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  • Paul Anthony Jones


(pref.) denoting a factor of 10^24

These days, practically everyone is familiar with hitherto mind-boggling quantities of data like kilobytes, megabytes, gigabytes and terabytes. But this system of ever-enlarging prefixes doesn’t end there. After tera– comes peta–, then exa–, zetta–, and finally yotta–, the largest prefix currently recognised in this system.

Each of these prefixes can then be attached to any one of a number of different base units to form quantities like petasecond (31.2 million years), exajoule (15 of which equates to the annual energy consumption of the entire United States), and yottameter (equal to around 105 million light years).

But who decides what these ‘official’ terms should be?

The kilo–, mega–, giga– system grew out of the metric system. By the 1900s, it was being officially overseen by a governing body called the General Conference on Weights and Measures, a branch of an even greater authority known as the BIPM (the Bureau international des poids et mesures, or International Bureau of Weights and Measures).

Although its literature is updated and published annually, the GCWM meets every four years or so to discuss all manner of issues relating to metrology—the science of weights and measurements. The most up-to-date data available to them is then used to define and redefine the units we use to measure everything from time and length to electrical flux capacitance and luminescence.

The most recent GCWM conference (the 28th) was held in 2018. But it was back in 1991 (at the 19th conference) that members decided the existing system of unit-forming prefixes needed updating. This was by no means without precedent: giga– and tera– had been added to the system in 1960, by which time technology had advanced sufficiently to make units of this size a necessary addition to the vocabulary. Peta– and exa– were then added in 1975, and so in 1991 zetta– and yotta– were added to round out the list.

With the world already dealing with zettabytes of data, it seems likely this list will have to be updated again in the coming future—but luckily, the GCWM are prepared. In fact, the last time they updated this system, steps were built into it to update it appropriately in the future.

Among the smallest units here, this system of prefixes is purely decimal: deca– (as in decibel), hecto– (as in hectare) and kilo– (as in kilogram) each derive from the Greek words for ten, hundred, and thousand. Mega– and giga– then buck this trend (as they both essentially mean ‘great’ or ‘giant’) while tera– acts as both as a corruption of the Greek word for monster, teras, and the Greek word for the number four (because tera– units are based around a factor of a billion, or 1 followed by four sets of three zeroes). Peta– and exa– then pick up where tera– leaves off, and derive from the Greek for five and six, while zetta– comes from the Latin for seven, and yotta– from the Greek for eight.

Admittedly, there’s a considerable amount of playing around with the spellings here, as it could be argued that peta– should rightly be spelled penta– (as it is in words like pentagon), while the more typical interpretation of the Latin word for seven would give us a prefix like septi– (as in septillion), not zetta–. But this variation is intentional, and largely driven by reasons of intelligibility between languages, a desire to avoid confusion with existing terms, and a further desire to separate these purely scientific prefixes from those used elsewhere.

But the initial Z and Y of zetta– and yotta– are intentional too. The GCWM envisage these as the first two steps in a now ever-extending list of unit-forming prefixes that will from here head backwards through the alphabet at each step. So whenever it’s next decided this list needs updating, it’ll likely be something like xenna– or xovi– (from the Greek or Latin for ‘nine’) that comes next.

But what if we don’t want to talk about ever larger units, but ever smaller ones? After all, for every kilometre (1,000 metres) there is a centimetre (1/100th of a metre). Well, not to worry—you’re covered there too.

The prefixes deci–, centi–, milli–, micro–, nano– and pico– will see you down through factors of one tenth, one hundredth, one thousandth, one millionth, one billionth, and one trillionth, respectively, to form units like millilitre, microjoule and nanosecond. Your fingernails grow at a rate of around one nanometer per second, for instance, while the radius of a carbon atom is approximately 77 picometres.

After that, the prefix femto– is used to designate a factor of one quadrillionth (10^–15); atto– marks one quintillionth (10^–18); zepto– is one sextillionth (10^–21); and yocto– is used for one septillionth (10^–24). The final two here, zepto– and yocto–, were likewise added to the list in 1991, and were similarly named as the first two steps in a sequence heading back through the alphabet.

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