It costs energy to get rid of it. The net mass-energy of equal positive and negative masses will be zero. Another question that we can now answer is whether, as Gene Wolfe's Autarch has implied in The Citadel of the Autarch , antimatter has negative mass. It does not. We know this from recent experiments with antiprotons at the LEAR facility at the CERN laboratory in Geneva in which antiprotons are scattered from normal matter protons at low energies.
Antiprotons have a negative electrical charge, or at least they appear to.
But if they had negative inertial mass of the type Bondi considered, they would really have positive electrical charges but would appear to be negatively charged because they respond perversely to every electrical force applied to them. In analogy with the case of gravity, the anti-proton would be attracted to a nearby proton, but the positively charged proton would be repelled by the positive charge of the antiproton. The two particles would both accelerate in the same direction, with the anti-proton chasing the proton. No such bizarre behavior is observed in the LEAR experiments, which can be taken as experimental evidence that antiprotons have positive mass. However, the problem mentioned above of attaching the rope to m - applies here too.