It’s been a while since I looked into it, but my impression was something like “general relativity allows it if you use some sort of exotic matter in a way that isn’t clearly possible but isn’t clearly crazy”. I could imagine that intelligent agents could create such conditions even if nature can’t. The Internet Encyclopedia of Philosophy has a decent overview of time travel in general relativity.
It’s been a while since I looked into it, but my impression was something like “general relativity allows it if you use some sort of exotic matter in a way that isn’t clearly possible but isn’t clearly crazy”. I could imagine that intelligent agents could create such conditions even if nature can’t. The Internet Encyclopedia of Philosophy has a decent overview of time travel in general relativity.