Part 12 — The String of Perfect Lightness

When the world had just begun there was a jackrabbit with very strong hind legs. In fact when he took off running he could push with main force itself — namely 10⁴⁰ tons.

Acceleration is proportional to force so you can imagine that he could take off with astounding quickness — in the twinkling of an eye he would be miles away going lickety split. But this jackrabbit never managed to achieve the ideal quickness because he had too much inertia. His mass was 100 ounces — I'm not talking about how much he weighed (your weight depends on what planet you're on) but about his inertia. Inertia is what determines the force needed for acceleration. Because the rabbit's inertia was 100 ounces and an ounce is a million times the universe's mite-sized mass unit, his inertia was 100 million time the natural unit and he could only come within a factor of 100 million of the ideal acceleration. He tried and tried but never could come closer than that. Even the main force of the universe, which he had in his hind legs, was not enough to do better than that.

Now this jackrabbit had a friend who was a frog and one day the frog came to him and said: "show me how to kick with main force and I'll take off quicker than a jackrabbit. I bet I'll even reach the ideal acceleration." The jackrabbit looked at the frog and squinted and sized him up and he said "You must be kidding." But, because he liked the frog, he showed him special stretches and knee-bends to make his legs strong and he trained him to where he could exert main force for just a brief instant. Main force is ten-to-the-forty tons so you can well believe that a brief instant is all it takes!

When the frog had done all his exercises and his hind legs were good and strong then he tried pushing off to a flying start. But his mass, that is to say his inertia, was 10 ounces (and an ounce is a million of the world's own mite-sized unit) so even though he could kick with genuine main force he could only get within a factor of 10 million of the ideal.

Here is how it works: if you get pushed by main force and your mass is N mites, then you will only accelerate a fraction 1/N of the absolute all-out world-class acceleration. Is there any animal so light — so apt for motion — that it could achieve world-class acceleration when boosted by main force? Or are all animals too inert for that?

Now the frog had a friend who was a flea and this flea had a daughter of whom he was very proud. She had six exceedingly beautiful legs, and you know a flea uses all its legs for jumping, not just its hind legs. She was small even for a flea, being (in metric terms) only 22 micrograms, but she made up for this by being sweet-tempered. The flea-father prevailed on the jackrabbit and the frog to take his daughter into training so that she might, like them (and pushing with all her six legs) exert the universe's main force for a brief moment: a force of ten-to-the-forty tons. And wouldn't you just know? Before long this flea-girl (whose inertia was truly the mite-sized natural unit, only a millionth of an ounce) was jumping all over the place with exquisite world-class acceleration!

She had the perfect lightness, and this too is a string of the lyre.