Antigravity In Anderson And Heinlein

I noticed the antigravity thread from Wells to Anderson and Blish only while writing the previous post. I should have mentioned Poul Anderson's Tales Of The Flying Mountains where "gyrogravitics" not only propels spaceships but also holds breathable atmospheres on colonized asteroids. In "- We Also Walk Dogs" in Heinlein's Future History, a means of controlling gravity has to be - and is - discovered or invented in order to enable extra-terrestrial delegates to confer comfortably on Earth but this seems to have been forgotten later in the series when interstellar spaceships simulate gravity with centrifugal force. There has been a Prophetic dictatorship and an interregnum of space travel between these instalments.

Gravity, electromagnetism and the strong and weak nuclear forces are the four forces of nature. Can there be a Unified Field Theory? Is it even theoretically possible to control gravity, surpass light speed or travel through time? Given enough time, a technological civilization should be able to reach the limits of the possible but what are those limits? There is some, although maybe a lot of, overlap between theoretical physics and speculative fiction.