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.”
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