Okay, only four and a half stars, but i opted for five not four because this is a truly magnificent, tho' difficult, book. Greg Bear has written some great SF; but i, at least, have found his recent work less exciting. This is Bear returning with a huge metaphysical vision, greater in its depth than even his 'The Way' series ('Eon', 'Eternity', and 'Legacy'). The story takes place in two time zones, now, and one hundred trillion years in the future in the eponymous city at the end of time; there is also a third arena, somewhere where there is no time, or, perhaps, all times - the chaos.
Bear has tried to imagine a universe where the Copenhagen interpretation of quantum mechanics is given its most literal reading: "All the possible pathways a particle can take - or a human - an infinite number, spread out through all space and time, weak where improbable, strong where probable - all, in the end, collapsing into a single, energy-efficient path, the most resourceful and simplest world-line." I'm not sure what he means by "in the end collapsing into a single [...] path" because three of the important characters from our time have the ability to jump to alternate realities in an attempt to improve their lot. But one can't blame Bear for a bit of fuzziness here, no-one can make sense of quantum mechanics when it comes to what it means (that's according to Richard Feynman in his 'QED' - and who are we to argue with him?).
At the end of time reality as we know it has almost been destroyed by the chaos - an empty horrific meaningless force which subverts all that we know as sanity and order. Indeed, it has devoured all bar the final city - the city at the end of time. Time, within such quantum multiverse, is not fixed - start in the middle of a story, go back to the beginning, return to where you started but you'll find it's no longer the same. The essence of this story is how one of the mighty beings at the end of time - the sort of being Bear imagines a person who had had tens of trillions of years in control of their own nature might turn into - is struggling against the chaos. It's giving nothing important away to say that the alternative world jumping characters in our time are, unbeknownst to them, part of his struggle.
Any hack can tell a story with `amazing' beings in it: "His mind was godlike - as beyond ours as ours is to a beetle's." Bear is a truly great SF writer because he goes so much further, giving us the feeling that we really have glimpsed something of the incomprehensible. I was worried, as the story progressed, that there were going to be too many loose ends - beings and objects named but not described. It's true that not everything is tied up by the end, but nearly everything is. What i would have liked, tho', is a lengthy appendix with a more detailed break down of the metaphysics and ontology of this great created world.
Apart from that little quibble, this is a wonderful wonderful work of hard SF from one of our greatest living visionaries.