Children of the Chronotron

An advanced planetary civilization extends its existence by creating a radioactive atmospheric shell, but the safeguard grows uncontrollable and prolonged lifespans lead to falling birthrates. Facing annihilation, survivors build a time-transmission device and dispatch a lone emissary into an earlier era to alter causation and seek refuge. The narrative alternates between the society's technological desperation and the emissary's encounters with contemporary humans, exploring the moral and practical costs of engineered immortality, the unintended consequences of technological salvation, and time travel as an extreme strategy for averting collective extinction.