 Delbrück’s arguments were based on the relativistic quantum mechanics of Dirac according to which the QED vacuum is filled with electrons of negative energy or - in modern terms – with electron-positron pairs.These electrons of negative energy should be capable of producing coherent-elastic photon scattering because the recoil momentum during absorption and emission of the photon is transferred to the total atom while the electrons remain in their state of negative energy.This process is the analog of atomic Rayleigh scattering with the only difference that in the latter case the electrons are bound in the electron cloud of the atom.