Man this is both a fun book, and one that gets a little too “in the weeds” later on. We start on Earth something like maybe a 1000 years into the future, with a very young feeling 200 year old man, Louie Wu, being approached by an alien (species known, individual not known) asking him if he’d like to go on a kind of adventure. He says yes because he’s bored on Earth, and in the last few nights on Earth he throws a big 200th birthday party, meets a 20 year old woman is the great great granddaughter of his ex girlfriend and she joins the trip. Also joining is a castoff member of a warlike feline race.
So the trip is to use an experimental hyperdrive ship to investigate a “ring world”, a constructed planet (kind of planet) that circles a star similar to the Earth’s sun. The ring world is a variation on a Dyson sphere, which is a gigantic sphere that encloses a star at the distance of a habitable planet’s orbit. So you have to imagine a planet like structure the size of the Earth’s orbit (but spheroid), and to do this, you have to remember that every planet in our solar system would fit comfortably in this space as well. Ok, so this thing is huge. It’s described as being about an Earth’s diameter wide, but then several million Earth’s in circumference, and is apparently designed to provide living space for trillions of trillions of beings. The designed apparently chose a ring, because of how much more efficient it would be and because if you built a sphere, the centrifugal force would still cause the densest parts of the matter to move to the equator.
So they visit it, and it’s huge, but also completely fallen and primitive. What was once an advanced society, has clearly collapsed into more primitive societies.
Scale is the most interesting and most impossible parts of the this novel, of course, given that every living being within “Known Space” could live on this structure, it’s hard to imagine there being anything outside of it.
But the novel is fun. Imagine a shadow version of Hitchhiker’s Guide to the Galaxy that takes itself at least a little seriously (and this came first by several years).
(Photo: https://www.amazon.com/Ringworld-Larry-Niven/dp/0575077026/ref=tmm_pap_swatch_0?_encoding=UTF8&qid=1574169800&sr=8-2)