I can say with reasonable certainty that there is no specific ruling on this in the book. Anything said here on the subject would be a house rule by the GM, or agreed upon by your Party.
Here's how I handled it.
I ran a campaign based in a weightless but air filled environment, where there was no gravity except made by magic, or by using centrifugal force. What I determined was the objects could not have negative mass, because physics does some seriously weird things when that happens(Source needed).
Objects with near zero mass would behave very similar to how light does, and time would probably be a bit wonky as well.
It basically boiled down to: OK. You can be weightless, but it also means that the only thing slowing you down once you get going will be air resistance.
The inverse essentially multiplied the damage associated with falling, and also made it rather hard to move (If not explicitly impossible) for low strength characters.
Bottom line is. This is a question entirely based on your GM's interpretation, and how much physics they care to bother with.
TLDR; The answer is, as far as I can tell that there isn't a definitive ruling on it. If it could be found in any of the books, I would look for it in the GM's book, or the extended combat rules in the Dominus Exxet. Neither of which it can be found in. I have read the Core Exxet, and can say that there are no mentions of anything remotely like this.