
© Illustration by Alan Jamison, MITThe new definition of the kilo, as Wolfgang Ketterle explains, is equivalent to the mass of a specified number of photons (particles of light), which could be trapped in a cavity between mirrors, as seen at right, so that they could be weighed on a standard scale.
For 130 years, a cylinder made of a platinum-iridium alloy and stored in a suburb of Paris called Saint Cloud has been the official definition of a kilogram, the internationally accepted basic unit of mass.
But that will change once and for all on May 20, when for the first time all of the basic units of measurement will be officially defined in terms of atomic properties and fundamental physics constants, rather than specific, human-made objects.The other objects on which physical standards are based, such as the standard meter, were already replaced years ago, but the kilogram - generally known as the kilo for short - turned out to be a harder unit to define in absolute terms. Physicists and engineers have been frustrated, however, by the inevitable imprecision of a unit based on a single physical object.
Despite the greatest of precautions, every time the standard kilo was handled - for example, to compare it to another unit that could then be used to calibrate instruments - it would shed some atoms and its mass would be slightly changed.
Over its lifetime, that standard kilo is estimated to have lost about 50 micrograms. A better way was needed.Now, instead of a particular lump of metal in a single location, a kilo is to be defined by fixing the numerical value of a fundamental constant of nature known as the Planck constant. This constant relates the energy of a photon to its frequency, and is referred to by the letter h. It is now defined as 6.62607015 times 10
-34 kilograms times square meters per second, thereby defining the kilogram in terms of the second and the meter. Since the second and meter are already defined completely in terms of physical constants, the kilogram is now also defined only in terms of fundamental physical constants.
Some may find this new definition complicated and difficult to understand, but Wolfgang Ketterle, a Nobel Prize winner and the John D. MacArthur Professor of Physics at MIT, doesn't see it that way. "Conceptually, the definition is very simple," he says.
Ketterle notes that the new definition of a kilogram corresponds to the mass of an exact number of particles - a very large number of particles. According to his formulation, it is 1.4755214 times 10
40 photons (particles of light) of a particular wavelength, which is that of cesium atoms used in atomic clocks.
Comment: Something out of a dystopian film is right! While this technology may be useful under very special circumstances, it is not a stretch to see it being used by a power-obsessed and totalitarian state as described by Margaret Atwood's novel, The Handmaid's Tale.