How do Eridians perceive their surroundings in Project Hail Mary?

Eridians perceive their surroundings through echolocation.

Rocky’s species, the Eridians, does not rely on human-style vision. Instead, they emit and interpret sound, using echoes to map nearby objects, spaces, and movement. This sensory difference is one of the most important early obstacles in Grace’s attempt to communicate with Rocky.

Because Rocky’s communication is built from musical, chord-like sounds rather than spoken human language, Grace must develop a translation system. Their friendship grows through repeated experiments, shared technical problems, and the gradual discovery that both species can reason scientifically despite radically different bodies and senses.

The common mistake is to assume that Rocky simply has unusually strong hearing. Echolocation is an active system: the Eridian produces signals and interprets returning echoes to construct a spatial picture. That makes Rocky’s perception fundamentally different from infrared vision, magnetoreception, or telepathy.