[Moderator's note - split off from this thread as a new topic. Well its probably off the topic in this thread [Moderator's note - you got that right], but perhaps its the way the statement above is structured. Can one really observe a photon to start? My understanding is , when a photon is absorbed by a detector or your eye, it no longer exists. If it is reflected no information passes on , and the photon continues on its merry way , unaltered at the same frequency. If absorbed however , information bits /energy from the source Atom(or particle for a better choice) has been transferred to the Absorbing Atom(or particle). Have we actually "observed" a photon here , I think not. The photon is the carrier of information , perhaps the information itself. And when it has been absorbed we gain some information about what created it. We can manipulate its properties and path, but we have not observed it in "flight". Perhaps the correct wording is , a photon can only be emitted/absorbed once. Which leads to more deeper issues , when teleporting an entangled photon. OR Perhaps the wording should be , a photon can only be "experienced" once when being absorbed. I'm not sure about the correct semantics? Its a matter of interpretation really. Unfortunately those interpretations can't all be correct , and they have deeply profound implications which differ in how wee see the world. If I roll a billiard ball along the table , I receive the information about the ball's path through the absorption of photons it reflects during its trajectory. So I can extrapolate its future path using Newton's laws . I am observing the ball in "flight" through continuous measurements/observations. In the microscopic world we '"experience" a photon when consuming It. Perhaps that is why we are still silent on how it gets from source to detector.