[AnemicOak]

The Clockwork Rocket (Orthogonal Book 1) by Greg Egan
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In Yalda's universe, light has no universal speed and its creation generates energy. On Yalda's world, plants make food by emitting their own light into the dark night sky. As a child, Yalda witnesses one of a series of strange meteors, the Hurtlers, that are entering the planetary system at an immense, unprecedented speed. It becomes apparent that her world is in imminent danger — and the task of dealing with the Hurtlers will require knowledge and technology far beyond anything her civilization has yet achieved! Only one solution seems tenable: if a spacecraft can be sent on a journey at sufficiently high speed, its trip will last many generations for those on board, but it will return after just a few years have passed at home. The travelers will have a chance to discover the science their planet urgently needs, and bring it back in time to avert disaster.