This idea has been more popular than the giant monster scenario in the world of written science fiction: examples include “Surface Tension,” by James Blish, Fantastic Voyage, by Isaac Asimov (which is actually a novelization of the movie), and of course The Shrinking Man (later filmed as The Incredible Shrinking Man), by Richard Matheson. And in the world of movies, in addition to Fantastic Voyage (starring Raquel Welch -- does anyone under the age of 50 even recognize that name? Fame is fleeting indeed!) and The Incredible Shrinking Man, there's Honey, I Shrunk the Kids, and the upcoming Ant-Man.
Of these, “Surface Tension” is probably the most interesting. Several different versions of the short story exist, and it was later incorporated into a novel. Scientists whose ship crashes on a watery world, knowing that they are doomed, create a race of microscopic humans to settle the freshwater ponds that dot the landscape. The story follows one of the microscopic descendants as he directs the building of a ship to penetrate the surface of the pond and explore the “space” beyond. It’s a stirring story, and it accurately treats the surface tension that would act as a barrier to the tiny ship as it labored to cross from water into air. But it does miss one key point.
While the square-cube law would not hinder these shrunken humans, there is another effect of rescaling that would come into play.Water itself behaves very differently on microscopic scales than it does at “human” sizes. This becomes clear if we look at the way that microscopic animals swim. Bacteria don’t have appendages that they push rhythmically through the water. Instead, they have flagella, which spin like tiny propellers (and, in the case of the spirochetes, the entire bacteria acts like a propeller).
The reason for this lies in a quantity called the Reynolds number, which measures the ratio between the forces exerted on an object moving through a liquid. The liquid exerts a viscous force on the object, which tends to slow it down, but a moving object also possesses inertia, which causes an object in motion to continue in motion. The Reynolds number is just the ratio of the inertial force to the viscous force. All other things being equal, this ratio is proportional to the length scale in question, so the Reynolds number decreases as you go to smaller and smaller length scales.
At “human” length scales, the Reynolds number is much bigger than 1, so inertia wins out over viscosity. A human swimmer will continue to glide through the water after he or she stops paddling, and a repeated cycle of motion, like the breast stroke, can propel the swimmer forward.
But pity the poor E. coli bacterium trying to do the breast stroke.The Reynolds number for microscopic organisms is much less than 1, so viscosity always wins over inertia. If the bacterium were equipped with flippers and tried to swim, it would grind to a halt every time it stopped moving its appendages. Worse than that, as the bacterium moved its flippers back and forth in the water, it would simply move back and forth itself, and it wouldn’t get anywhere.The human form of swimming relies on the fact that the Reynolds number is large, and it just won’t work at microscopic scales. And that’s why bacteria use a different means of locomotion.
All of which means that shrunken humans are as problematic as giant ants. The humans in “Surface Tension” would not have experienced water the same way that we do, gliding gracefully through it. Instead, the water would seem to them more like honey, or even Jello, making every motion a struggle, and swimming nearly impossible. The moral here is that a writer who wants to change the size of people or animals should proceed with extreme caution. In the words of the biologist J.B.S. Haldane, we really are just the “right size.”