4/8/2000, revised 5/19/02
Any two masses attract each other through the very very weak force of gravity. How does this force manifest itself? It can be attributed to the mutual attraction between the gravitons that are embedded in each of the masses. The gravitons can be thought of as key blocks that bind energy into mass and hold the particle pieces together!
Production and Destruction of Gravity in Everyday Situations
We have a common example where gravity is created and subsequently destroyed. This is the process of positron/negatron pair production caused by capture, in the vicinity of a nucleus, of a photon having an energy greater than the rest masses of two electrons, or 1.022 MeV. Two oppositely charged particles appear, each having an amount of kinetic energy equal to half the excess photon energy above the rest masses. Both charge and mass, and consequently gravity, have been created from energy. The electrostatic force between particles is very much larger than the gravitational force, so that gravity is usually ignored in the process.
When the positron loses all of its kinetic energy by collisions, it is attracted to an atomic electron by electrostatic force, and the two antiparticles annihilate to form a pair of 0.511 MeV photons emitted back-to-back to conserve momentum. Both charge and mass, and consequently gravity, have been destroyed.
It can be argued that the process of annihilation begins by electrostatic attraction but is completed by gravitational attraction! For particles of the same type, the Pauli exclusion principle acts like a repulsive force and prevents merger. But for antiparticles, there is no Pauli exclusion and the particles can continue to merge. The particles merge sufficiently until the gravitational force between their centers is unbearably strong, causing whatever binds the mass together to fail to do its job, which releases it to form pure energy.
In general, this is probably what happens to eliminate gravity when any two antiparticles, such as a proton and an antiproton annihilate. I argue that the same process can happen in a sufficiently large black hole, where the process is driven by gravity.
Gravitons and Graviphotons?
What is the mechanism for the production and destruction of gravity? One possibility is that the graviton can exist in two separate states. When two particles annihilate, the bound graviton particles dissociate by emitting "graviphoton" waves, thus releasing the mass as its photon equivalent. In contrast, when a pair of particles is created from an energetic photon, graviphotons have to be pulled out of the wave continuum to create the gravitons that bind the photon energy into mass! The Heisenberg Uncertainty Principle would hide the actual locations of the graviphotons, which in general have very small reaction cross sections, thus spreading out their range of interaction. This implies that there is a tremendous and essentially undetectable graviphoton flux everywhere in space, available to foster these reactions. This is analogous to the extremely large neutrino flux that continuously passes through us almost completely undetected!
Hence, gravitational force exists when gravitons are bound particles, and the force disappears when the gravitons have disassociated and have emitted graviphoton waves. The exchange between the two states goes on naturally on the microscopic scale when matter is created or destroyed.
Van Flandern  argues that the attraction of gravity between two distant bodies is due to the interaction between the gravitons embedded in the matter and some related entities that he calls c-gravitons. In producing this attraction, he has independently proposed that there must exist a tremendous flux of c-gravitons that produce the attractive force by a shadowing process! Hence, c-gravitons are apparently equal to graviphotons.
One can even argue that these c-gravitons/graviphotons may also be the equivalent of the long postulated aether that light and electromagnetic waves propagate through.
He further demonstrates that gravity acts essentially instantaneously at a speed orders of magnitude greater than light speed, because the measured motion of distant celestial bodies is not affected by delays in the transport of light between them. Hence, gravity acts by a different mechanism than electromagnetic attraction/repulsion, which both act at the speed of light. He then argues that wave/particle duality is a consequence of the interaction between light-speed entities and non-light-speed entities. An implied consequence is that a Grand Unification of all four forces, weak, strong, electromagnetic, and gravitational, is not possible.
I'm not sure that anyone has yet succeeded in fully explaining gravitational attraction. It acts as if there was a background field that instantaneously knows the positions of all masses even though the bodies are located a universe apart. It may operate by a bootstrap effect, a kind of whispering gallery where each mass continuously tells its neighbors what it knows in terms of instantaneous positions and higher space-time derivatives, and this gives it the effect of faster-than-light-speed analytic continuation to continuously map the entire space. One can postulate that the intensity of the graviphoton field at any point is a measure of the summed distribution of mass around that point, and that it is possible to anticipate the effect of motion of these masses by the time behavior of the resultant field.
I have argued that the Big Wave Model of a new Big Bang requires a coherent wave of photons and graviphotons moving out from the origin at the speed of light, creating neutrons as they go. If graviphotons moved faster than the speed of light, then they would get out of coherence with the photons. Hence, graviphotons must be components of a gravity wave that moves at light speed. Their role in instantaneous gravitational attraction has to have a more complicated explanation.
The above arguments raise an interesting new question. What are the effects of the leakage loss of photons, neutrinos and graviphotons at the edge of the universe on the reversibility of a closed universe? The only thing that makes sense is to assume that there is essentially a zero current boundary condition at the edge that exactly compensates for this loss. This implies that there are in fact billions of other universes distributed throughout the Cosmos, bumping up against one another, undetected, such that the leakage out of each one is exactly balanced by leakage in from its adjacent neighbors. On the grand scale, nothing is lost.
1) A. Karel Velan, The Multi-Universe Cosmos, Plenum Press, 1992.
2) Tom Van Flandern, Dark Matter, Missing Planets, & New Comets, North Atlantic Books, Berkeley, California, 1998.