Exotic matter – matter with negative mass - has long been the pipedream of science fiction writers, futurists, and certain rather. . . optimistic researchers. It’s the key to faster than light travel because it’s the only stuff that can curve space in the right way to hold open wormholes and construct warp fields. And if you can travel faster than light you can also travel backwards in time. We’ve been over those already. And we also recently covered a very new use for negative mass: as “dark fluid”, a proposed explanation for both dark matter and dark energy. That episode really got me thinking about the subtleties of negative mass and how it should really behave gravitationally. Turns out it’s complicated, and to answer it we really have to question the very definition of mass.