I have not yet finished this book but it is living up what I have come to expect from Le Guin-a complete world governed by different social structures, deep characters, and the painful feeling that I am missing out on something whenever I am forced to put the book down.
The book is structured both sequentially and simultanouesly according to concepts in phsyics explored by Shevek, the protagonist. This concept is not so difficult to me because it is at the heart of the Jewish calendar, or more correctly the heart of the conflict between the Jewish and the secular calendars. On the face of things, the calendars are only different in that the former is a lunar calendar while the latter is a solar calendar. The both have seven day week cycles, they are marked by (semi)lunar events called months, each annual orbit of the sun begins a new calendar year, and the months are sprinkled with secular and/or religious holidays. However, if you live with both, they begin to take on different meanings. The Jewish calendar becomes one of tradition, of repeating cycles. The Sabbath grounds the week and returns one constantly to peace, to the celebration of life, and to God. The secular calendar marks the forward movement of time through life, the daily grind. The cycle and the progression are part of each other.
Oh travel through the heavens! This is not what I meant to write about but I have wandered into time and physics. I will give it a go.
Shevek tries to explain time like a book. Everything is in the book already but we must move through it from one end to the other. We are only readers of a small portion and we are only able to understand our portion if we move from beinning to end. If you have read Slaughterhouse Five, you will be familiar with this understanding of time as this is how Tralfalmadorians view time, except that they can see the end as easily as the beginning and move fluidly through it. This naturally becomes determinist, lackign free will. Shevek tries (maybe he eventually succeeds) in reconciling his theories of physics and time with free will. I would struggle to put the two together, too, but I am not worried about free will. It doesn't seem worth worrying about since, unless you are Kilgore Trout, you don't get to change whatever the status quo is.
The two worlds, Anarres and Urras, revolve around each other, referring to one another as moons of their earth/world. So, too, do their civilizations and governments pull and repel each other while travelling through their orbits. I haven't developed this thought enough to comment on its relationship to the theories of physics and time that are central to the novel.