Jun 7, 2007
After such a long, serious, and scientific post on genetics and disease yesterday, I thought it was time to post a slightly less serious, shorter, but hopefully useful item on length. As a science journalist, I often need to explain the scale of nanometres, picometres, and very, very rarely yoctometers (okay never) to a non-scientific audience in my writing. Similarly, visitors to this site often ask questions relating to size and the relative scale of something like a femtometer. For more on the definitions of the prefixes you can check out this earlier article on yotta to yocto. Meanwhile, here’s a digest of some of the more common size comparisons:
One metre (1 m), that’s about the length of our dog, the height of a two-year old toddler, or roughly the length of a six-foot adult male’s arm, give or take a couple of inches.
One millimetre (1 mm) a sheet of fairly stiff, but plain, cardboard is about 1 mm. A pinhead measures up to approximately 1.7 mm.
One micrometre (1 um, the u should be a Greek letter mu) is the size at which things start to get a bit tricky. Because of the “micro”, these things are by definition microscopic: a grain of pollen, a red blood cell, are 1 micrometre across. A human hair is about 200 micrometres thick, for comparison.
One nanometre (1 nm). Now comes the really interesting bit, a nanometre is a billionth of a metre, viruses are on this scale as to are the breadth of a strand of DNA. Cell membranes too are about one nanometre thick. However, when researchers talk about nanotechnology, the scale of the objects they are discussing can stretch from this very large molecule size all the way up to several hundred nanometres…which strictly speaking is probably better thought of as a few tenths of a micrometre instead.
The Sense of Scale site has some additional comparisons, although they seriously let themselves down by talking of flourine, as opposed to fluorine atomic nuclei. Nevertheless, they do offer some interesting size comparisons, such as 260 nm being the length of the smallest transistor in a Pentium 3 chip. A Pentium 3 chip, you say? Well, presumably the site was produced when those chips were cutting edge and long before 65 nm and 45 nm processes in microelectronics had become reality. A grain of salt is about 100 micrometres, meanwhile, which given it is a near-perfect cube means it is a million cubic micrometres.
All of this relates, of course, to the orders of magnitude primer on Sciencebase some time ago, which is visualised very well in the FSU’s Powers of Ten movie. The interactive clip stretches from 100 attometres (10 to the minus 16 metres) to 10 million light years (10 to 23 metres, which is a tenth of a yottametre).
Of course, I’ve only touched on length in this post. Sometimes I need size comparisons for mass, time, density, in fact most physical properties. If you have any good indicators, leave a comment to tell us about them.