Permutation City† by Greg Egan
I've read a number of SF books and stories about virtual people in a virtual reality, AI's acting in a computer system with analogs of human bodies in an analog of physical surroundings, etc.† But I don't recall ever reading one that delves so much into the practicalities of such settings.† How would a human consciousness transferred into a virtual world respond to the feeling it was in an imitation world, "not the real thing"?† How would the simulation be simplified to avoid the computational burden of accurately representing every down to quantum events, yet be sufficiently realistic?†
Don't get me wrong, it's not an advanced college lecture, although some readers may find the quantity of ideas to be more than they prefer.† And the strength of the book is the ideas, not its literary prowess nor the quantity of story compared to the quantity of ideas.
Paul has been trying to resolve problems in virtual lives in virtual realities.† Most people who try a virtual life decide to abandon the virtual world.† Most people who remain are wealthy people whose age or illness will not allow them to live much longer outside the virtual world.† Paul uses copies of his own mind in experiments.† However, after his copies abandon the virtual world several times, he is faced with trying to solve the problem.
Maria has played with a kind of virtual world.† In order to minimize the necessary computational power to simulate a world, this virtual reality was designed with a simplified physics.† The simplified physics has various implications that make it behave differently than our universe.† Maria, and some others, has puzzled over how to represent forms of life capable of evolving within the simplified physics.† Simple bacteria-like life-forms have been created, but producing anything more sophisticated has been such an intractable problem that most researchers have given up and switched to working on other projects.
After Maria announces she designed a virtual microbe that evolved to a new kind of microbe, Paul hires her to design a simple virtual planet with life forms capable of evolving.† While Maria is working on the project, the police visit her.† They tell her that Paul is involved in a scam - selling rich virtual people what he claims is another life in a place safe from social and technological upheavals...
One theme of the book might be thought of as "reality" and simulation.† Should a virtual person in a virtual world feel it lacks "reality"?† How can you be sure if you're a "copy" in a virtual reality or an "original" in the outside world?† Are Paulís dust universes real?
The book is worth reading for the speculation - both the speculation about what might be possible in designing virtual worlds, but also the speculation about what the limitations, work-arounds and simplifications that may be needed to implement them.† However, as a story, it may be rather slim.† You may be disappointed if youíre expecting a novelís worth rather than accepting the story component of the book to be on the scale of a novelette or novella.