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## Main Question or Discussion Point

Hi,

Etherington't reciprocity theorem states that distances measured by angular separation and by luminosity differ. My question is which one (if any of them) is the

Etherington't reciprocity theorem states that distances measured by angular separation and by luminosity differ. My question is which one (if any of them) is the

*actual*distance. I can understand they might differ in an expanding universe, but there's still a physical distance in such one, so... yeah, which one is it?