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 is entering the planetary system at an immense, unprecedented speed. It becomes apparent that her world is in imminent danger -- and that 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.

Orthogonal is the story of Yalda and her descendants, trying to survive the perils of their long mission and carve out meaningful lives for themselves, while the threat of annihilation hangs over the world they left behind.