3 ms·
So, do I understand you correctly: relative to the "static observer", the rods are both moving "sideways", as if the x-axis of a graph were moving toward +y and
by Steuard 11y ago
So, do I understand you correctly: relative to the "static observer", the rods are both moving "sideways", as if the x-axis of a graph were moving toward +y and the y-axis were moving toward +x?
In that case, the "static observer" would see the point of contact moving at very nearly sqrt(2) times the speed of light (along the x-y diagonal). But that's okay, because no physical object (or bit of information) is being carried along at that intersection point. (If you put some little spindle at the intersection to be carried along, you'd suddenly have Problems.)
On the other hand, from the reference frame of one moving bar, the intersection point appears to be moving almost infinitely fast! If the "static observer" measured speeds u and v for the two rods (as fractions of light speed, so u and v are both close to 1), then an observer moving with one rod will see the intersection point move at speed v/sqrt(1-u^2). If we write u = 1 - ε for some very small ε, then the intersection point's speed in this frame is roughly v/sqrt(2ε). (The intuitive reason for this result is interesting: an observer on one moving rod will consider the other rod to be almost parallel, at a very small angle (rather than perpendicular), and still rushing toward her at nearly the speed of light.)
- madaxe_again 11y agoTold you it was a nice one to muse on! I'm a physics grad, so know the answer as you've stated it. Gotta love Lorentz contraction. :)
- Steuard 11y agoI should've guessed that you had a physics background, since that turned out to be an unexpectedly interesting problem. (The view from one of the moving rods in particular was more subtle than I anticipated. As usual, once I stopped trying to intuit the answer and finally set it up carefully with 4-vectors the answer came out pretty cleanly.) I wonder if there's a good place to use this in my classes. (It was a bit more subtle than I'd usually throw at my sophomores...) What's your field?