Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: rotating sticks faster than light speed?

Learn more physics!

Most recent answer: 05/05/2013
Q:
Since nothing that has mass can travel faster than the speed of light. What would happen if you had a stick or something that was several light years long rotating from a central hub at a very fast speed? How could the very end of that stick not exceed the speed of light? Or even the stream of light that comes from a Pulsar that rotates at a thousand times a second. How fast would the ends of that stream be traveling?
- Russell Barker (age 53)
Liberty MO. USA
A:
That's an interesting pair of questions, because the answers are completely different.
The ordinary stick simply cannot rotate fast enough for the ends to go faster than c in anybody's frame. As the speed approached c, the momentum would approach infinity and thus the force required to keep the end in circular motion would become infinite. There's no way around it- the stick just can't rotate that fast.

A beam of light can rotate that fast. However, it's not quite an object in the same sense as the stick is.

For example, if you do something to the end of the stick (maybe leave a sticky note on it) the effect propagates along with the stick carrying information. Information can't travel faster than c, so neither can the end of the stick.

If you do something to the end of the light beam it does not propagate along with the beam. The end of the beam is constantly being replaced by new light coming from the source (traveling at c), unaffected by whatever you did to the old end. So no information travels faster than c, even though the spot on some distant screen may travel faster than c.

Mike W.

(published on 05/05/2013)

Follow-up on this answer.