IT SOUNDS like a creature from a sci-fi movie, but yocto is both more and less exotic than that: it鈥檚 a numerical prefix denoting an extremely small number.
To save writing all those zeros when noting very small or large amounts, scientists devised some short-cut prefixes. But while we鈥檝e all heard of a milligram (a thousandth of a gram), a kilometre (a thousand metres) and perhaps a gigawatt (a billion watts), what about a yoctogram? It is 10-24 of a gram, about 60 per cent of the mass of a neutron. And yocto, along with yotta (1024 鈥� the digit 1 followed by 24 zeros), are the most extreme prefixes used as multipliers for metric units.
Where did the name come from? The older prefixes 鈥� from milli to kilo 鈥� date back to 1795, when the French Revolution gave birth to metric units. Milli and kilo come from the Latin and Greek terms for a thousand. In 1875, the International Bureau of Weights and Measures was established in Paris to standardise this as a global scientific system.
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As science marched on to smaller and bigger things, more prefixes were needed. By 1975 the Paris bureau had authorised prefixes from peta (1015) down to atto (10-18). After giga (109), tera (1012) and nano (10-9) had been coined from the Greek for giant, monster and dwarf, the bureau鈥檚 sages were running out of classically derived terms. So they used the Nordic words for 15 and 18 for femto (10-15) and atto.
鈥淎s science moved on to smaller and bigger things, more prefixes were needed鈥�
After that, some lateral thinking was needed. Take 1015. This can be written as 10005, so it was called peta, from the Greek for five, penta.
Yocto got its name in an even more convoluted way: 10-24 is equivalent to 1000-8, and the Greek term for 8 is okto. The 鈥渙鈥� from okto, however, could be confused with a zero, so the sages added a 鈥測鈥�. At the big end of the prefixes, yotta is a variation of the Latin or Italian for eight.
At the moment yotta and yocta are the largest and smallest official prefixes, but will they continue to hold this position? An electron weighs around a thousandth of a yoctogram. Could this be named a xontogram, moving one letter along from yotta and based on nonus, Latin for nine? Earth鈥檚 mass is 6000 yottagrams 鈥� or perhaps one day, Paris willing, 6 xonagrams? This model allows plenty of alphabetic alterations of Latin numbers to expand into. But will science ever work at a small enough level for a pekrogram (10-51) or big enough for sortagrams (1042)?
