Beyond Ultra Large and Extra Small

Metric (SI) prefixes come in handy for expressing both large and small quantities. So we have gigawatt power systems and terabyte hard drives at the high end, and we have nanoscale electronics and femtosecond lasers at the low end.

Advances in scientific research and technology keep pushing us to new frontiers in the domain of metric prefixes.

In a 1998 article in Scientific American, Philip Morrison and Phylis Morrison described the escalation in the amount of stored knowledge since the days of the famed library in ancient Alexandria. The library probably shelved the equivalent of 50,000 books today. Counting the literature of China, India, Iraq, and Iran, the total number of distinct books available 2,000 years ago might have been about 100,000.

In 1945, a librarian at Harvard estimated that "worthwhile" printed material amounted to about 10 million books. With one byte as a unit of information representing roughly the equivalent of a single character of English text, it's possible to store that text as 10 trillion bytes, or 10 terabytes, of data. Images, drawings, photographs, audio and video recordings, and huge databases, however, require much larger amounts of storage space.

The U.S. Library of Congress now holds more than 34.5 million books and other print materials. Because it also has 3.3 million sound recordings, 13.4 million photographs, 5.4 million maps, and other byte-hungry material, however, the total store comes to a couple of petabytes (a million billion bytes).

One can plausibly estimate that fresh text, including newspapers, amounts to less than 100 terabytes annually. Recorded music, films, photographs (including family snapshots), and other materials add considerably to that accumulation.

"The biggest byte makers are the television stations of the world," the Morrisons wrote. "Although it is hard to correct for innumerable repeats, our best source puts their originality at one tenth of all they send out and so allots them under 100 petabytes annually."

To assess the totality of information production, one can also try to include ephemeral signals conveyed from one person to another. The sounds of telephone calls add up to some 1,000 petabytes worldwide, or a few exabytes. Face-to-face speech provides several more exabytes of data.

At this stage, the summing gets trickier. The Morrisons simply concluded, "No estimate of the eventual human store seems quite credible as yet."

In a 2011 online article in Science Express, Martin Hilbert and Priscila Lopez provided new estimates of the world’s technological capacity to store, communicate, and compute information. They estimated that in 2007 humankind was able to store 2.9 x 10²⁰ optimally compressed bytes, communicate nearly 2 x 10²¹ bytes, and carry out 6.4 x 10¹⁸ instructions per second on general-purpose computers.

These estimates put us in a realm of metric prefixes that relatively few people yet know. In 1991 as part of the International System of Units (SI), the General Conference on Weights and Measures (Conférence Générale des Poids et Mesures) adopted new prefixes representing 10²¹, 10²⁴, 10^-21, and 10^-24. This means that 1,000 exabytes equals 1 zettabyte (ZB), and 1,000 zettabytes equals 1 yottabyte (YB).

I first encountered the new prefixes in 1993 when researchers measured voltages in a superconducting circuit so small that they had to use the term milliattovolt, where "atto" stands for 10^-18. The proper term is zeptovolt.

Here's the current table of official metric (SI) prefixes:

Power of 10	Prefix	Symbol	Power of 10	Prefix	Symbol
24	yotta	Y	-1	deci	d
21	zetta	Z	-2	centi	c
18	exa	E	-3	milli	m
15	peta	P	-6	micro	m
12	tera	T	-9	nano	n
9	giga	G	-12	pico	p
6	mega	M	-15	femto	f
3	kilo	k	-18	atto	a
2	hecto	h	-21	zepto	z
1	deka	da	-24	yocto	y

Many of the prefixes come from Greek and Latin words, often via French. "Zepto" is derived from the Latin septem, meaning 7, because this is the seventh prefix in the system of metric prefixes. The s was replaced by z to avoid confusion with the abbreviation for the second. The prefix "zetta" was coined to parallel "zepto." Similarly, "yocto" is derived from the Latin octo, meaning 8, and "yotta" parallels that term.

You can find a guide to units of measurements and their history at A Dictionary of Units of Measurement.

You might be interested to know that an attoparsec is a distance of about one inch (3.1 centimeters). The distance for Earth of the most remote object yet observed in the universe is about 125 yottameters (13.2 billion light-years). The diameter of the largest known galaxy is about 53 zettameters. At the other end, an atomic mass unit equals 1.66 yoctograms.

The Convert Auto page (a student project) offers a handy tool for converting from one unit of measurement to another in a wide range of fields.

Now you can really start talking ultra large and extra small.

Originally posted July 27, 1998

References:

Hilbert, M., and P. Lopez. The world’s technological capacity to store, communicate, and compute information. Science Express (Feb. 10, 2011).

Morrison, P., and P. Morrison. 1998. Wonders: The sum of human knowledge? Scientific American 279(July):115.

Peterson, I. 1993. Measuring superconductor magnetic noise. Science News 143(Jan. 16):37.