Pohl, Frederik. The World at the End of Time.

NY: Ballantine, 1990.

I’ve been a fan of Fred Pohl since The Space Merchants in 1952 and I had assumed I had read, over the years, every novel (and most short stories) he ever published — but somehow I seem to have missed this one. There are two main characters here, one human, the other very much not, but both of them extraordinarily long-lived — again, one in the normal course of things, the other as a result of screwing around with relativistic effects.