where E R (0.002 eV for Fe) is the energy lost as recoil, E γ is the energy of the gamma ray (14.4 keV for Fe), M (56.9354 u for Fe) is the mass of the emitting or absorbing body, and c is the speed of light. In the case of a gas the emitting and absorbing bodies are atoms, so the mass is relatively small, resulting in a large recoil energy, which prevents resonance. (Note that the same equation applies for recoil energy losses in x-rays, but the photon energy is much less, resulting in a lower energy loss, which is why gas-phase resonance could be observed with x-rays.)