinitesimally small number, which starts with 33 zeros after its decimal point, describes the behavior of elementary packets of light known as photons.
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- The kilogram is one of seven base units in the International System of Units, which defines all other measurements. The other six base units are the meter, the second, the mole, the ampere, the Kelvin, and the candela. It's easy to overlook the importance of these units that ensure stability in manufacturing, commerce, scientific innovation, and more.
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- The metric system was conceived in the late 1700s as a way to make measurements “something for all times, for all people”. Many of these early metric units were based on things in nature, but, these proved impractical to use. For example, the meter was defined as 1/10,000,000 the distance from the North Pole to the equator, passing through Paris. The kilogram was the mass of a liter of distilled water at its freezing point.
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- In the intervening century, these physical objects have one by one been replaced with fundamental constants. The kilogram was the final holdout.
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- Big K got the job done. Scientists forged a series of copies for researchers around the world to use. Only three times in its nearly 130 years did researchers release Big K from its vault to compare the precious cylinder with its secondary standards.
- With each of these comparisons, scientists became increasingly concerned: Big K seemed to be losing weight. Compared to its copies, the tiny cylinder appeared to be getting progressively lighter. That, or its copies were getting progressively heavier. It's impossible say which, since Big K, by definition, is exactly one kilogram. Even if someone took a file and shaved off a corner, Big K would still weigh one kilogram, and kilograms around the world would have to adjust.
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- In total, Big K’s mass differs from its copies by about 50 micrograms. This is almost the mass of a grain of salt. And though this might not seem like a lot, it's a tremendous issue for exacting fields like medicine. This loss doesn't just affect mass, it affects any other units, like the Newton, which are defined in relation to mass.
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- To resolve this weight loss a redefinition of the kilogram and three additional units—the ampere, the kelvin and the mole were based on “invariants of nature.”
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- Two different possibilities for the kilogram emerged, both of which are tied to Planck's constant. The first is based on something known as a Kibble balance. It's a little like the classic beam balance, a bar with a hanging pan on either side.
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- To measure the weight of something, place a known mass on one side and the object of interest on the other. Thanks to the gravitational force, you can tell how much that object weighs in relation to the known mass.
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- For a Kibble balance, however, one of these pans is essentially replaced with a coil in a magnetic field. And instead of using a gravitational force to balance the mass, it uses an electromagnetic force. By comparing a mass with aspects of this electromagnetic force, scientists can make exacting measurements of Planck's constant.
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- The other solution is based on crafting another gleaming object: a perfect sphere of crystalline silicon-28. This idea is based on a constant known as Avogadro's number, which defines the number of atoms in a mole to be roughly 602,214,000,000,000,000,000,000. By counting the number of atoms in a silicon sphere that is exactly 1 kilogram, scientists can figure out Avogadro's number with extreme accuracy. That can then be converted to Planck's constant.
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- The final value of Planck's constant is unimaginably small:
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- 0.000000000000000000000000000000000662607015 meter-squared-kilograms per second.
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- With the two methods, scientists can now measure a kilogram with an uncertainty of one part in 100,000,000. This is a difference that is about a quarter of the weight of an eyelash. The new standard makes a world of difference for things like manufacturing car components, developing new drugs, and crafting scientific instrumentation.
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- November 23, 2018. An Index of recent Reviews is available.
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--------------------- Saturday, November 24, 2018 -------------------------
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