Phase Space is a collection of 25 SF stories by Stephen Baxter, many thematically linked to his "Manifold" trilogy (Time, Space and Origin) and other novels of cosmic scope.
"The phase space of a system is the set of all conceivable states of that system," says the first page. As with "Manifold" these stories explore possible (and significantly linked) states of Earth and the universe, alternate timelines offering different solutions to Baxter's favourite cosmological question--the Fermi Paradox.
It's a simple idea. According to our best scientific theories there's nothing special about Earth or the Solar System. Intelligent life has evolved here--ourselves. It's likely to evolve elsewhere. The skies should be full of other intelligences. Where are they?
Perhaps our theories are wrong and we're in a galactic quarantine. Perhaps what we see through our telescopes is a clever fake--but supposing we overload the capabilities of the fakers? Maybe intelligence always destroys itself before crossing interstellar space, or something kindly takes emerging life away to a safer place. Perhaps there's teeming intelligence out there, but we're not listening on the right wavelength. Perhaps they're hiding...?
Another Baxter theme revisited again in this mind-stretching collection is the high-tech romance of the space programme and walking on the Moon. Alternate histories of space exploration are deftly conjured up, some of them wonderfully paranoid. Yet another theme is deep time--the unthinkable gulf from Big Bang to the final extinction of the universe and possibilities of life at both extremes.
Baxter at his best has a bleakly lyrical view of the remote future, reminiscent of Arthur C Clarke. There are homages to other classics, including Asimov's "Nightfall" and even Dante's Divine Comedy whose final vision of paradise takes on a highly unexpected SF meaning. --David Langford