I'm at a bit stumped with respect to simple thought experiment. Imagine I fire a photon at a mirror and observe the photon returned. First, I send off a photon. It traverses space to the mirror. It bumps into an electron in the mirror, gets absorbed, the electron has no stable orbit for the new energy level, so returns to its previous state emitting the excess energy as a photon. This new photon happens to come back at me, and I observe it. Lets simplify it to three stages: travel1, bump&grind, travel2. Now, when I actually observe the returned photon, is when the mirror emitted it. In other words, travel2 is zero (by definition). There is no distinction in my timeframe. If I measure the total time that passed however, and subtract the time it takes for the mirror to send me a photon in return (i.e. subtract the b&g part), I know this equals travel1+travel2. But travel2 is 0, leaving me with a travel1 too big (by a factor of 2) and hence an overestimate the distance to the mirror. I'm not sure where I'm failing. Is it simply that I didn't bother to Lorentz transform? If I transformed my and the mirror frames, would I find that the distance is half that which would be suggested by 'travel1'? But then, wouldn't I also find that time has elapsed between the mirror's emission event and my observing it? Wouldn't this contradict defining travel2 as zero? Note that I'm not so much interested in actual measurements, or making it add up, as I am in figuring out the correct relative timeline for the events - in my time frame.