This seems to be a pretty old thread but i guess this is one i should choose to be my first post(ever)!!!
REgarding gravity, the basic assumption of calling all forces dealing with heavenly objects or otherwise gravity is wrong since the word itself is referred to the force of attraction b/w earth and any object within its gravitational field. all other forces may be referred to as gravitational forces. the thing with the plane that simulates zero gravity is nothing but the counteraction of free fall on "gravity".u may simulate the same experience in an elevator if u have the guts that is!!
Regarding simulation of gravitation b/w 2 object or for that matter even creating a zero gravity spot on earth, we have to goto the fundamental level of interaction b/w 2 bodies.gravitation like many other forces including strong,weak forces etc. is a particle transfer force that is particles are exchanged when a force leads to interaction b/w 2 objects.for gravitation it is merely called a graviton(whose anti particle is also predicted in the Standard Model).but the problem of actually observing a graviton is more complicated and efforts have been going on for the past few decades(in CERN and Fermilab) to actually observe one in transit or interaction.for this the nuances of the string theory must be explained and the ideas of multiple dimensions and membrane-like behaviour be clear.
here the inrteraction b/w heavy particles(certain bosons) is used for detection(actually the presence of the graviton is predicted by its absence!)
the gravitational force is fundamental and applicable to all masses in nature (even us! but our mass product is too low to counteract the small G value)but it so happens that sometimes gets over ridden by a dominant force.for eg: in case of particles less than 10^-8m the molecular forces take charge.
regardin repulsive forces b/w masses there can be as is the case with magnets of like polarity or charges.
Thus in conclusion if we want to produce gravitation it may be solely by being able to produce AND control a very large mass within a very small area(as in the case of neutron stars or black holes).
but if it is merely to create a force of attraction then we may resort ourselves to a standing on the edge (periphery)of a rotating body.
also note that there is no repulsion b/w planets but merely a balancing of forces ie. centripetal counteracts gravitational.
i could keep on writin but i guess this is enough!!!
no sources referred so please correct me if any of my points are wrong(even seemingly so).
thank u.
