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The kilogram is the basic unit of mass in the metric system, but there’s a serious problem: the standard that defines how much mass a kilogram actually has isn’t reliable anymore...

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Sources:
http://www.npr.org/templates/story/story.php?storyId=112003322
http://www.bipm.org/en/bipm/mass/ipk/
http://news.bbc.co.uk/2/hi/science/nature/7084099.stm
http://ukma.org.uk/first-kg-prototype

QQs: Why isn't a Kilogram a Kilogram? 

The kilogram is the basic unit of mass in the metric system - the system of measurement used by almost every country in the world. But there's a serious problem: The standard that defines how much mass a kilogram actually has, isn't reliable anymore. 

After the French Revolution, the new government of France proposed a unit of mass based on the weight of water, replacing older units that were based on less reliable things like the weight of grain. In 1875, seventeen European countries signed a treaty agreeing to use it, and the kilogram went international. 

The newly created International Bureau of Weights and Measures, abbreviated the BIPM in French, got the job of creating a prototype kilogram to be the worldwide standard and making a copy for each member nation. They made a metal cylinder out of an iridium-platinum alloy chosen for its density, stability, and resistance to corrosion. The cylinder was completed in 1899, and it's still the official definition of the kilogram today. They stored it in a sealed vault in Paris and ever since, every few decades copies around the world have been weighed and compared against the prototype to make sure everything is still accurate. 

Then, about 30 years ago, scientists realized that something weird was going on: The masses of the original and the copies were starting to drift apart, and they didn't know why. It's possible periodic washings are removing tiny amounts of material from the surface of the prototype, or maybe pollutants from the air were incorporated into the cylinder when it was originally made, and are gradually escaping - no one really knows. We're not even sure whether the original is getting lighter or the copies are getting heavier.

So far the change is only about the weight of a single greasy fingerprint, but even that can be a problem for scientists who rely on precise measurements. The best solution would be to redefine the kilogram using universal constants, which the BIPM has already done for the other base units that make up the International System of Units. A meter, for example, is now officially the distance light travels in a specific fraction of a second. 

Today the race is on to do the same for the kilogram using the Planck constant, which is a quantum physics concept that relates a photon's energy to its frequency. The most promising approach involves a watt balance, a super-sensitive scale that measures an object's weight using electric current and voltage, both of which are already measured in units based on the Planck constant. The plan is to weigh an official kilogram cylinder using this watt balance, and use that to establish the official mass of a kilogram in terms of the Planck constant, forever. After that, scientists won't need to use unreliable physical kilogram weights anymore.

The BIPM's goal is to make a new definition official at their next conference in 2018. So hopefully, we'll soon be measuring mass in terms of photons. 

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