Einstein could not fully justify his rate equations, but claimed that it should be possible to calculate the coefficients A i j , B j i and B i j once physicists had obtained "mechanics and electrodynamics modified to accommodate the quantum hypothesis". Not long thereafter, in 1926, Paul Dirac derived the B i j rate constants by using a semiclassical approach, and, in 1927, succeeded in deriving all the rate constants from first principles within the framework of quantum theory. Dirac's work was the foundation of quantum electrodynamics, i.e., the quantization of the electromagnetic field itself. Dirac's approach is also called second quantization or quantum field theory ; earlier quantum mechanical treatments only treat material particles as quantum mechanical, not the electromagnetic field.