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Q & A: Turning really really long pole

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Most recent answer: 10/22/2007
Q:
If i had a pole that was 300 million miles long,and i was in space,and i held it at 0 degrees then i turned it 90 degrees in one second wouldnt the end of the pole be traveling faster then the speed of light?
- Tim (age 20)
Manchester,England
A:
If you did that, you will find out that your pole will not remain rigid. If you hold onto one end and try to turn it 90 degrees in one second, you will snap your pole if it's brittle, or just bend it out of shape if it's not quite as brittle. Imagine doing this with some cooked spaghetti on your plate. Turning one end by 90 degrees simply puts a kink in it and pulls most of the spaghetti a little closer. Even pulling the pole a little closer cannot affect the whole pole all at once. Mechanical disturbances can travel in poles at no faster than the speed of sound in the pole, which is quite a lot slower than the speed of light. Even if your pole is very rigid and does not break, you will only be sending a wave from one end of the pole, traveling along the pole at the speed of sound in the pole (waves which have side-to-side motion typically travel slower than compression waves in most materials). So the point that you're getting at, that the speed limit of relaticity doesn't fit well with some familiar ideas, like the existence of rigid bodies, is correct. It's the familiar ideas which have to go. Tom (w Mike)

(published on 10/22/2007)

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