Photon–photon scattering limits the spectrum of observed gammas to a photon energy below 80 TeV, that is, a wavelength of more than ~ 1.5×10−20 m. The other photon is one of the many photons of the cosmic microwave background. In the frame of reference where the invariant mass of the two photons is at rest, both photons are gammas with just enough energy to pair-produce an electron–positron pair.
80TeV means frequency in the area of 10^27 Hz, can you explain the meaning of "observed" how, where? what I do not understand is if they are sayng that scattering can take place below that threshold, can you explain if that is the case and why?
One would expect that such scattering is only possible at very high frequencies, because ordinarily photons do not interfere with one another. What am I missing?