2018 Locus Award, Best Science Fiction Novel
Our universe is ruled by physics, and faster-than-light travel is not possible - until the discovery of The Flow, an extradimensional field we can access at certain points in space-time that transports us to other worlds, around other stars.
Humanity flows away from Earth, into space, and in time forgets our home world and creates a new empire, the Interdependency, whose ethos requires that no one human outpost can survive without the others. It's a hedge against interstellar war - and a system of control for the rulers of the empire.
The Flow is eternal - but it is not static. Just as a river changes course, The Flow changes as well, cutting off worlds from the rest of humanity. When it's discovered that The Flow is moving, possibly cutting off all human worlds from faster-than-light travel forever, three individuals - a scientist, a starship captain, and the empress of the Interdependency - are in a race against time to discover what, if anything, can be salvaged from an interstellar empire on the brink of collapse.