Big K is history, and it’s a big deal

When the kilogram was redefined recently, neither the world nor anything in it weighed more or less, but it was ensured that the weight of everything would remain the same, correct to the smallest part of a gram, for all time to come. The kilogram has been defined since 1889 by the International Prototype Kilogram, or the Big K, which is a platinum-iridium cylinder kept inside a glass jar in an underground vault under high security in Paris. All weights in the world are measured against it, but scientists who gathered recently in Paris decided that a more accurate idea of the kilogram is needed. Even under strict security, the Big K has been losing weight, perhaps only infinitesimally, by losing atoms through radiation or when being cleaned of an impossible speck of dust that found its way to it. This will not happen now, and Big K is history.

The kilogram will now be calibrated by a device called Kibble balance using Planck’s constant, which is an unimaginably small, fundamental and unchanging value of nature. Instead of weighing one mass against another, the new balance will weigh a mass against an electromagnetic force which can be measured with the greatest accuracy. It will not solve the problem of the smaller kilogram at the grocer’s shop or the bigger kilogram gained by a lazy body. But scientists need the most accurate measurements for their calculations and even an infinitesimal variation in the values may lead to wrong results. That is why they have opted for a universal constant and done away with the physical constraints imposed by a 130-year-old way of weighing things. The redefined kilogram will remain just that as long as Planck’s constant is constant.

The kilogram was among the last poorly defined units to be scientifically redefined. The second was redefined in 1967 in terms of atomic oscillations. It facilitated global communications through the GPS and the internet by measuring time to the billionth of a second, but scientists are now looking up to the more accurate optical clocks. The metre, which was once measured by a metal rod kept in Paris, has now been redefined in terms of the distance travelled by light. The candela, the unit of luminous intensity, was redefined in 1979. With the redefinition of three other units — the ampere, the kelvin and the mole — coming into force next year, all seven basic units of the metric system will now be linked to a fundamental constant of nature. The changes will certainly have an impact, though they may not immediately be seen in our daily lives and its transactions.