At least thirty years back, there were some papers published discussing the possiblity. I mean scholarly papers, from physicists, who were looking at the mathematical models in Einstein's theory of relativity and examining them under unnatural, possibly impossible conditions.

Google for "Tipler machine," "Kerr metric space warp," or "closed timelike curve" and you will see some very brilliant stuff.

I don't know whether any of it was ever found to be mathematically inconsistent with relativity, or proven to be mathematically impossible, though the reduction-to-practice aspects are daunting.

A Kerr metric space warp, for instance, would require the mass of a star crammed into a superdense black whole whose core was not in the natural spherical shape that gravity would push it into, but, rather, a torus--a ring, in other words--that was spinning at almost the speed of light. Pass through the center, go into the past. Or something like that. It's a fascinating idea but you can't exactly build one in the garage.