Thanks. Yeah it’s funky stuff. Turns out I called it wrong earlier too.
I said the mirror’s clock would be sped up compared to laser guy’s, but if that were true then our clocks would be sped up compared to a light beam’s, and light would take infinite time to get anywhere (from our perspective). But actually, when time slows for something moving fast, that means their clock slows.
Getting back to the example of the guy on the train, if he shines the laser forward, and the train is 186,000 miles long from his perspective, we on the ground would see the train as shorter, so by our clocks it would take less than a second for the beam to reach the end. From train-guy’s perspective it would still take a second. So we consider his clock to be running slower than ours, because the same event takes longer to occur by his clock.
So a mirror moving away from us at the speed of light would, from our perspective, have a stopped clock. Similarly, from the point of view of the wavefront of a lightbeam, everything else is moving past it at the speed of light, so everything else should have clocks that are stopped. So the lightbeam should be able to get anywhere before those clocks register any time as having passed. So if light appears to require time to get somewhere, it must be because of differences in what constitutes “simultaneity” in different locations rather than because of travel time.
This means that movement through time and space is kind of zero-sum. From my perspective my position is always the “origin”, so I move through time but not space. But the faster I see you moving through space, the more your clock slows and the slower you move through time. Light beams move through space but not time.