The kilogram is forever changed.
Why the change is necessary?
Currently, the kilogram has a very simple definition: It’s the mass of a hunk of platinum-iridium alloy that’s been housed at the International Bureau of Weights and Measures in Sevres, France since 1889. It’s called the International Prototype Kilogram (a.k.a. Big K, or Le Grand K), and it has many copies around the world — including seven at NIST in Gaithersburg that are used to calibrate scales and make sure the whole world is on one system of measurement.
Sealed under a trio of nested glass bell jars, a gleaming metal cylinder sits in a temperature-controlled vault in the bowels of the International Bureau of Weights and Measures in Sèvres, France. No one is allowed to touch the weight standards even the keepers have to resist the urge to touch them, as it could contaminate them with oil of their skin.
Representatives from more than 60 countries voted during the 26th meeting of the General Conference on Weights and Measures in Versailles, France to redefine the kilogram and voted to change the definition of a kilogram, tying it to a universal constant in nature. The change will go into effect on May 20, 2019. "On that day, you won't see any change in our in our daily lives," Davis says.
How is this happening?
One important reason for the change is that Big K is not constant. It has lost around 50 micrograms (about the mass of an eyelash) since it was created. But, frustratingly, when Big K loses mass, it’s still exactly one kilogram, per the current definition. When Big K changes, everything else has to adjust. Or even worse: If Big K were stolen, our world’s system of mass measurement would be thrown into chaos. Rather than basing the unit on this physical object, henceforth, 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, in everything from the flicker of a candle flame to the twinkle of stars overhead.
“That fundamental constant is woven into the fabric of the universe,” says Stephan Schlamminger, leader of the National Institute of Standards and Technology team who, along with an international cohort of scientists, worked to refine Planck's constant for the kilogram redefinition. Most importantly, this value will remain the same for all time, no matter the location.
“Imagine a world where every time you traveled you had to use different conversions for measurements, as we do for currency,” Madhvi Ramani of the BBC explains. “This was the case before the French Revolution in the late 18th Century, where weights and measures varied not only from nation to nation but also within nations.”
Inspired by the revolution, scientists at the time wanted to start fresh on a new, consistent system of measurement, basing units not on arbitrary mandates from kings, but on nature. The goal was to create a system of measurement “for all time, for all people.”
Here’s how the kilogram will be defined in the International System of Units:
The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10-34 when expressed in the unit Js, which is equal to kg m2 s - 1, where the meter and the second are defined in terms of c and ∆νCs.
To resolve this weight loss, the General Conference on Weights and Measures unanimously passed a resolution in 2011 to redefine the kilogram and three additional units—the ampere, the Kelvin and the mole—based on “invariants of nature.” Since then, scientists around the world have raced to find a solution.
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, which is, in essence, a bar with a hanging pan on either side. 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.
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.
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
The final value of Planck's constant is unimaginably small: 0.000000000000000000000000000000000662607015 meter-squared-kilograms per second.
Sébastien Candel, president of the French Academy of Sciences concluded: "I hope that such will also be possible for many other issues for the world."
Source:- National Geographic
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