If I turn the tip of a 100 light years long pencil, the other end instantly turns , or it takes 100 years?

Question

If a civilization could build a strong , lightweight tube, hundreds of thousands of kilometers long , could theoretically use it to communicate faster than light, simply by rotating it at one end , the other rotates at the same time , or not ?

Answer 1

Information travels at the speed of sound within any rigid body; thus your tube will take way longer than 100 years to rotate from one end to the other.

Note that the material with the highest speed of sound is about 12,000 meters per second; this is diamond. Other values are listed here: http://hyperphysics.phy-astr.gsu.edu/hbase/tables/soundv.html

If you were able to construct a material that could transmit sound at the speed of light then it would take 100 years for the rotation.

So the answer is not.