Not that I believe this would work, but I have a perpetual motion machine idea.

I'm told you can convert between energy and mass. So first step, take a bunch of mass on Earth's surface, turn it into energy. Shine it as light up to a space station in Earth orbit. Collect the light, turn it back into mass, drop it. Collect kinetic energy, repeat.

Why wouldn't this work? Is there a slightly different energy-to-mass ratio depending on where you do the conversion? (Edit: I just realized this would give a way to tell the difference between "You're in an elevator accelerating upward" and "You're in an elevator standing 'still' on Earth" from the inside, which if I remember correctly you're not supposed to be able to do) Would the light lose energy as it traveled upward (Does differently-shaped space redshift it)? Is the answer the same if instead of gravity you used another force? (Say Earth was positively charged, and you converted negatively charged mass to energy, and back)