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Name: Peter R.
Status: student
Age: 16
Location: N/A
Country: N/A
Date: 3/8/2004


Question:
If I have a pole 1 light year long and rotate it at one end will the other end spin at the same time or in 1 year?


Replies:
Peter,

What do you mean by "rotate" and "spin?" Do you mean the pole is spun along its own longitudinal axis, or do you mean swung in a circle with you at the center and the far end of the pole pointing away from you? I shall assume you mean the latter.

In that case, the far end of the pole would take the same time to make a full circle as the end near you. Consider a CD that contains a scratch that radiates from the center to the edge along the CD's radius. When the CD spins on its drive, both ends of the scratch make one revolution in the same time interval. However, the linear velocity of the part on the periphery is traveling faster that at the center because it must travel further to complete one trip around the full 360 degrees.

Regards,
ProfHoff 822


If you have a pole 1 light year long, you have a very long pole, indeed. It is, in fact, about 9.4 E15 m long. (The E15 means move the decimal place 15 places to the right, so the length is 9,400,000,000,000,000 m. If your pole has a square cross section 10 cm on a side (about 4 inches), it has a volume of 9.4 E13 m^3 (94,000,000,000,000 cubic meters). If it is made of iron, it will have a mass of about 7.4 E17 kg. You will find it difficult to make such a pole and not at all easy to push it around!

But to answer your question. Nothing, including information, can travel faster than the speed of light, so you have to wait at least a year. In fact, the information that the pole was twisted at one end cannot move down the pole faster than the speed of sound in that medium. Since the speed of sound in iron is 5130 m/s, it will take about 58,500 years before the far end realizes the near end was twisted.

I do not recommend this as an extra credit experiment for your science course...

Best, Dick Plano, Professor of Physics emeritus, Rutgers University


Assuming that your pole is not a quantum mechanical pole, it would take 1 year minimum. Depending upon the material your hypothetical pole is made from, a twist at one end will propagate at a speed of the order of magnitude of the speed of sound (+/- 100) and that is much slower than the speed of light. Even an idealized pole the atoms cannot move at a speed exceeding the speed of light, their mass would increase as their speed approached that of the speed of light.

Vince Calder


Peter,

The pole moves as one object. It takes a year for the light from the end other end of the pole to reach your eyes. You don't see the motion until a year after it happens.

Dr. Ken Mellendorf
Physics Professor
Illinois Central College



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