During a meeting with Jim Al-Khalili and our project student, Spencer (Jim mentions our project here) we talked about the appropriate length units with which to discuss different physical objects. When talking about things of human dimensions, metres are sensible units, but when talking about distances between stars, light years are more reasonable, as we can discuss the numbers while sticking to values somewhere vaguely near one unit – i.e. one metre or one light year.
There’s a convention with the normal scheme of units that there are base units; metres, seconds etcetera, and then prefixes, like mega-, nano-, kilo- and so on. The convention about these prefixes is that they appear every three orders of magnitude, so every factor of 1,000. That means we use kilo (1,000) then mega (1,000 times 1,000) then giga (1,000 times 1,000 times 1,000) up and up to the highest defined prefix, and likewise down in thousandths to go to tiny units.
Now, I’m a nuclear physicist, which means my favourite length units are femtometres (1/1,000,000,000,000,000 m) which are about the size of nuclei. The next thing up on the length scale of things that are found in nature are atoms. They’re closest in the naming scheme to nanometres in size. Between these, though, are picometres. That’s a length which is 10-12m = 1/1,000,000,000,000 m). During the meeting, I figured that actually there is nothing in nature that is around a picometre in length since it falls between nuclei and atoms in size.
Naturally after the meeting, I followed this up by using a reasonably scientific method: I googled “femtometre”, “picometre” and “nanometre” to see how many hits there were for each. It turns out that the ratio of femto:pico:nano is about 1:2:5. This surprised me a bit. Picometres are more commonly used than I thought. It turns out that the reason is probably in part that nuclear physicists actually call femtometres fermis; with a lack of deference to the S.I. unit standard, but with deference to Enrico Fermi and therefore use that word in the literature and also that atoms of size 0.1 nanometres are also 100 picometres in size, so one can use either nanometres or picometres when describing them. The main reason, though, is that although no matter is naturally at the picometre length scale, the wavelength of X-rays and gamma rays are exactly around there, and these are, of course, used all the time! While talking about units, let me mention an easy one to remember: It turns out that the speed of light is very close to exactly 1 foot per nanosecond 🙂