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Century Rain (Alastair Reynolds) starts out giving the impression of a time travel story, with a detective/musician in an alternative late 1950s French and an archaeologist living on an large habitat complex orbiting a post-nanocaust earth.
 
Plot (Spoiler):
 
It turns out that the 20th century earth is in fact a replica of a pre-second world war original (made by an unknown entity) that has now been 'woken' from a form of stasis. It is presumed to be encased in one of the many giant, near impenetrable, spheres dotted around the galaxy. These were discovered during exploration of an abandoned wormhole network which turned up no other signs of intelligent life.
 
 
Access to the artefact (earth two or E2 as they dub it) is only possible using a unique wormhole from under the surface of Phobos (a Martian moon). The point at which the artefact's timeline diverges from that of the real thing coincides with past occupation of the moon by enemy forces: Slashers. Things are made inexorably more difficult and risky because no technology beyond the level already present inside the artefact is permitted to enter it via the wormhole gateway courtesy of a mysterious "Censor" membrane.
 
The crux of the plot is that Slasher "War Babies" (a group of genetically engineered, warrior creatures, originally with the appearance of small children) have influenced the artefact earth's events to avoid the second world war and therefore the resulting sci/tech progress in rocketry, atomic physics, etc to reduce the copies' chances of defending themselves or discovering their encased situation. Their mission appears to be to be to use build and locate the artefact's position by operating large detectors to pick up the only signals that can penetrate the shell: gravity waves.
 
Clever subterfuge allows the War Babies to send the necessary information back through the gateway in the possession of unwitting Thresher operatives. Although there appears to be a gaping plot hole in that the map with the exact positions of the 3 gravity wave sensors (which is mentioned repeatedly) never ends up being used, despite it's vital necessity in the triangulation process, and previous convenient exclusion from theft (most annoying bit of the book).
 
With the location of the correct sphere, Slashers then break it open and attempt to wipe it clean of humans using a self replicating nano-weapon called "Silver Rain" (which is of course averted by the protagonists of both time lines working together).
 
 
Critique:
 
As in his Revelation Space universe, Alastair Reynolds is successful in rendering a future of disparate factions of humanity. Not only the Thresher-Slasher split, but the chaotic political organisation of the Slashers: with moderates fighting to protect the antiquated and practically defenceless Thresher society, and hard liners (who are the enemy here).
 
What is lacking (again, as in all of his novels to date) is any mention of Trans Human AI, or in fact any AI beyond the sophistication of now. Perhaps Reynolds is in the Penrose camp; denying the possibility of Strong AI, or maybe it's just convenient to ignore for the sake of his plot lines...
 
Anyhow, he at least embraces the first two of the three coming technological revolutions (as extolled by the likes of Ray Kurzweil). The Slashers personal appearance and nanobot symbiosis is cool, but limited to their human form. The argument presented is that, although the resident nanobots can hold many of the memories of a host, a personality (a person) is a delicate thing [which is kinda true in that our brains are only the slinging together of memories, learned skills and chemical biases] and will be lost without a biological body to be anchored to [kinda lame].
 
The Slasher ships, however, are extremely amorphous (dynamically reshapeable thru-out) and self repairing with instant manufacturing capabilities; A natural product of nanotechnological progress that seems reassuringly realistic to me.
 
I like the extrapolation of a modern phenomenon: Slashdot (the website and it's participants) into the "Slasher" faction, at least in part as it makes me feel personally involved in the story. 
 
The way the more significant events in the past of the future time line are gradually pieced together is fairly artful, gradually presenting a scenario in which nanotechnology has run amok and the worst case scenario has been realised: grey goo! (well, to an extent). The not so immediately obvious result of mass information loss is also visited here, with the Threshers delicately picking though the wreckage of earth in a vain attempt to reconstruct and preserve history (somewhat "12 Monkeys" esque). 
 
Some interesting concepts raised:
 
The Thresher faction in the book directly represent an argument against increased digitisation and the misplaced faith in information stored this way. Although the usefulness of buried 'time capsules' containing analogue information and artefacts has long (since at least age 12) seemed humorously pointless to me (due to the increasing amount of records and information kept on current history), the possibility for large scale history loss now seems plausible. Given a sufficiently catastrophic event it is feasible that modern civilisations could again be lost to the degree of Aztecs, Incas, Egyptians (or even mythological Atlantans!). Even now vast tracks of digital information are being lost daily from old web pages being removed without being backed up to internet achieves to documents and photos lost to hard drive crashes. 
 
The nanocaust forms the Thresher society who neurotically cling to safe, if antiquated, technologies to try to avoid these dangers. In so doing they make great sacrifice, progress wise, and live backward, relatively uncomfortable, unextended lives.
 
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In the story, during the time when the worm hole connection is severed, no time passes on E2 (as observed by the rest of the universe). This would suggest that the sealed shell is holding the contents in complete quantum isolation. Because no information can reach or escape the object (excluding gravity waves, which is slightly dodgy) then the quantum state of the interior can remain absolutely static. This is somewhat like the old chestnut: if a tree falls in a forest but no one hears it does it make a noise?. I.e. in reality, do objects have to share some form of interaction to share a common time frame? It seems unlikely that quantum isolation could be artificially achieved in the real universe. Although the event horizon of a black hole or such like might suffice.
 
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The main future character (Verity) forwards the idea that the entire E2 could be within a computer simulation. This is because they have to pass through the discontinuity of the censor barrier which will not permit them to take through any advanced equipment that could be used to probe the environment beyond the barrier for flaws that would distinguish it from reality. But even if it let everything through there would still be no way to know for certain if it was a simulation as long as it was realistic enough to fool whatever tests they came up with. 
 
The characters presumed that the inhabitants of E2 would find out they were in a shell sooner or later with sufficient progress. But there need not be an obviously visible boarder change over point from reality to simulation (like the censor might have been). Provided the boundary interacts with reality as would be expected of any other open plane of space there would be no way to tell (i.e. if the boundary could produce the necessary holographic EM radiation, particle creation, forces, etc, then the boundary could be placed anywhere). Any space probes that reached the interior surface of the sphere could simply have been disassembled into simulation and their simulated behaviour used to send back the necessary signals back to E2 to convince them it was still passing through real space. Even real life astronauts would not be able to tell they had been disassembled and converted to pure information, being reassembled upon return to the real.
 
The same question can be applied more generally: how do we know we're not in a simulation now? Well, we don't and we can't know for sure if the entire universe we perceive is actually running on some massive computer. That the universe is a simulation is the belief of a dominant, galactic, religious movement in Iain M Banks' Algebraist. There the movement aims to convert a sufficient percentage of conscious beings to the same belief in order that the entities running the simulation end it and allow them up to a high level of implementation (i.e. out of the simulation). 
 
This is an intriguing possibility, but one that is futile to pursue. Even if the universe is run on a divine computer we may as well carry on regardless as it makes no practical difference if no observations can be made to determine evidence of it. 

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