I’m close to finishing the Audible version of Clockwork Rocket by Greg Egan 📚 and have already bought the sequel. This is “hard” science fiction at its best. Egan has built a universe with remarkable properties simply by changing a minus sign to a plus in one equation that controls space-time geometry. As a result light’s speed varies with frequency, high speed travel causes travellers to age faster rather than slower, and plants emit light.
The physics has been worked out with great care and there is a lot of explanation in the book, backed up by Greg Egan’s website which has over 80,000 words of additional explanation, along with a hundred or more diagrams demonstrating how the orthogonal universe works.
However inventive the “Riemannian” orthogonal universe is, the book wouldn’t hold the reader’s attention without some interesting characters and an exciting plot - and it has those in spades. The dominant species in this “powderpunk” world are physically very different from us but have very recognisable cares, ambitions, and interests. (I was going to say “steampunk” but given the noticeable lack of any liquids in their universe this wouldn’t makes sense.) They are highly intelligent and appear to have no religion making their unique form of reproduction an interesting and rather disturbing social dynamic.
I’ve really enjoyed this book and can thoroughly recommend it to anyone who likes some light physics with their entertainment.