Negative mass is a hypothetical counterpart to ordinary (positive) mass. Such matter would violate one or more energy conditions and show some strange properties, stemming from the ambiguity as to whether attraction should refer to force or the oppositely oriented acceleration for negative mass.
Although it is not known if negative mass exists, or even if its existence is theoretically possible, several scientists have speculated on its properties. Among these are Hermann Bondi in the 1950s, Banesh Hoffman (1906-1986), of the City University, New York, in the 1960s and ’70s, and Robert Forward, in the context of spacecraft propulsion, in the 1980s. In both Newton’s and Einstein’s theories of gravity, negative mass is a requirement for antigravity to exist.
The concept of negative mass arises in the first instance by analogy with electric charges, of which there are both positive and negative varieties. Just as a positive electric charge can be canceled by a negative charge, thus giving rise to the possibility of screening against electric forces, so we can envisage the possibility of “gravity screens,” if negative mass existed to neutralize ordinary, positive mass.