Interesting that everyone automatically defaults to Greg Egan when thinking of hard mind-bending sci-fi that explores concepts like these. I don't even remember how I got into Greg Egan, but do you have any other favorite authors who play with concepts like these?

In my head, Greg Egan is filed under the "Math-Fi" directory, as that's what I generally associate him with but he's very well versed in pretty much everything (Biology, Physics, etc).

>but do you have any other favorite authors who play with concepts like these?

Maybe Philip K Dick, although his books are not exactly hard science fiction. But they explore the nature of reality in often mind-bending ways.