            By 1912, however, Einstein had reached an impasse  , realizing that he needed to go beyond the mathematics that he knew and was familiar with.
<meaning-changed>             By 1912, however, Einstein had reached an impasse  , realizing that he needed to go beyond the mathematics that he knew and was familiar with.
 The energy arriving at is greater than the energy emitted by by the potential energy of the mass in the gravitational field. Hence corresponds to the gravitational mass as well as the inertial mass of a quantity of energy. By 1912, however, Einstein had reached an impasse , realizing that he needed to go beyond the mathematics that he knew and was familiar with.