There's a lot of issues with these kinds of examples. The theoretical math/physics examples like these make me laugh. Like you stated, why is the negative mass object affected by gravity at all? The concept of parallel universes just because the math works out, but only if you change something to a negative value. That's fun to talk about over a couple of beers while puff puff passing, but people start to take things seriously and then we get all sorts of problems.

> then we get all sorts of problems.

Like what?

String theory