The revision of the metric system will dethrone that, true kilogram | Science

By Adrian ChoNovember 6, 2018, 4:05 pm

As an aging monarch, The Big K is about to bow to modernity. For 130 years, this sparkling cylinder platinum-iridium alloy is the world reference in mass. Kept in a bell and locked at the International Bureau of Weights and Measures (BIPM) in Sèvres, France, the weight is removed every 40 years or so to calibrate similar weights around the world. Now, in a revolution much less bloody than that which cost King Louis XVI his head, he will yield his throne like a real kilogram.

When the 26th General Conference of Weights and Measures (GFCM) convenes next week in Versailles, France, representatives of the 60 member countries should vote to redefine the International System of Units (SI) so that four of its units base, kilogram, ampere, kelvin and mole – are defined indirectly, in terms of physical constants that will be set by fiat. They will join the other three basic units – the second, the meter and the candela (a measure of the perceived luminosity of a light) – which are already defined in this way. Rewriting eliminates the last physical artifact used to define a unit, The Great K.

This change aims to make the units more stable and to allow investigators to develop ever more precise and flexible techniques for converting constants into units of measurement. "It's the beauty of the redefinition," says Estefanía de Mirandés, a physicist at the BIPM. "You're not limited to just one technology." But even proponents of obscure changes recognize that they can disconcert non-experts. "The coldest people said, 'What are we going to do to teach people to use this?'", Says Jon Pratt, a physicist at NIST (National Institute of Standards and Technology) in Gaithersburg. Maryland.

The new SI generalizes the compromise already exploited to define the meter more precisely in terms of speed of light. Until 1983, the speed of light was measurable in meters and seconds independently defined. However, that year, the 17th CGPM defined the speed of light at exactly 299,792,458 meters per second. The meter then became the measurable thing: the distance traveled by the light in 1/299 792 458 seconds. (The second was related to oscillations of microwave radiation of cesium atoms in 1967).

The new SI plays the same game with the other units. For example, he defines the kilogram in terms of Planck's constant, which appears in all quantum mechanics. The constant is now fixed exactly at 6.62607015×10^-34 kilograms square meters per second. As the kilogram appears in this definition, any experience that previously measured the constant becomes a way to measure a kilogram.

Such experiences are much more difficult than the speed of light, an essential element of undergraduate physics. A technique uses a device called Kibble balance, a bit like the mythical balances of justice. A mass on one side is counterbalanced by the electrical force generated by an electric coil on the other side, suspended in a magnetic field. To balance the weight, a current must flow through the coil. Researchers can equate the mass with the current time generated by an independent voltage when they remove the mass and move the coil up and down in the magnetic field.

Metric makeover

Metric unit	Amount	Definition constant
Kilogram	Mass	Planck's constant
Metre	Speed	Speed ​​of light
Second	Time	Frequency of cesium radiation
Ampere	Current	Electron Charge
Kelvin	Temperature	Boltzmann constant
Mole	A quantity of substance	Constant of Avogadro
Candela	Light intensity	Effectiveness of light of a specific frequency

The real difficulty lies in the dimensioning of the current and the voltage, with devices of quantum mechanics that do it in terms of charge of the electron and the Planck constant. Now that the new SI has set these constants, the scale can be used to calculate a slug with a mass of exactly 1 kilogram. Redefining also makes quantum technology the standard of the system information system (SI) for measuring voltages and currents, says James Olthoff, a physicist at NIST. Until now, the SI has defined the amp in an inconvenient way, in terms of force between infinitely long current carrying wires separated by a meter.

However, the application of the new complex definitions will disconcert anyone who does not have a higher degree in physics, says Gary Price, a metrologist in Sydney, Australia, who advised the Australian National Standards Commission. In fact, he argues, the new SI does not meet one of the basic requirements of a system of units, namely to specify the amount of mass with which to measure the masses, the length with which to measure the lengths, etc. "The new SI does not represent weights and measures at all," says Price.

Metrologists have considered more intuitive redefinitions, says Olthoff. For example, you can define the kilogram as the mass of a large number of a particular atom. But such a standard would be impractical, says Olthoff. Somewhat ironically, the researchers have already counted the atoms in extremely rounded 1-kilogram silicon-28 spheres in order to set an exact value for the mole, previously defined as the measurable number of carbon atoms 12 in 12 grams of substance.

If approved, the new IS will come into effect in May 2019. In the short term, little will change, says Pratt. NIST will continue to broadcast the weight standards by calibrating its weight in kilograms – although now it will do so with its kibble scale. Researchers could eventually develop table scales that companies could use to calibrate their own weights in micrograms.

The next step is to rethink the second. Metrologists are developing more accurate atomic clocks that use optical radiation with higher frequencies than the current cesium standard. They should serve as a basis for a finer definition of the second, says De Mirandés, perhaps in 2030.

As for Le Grand K, the BIPM will keep it and periodically calibrate it as a secondary mass standard, explains De Mirandés. It is an honorable end for a fallen French king.