I think the portal thing is plausible, as I imagine it, the energy required to maintain the portal would depend on what is transported, so the faster the magnet falls, the more energy it would have and the more costly the portal would become to maintain, in such a way that if you were to build such a generator it would (at the impossible 100% efficiency limit, with perfect wires, no heat loss) only be able to power the portal itself.<br><br>(It would not accelerate to infinity because to reach 100% efficiency you would at each pass also need to recupe 100% of the object's added kinetic energy, so it would always reenter the other end at it's initial velocity, ie the load on the coil would push against the falling object with the exact same force as gravity does),(reply>), What Tom envisioned isn't a portal, it's a teleport shredder. A portal would have a continuous potential energy function so you can't extract energy out of it.