What happens when I move a very long bar?

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Faster than light information

Hey there,

I just want to make clear that I'm new here and I don't know whether this is the right place to ask such a hypothetical question. So if it's not, I'd be glad if someone could tell me where to ask. :)

Imagine you and a friend of yours were very far away from each other, let's say the distance is 1 light-year. Now you're holding a bar which measures exactly 1 light-year and currently touches your friend's shoulder. What happens when you move that bar (in order to poke your friend)? Will there be a delay? And what magnitude will that delay have? A year? Several years?

Looking forward to your answers. :)

In my opinion, this is a duplicate of this question: http://physics.stackexchange.com/questions/2175/faster-than-light-information — Marek, Dec 26 '10 at 19:18
Definitely a duplicate. The title is much better on this one, however. — dmckee --- ex-moderator kitten, Dec 26 '10 at 19:25
@dmckee: yes; I like this question much better too. It would be best if we could move answers from the other question under this one. — Marek, Dec 26 '10 at 19:32
@Marek: Nice idea in principle; some of the answers to the other question wouldn't make sense on this one, though. I suppose someone could try to edit the other question a little. — David Z, Dec 26 '10 at 21:14

score 1 · Accepted Answer · answered Dec 26 '10 at 19:15

The change would transmit at the P wave velocity (look up elastic waves). This is typically a few kilometers per second for most materials you would be likely to make a bar out of. You can maximize this speed by going for low density at high strength, something like titanium comes to mind. In any case the result would be many thousands of years.

What happens when I move a very long bar?

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