Photon in QFT is e.g. a coupling between two electrons using below Feynman diagram - requires both emitter and absorber, they are switched in perspective of CPT symmetry.

So can photon be emitted if there is no absorber on the way?

A practical example: from radiotelescope with thermally excited resonator - while it is focused on absorption, in theory its antenna could also emit e.g. thermal excitations (seen as negative signal), but used frequencies are very difficult to absorb, often such directional EM wave would just travel to infinity through space, so from CPT perspective would have no emission mechanism.

Can radiotelesope emit if such EM wave would just travel to infinity?

Some would say that the necessary advanced wave handshaking from absorber to emitter is retrocausal in nature. ie actions in the present being influenced by future microstates.

This idea is central to the Transactional Interpretation (TI) of quantum mechanics. The jury is still considering the matter.

2 hours ago, Duda Jarek said:

Photon in QFT is e.g. a coupling between two electrons using below Feynman diagram - requires both emitter and absorber, they are switched in perspective of CPT symmetry.

So can photon be emitted if there is no absorber on the way?

A practical example: from radiotelescope with thermally excited resonator - while it is focused on absorption, in theory its antenna could also emit e.g. thermal excitations (seen as negative signal), but used frequencies are very difficult to absorb, often such directional EM wave would just travel to infinity through space, so from CPT perspective would have no emission mechanism.

Can radiotelesope emit if such EM wave would just travel to infinity?

I don't know much about QFT - and no doubt others will weigh in - but I had always thought the EM interaction between two electrons in QFT is mediated by virtual photons rather than real ones, and that a virtual photon is not a photon. There is what seemed to me, as a non-specialist, a fairly clear review of the distinction by Prof. Matt Strassler here: https://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/

I can't imagine there is anything in QFT that precludes a free (i.e. real) photon being emitted unless an absorber is identified. That seems nuts - what about the CMBR?

As exchemist noted, the interaction between electrons involves a virtual photon.

There’s no violation of CPT if you emit a photon with no subsequent absorption.

You keep trying to apply CPT when it doesn’t apply (thermodynamics), we’ve been down that path before, and you were told not to bring it up again since you show no interest in correcting your misconceptions