Yeah, I remember that. It was kind of a mind-numbing thought. It took me longer than it probably did most to get thru that book, which made me think I was ready for Kip Thorne. Hardy har har! Simpler for me to follow, but exact same thing and we can observe it ourselves in the wild is to replace the light clock in that experiment with a person sitting in the passenger seat of a car tossing and catching a tennis ball a few inches between their lap and the roof of the car. Just a simple idle thing a kid on the way to a game might be doing while dad drives. To the kid, it’s going straight up and down. To someone they pass as they drive by, not so. Pretty cool.Reminds me of Einstein's thought experiment that Hawking talked about in 'A Brief History of Time':
A man bounces a ball on flatcar on a moving train. To him the ball is bouncing straight up and down. Another man is standing beside the tracks and watches the man bouncing the ball as he goes by. To him the ball is flying in a long arc. Both things are true. The ball IS bouncing straight up and down, and it IS bouncing in an arc. All depends on your frame of reference....
Speaking of tennis balls *and* physics, there is such a thing as the “hairy ball theorem”, which can explain and prove why the wind conditions on at least 2 places on Earth will always be identical to one another, and this must always be true.
ETA: I got thinking of and talking about the light clock when in fact you used the same bouncing ball illustration I had in mind and *then* started talking about. Sorry for geeking out.