Let's assume that negative mass is possible. How would it react with gravity? Since F=ma, the acceleration would be in the opposite direction to the force applied. Keep this in mind Now, the gravitational force, F=GMm/r2. Usually this force acts towards the centre of the Earth since 'G' is negative, which makes the overall force negative. Negative forces attract. If mass is negative, then the force would be positive, and act away from the Earth. Positive forces repel. But since the object has negative mass, it accelerates in the opposite direction to the force, so fall towards the Earth! This makes sense if you think about it because all objects fall at the same rate regardless of mass, even massless objects Am I right?