Rigid Rods and Relativity

Teacher question. One of my students has some questions about relativity: 1) If we had a pole that was one light year long, and we pushed it from one end, what would happen? 2) Would we see the pole move before it actually moved? If so, does that mean that the mass is getting compressed along the pole like a longitudinal wave? Does that also mean, to some small degree in our daily lives, whenever we move something, we see it move before it actually does? 3) Would we see the pole move at the same time it actually moved? If so, then mass could travel at the speed of light. so, theoretically, we could travel at the speed of light by being pushed by the pole. 4) Would the pole move before we see it? If so, how? This defies the law that nothing could travel faster than the speed of light.
- Jon Scott (age 49)
Normal, IL USA

Nice questions! We've discussed some of this before: https://van.physics.illinois.edu/qa/listing.php?id=1398.

1. When you push it, the forces that cause it to move only transmit at the speed of light. So it would be a year before the far end would start to move. That means that during that year the rod would have to distort. It can't be rigid.

2. No, you'd see parts of the pole move after hey actually start to move because it takes time for light to reach you from them. Even after you allow for that delay, to figure out when (in your frame) they actually started to move, you'd still see a delay between when you pushed and when distant parts started to move.

3. See (2).

4. The distant parts of the pole would move before you see them move, just due to the time it takes light to reach you. However, they would not move instantly after the push, and they would never move at the speed of light.

Mike W.

(published on 04/01/2016)