The nine billion names of God

This book has some great short stories, one of them being "The Wall of Darkness". It is about a strange and very small universe and what that could look like for the inhabitants exploring it. Any time AC Clarke writes about a fantastic science fiction, I know it to be realistic in the sense that it is within the realms of the scrutiny of science possible. That's what makes his stories so enjoyable.For those of you who love hard science fiction (science fiction stories with some existing science