In an empty hall, sound bounces between walls, turning into a booming echo.

For hydrogen (H₂), stability and excitation swap roles, as if the seesaw has flipped. Ring-shaped molecules like H₄ or C₄H₄ (carbon and hydrogen) usually bend to survive. But the echo chamber straightens them out to perfect symmetry.

The more molecules inside, the stronger the effect — the crowd doesn't dampen the anomaly, it fuels it.

Spectroscopy — analyzing the outgoing light — lets us see these metamorphoses. So light echo teaches us to sculpt molecules with unimaginable properties — from ultrasensitive sensors to world-changing materials.