James Cameron spoke about the science of Avatar at Caltech last month; Discovery has a summary.
“We tried to make it not completely fanciful,” Cameron told the crowd, which filled the auditorium. “If it was too outlandish, there would be a believability gap.” So while Pandora features floating mountains, that might not be so far-fetched, Cameron said, considering Earth has developed high-speed “bullet” trains that levitate on magnetic fields. Of course, the “reality-based” scenario did have its limits. “We figured that to actually lift mountains, the magnetic field would have to be strong enough to rip the hemoglobin out of your blood,” says Cameron. “But we decided not to go there.”
No word on whether he addressed the Na’vi’s lack of technological advancement.