Photons which have experienced a change in frequency (red shift) due to gravity(or other red shifting affects), have necessarily lost energy. But total energy is conserved. Can someone please explain in what form is the energy lost? Also related to the red shifted light subject: Electrons emit and absorb energy in quantum values. However, after an adequate amount of red shifting, photons of a given emission will lose the ability to excite the same (equivalent) electron to that higher energy state. What is the tolerance? For example say an electron in the outer shell of some atom can be excited to higher state by photon of 5500 angstroms. What would happen when the electron encounters a red shifted photon of 5505 angstroms. Will the electron be excited to a certain extent, but be unstable?