Goodbye to kilo as we knew it

The global scientific community has approved the first major revision of the International System of Units from now on, the kilogram is not defined by a phy

Goodbye to kilo as we knew it

The global scientific community has approved the first major revision of the International System of Units

from now on, the kilogram is not defined by a physical object

In their day-to-day, the consumer will not notice changes to make the purchase

From now on the units of measure of the two magnitudes more basic -length and mass - derived from two fundamental theories of physics: quantum mechanics and relativity. The sixty delegates of the General Conference of Weights and Measures (CGPM) have approved this Friday in Versailles to change the definition of kilogram; in place of be measured in relationship to a standard object will be defined in reference to Planck's constant, a value of the same physical processes. The metro, for its part, already calculated since 1983 with respect to the speed of light in vacuum, whose constancy is one of the foundations of the theory of special relativity.

The reform will enter into force on the 20th of may, at which time the system will be guided by the reference current, a cylinder of iridium and platinum 39 mm nicknamed le grand K. Since 1899, le grand K was kept under three keys in an underground vault at the Pavillon de Breteuil, in a building on the outskirts of Paris, and has been the standard with which to compare all the weights of the world. However, experts explain that it is difficult to ensure that it is not modified with the time, as it could degrade or be manipulated.

In fact the latest measurements have confirmed a variation of about 50 micrograms produced over the last century. "If I cogiese in my hand and that object," he explained during the presentation the nobel prize in Physics William Phillips, of the National Institute of Standards and Technology, u.s. (NIST), "my fingerprints would increase their mass." The problem is that the cylinder can not increase mass as it weighs, by definition, a kilo. "So all you would lose weight," he added.

This revolution in the way we calculate the unit does not change anything in the scales, butchers and fruit of the world, nor in the majority of professional instruments. Nor will it change the global system of measures, that ensures that Madrid, Tokyo and Washington share the same references, but its creators explain that the change ensures a more stable, more universal and more practical. Trust in an object that wears out with the passage of time and that is based on a single specimen (made replicas) presented risks and inaccuracies. "We cannot assign properties to the objects," said Jean-Philippe Uzan, Director of Research of the National Center of French Research, "we have to create new definitions by observing nature."

Universal and accessible to all

that's why scientists have opted for a system based on Planck's constant, unchanging, and accessible to everyone. In this way, from now on the measurement of small masses may be more direct and precise. Although for the moment only three laboratories in the world have exceeded the level of accuracy required by the Committee on Data for Science and Technology (CODATA) to be approved: one in Canada, another in the united States and the last in France. These three teams have been the only ones up to now in measuring Planck's constant with an accuracy of at least seven decimal places.

Another practical advantage is that now it will be easier to measure fractions of a kilo, as nanograms (a billionth of a gram), the use of which is crucial in many technical fields, and scientists. In addition, the kilo is used to redefine the other three basic units of the International System, such as the candela, in addition to other derivatives, such as the newton and the joule. Throughout the week, the GFCM has also decided to review the kelvin, unit of temperature, which from now on will be linked to the Boltzmann constant (which establishes a proportionality between the thermal energy and the temperature). For its part, the mole, amount of matter, it will be calculated following the constant of Avogadro and ampere is linked to the electric charge of the electron.

scientific Revolution

The system of weights and measures world is a legacy of the French Revolution. "At all times, for all peoples", proclaimed the Convention in 1795 to make official the new units. In this way through the interest of scientists, such as Condorcet, Lagrange, Laplace or Monge to create a system that is more rational and universal. The kilogram was originally defined as the mass of a decimeter cubic of water. The metro was calculated as a fraction of the distance between the North Pole and the Equator.

but one hundred years later, in 1889, the CGPM agreed in addition to create physical objects of reference; so appeared the grand K and a rod of one meter length made in the same materials. For more than three decades, the metro would be the first unit to be bound to a constant; from that moment is established as the distance traveled by light in vacuum during a time interval of 1/299.792.458 of a second.

Long process

"The definition created for the metro was at once bright and pretty, we wanted to apply that same beauty to the kilo," said Phillips. However, the work of redefining the kilogram using universal constants has been more complicated and has required the development of instruments and accurate knowledge. In the center is the so called balance of Kibble (or balance of Watt), invented in 1975 by the british physicist Bryan Kibble.

this Is a measuring instrument which weighs the mass and the electromagnetic force by recording electrical current and voltage. In the balance of Kibble measuring the current required to support the weight, thus determining the mass. And given that the units that define electric current and the voltage are marked in function of fundamental constants (the speed of light and Planck's constant), the unit of mass can be established in function of constant absolute.

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Updated Date: 24 November 2018, 20:00



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