A combination mainstream and extremely non-mainstream scientist and philospher, Freeman, longtime professor of physics (now emeritus) at the Institute of Advanced Study in Princeton, has been being variously honored as the recipient of the Enrico Fermi Award (awarded by the President of the United States) and the Templeon Prize for Progress in Religion (in a nomination arising from the Southern Nazarene University in Bethany, Oklahoma).
But probably most sci-fi fans will know Freeman as inventing (conceptionally at least) the Dyson sphere. And those who know this tidbit of information most likely learned it from reading Larry Niven's best seller Ringworld.
A Dyson sphere is a hollow sphere of super strong material surrounding a star. The size of the sphere is just right so that life can exist on the inside surface. The point is that it is the ultimate cure for population growth - or at least it postpones the problems longer.
There is a problem with creating gravity, though. So Larry's idea - a rotating ring where centrifugal force (yes, yes, we know it's really centripetal force) keeps everyone on the surface - was a relatively simple solution. There is also just a wee bit of a problem on how you go about building such a contraption. There are various versions of Dyson spheres, the building of some (so we hear) can be started today. Freeman thought the spheres could be built in a few thousand years after civilization reaches a state of high technology. But as yet no one has started the construction of a Dyson sphere.