Virtually all of Larry Niven's novels that don't have FTL touch on this. Also shows up in most of Alastair Reynolds' novels, and in the Three Body Problem series.

I know of Niven's "The Legacy of Heorot", but it only takes place on one planet.

Most of Alastair Reynold's novels have FTL - I only know of one that counts, which is Pushing Ice.

The 3-body problem has FTL also.

> I know of Niven's "The Legacy of Heorot", but it only takes place on one planet.

The chronologically earlier novels in his Known Space universe (the Ringworld one) are mostly around near-c Bussard ramjets.

> Most of Alastair Reynold's novels have FTL

Hmm, wait, which ones? Only one I can think of is House of Suns, in a very restricted manner.

> The 3-body problem has FTL also.

It has FTL communications, but not travel.

FTL communications is not less problematic than travel.

Niven's "A World Out of Time" is sublight, with time-dilation.

alastair reynold's revelation space series certainly doesn't have ftl, apart from maybe [jumper clowns spoiler deleted]; the conjoiner drives use [science] to achieve 1g acceleration to get to near light speed, and time dilation is part of many of the plots.

> the conjoiner drives use [science] to achieve 1g acceleration to get to near light speed

For [science] read [magic], more or less, but yeah. In the very first stories they were Bussard ramjets, but this got retconned out (and a Bussard ramjet actually shows up in a later book as a failed experiment). In most of the books they're more or less applied magic, though.

This, incidentally, seems to be a common theme, as later discoveries tended to fall down on the side of Bussard ramjets not working (due to insufficient density etc). The last of Niven's Known Space books have some special pleading for how the pilot has to carefully direct the ramjet to get sufficient combustion volume, a detail that was never present in the old ones.

Time for the Stars has FTL at the end. (Physical FTL, that is. Telepathic FTL exists at the beginning.) But it's otherwise it's deliberately structured around the twin paradox - the main character is even a twin.

Indeed, while I remembered the importance of time dialation in Forever War , I didn't remember the FTL travel between collapsars.

Spider Robinson's "Variable Star" is not hard sci-fi, but relativistic speeds and time dilation are relevant to the plot.