Rocky Has Sonar

Humans perceive the environment with photons (light). Rocky uses sound waves. On his planet, with 29 atm of ammonia and possibly no surface light, optical vision never evolved.

Unlike bats and dolphins (active sonar: emit pulses and listen for echoes), Rocky uses passive sonar: his brain processes ambient sound bouncing off objects.

A thin aluminum sheet blocks nearly all light but transmits sound perfectly. To Rocky, the Hail Mary's metal hull is as transparent as a glass window to us.

Ink marks on a measuring tape produce no relief, so sonar can't detect them. Only physical relief is "visible" to sonar. That's why the Eridian clock has raised numbers (3 mm high).

The Fourier transform is an algorithm that decomposes any complex signal into its component frequencies. Record a piano chord, and the algorithm tells you exactly which notes compose it.

Human speech is chaotic: constantly changing formants, noisy consonants, blending vowels. It's hard to cleanly decompose.

Eridian chords have discrete, stable frequencies: each "phoneme" is a precise combination of notes. The Fourier algorithm identifies them unambiguously.

5 simultaneous notes in a chord implies 5 independent "vocal cords." Grace can transcribe each word as a list of numerical frequencies and build a dictionary.