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If a pole rotating on an axle is long enough, what happens to the outer point when it reaches the speed of light?

If you attach a straight pole to an axle so that it revolves at a right angle to the axle (like spokes on a bicycle wheel) and spin it, the outside of the pole will move faster than the inside. The longer the pole, the faster the movement around the circumference.

If I built a pole that was long enough, it would get to a point where the end -if spun - should reach the speed of light, or even exceed the speed of light. However, I know that nothing can travel as fast as the speed of light. What would happen to the pole?

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Categories: Our universe.

Tags: relativity, spacetime, rotation, Speed-Of-Light.

 

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JimL status says:

Your question is essentially a version of the situation described by what's called the Ehrenfest paradox.   Here's a summary article on wiki:

http://en.wikipedia.org/wiki/Ehrenfest_paradox

Here's a key point quoted from therein:

"It must be noted that any rigid object from real materials, which is rotating with the transverse velocity close to the speed of sound in this material, must exceed the point of rupture due to centrifugal force. Thus, in the case of speed of light, it is only a thought experiment (The neutron-degenerate matter allows velocities close to speed of light, because e.g. the speed of neutron-star oscillations is relativistic)."

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Tags: relativity, spacetime, rotation, Speed-Of-Light.

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posted on 2010-07-29 18:11:08 | Report abuse

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JonMann says:

Hullo JimL/AudioBlitz,

I've also wondered about this, but having an acute lack of physics knowledge, I've struggled to find out. I've had a look at the link Jim provided and the wiki explanation is sadly beyond my comprehension...

The way I see it:

At any angular velocity, in theory there will be a point along the radius of the pole at which light speed will be reached, and therefore by extension, exceeded. Needless to say, this is rather impractical, but how does the absolute nature of the speed of light account for the possibility, even in theory, that it could be exceeded in such a simple way?

Apologies in advance for the ignorance, but if anyone could explain this in laymans terms, I would be grateful.

Cheers,

Jon

 

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posted on 2010-08-04 10:33:33 | Report abuse


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aturnb says:

When you accelerate any object with mass you actually increase its mass according to the Special Theory of Relativity. This theory also tells you that it requires an infinite amount of energy to accelerate an object with mass to the speed of light.

So, assuming you had a power source capable of supplying infinite energy to the pole and that your pole was capable of withstanding the centripetal forces,  you would find that as the pole rotated faster and faster the mass of the pole would change along it's length until the tip approached that of infinity when it reached the speed of light. As you cannot accelerate an infinite mass any futher if you were to continue to put more energy into the pole you would find that the remainder of the pole would accelerate up to the speed of light. At no time would any part of the pole exceed the speed of light.

Bear in mind as well that long before that happened you would actually have created a black hole at the tip which would have sucked you and your pole into it.

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Tags: relativity, spacetime, rotation, Speed-Of-Light.

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posted on 2010-08-10 19:49:27 | Report abuse


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