The title sounds impossible and that is exactly what I presume this idea is which is why I am here... I thought of this the other night and it should be impossible but I am not sure which of the many "laws" which collectively tell us there is no such thing as a free lunch prohibit this from working. To clarify: This is just conjecture like Maxwell's demon or Schrodinger's (only much less clever I am sure)... I am not suggesting such a thing could ever be practically constructed... I'll start with my assumptions that way someone can give me a quick answer if I am simply starting with a wrong assumption: Accelerting an object that has mass to the speed of light would require infinite energy. Any energy used in setting an object in motion reletive to another could be recovered by stopping the movment... Ie. coil used to move a magnet could also recover energy by stopping the magnet. Black holes have an event horizon inside of which the escape velocity exceeds c. Black holes evaporate over time if they are small enough that their "tempature" is higher than the cosmic background radiation (assuming no other "stuff" added to them). So... Lets assume some advanced race were to desire a means for storing a great deal of energy. They take a black hole having a mass that puts its tempature just a tiny bit above the cosmic background radiation (thus it evaporates but not very quickly... This would be about as massive as the moon and have a diameter of about .1mm IIRC). Then they take a particle (or large number of particles) and set them orbiting around the black hole. Then they begin adding mass to the black hole... Not enough to make it quit evaporating and start growing but enough to expand the event horizon closer to the orbiting particles. At the same time they add mass to the black hole they begin increasing the speed of the particles orbit using remote means (such as magnetic fields if the particles are ferrous but I am sure other means are available). The increase in speed is enough to prevent the particle's orbit from decaying as a result of the increased mass but NOT enough to cause them to fly away... See where I am going with this yet? What is to stop them from increasing the mass of the black hole and increasing the speed of the particles orbit until the particle have reached, for example, 99.9999999% the speed of light and are just on the edge of the event horizon? Once they have done this the particles have a great deal of potential energy (depending on their mass which would obviously be quite small given the problems with the gradient of the black hole that would arise if they were large). Now if this hypothetical being were to want to recover a portion of their substaintial energy investment they could allow the black hole to evaporate while slowing the speed of this particle thus recovering energy from it. In this way could arbitary amounts of energy not be stored? And if so how is this possible? Energy and mass are equivelent and this particle or group of particles would have more mass the more energy you stored (reletively speaking... Things become more massive as they appoarch C) so it ok there but the wierd thing is the volume... It would be a device with a finite volume but capable of storing arbitrary amounts of energy? This makes no sense... What am I missing? Thanks.