The Kilo Lose Weight

On Monday, stock and other 20 experts in Europe and the U.S. measures met in London to plan a redefinition of the kilo that will have to accept the 54 countries, including Spain, using the International System of Units. "We need a definition that is stable and accessible to everyone," says Stock, commanding one of the world's most accurate scales with which it is reweighed the kilo to express their value based on a universal constant, rather than a weighs the nineteenth century.

Each year, three men descend to the basement where the kilogram, open the door and check that remains intact. Is the director of the BIPM, the president of the International Committee of Weights and Measures and the director of the Archives of France. Each custody of the three keys that open the door.

"The first time I saw all that process I thought was very funny," says Ian Mills, microscopy expert at the University of Reading (UK) and one of the organizers of the meeting last week. Only three times has taken the kilo of your camera. Was to verify that their weight remained the same. The results of the last two weighings, in 1946 and 1989, showed a disturbing result: the kilo had thinned about 0.00005 grams.

Planck against Avogadro

"I do not know what happened to the weight over the last century," says Stock. Like him, other teams in the U.S., Switzerland, Canada, China and six other countries are doing their own calculations based on universal constants.

The efforts are intended to advise the political delegates of the General Conference of Weights and Measures (CGPM), which groups 54 countries of the Metre Convention. Signed in 1875, the convention was designed to standardize weight (kilo), time (second), length (meters), intensity of current (ampere), temperature (kelvin), amount of substance (mole) and luminous intensity (candela ). After the meeting last week in London, has become apparent that the agreement on the new kilo can not be achieved at the next meeting of the CGPM in October.

The reason is the discrepancy between the two major experiments that have reweighed the kilo in terms of two constants. One is that Max Planck discovered in 1900 and lays the foundations of quantum physics. Another is the Amedeo Avogadro proposed almost a century earlier to calculate the number of atoms in a gas in terms of its volume.

Applied to the kilo, the proposals of both authors disagree. "The exact extent of dissent is 0.00000017," said Stephanie Miranda, who works in Barcelona physical redefining the kilogram in the IBMP. This disagreement will be sufficient to delay the International Covenant until the next meeting of the CIPM, within four years.

"Trouble Ahead"

"There is a reasonable possibility that the new definition was adopted in 2015, but there is no certainty," says Stock. Three other measures of the international system will have to wait until 2015 to be redefined: the mole, the ampere and the kelvin, which is not like all the experts involved.

"This issue could cause problems in the future," says this newspaper physicist John Hall, an expert in laser measuring magnitudes and winner of the Nobel Prize for Physics in 2005. According to Hall, the redefinition can cause a "disconnect with the past."

The French weight was forged 131 years ago in London to accurately reproduce the so-called "kilogram of the archives." It was a measure equivalent to the weight of a liter of water was set by the Government of France in 1795, while the country was waging a war against the European powers who wanted to crush the revolution.

The ampere, the kelvin and the mole will also be reviewed

The British company Johnson Matthey, still in operation, carefully polished cylinder 39 mm high made of platinum so that your weight is adjusted as possible to a liter of water. The weight is kept in the chamber since the BIPM with six copies. The weights of the original and copies, more than a century later, are mysteriously different. "I do not know if some are gaining mass and other mass loss or how exactly we are talking about," says Mills. The difference is so small that it causes fights between fruit vendors and customers, but scientific debates in laboratories around the world and problems for students. "The goal is not to redefine the concept as simple as possible, but to set one mass unit that is as stable as possible over time," argues Stock.

Stability and accuracy

The reason for the change is that nothing is more stable that can compare the original kilo to fix its true weight. This is where Planck's constant, unchanging value in nature that is used as a reference to express the value of a kilo. In 1983, already used another constant of nature, the speed of light, to redefine the metro.

There are two ways a kilo reps with maximum precision. One is to use a balance of power. On one side lies the kilogram and in another a bovine by circulating electric currents which reproduce the Planck constant, close but not equal to the weight of an electron.

Of the five scales that are in the world (USA, Canada, Switzerland, France and the BIPM), only the U.S. has brought results so far. The other way is to use the Avogadro constant, which is the number of atoms of carbon 12 is in 12 grams of the substance. Since both the Planck and Avogadro's constants of nature are, their estimates of kilo should be identical, but are not.

"The discrepancy is one part in ten million and the target is no more than two or three parts per hundred million," summarizes Mills. It is not the only problem. The redefinition need additional confirmation from other experiments that did not arrive for another two years, in the case of the balance of power used by Stock and other even more precise weights that are still under construction.

The Nobel Hall further calls kilo heavier than replicas, including those used by businesses before adopting the new value. In total, the redefinition process has been underway since 1976, according to Stock. Long before the end, maybe this year, the three custodians of kilo reopen the camera and take the weight for a new weighing which could give new surprises.