On the idea of gravity, modern physics suggests there being no way of having a perpetual source of energy
may i derict you to the wikipedia article which states
"In the macroscopic world, energy spreads among more than one particle and therefore is shared – so loss of energy is always larger than zero, therefore an object in perpetual motion implies work being continuously done to overcome this natural retarding process"
in laymans terms, there is no way of moving something bigger than an atom forever.
This states that there would be no source of energy that would be able to create the force required to generate the sheer force required to keep a massive object such as the earth movie
Now in the RE theory, this is explained, as orbit, a state in which no energy is being used, it is a dual falling system (one which i only understand the basics of), and therefore no energy is actually wasted in its use