Timelike Infinity

Timelike Infinity is a 1992 science fiction book by Stephen Baxter. The second book in the Xeelee Sequence, Timelike Infinity introduces a universe of powerful alien species and technologies which manages to maintain a realistic edge due to Baxter's physics background; it largely sets the stage for the magnum opus of the Xeelee Sequence, Ring (as opposed to Vacuum Diagrams, Flux, or Raft, which concern themselves with side-stories).

Plot summary

Set thousands of years in the future (AD 5407), the human race has been conquered by the Qax, a truly alien turbulent-liquid form of life, who now rule over the few star systems of human space – adopting processes from human history to effectively oppress the resentful race. Humans have encountered a few other races, including the astoundingly advanced Xeelee, and been conquered once before – by the Squeem – but successfully recovered.

A human-built device, the Interface project, returns to the solar system after 1,500 years. The project, towed by the spaceship Cauchy, returns a wormhole gate, appearing to offer time travel due to the time 'difference' between the exits of the wormhole (relativistic time dilation), with one end having remained in the solar system and the other travelling at near lightspeed for a century. The Qax had destroyed the solar system gate, but a lashed-up human ship (a great chunk of soil including Stonehenge, crewed by a group called the Friends of Wigner) passes through the returning gate, travelling back to the unconquered humanity of 1,500 years ago.