One of the best ways to bring science writing to life is to make a vivid size comparison between whatever you’re writing about and something else readers know well. An 8-inch tarantula indeed sounds huge, but when you write that it’s the size of your face, that’s enough to get Stephen Colbert’s attention.
As scales get really small, such comparisons become even more helpful. Many people have a rough sense of how long 15 feet is (it’s the distance from the free throw line to the hoop in basketball), but few can really gauge 15 nanometers (it’s roughly the size of an antibody, or about a thousand times smaller than a red blood cell.)
Earlier this week I wrote about size scaling for things bigger than a breadbox. In today’s post, I’m tackling scale comparisons for very small things. On Tuesday, look for tips on how to make sense of the immense scales of space.
The tendency with the tiny is to compare small things to the width of a human hair or the period at the end of this sentence. These have the advantage of being easily graspable, but they are overused. Helen Fields (bio) notes that the human hair comparison is fraught by huge variation. A human hair can be between 18 and 80 micrometers wide, according to Wolfram Alpha, so how do you decide if the measles virus is a hundredth or three-thousandths the size of a hair’s width?
Gisela Telis (bio) branched out recently, “I resisted hair, and described an amoeba as ‘a tenth the size of a dust mite.’ Of course, now my poor reader has to have a sense of a dust mite’s size.”
One way to get a sense of the scale of a dust mite is through this animation, notes Jill U Adams (bio). Zoom in on the pinhead (2 millimeters) and see the human hair (18-80 micrometers) with dust mites on it (~420 micrometers). Zoom in further and you’ll see an assemblage of even smaller items including a ragweed pollen grain, a red blood cell, baker’s yeast, and ending with rhinovirus.
Another nice source comes from the Genetic Science Learning Center at the University of Utah. Their graphic starts with a coffee bean and goes down through a grain of salt, a human sperm, and ends with a carbon atom, covering more than 8 orders of magnitude: a carbon atom is to a coffee bean as a second is to a decade.
Which brings me to another point. For things the mind can’t easily grasp, rescaling to things it does have some intution about can be useful. The National Nanotechnology Initiative has some nice examples of this on their site about the nanoscale.
Of course lengths aren’t the only quantities out there. A concentration of a part per billion is equivalent to a half teaspoon in an Olympic-sized swimming pool. (Here are a few more comparisons for these amounts.) Doug Fox (bio) once wrote that the power consumption of the human brain is the same as that of the light bulb in your fridge.
As scientists develop techniques with ever-finer resolution that reach into ever-more-diminutive scales of all types, old-time ways of evoking the small — whether with peas or fleas or blinks of eyes — just don’t cut it. Do you have favorite ways of making sense of the minute?
Image credit: CDC/Janice Haney Carr via Wikimedia Commons