Looking at gravitational and electrical interation relations, I had an idea: F = kQq/r^2 F = GMm/r^2 Couldn't it exist negative mass? First formula says that electrical charges reject each other if they have same sign, and they pull if they've different sign. We know that masses with same sign attracts; so, second formula "should say" that masses with different sign reject. How could we demonstrate negative masses do exist? How would a "negative mass black hole" behave?!?