To show that the equivalence principle necessarily implies the gravitation of energy, Einstein considered a light source S 2 separated along the z -axis by a distance h above a receiver S 1 in a homogeneous gravitational field having a force per unit mass of 1 g . A certain amount of electromagnetic energy E is emitted by S 2 towards S 1 . According to the equivalence principle, this system is equivalent to a gravitation-free system which moves with uniform acceleration g in the direction of the positive z -axis, with S 2 separated by a constant distance h from S 1 .