I remember one of the speculative ideas kicking around a while ago was the notion that mass, like charge, could be "positive" or "negative." This was different from matter and anti-matter in that "negative mass" would be made up of "negative energy." I know that negative energy can be produced in tiny quantities, and I also know that negative energy is part of Kip Thorne's theory of expanding quantum foam into "time portals." So what is the theoretical status of negative mass? Still speculative? Unsupportable? totally bogus?