"Phase Space" by Stephen Baxter is a collection of 25 loosely-related short stories linked to - and expanding on themes introduced in - his Manifold novel trilogy of "Time", "Space" and "Origin". At its heart is the question posed by the physicist Enrico Fermi: given that the universe is billions of years old, if life exists out in the cosmos, why don't we see the evidence of it all about us? These stories represent the author's attempts to try to make sense of this paradox.
Pieces such as 'Open Loops', 'Sun-Cloud', 'The We Who Sing' and 'The Gravity Mine' explore the idea that other forms of intelligent life might exist or once have existed out in the cosmos. Baxter takes various scenarios, from the distant past - a mere few hundred thousand years post-Big Bang - to the distant future - trillions of years from now, when the universe is cold and dark - and supposes various alien civilisations, each of them coming to terms with their world and each of them with their own versions of the Fermi Paradox.
One of Baxter's favourite themes is space exploration and it is no accident that many of the stories take astronauts as their main characters: including 'Poyekhali 3201', an imagining of Yuri Gagarin's experience as the first man in space. One of the best stories in the collection is 'War Birds', in which the Cold War has escalated into space and the Shuttle fleet has fallen under the control of USAF, becoming an agent of destruction for a militaristic US government intent on demonstrating its capabilities to the rest of the world.
As interesting as such alternate histories are, however, they are unrelated to the main theme of the collection. Indeed the most intriguing stories are those which explore the idea that humanity exists either in some kind of simulation - as in 'Tracks' - or within a bubble or quarantine zone - 'Barrier' - in both cases set up by a higher intelligence, seemingly to foster or protect our species from true reality, but for what purpose is not clear. The collection's main piece, 'Touching Centauri', is perhaps the most exciting of all, as it examines what might happen when we begin to stretch that simulation to its limits.
Baxter's imagination surpasses that of most other writers of modern science fiction. But while the ideas are wide-ranging, the writing itself can occasionally feel awkward or disjointed. Often one feels that the themes have been compressed to fit the medium of the short story, when in fact there are enough ideas to sustain several longer pieces, perhaps of novella- or even novel-length. A more minor disappointment is that more of the stories do not feature the main characters from the Manifold arc; indeed Reid Malenfant appears in only one of the pieces.
That said, however, "Phase Space" is an exciting collection of work with tremendous vision, and an excellent companion to the Manifold trilogy.