As far as I understand it, the Doppler redshift requires either a moving physical source or physical observer. Does this mean that a photon moving in a vacuum can not be redshifted UNTIL it comes into contact with some form of moving matter (the observer)? And presumably a redshifted photon in a vacuum must have been produced by a moving physical sources?

Is there any energy exchange with the source or observer (radiative transfer?).

Regards,
Ian Tresman