In
theoretical physics,
negative mass is a hypothetical concept of matter whose
mass is of opposite sign to the mass of normal matter, e.g. -2 kg. 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. It is used in certain speculative theories, such as on the construction of
wormholes. The closest known real representative of such exotic matter is a region of pseudo-
negative pressure density produced by the
Casimir effect. Although
general relativity well describes
gravity and the
laws of motion for both positive and
negative energy particles, hence negative mass, it does not include the other
fundamental forces. On the other hand, although the
Standard Model well describes
elementary particles and the other fundamental forces, it does not include gravity, even though gravity is intimately involved in the origin of mass and
inertia. A model that explicitly includes gravity along with the other fundamental forces may be needed for a better understanding of the concept of negative mass.