Worth its weight

The redefinition of kilogram is timely

The “Travels of Marco Polo” is an eternal bestseller due to its (rather fanciful) descriptions of life in 13th-century China. But Polo conceptualised it as a handbook of weights and measures, making it invaluable for merchants hampered by the lack of uniformity in measurements. It was the “Metre Convention” of 1875 that established the International Bureau of Weights and Measures and led to universal measurement standards being set for the first time. On November 16, 2018, however, the member-nations of the Bureau unanimously agreed to change the definition of a kilogram (kg). This is the last of seven standard Systeme Internationale, or SI units, to be redefined. The change of definition is of huge importance. Instead of using an arbitrary physical mass that changes with temperature, or simply due to picking up dirt, the new definition is based on invariant physical laws. It will enable scientists to fine-tune calculations ranging from the quantum to the cosmic.

The SI units were originally defined in terms of terrestrial relationships. A metre, for example, was a fraction of the distance from the Equator to the North Pole, and a second was a fraction of the average day. But these definitions were inexact and variable. The other six SI units have long been redefined in fundamental, invariant terms. A metre is now the distance travelled by light in vacuum in 1/299792.458 of a second. A second is defined in terms of the number of oscillation of electrons in the caesium-133 atom. Such accuracy is important. In satellite navigation and space travel, relativistic effects are visible. An astronaut at the International Space Station ages ever so slightly slower than her sibling on Earth.

The kilogram is the mass of a cylinder (“Le grande K”) in the vault of the Bureau's headquarters in Sèvres, France. It will be redefined in terms of Planck’s Constant (“h” in physics jargon). The constant links the energy of a photon, or light particle, to the frequency of light. It is a very small number. In the SI definition of h, there are 33 zeros after a decimal point. It is usually written in terms of energy multiplied by time, and it is invariant under all conditions, everywhere. Max Planck worked it out in 1900. His calculation, E=hv [where the Energy output of a photon is equal to Planck’s Constant (h) multiplied by the frequency (v) of the photon] is the second-most famous equation in science. In 1905, Albert Einstein generated the most famous equation, E=mc2, when he proved energy was equivalent to the mass of an object, multiplied by the square of the speed of light. This energy-mass equivalence allows Planck’s Constant to be used to define mass. But the redefinition is not trivial. We’re dealing with very small numbers and extreme accuracy is necessary.

Two experiments are being done to derive the exact values. The first uses an apparatus called the Kibble Balance. This measures a mass versus the energy of an electromagnetic field. The second experiment involves a sphere weighing 1 kg, made from silicon-28. Silicon-28 has spherical crystals of known volume. So, the exact number of atoms in the 1 kg sphere may be calculated. Using Avogadro’s Number (another fundamental constant), the number of atoms can be expressed in terms of Planck’s Constant. These two results are matched to derive an exact value of the kg. This may differ from the current definition by less than the mass of an eyelash.

The changed definition comes into effect on May 20, 2019. It will be important in terms of making components for aircraft, pacemakers, developing molecular drug dosages, and calibrating scientific instruments to measure the tiny perturbations caused by gravity waves. A fundamental definition of mass will also help us communicate with sophisticated aliens, if we encounter them. While they would not know the mass of a random metal cylinder, they will surely be able to map Planck’s Constant, whatever they call it.