To further clarify that the energy of gravitational mass must equal the energy of inertial mass, Einstein proposed the following cyclic process: (a) A light source S 2 is situated a distance h above a receiver S 1 in a uniform gravitational field. A movable mass M can shuttle between S 2 and S 1 . (b) A pulse of electromagnetic energy E is sent from S 2 to S 1 . The energy E ( 1 + g h / c 2 ) is absorbed by S 1 . (c) Mass M is lowered from S 2 to S 1 , releasing an amount of work equal to M g h . (d) The energy absorbed by S 1 is transferred to M . This increases the gravitational mass of M to a new value M ′ . (e) The mass is lifted back to S 2 , requiring the input of work M ′ g h . (e) The energy carried by the mass is then transferred to S 2 , completing the cycle.