Clockwork Rocket by Greg Egan is specifically about physics which does not exist here. "Hell is the Absence of God" by Ted Chiang deals with a world in which g

Clockwork Rocket by Greg Egan is specifically about physics which does not exist here. "Hell is the Absence of God" by Ted Chiang deals with a world in which god, heaven and hell are unambiguously real. Both are definitely "hard sci fi"