Speed of Push vs Speed of Light

[PCNerdz]

TL;DR Summary

Lightyear long pole, button activates light on the other end. How long does it take to reach you?

Been thinking about this since I was 12. Suppose you had a 1 lightyear long pole, and on the other end was a button 1 inch away that activates a light. The light is powerful enough for you to see it 1 lightyear away. You push the pole 1 inch forward. How long does it take for the light to reach you? Would you even be able to push the pole 1 inch?

My guess is it all depends on the speed of push, so anywhere from 2 years to many centuries, and it depends on how the pole is set up for the 2nd Question. We will say Gravity, Friction, air resistance and all that is negligible to help simplify it. An isolated system.

I think this requires you to know the speed of push, but I can’t find much on the internet on that subject. An exact answer would be great, but I’m mainly trying to create a discussion to see what some of you think about it.

 

[Baluncore]

Welcome to PF.
It actually depends largely on the speed of sound in the pole.

 

[Dale]

It would be roughly the length divided by the speed of sound in the rod plus one year.

 

[Mister T]

I think this requires you to know the speed of push, but I can’t find much on the internet on that subject.

When you push on one end of the rod, the other end doesn't instantaneously move. It takes a while for the push to travel. This push is just a sound wave pulse.

 

[PCNerdz]

Thanks, I didn’t know the “speed of push” had to do with the speed of sound. This has been in the back of my mind for years as a question I wasn’t sure if I would ever find the answer to. I learned something new today. Thank you for taking the time to reply, have a good day.

 

[sophiecentaur]

When you push on one end of the rod, the other end doesn't instantaneously move. It takes a while for the push to travel. This push is just a sound wave pulse.

These things need to be said.