section_path: Quantum field theory/Contributions to the mass of a system
section_name: Contributions to the mass of a system
paragraph_text: Contributions to the mass of a system
  
  The energy of a system that emits a photon is decreased by the energy E {\displaystyle E} of the photon as measured in the rest frame of the emitting system, which may result in a reduction in mass in the amount E / c 2 {\displaystyle {E}/{c^{2}}}. Similarly, the mass of a system that absorbs a photon is increased by a corresponding amount. As an application, the energy balance of nuclear reactions involving photons is commonly written in terms of the masses of the nuclei involved, and terms of the form E / c 2 {\displaystyle {E}/{c^{2}}} for the gamma photons (and for other relevant energies, such as the recoil energy of nuclei).
  
  This concept is applied in key predictions of quantum electrodynamics (QED, see above). In that theory, the mass of electrons (or, more generally, leptons) is modified by including the mass contributions of virtual photons, in a technique known as renormalization. Such " radiative corrections " contribute to a number of predictions of QED, such as the magnetic dipole moment of leptons, the Lamb shift, and the hyperfine structure of bound lepton pairs, such as muonium and positronium.
  
  Since photons contribute to the stress–energy tensor, they exert a gravitational attraction on other objects, according to the theory of general relativity. Conversely, photons are themselves affected by gravity; their normally straight trajectories may be bent by warped spacetime, as in gravitational lensing, and their frequencies may be lowered by moving to a higher gravitational potential, as in the Pound–Rebka experiment. However, these effects are not specific to photons; exactly the same effects would be predicted for classical electromagnetic waves.