Other way is rotor that is placing periodically the metal part in the area between magnet and coil of the generator to create alternating field instead of permanent field. It is so called alternator. So, output power is free if the permanent magnet field source is used to make the changes of energy density in the point of the power extraction. Antiphoton is reverse-time process. So, the power can be produced by means of potential field only as inner field structure disbalance.
Space power is unlimited.
In general view it can be named as balance law. Note that zero is not nothing. It is balanced structure. It is known from ancient mathematics. Cause and Two Contra-directional Effects for Spatial Separation Simple example for two spatially separated effects are operating from one cause is this magnet device, Fig.