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If I'm understanding some of Feynman's work correctly, an emitted photon must eventually be absorbed - i.e. it will not be emitted in the first place if it cannot be eventually absorbed. (A hand-waving explanation for this seems to be that light is constantly exploring possible routes with 'virtual' particles and since light travels at c, from its point of view its start and finish occur at the same 'time', and only when it finds a plausible route from start to finish do you ever get a real photon.)

My question is, can this no-emission-without-absorption phenomena be used to do some sort of reverse-telescopy in astronomy? i.e. Attempt to shine light at different parts of the sky and parts of the sky that cannot absorb that frequency of light (because there will never be anything in that direction that will intersect with and absorb the photon) won't be able to have light shone at them. Could we map out the future sky (after all, what we would be seeing was where objects able to absorb light would be in the future when our emitted light reached them in millions of years) by measuring different levels of emissions from our laser in different directions and frequencies?

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Unfortunately, in order to determine if a photon is emitted or not, a detector needs to be put in the path. The detector absorbs the photon, confounding your experiment.

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  • $\begingroup$ Actually, in principle, you could measure recoil from the photon, not necessarily absorb the it, to know that it has been emitted. $\endgroup$ – Ruslan Apr 11 at 6:12

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