No one really thinks so.
Unlike the other forces, gravity does not act on matter in different charge states. Electric forces attract or repel because charged matter can be either positive or negative. Gluons which carry the strong nuclear force interact according to the 'color charge' of a particle.
Gravity acts only on the total energy of an object which includes its rest mass energy and its kinetic energy, along with the energy contained in its various fields. Total energy can only be positive for un-bound particles moving in free space.
Nevertheless, physicists have still to determine whether anti-matter 'falls' the same way as ordinary matter because it differs in its charge compared to its ordinary matter state. These experiments are being conducted even as I write this! Stay tuned! There will be a lot of excitement if an anti- electron is discovered to 'fall up' relative to the ordinary electron in the same gravitational field of the Earth.
Copyright 1997 Dr. Sten Odenwald
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