However, there is nothing in physics actually forbidding the construction of these devices. Since (as every fule kno) E=mc^2, it is theoretically possible to completely convert light->mass and vice versa at no cost. We just don't know how yet :-). Similarly, it is theoretically possible to have a 100% perfect mirror.
The mass is converted into light by the mass->light device. The light is collected using the perfect mirror and reflected upwards. At the top of its journey, the light is then converted back into mass. The mass is then attached to a pulley and dropped back to earth where the process begins again. This process can continue indefinitely and produce power.
How it doesn't work.