I recently posted an abbreviated explanation of Paul Steinhardt's and Neil Turok's (Princeton, Cambridge Universities,respectively) cyclic model of the universe under the COSMOLOGY forum here. That model involves perpetually colliding parallel branes with our universe enclosed between them. The following is the author's explanation as to why the energy doesn't run out, and I'd like to know how gravity provides such an endless source of energy since it would appear to be possible to power many possible "perpetual motion" applications...Or maybe my question should be the opposite: How can negative gravitational potential energy power a positive energy process?? If gravity is a "bottomless pit" of energy why can't we tap it??