  More recently, the ALPHA-g experiment has shown that antiatoms fall in the
Earth's gravitational field like ordinary atoms, with the non-negligible
possibility that antiatoms fall with a slightly lower acceleration. A possible
lower acceleration of antiatoms (which could be revealed with two orders of
magnitude higher precision measurements) would be an epoch-making discovery and
a window to a new physics, suggesting among other things that quarks and
antiquarks have gravitational charges of opposite sign. If gravitational
charges of opposite sign exist, it implies the existence of virtual
gravitational dipoles in the quantum vacuum, and opens up the possibility of
including the quantum vacuum as an important source of gravity in the Universe.
