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negative mass
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.
==In general relativity==
Negative mass is generalized to refer to any region of space in which for some observers the mass density is measured to be negative. This could occur due to a region of space in which the stress component of the Einstein stress–energy tensor is larger in magnitude than the mass density. All of these are violations of one or another variant of the positive energy condition of Einstein's general theory of relativity; however, the positive energy condition is not a required condition for the mathematical consistency of the theory. Various versions of the positive energy condition, weak energy condition, dominant energy condition, ''etc.'', are discussed in mathematical detail by Matt Visser.
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