Although no particles are known to have negative mass, physicists (primarily Hermann Bondi and Robert L. Forward) have been able to describe some of the anticipated properties such particles may have.
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Later, by slightly modifying the Van den Broeck metric, Serguei Krasnikov reduced the necessary total amount of negative mass to a few milligrams.
In 1995, Visser et al. proposed that cosmic string could theoretically also exist with an angle excess, and thus negative tension and hence negative mass.
Working in collaboration with, among others, science fiction writers Cramer, Forward, and Landis, Benford worked on a theoretical study of the physics of wormholes, which pointed out that wormholes, if formed in the early universe, could still exist in the present day if they were wrapped in a negative-mass cosmic string.
Some phlogiston proponents explained this by concluding that phlogiston had negative mass; others, such as Louis-Bernard Guyton de Morveau, gave the more conventional argument that it was lighter than air.