Gravitational dipoles

The physicist Robert L. Forward proposed in one of his books the idea of a disc of degenerate matter (found in white dwarfs) or neutron matter (found in neutron stars) held suspended above the ground.

Degenerate matter is extremely dense, and the constituents do not occupy the same quantum state only because of the degeneracy pressure due to the Pauli exclusion principle. Neutronium is also extremely dense, and it is formed when the electrons of an atom combine with protons under great pressure until only neutrons are left behind.

The disc would be made up of matter so dense that it would have its own appreciable gravitational field, which would combine with that of the Earth above the disc, while a truly zero-gravity zone would be created below it.

A similar idea consisted in running the neutronium through coils to create a zone, within such a ring, where the total net gravitational field would be zero.

The idea is theoretically feasible, were it not for the fact that both degenerate matter and neutronium are currently impossible to find or create, as well as being extremely unstable at the pressures found on our planet.