Broken off from another thread: So here's a simple question for which the answer is not so simple using current theory: Do the photons from the CMBR exist free and independent of observation? Because some people say that photons are mediators of EM force but are otherwise abstractions. For example, this quoted from Mentz114 in an older thread: "Photons" only exist at the moment they are emitted or absorbed i.e. when they interact with matter. There is no evidence ( nor any way of getting any ) that photons exist in the EM field when it is not interacting with matter. Speculations about 'free' photons usually lead to apparent contradictions, as evidenced by your question." 1. If the answer is "NO, there are no free photons" then that means that CMBR "knew" (at the time of emission) we would be here 13 billion years later to witness those particular photons. Pretty amazing feat. And yet fully consistent with QED. 2. If the answer is "YES, many CMBR photons are free and will never be detected" then this implies: a) that those photons could be considered "dark" (in the sense they will never land anywhere); and b) that in an accelerating/expanding cosmology, more and more energy from stars will end up as similarly "dark" over time. It seems to me that an experiment would be possible to discern these 2 scenarios as well. Imagine a heat source in space surrounded by detectors. Now remove one of the detectors such that in one direction, the source is exposed to darkest space. In scenario 2 (Yes there are free photons), you expect no difference to the rest of the apparatus. But in scenario 1 (No free photons are possible), you might expect that photon emission would be suppressed in a particular direction (where there is no matter to absorb them). You ought to be able to account for that difference using conventional means. Any thoughts?