The ClockWork Rocket - Greg Egan

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.