Slow start, hard to get into, very heady, but worth sticking around for if you like sci fi that makes you think and goes beyond pop fic. Really fascinating and well-written. Takes on science and human nature in equal turns and ends up very literary. Probably the most believable time travel work I've ever encountered in print or film - and I love time travel works. I frequently get frustrated by writers' inability to maintain a coherent logic with time travel narratives and multiversal madness, but Palmer completely sidesteps that by sticking to a basic set of the laws of physics and discounting the multiverse in favor of a single contained-system universe and working through how time travel would thusly work. I kind of wanted to rate it more like 4.25-4.75 stars because of how long it took me to get into it and stay with it and for some of the more "out there" aspects of the book, but the way it's gonna sit in my mind for a while and make me ponder the universe definitely feels like a 5-star read. Very magisterial work on the whole.