Counterintuitive Facts (94): Why Isn't the Galaxy Filled with Spaceships?

I. Suppose we built a spaceship: Von Neumann Probe. Named after mathematician John von Neumann, because he proposed the concept of "self replicating machines." This spaceship has two functions: 1. Fly to a planet, use local resources to replicate itself (produce two new ships). 2. Let new ships fly to next planets, keep replicating.

II. This is exponential growth. 1 becomes 2, 2 becomes 4, 4 becomes 8, 8 becomes 16... Generation N ships number 2^N. Only 50 generations of replication, and you have over 1 quadrillion ships.

III. Even if ship speed is only 1% of light speed. This is completely physically feasible. No subspace engines needed. This fleet only needs a few million years to fill the entire Milky Way. The Milky Way's diameter is 100,000 light years. Travel at 1% light speed takes 1