I understand that energy of photons is defined by their wavelength/frequency. This frequency (and so energy) will be different for different observers: observer moving towards the photon will see higher frequency than observer moving away from the photon. So basically, the photon might look more energetic from one reference frame and less energetic from some other. It means that photon's energy is based on the observer?
When there are two highly-energetic photons, pair production of electron/positron might take place. How can this actually happen when the energy is based on the observer? It would mean that some observer might see pair production and another one wouldn't see it at all?