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The Kilogram Standard Loses Mass

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This will undoubtedly throw intrinsicists for a loop.

Kilo prototype mysteriously loses weight

PARIS - A kilogram just isn't what it used to be.

The 118-year-old cylinder that is the international prototype for the metric mass, kept tightly under lock and key outside Paris, is mysteriously losing weight — if ever so slightly. Physicist Richard Davis of the International Bureau of Weights and Measures in Sevres, southwest of Paris, says the reference kilo appears to have lost 50 micrograms compared with the average of dozens of copies.

"The mystery is that they were all made of the same material, and many were made at the same time and kept under the same conditions, and yet the masses among them are slowly drifting apart," he said. "We don't really have a good hypothesis for it."

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Of all the world's kilograms, only the one in Sevres really counts. It is kept in a triple-locked safe at a chateau and rarely sees the light of day — mostly for comparison with other cylinders shipped in periodically from around the world.

"It's not clear whether the original has become lighter, or the national prototypes have become heavier," said Michael Borys, a senior researcher with Germany's national measures institute in Braunschweig. "But by definition, only the original represents exactly a kilogram."

The kilogram's fluctuation shows how technological progress is leaving science's most basic measurements in its dust. The cylinder was high-tech for its day in 1889 when cast from a platinum and iridium alloy, measuring 1.54 inches in diameter and height.

At a November meeting of scientists in Paris, an advisory panel on measurements will present possible steps toward basing the kilogram and other measures — like Kelvin for temperature, and the mole for amount — on more precise calculations. Ultimately, policy makers from around the world would have to agree to any change.

Many measurements have undergone makeovers over the years. The meter was once defined as roughly the distance between scratches on a bar, a far cry from today's high-tech standard involving the distance that light travels in a vacuum.

One of the leading alternatives for a 21st-century kilogram is a sphere made out of a Silicon-28 isotope crystal, which would involve a single type of atom and have a fixed mass.

"We could obviously use a better definition," Davis said.

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I'm a bit skeptical of the idea that it's a big mystery. Surely even a microscopically tiny scratch or chip could easily account for the loss of 50 millionths of a gram.

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I'm a bit skeptical of the idea that it's a big mystery. Surely even a microscopically tiny scratch or chip could easily account for the loss of 50 millionths of a gram.

Don't worry, they'll think of that after having spent a couple of years studying the "mystery," financed by the taxpayers of course.

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I'm a bit skeptical of the idea that it's a big mystery. Surely even a microscopically tiny scratch or chip could easily account for the loss of 50 millionths of a gram.

The standard is never touched by anything that would do that.

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I'm a bit skeptical of the idea that it's a big mystery. Surely even a microscopically tiny scratch or chip could easily account for the loss of 50 millionths of a gram.

The standard is never touched by anything that would do that.

I should add that dust and cleaning could be a problem. Alternate ways of measuring mass are being considered.

See http://physics.nist.gov/News/TechBeat/9501beat.html

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I'm a bit skeptical of the idea that it's a big mystery. Surely even a microscopically tiny scratch or chip could easily account for the loss of 50 millionths of a gram

Don't worry, they'll think of that after having spent a couple of years studying the "mystery," financed by the taxpayers of course.

It's more likely the government team will blame it on global warming.

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Or maybe the masses aren't changing, just gravity is spontaneously changing in different places :D

(assuming they measure the mass by weighing it).

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Or maybe the masses aren't changing, just gravity is spontaneously changing in different places :D

(assuming they measure the mass by weighing it).

I thought of that, but I'm sure they would weigh both the standard and the comparison sample at the same time in the same place. I'm sure gravity is not completely uniform around the earth as the density of the earth varies throughout. Plus the moon would have an effect on gravity.

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I'm shocked that they still use the prototype to define a kg. We've moved to much more stable definitions in many other areas.

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I'm shocked that they still use the prototype to define a kg. We've moved to much more stable definitions in many other areas.

Both distance and time are related to frequency.

The meter is the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second.

A second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium atom.

If you can come up with a way to relate mass to that or some other property, it would be great. Even if mass could be related to some unchanging property or predefined physical constant (like x times Avogadro's number of atoms in a cubic meter of a specific element), that mass would still have to be constantly measured because any material object will change over time do to physical or chemical processes.

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If you can come up with a way to relate mass to that or some other property, it would be great.

Paul - I'm no phycisist. Are you saying that such a definition has been researched but not found?

Maybe a given number of moles of cesium atoms after a given number of seconds would see a change in mass that is a constant 1 gram?

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If you can come up with a way to relate mass to that or some other property, it would be great.

Paul - I'm no phycisist. Are you saying that such a definition has been researched but not found?

Maybe a given number of moles of cesium atoms after a given number of seconds would see a change in mass that is a constant 1 gram?

Sorry, but that is backwards. Right now, we determine how many moles of atoms exist in an object by their mass. Until you can count 1023 atoms...

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