The argument is that they're impossible, because they require negative energy (which not only doesn't exist but doesn't make sense), so it's fine. In the prese

The argument is that they're impossible, because they require negative energy (which not only doesn't exist but doesn't make sense), so it's fine. In the presence of negative energy you can form closed time-like curves in general relativity, which would violate causality, but these are not valid solutions because negative energy is not a thing. It's just like saying you can travel faster than the speed of light in special relativity, you just need imaginary mass (ie. m^2<0), but mass is unfortunately a real quantity rather than a complex quantity so you can't.